Hi All

I'm trying to set up my 750 holley 4160 carb for my 355 chevy engine , Reading what Holley says I should divide by two my vacuum reading wiich is 9 in gear ( auto trans) so that would bring me at a 4.5 power valve instead of my 6.5 . But Barry Grant and others says that the vacuum should be what the poser valve shows ... So who's right or wrong?

 

See this sticky for how to tune a Holley. After you've done several other things, you'll probably find that (a) your idle vacuum will be ALOT higher than it is now, and (b) Holley's PV instruction doesn't produce the best results for a street-driven car in this day and age.

https://www.thirdgen.org/forums/carb...ey-tuning.html

 

my car runs rich at idle , eyes burn, smells hard etc... and still it should be a 65 power valve and 70 main jets.. Tuned the carb to get the higher vacuum and ignition is set at 12° initial.. What's wrong then if I don't have to change the power valve??...

 

Now that you've completed all of the steps I outlined in the "sticky", what were the results? Any better?

 

Thanks lot for your help!!

Ya I've read it all! And sure that's sounds to be the right path! Now I'm looking how to adjust those transfer slots..Is there any screw to adjust those 4 together???

 

No....

There are only 2, there aren't any transfer slots on the sec side. What you have to do, is set the sec throttle return stop ("idle speed", as it were) such that the primaries are within their correct range (.030" or so of the slot showing) when the curb idle speed is set correctly.

On carbs with 4-corner idle, you end up having to drill holes in the throttle blades to do this. I don't know offhand of any street carbs that have that though.

On some carbs, it's possible to turn the screw around backwards, and get a tool on it from above. Depends on how the linkage is made.

Holley's instructions and factory calibrations SUCK. They're leftovers from the 60s, when people would buy a HUGE carb and a HUGE cam, and slap it all on a low-compression 283 or 327 or 260 or 289. AFAIK they haven't been updated in 35 years, that I've noticed anyway. They're sort of implicitly assuming that everbody that buys their product is making that set of classic hot-rodder mistakes: too much cam, too big of a carb, not enough compression, in a HEAVY car, coupled with cheap gas and generally shorter drives (cities smaller and suburbs closer to them and so on) so nobody cared about mileage like they do now. What inexperienced people typically built back then, was a motor that made NO vacuum, had NO leave, and a weak "pull" on the venturis at cruise, but ran like a bat outta hell when you stomped on it. The calibration that Holley supplies ends up therefore being WAY rich at cruise (to make up for the weak "signal") and has too low number of a power valve (to make up for the low cruise vacuum), and drank gas like a hole in the bottom of the tank. But, at least it RUNS at all, when put onto a motor with the above combo mistakes. The end result is that the plugs run black, the idle is burn-your-eyes stinky rich, fuel consumption is measured in gallons per mile, and there's a LLLLLOOOOOOONNNNNNNNNNNGGGGGGGGGGGG flat spot when you give the car light throttle at cruise as well as a flat spot or a bog when taking off lightly from a stop especially with an auto trans (although it'll make a sort of stumble when you let out the clutch on a stick).

Sound familiar?

 

man.... I'm impressed... Looks like you know my car..!!!! , So ifI'm following what Holley says With my actual 9 vacuum I should change my 6.5 power valve for a 4.5, leave my 70 jets where they are and change the black diaphragm spring for a purple or plain . But so funny that they don't say a thing about the transfert slots ... Tell me please what is right or wrong in those intructions please!!

 

Yeah I've seen something like what I describe happen once or twice before, maybe.Everything.

Ignore them. Follow the steps in the "sticky", and go from there. Don't worry about the secondaries until you get the primaries right. But yes, most engines will eventually turn out to run best with the black, brown, or plain spring in them. Especially if the carb is on the large-ish side, which yours is. But you're not ready for that yet. Dial the primaries in first.

 

Thanks again for your help!

OK , so far tell me if I get it wrong , so you say First step will be my jets , I have now the 70's So I have to lean those down as posible and then go with the power valve that as you say I will probably then have to pick up a bigger one then the 6.5 I have now and that shoudl stop the burn eyes effect right? .

I'm surprised that you never mention the use of a vacuum gauge..

Then I have to go with the transition slots and here I don't get it ... , Under the carb ( under the diaphragm ) should be a little screw right? I have to adjust the secondary blades with that screw to put between 0.20 to 0.40 " ??? What about the primary blades????

Then I Should end with a choice of primary jets for the best 1/4 mile?? Then what about the first tuning ???

Sorry for all those questions , I'm just not as used to those stuffs for sure as you are!

 

If your vacuum gauge is showing 9 in./hg. at idle, you will be fine with a 6.5
PV.

That's pretty much perfect as a typical performance calibration is 2 in./hg.
lower than idle vac.

If you install the 4.5, the fuel curve will be delayed and it will cost you
response and power.

The rich idle has nothing to do with the PV selection unless it's faulty, or
too close to the manifold pressure.

You should focus on the idle mix, emulsion, idle bleeds, float levels first
as these are part of the main idle circuit.

High pressure (floats set too high) can cause a rich idle. incorrect air
bleed size will do the same.

When I set up carbs that are far out of adjustment, the first thing I do is
disable the secondary side, and start with the primaries.

The transfer slots should show about 0.020" for initial setting. The slot is
about 0.020" wide, so you will see a square when it's set close to where it should be.

The primary float level should just creep below the site glass/threads on
level ground.

Make sure the inlet pressure is not more than 8 PSI. If the fuel spills over,
you could have a bad needle/seat assembly, or a hole in the float. This
will also cause rich conditions.

When setting the idle mix, adjust for highest vacuum. You'll have to go
back and forth between sides to get it right.

If the cam has a tight LCA, you might want to disable vacuum advance to
stabilize the timing. Increase the base timing, and check total for best
power. You don't want to over-shoot total timing as you increase the base
timing. The mechanical advance assembly might need tweaking to get it proper.

Too much advance, or fluctuating timing at idle can be the cause of the burning eye sensation. Use the ported vac on the carb, and adjust the
vac cannister (if equipped) to keep vacuum advance for the street.

 

Right:

Like the sticky says, tune on the main jets until the car just barely begins to surge in high gear, at relatively low speed, about as slow as you would go in high gear (in OD & TCC engaged if auto), maybe 50 mph or so; keep leaning the jets down (smaller) until the car starts to surge. Go back up 2 jet sizes from there.

Yes you will ALMOST CERTAINLY need a higher number PV than a 45, or even a 65; more likely, if you have anything resembling a street cam, it will end up being between 85 and 105.

That sticky is composed of pieces copied and pasted from a couple of other posts IIRC; I may have mentioned a gauge in one, but it just not got included in the sticky. In the idle speed & mixture part, use it to set your idle mixture, for the highest vacuum.

Yes, under the carb, maybe under the diaphragm but more likely on the other side, the sec linkage will come to rest against a screw, that is accessible from under the carb. The goal of that is, to adjust the sec throttle opening so that the idle speed is correct when the pri blades are in their correct relation with the slots. It'll take a couple of tries: when the idle speed is right, take the carb off and turn it upside down; adjust the pri idle speed screw while wtching the pri blades and observing how far the blades move; if they move, say 1/16", then adjust the screw on the sec throttles to move the blades the same amount, whihc will thereby let you close the pri throttles but the idle speed will stay the same. It may take a couple of tries. This is why it's handy to turn the screw upside down, if your carb allows it, so that you don't have to take the carb off every time to make this adjustment.

You'll probably find tha tthe idle speed and mixture interact somewhat, so when you get the slots right, adjust the mixture again, and if the idle speed is different, re-set it with the sec stop screw, and then go back to the mixture to fine-tune it again, then the idle sleed, etc.

Then when you get done with the primaries, tune the secondary jets for best mph in the ¼; and choose the spring such that if you floor it from a 2000 RPM roll in 3rd gear, it accelerates smoothly and evenly up to 4000 RPM. If you start with the black one and get a feel for it, then put in the brown one, then the plain one, then the purple one, then the green one, then the yellow one, it'll become pretty obvious what "right" and "wrong" feels like, and which one is best.

Incidentally, this is all assuming that you have your ignition timing set up correctly; which would be, about 36° at 3000 RPM with the vac advance hose disconnected from the dist. Make a note of what the timing is at idle then: that will be your new "setting". Could be anywhere from 10° to 18°; doesn't matter, except that whatever it is, that's "correct", not whatever the "spec" book calls out for in a stock motor. If you hook the vac advance hose back up with the engine at 3000 RPM, you should see the timing jump another 12° or so, to near 48-50° BTDC. But, that 36° at 3000 RPM with no vac advance, should be THE VERY FIRST THING you set, even before touching the carb.

The slot relationship is what causes the burn-your-eyes idle; the large main jets is what causes the black plugs and high fuel consumption; the too-low PV is what causes the flat spots; and the wrong spring will make it either run too rich in the mid-range of RPMs because the sec isn't opening, or run too lean and bog or sputter when the sec opens, from opening too soon.

 

Man ... reading you guys make me feel crap... , Ok got it !!!! Just my last question please ,wich screw moves the primary blades? Loved the last lines explaining wich part gives wich problem!!!!

AGAIN !!! a HUGE THANK!!!!!!!!!!!!!

 

It's the big screw on the throttle linkage next to the accelerator pump cam
(driver's side of the carb).

EDIT: Here's a good picture. It's the screw near
the middle of the carb angled backward

 

Ohhhhh OK so that's simply with the idle screw then. So then better never touch then this one once we've set up those transfer slots right and adjust idling only with mixture screws and vacuum gauge . ?

 

Incidentally, note that the carb in the pic, is a HP series carb... not a street one at all, pure racing.... note that it has 4-corner idle. Idle mixture screws in both pri and sec metering blocks and downleg boosters and interchangeable fuel inlet fittings. No choke either.

If your carb has a sec metering plate instead of a block (sec fuel bowl up against the main body, instead of looking like the pri side) then tuning the sec side is a little different. Don't worry about that until you get there though.

The sec throttle stop screw is a little bitty thing, either a small Allen head or a small slot. Doesn't even have a "head", looks like just a short piece of threaded rod screwed into the casting.

Right; once you get the transfer slot relationship right, avoid disturbing it if at all possible. Usually not a problem anyway, as the idle will be QUITE stable once you get it set up right. There won't be any need to be moving it.

 

My carb is just the regular Holley 4160 750 , vacuum secondary ,electric choke , ok so I'll go from there then, I'm impressed that you guys go all opposit way then Holley tech support!! But your way sounds much more logical to me as a power valve at 6.5 would only open at 6.5 vacuum and my idle is only 9" in gear ( TH 700R4 ) so what would be the improvment to be running a 4.5 instead of 6.5 as both would anyway be closed with a 9" idle.. , but primary jets 70 on a 750 carb for 355 ci sounds logical to lean it down a bit wich would not be needed on a performance 406 ci , I'm I right here?

 

Yes, for the most part. It's all inter-related...one thing may affect the other.
You may need to tweak other circuits if it's way out of adjustment.

It depends on the fuel curve, jets and bleed sizes. The power valve responds
to the manifold pressure. That additional fuel is added to whatever is
flowing from the idle and/or main cicruit at the time (+ accel pump shot).

Generally when you hammer the throttle, the pressure change is so sudden
that installing a power valve that is way under the manifold pressure will
result in a lag for power enrichment.

The closer the PV number is to the manifold pressure, the sooner the fuel
comes in.

Keep in mind, the Holley site is probably steering you toward streetability
and economy whereas I am (we are) tuning for best throttle response and
power.

Holley guidelines work well for baseline tuning. For performance, you have
to toss all that stuff out the window and test on the dyno and track.
I'm not sure what you mean here. The 406 would demand more air per
stroke which requires more fuel to make the same (or more) power.

The same carb on a bigger cube motor might need less jet at cruise,
but more at wide open...because the fuel curve will begin to flow sooner
due to the lower pressure created by the bigger displacement.

I'm not sure if that makes sense, but a longer stroke creates faster piston
speeds at the same RPM. This lowers the manifold pressure and increases
the air speed through the venturi.

Faster air speed through the venturi , means a lower pressure in the
venturi. ..and that ultimately means the fuel is presented at a higher
rate.

So, you could have proper jetting at one RPM, and throw off the ratio
at higher RPM. That's where the bleeds come into play. The jets and
air bleeds work as a team to dial in the fuel curve for each circuit.

Bigger bleed + bigger jet = later fuel response at a higher RPM.

Confusing? Yeah! If you stuck a wideband on the exhaust, you could
play with these parts and it would click a lot faster for you.

 

All of a sudden I'm more confused.. , My goal is maximum power out that engine not dying inhaling those fumes ....and blowing out my bank account on gas as this car runs now about under 10m/gallon.. I'd like a car steetable and powerful but no racing , so shoudl I go with Holley or what you guys tell me???

 

The Holley is fine. Sorry to confuse you.

All of the tips throughout this thread will give you better idle quality
and reduce the rich condition.

If you tell us what is confusing, we can try to help further.
----------
Here is condensed version of my last post:

Yes, for the most part.


The closer the PV number is to the manifold pressure, the sooner the fuel
comes in.


YOu would have to check the AFR in the RPM range you want to tune for.

Different size motors will change how the carburetor responds with jetting.

 

Ignore Holley's instructions. They are wrong. They will produce an engine that runs rich at cruise and blackens the plugs, has burn-your-eyes stinky rich idle, has a flat spot off idle, and drinks gas like somebody punched a hole in the bottom of your tank. IOW, you'll end up right back where you are right now. That's how you got here in the first place: your carb is "calibrated" that way now.

Follow the sticky.

You might want to read this document http://www.holley.com/data/TechServi...ech%20Info.pdf It's Holley's "how it works" document. Once you read it, you'll see what the problems are with the instructions they give you: their tuning "instructions" aren't even consistent with their own functional description.

 

I'm going with what sofakingdom vbmenu_register("postmenu_3637469", true); said as I'm really impressed by his explanations and every one and single thing said is exactely the problems I have on the car ! Will get some Holley parts and begin from there , having those 70 primeray jets now wich numbers shouldl I begin with?

HUGE THANKSSSSSSSSSSSSS TO ALL YOU !!!!!!!!!!!!!!!!!

 

You'd think, if you didn't know better, that I might have seen this before, maybe even more than once...

From the "sticky":So that means if you have 70s now, you need 68, 66, 65, 64, and 72. Although, instinct tells me you likely won't be needing the 72s.

You'll be having to eventually buy some other stuff too, notably some power valves, and maybe some gaskets. (they can be hard to get off sometimes) YOu might want to look into the Holley "trick kits" such as this one http://store.summitracing.com/partde...HLY%2D37%2D933 because even though it might look expensive now, by the time you finish buying al the small parts you'll need throughout the process, it might end up saving you A TON of money. I keep all the pieces in stock for my own use, but for somebody who doesn't have that built up from their history, it's a great place to start. Doesn't have EVERYTHING, notably not a wide selection of power valves; but it has ALOT of everything else.

Put a bit of motor oil or spray silicone on your gaskets when you put it back together, so the glue stuff on them doesn't stick. The blue ones don't have the glue so they don't need that. If yours stick, MAKE SURE you get every last little bit of that gook off, but DO NOT under any circumstances scratch or gouge the metal!!! Also, don't overtighten the fuel bowl screws; that'll bend the corners of the bowls in, and they'll be a pain to get to seal forevermore afterwards.

Lukn's description of what the PV does is exactly right. Think of it as merely a valve, which remains closed whenever the vacuum is high; but then as vacuum drops, eventually its spring overcomes the pull of the vacuum, and it opens. Then, it adds fuel in "parallel" with the jets, through an orifice that YOU can't change. Ideally, that orifice should be about 20-25% of the total flow of the main jets, but that's fixed in mfg of the carb. All you can change, is the point in the vacuum range at which it opens, by changing the valve. It doesn't make the carb "rich" or "lean", as such: all it does is change the transition point in the carb's operation between lean (cruise) and rich (power).

Also, as he says, a wideband AFR will tell you , in numbers, what changes you need to make, and how much. For instance, if at WOT when the AFR needs to be about 12.8, if you see 14.1, then you know that your AFR is about 10% lean; so you can simply increase your jet size by 10% cross-sectional-area, which is of course proprtional to THE SQUARE of the jet size, by 10%; and you'll probably come out right on the money.

Without the gauge, you have to do it by the car's driveability. IOW, listen to the engine, learn its language, and let it tell you what it wants.

Get your parts, run through the process, and let us know how it turns out.

 

man..! You guys are GREAT!!!!!!!!!!! , By the way I'm in Canada Toronto area and we still can't order on line anything from here so has to come from US or we have to pull 30 miles to the closest performance shop to get our parts..Sucks..I'll begin work on that carb this WE and sure let you know !Talk to you soon!!

 

OK guys I'm back!

First I've checked the fuel level on both bowls with car idling and were perfect so did not have to touch here . Then openned primeray bowl to be sure wich jets and power valve were in and they are stock 70 jets et 6.5 valve. I then took off the carb and looked under to check the transfer slots and was surprised to find that secondaries were totally closed,so I've tried to get those 0.20" by screwing in the little adjustment screw to opem them but even at the maximum it was showing about 0.5 to 0.10", primeraries were about 0.40" , I've adjusted them to 0.20 with the idling screw and put back everything . Fired up the car and could not get any corerct adjustment with mixture screws without touching the idle screw so went back with an adjustment with vacuum gage and again the same hard gas smell ( burn eyes) etc... , OK so nect step sounds like I'm gonna change those main jets from 70 to 64 and 67 to see what it does but then if no better what lese can I do...????

THANKSSSSSS!

 

Right: OTOH I'm not the least bit surprised. Pretty typical actually. That's why it's a mess.

Don't worry about how much of the secondary one is showing, it doesn't work the same way. The PRIMARY one is the one you want to get to .020" - .030". But you're not ready for that yet.

Follow the procedure, as outlined in the "sticky". Primary main jets first; then primary power valve; then PRIMARY transition slot by adjusting the SECONDARY throttle stop.

The deal is, idle air can be let into the engine partly by the pri throtlles, and partly by the sec throttles. You want to open the sec ones far enough that when the speed is right, the pri side is in the correct relation with the transfer slot. Which is .020" - .030" showing; the slot should be more or less "square" or only very slightly taller than it is wide. Note that that's .020", not .20". .020" is between 1/32" (.03125) and 1/64" (.015625"). Not very big. .030" is almost exactly 1/32". About the diameter of the wire of a paper clip.

 

OK so then I'm gonna get my hand on some pairs of jets beginning with 64 , sorry I didn't get what you mean by that ??? " then PRIMARY transition slot by adjusting the SECONDARY throttle stop ".

 

just go buy a holley jet kit for $30 it'll probably be cheaper than buying 4 different jet sizes at $5-8 a set

 

 

So far I can perfectly adjust mixture screws to read the highest vacuum and the engine would rev perfectly as well as idling great but it will still smell gas terribly and kill me on gas...So something must be wrong somewhere...

 

What are your cam specs? Maybe somebody with more experience can chime in, but I think a cam with lots of overlap will give you crappy gas mileage and that eye burning sensation from all the unburnt fuel getting pushed out when both valves are open at the same time. Just a thought.

 

I just had my first power valve failure in a 650 Demon mech. secondary. I found this thread and followed the sticky and I am impressed. I wound up with a 95 pv and it seems to have fixxed some off idle light throttle issues I have always had. Pretty nice write up though.

One question though, I set the timing to 36* total at 2000 rpm with the vacume advance unhooked.(Only 10* mech advance and it comes in pretty fast) When I hooked the can back up I have 55* at idle and about 65* at part throttle cruise. How much is too much? Do I base that off of any detonation? There is a cam deal that goes on the end of the vacume advance can that I can limit it with, just not sure what I should be aiming for.

 

36 total at 3500rpm not 2000

 

Right now the base is 26* with 10* mechanical advance. When I modified the distributor to limit the mechanical advance the weights never open up very far and none of the weight springs work right. I welded the slots so that basically it only has the first 10*, if I was to do it again I would do the opposite and use the last 10* of advance. Then the springs would work right. This was never ment to be a long term solution, just a free experement to get the timing right until I buy an aftermarket distributor. That was about three years ago and short of the advance coming in real fast, it works fine.

 

well if you have 26 at idle, you would want another 10-15 out of the vacuum can. Sounds like you have 22+ of vacuum can advance which is too much, but I guess if it doesn't ping it works for you. 65 is WAY too much, maybe 50 at the most at light light load.

 

Thanks, that's kinda what I'm thinking. I think I'll pull the cap and put that advance limiter on the vacume can. Limit it to around 15*.

 

For the question above regarding cam specs, I would suspect the duration
and lobe centers are quite tame for having 9 in. vacuum at idle in gear.
This sort of reading would not be suspect to having excessive overlap and
charge getting pushed into the exhaust.

You would base the full advance timing on best average HP (or peak HP) on
a dyno, and highest MPH through the traps on the track.

For cruising, and part load, the vacuum advance can be quite high as the
throttle blade angle is not allowing much charge into the cylinder (therefore
less peak cylinder pressure and less chance to detonate).

You can base your part throttle timing on detonation, fuel economy and
exhaust temperatures. Too much timing at cruise will cost you miles per
gallon, and not enough timing will increase the exhaust temperatures.

Without any sort of measuring device, or monitor you'll have a tough time
determining where to set the amount of vacuum from the cannister. The
most important is the total timing at wide open. This must be checked
with the vacuum advance connected and disconnted depending on which
port the vacuum line is connected to.

As the throttle blades open , the manifold port (manifold pressure) will decrease
and so will the vacuum signal to the cannister. This means, as the throttle
blades open, the vacuum advance will roll off and the mechanical advance
increases. If you set it up properly, the cross over point for vacuum roll
off will be close to where the total mechanical advance kicks in ~ 3000 -
3500 RPM.

Alternatively, if you have the vacuum connected to the "timed port" which
is above the throttle blade, then vacuum signal to the distributor increases
as mechanical increases until a certain point near wide open.

This is because the "timed port" (< I hate that term) takes the vacuum
signal at the venturi , not the manifold.

I can draw a graph if I confused anyone with my explanation.

 

The timed vacuum port isn't a venturi vacuum signal.

 

That's just WRONG, plain and simple.

The ONLY thing besides the venturi itself, that uses venturi vacuum, is the vacuum secondary actuator on Holley carbs and similar designs. That diaphragm opens the secondaries as the "signal" in the primary venturis, which is a measure of flow, nears the capacity of the primaries.

The "timed" port takes vacuum from a port on the side of the throttle bore that is located just above where the blades sit at idle, such that there will be no vac adv at idle, but rather, only when the engine is at some kind of working speed.

There is also no "relationship" or "taking over" between mechanical and vacuum advance. The 2 systems do different things in response to different aspects of engine operation altogether. Centrifugal (RPM dependent) advance is based on the simple fact that combustion requires some finite non-zero time to occur, and so must be started somewhat before the time at which power i needed, which is of course just as the piston passes through TDC. Vac advance is helpful because combustion occurs even MORE slowly with the small amount of cyl charge that occurs during cruising (high speed, throttle only slightly open, high vacuum, and lean mixture besides). That part is at least correct.

And, the very REASON for vac advance in the first place, is gas mileage. Having too much will not "decrease gas mileage". That's WRONG as well. That won't happen until there's SO much that you are LONG PAST the point where other problems have already occurred and you've already had to back it off; notably, pinging while driving down the road at a steady speed.

Virtually all gasoline engines will produce max power with the spark at somewhere between about 32 and about 38° BTDC. Higher compression and more efficient chamber shapes reduce the required amount; low compression, dome pistons, and other inefficient chamber shapes, require more. A typical flat-top SBC requires about 35-36° of "total" (static + mech) timing.

A typical "good" high-perf curve is about 18-20° of "static" advance (the kind you get by twisting the dsitributor around) and about 15-18° of centrifugal advance (this amount is determind by the length of the "slots" in the mechanism, the shape of the weights, and the shape of the cam they work against). This gives about the "ideal" 35° once it's "all the way in". The curve should start just a little off idle, maybe 1200 RPM or so, and should be "all the way in" by 3000 RPM at the latest.

Vac advance should be set to disappear as soon as the vac drops due to the engine trying to produce power. You can use as much as you want, up to nearly the point that it starts to ping when driving down the road at the lowest speed you can reasonably go at in high gear, and you go up a slight incline. Factory settings will produce around 48° of the sum of all 3; static, plus centrifugal, plus vacuum.

 

Settle down boys, and read again.

If you interepreted "at the venturi" to mean the booster area I stand corrected.

Otherwise, every carburetor I've tuned over the past 12 years takes the
timed port just over the throttle blades.

The signal here is atmospheric pressure - manifold pressure (and any pressure
drop loss through the venturi channel (IOW carb 'throat')

 

The signal there is atmospheric pressure until the throttle blades have opened enough to expose the port to the manifold, at which point the signal snaps up to full manifold vacuum and then decreases with throttle opening just as manifold vacuum does. It doesn't continue to increase until near WOT.

 

I respectfully disagree. There is a relationship between mechanical and
vacuum at part throttle as vacuum is still present.

You can easily measure this and plot it with a timing gun vs. RPM.

In order to tune the mechanial rate of advance, and when the vacuum
advance rolls off, allows better throttle response, acceleration and power.

That is why we can adjust initial (base) timing, mechanical rate, and
vacuum at the distributor cannister.

I don't agree with this, or maybe it needs to clarified. Are you stating
that mechanical advance should be adjusted so that the spark iginites the
mixture "as the piston passes through TDC"?


OK.

I disagree once again. If you ignite the mixture too soon, the peak pressure
will work against the piston rise. This will reduce the piston speed and
increase the work required to complete a cylce. More work requires more
engery, therefore BFSC #'s will rise and/or fuel economy will decrease.

There is a point where too much advance is working against you.


OK

Mostly agree, but from what I've seen it's fairly consistent with combo's
I've worked with.

Somewhat disagree based on my points above about BSFC
----------
not entirely correct.

The signal is slightly less than atmospheric as there must be a pressure
drop to move air and fuel into the manifold at idle, or part throttle.

Bold section:

There is a slight increase until the pressure above the blade equals the pressure at the port.

At a certain throttle angle, pressure above the blade and below the blade
will be equal making the port 'dead'.

 

Thanks sofa, I think I'll still drop the total from the vacume can to 15*. I'll aim for 50* total for my part throttle high vacume cruise. I don't trust myself to hear any faint detonation, and I have a bad habit of driving it like a diesel on the highway.

 

Right.... ANY detonation is bad, and even a little pinging is to be avoided if possible.

A strategy you might want to try, is to tune the vac adv for max vac along a familiar, repeatable stretch of highway; since max vac = best efficiency, under cruise conditions. If that turns out to be within a range that passes the smell test (say, between 10 and 16°), then you can use it as a sanity check, if nothing else.

It's possible to tune the main jets the same way, but there are other things that affect them and their "ideal" setting, so it's harder to really be sure you get those right under all circumstances that way.

 

Do you mean pinging as in detonation, or is there another term to be defined here?

 

Pinging = spark knock = pre-ignition
Detonation = something else all its own

 

How about :

Detonation = Pinging = spark knock = knock retard
pre-ignition = something else all its own

I'd also like to get your clarification on the questions above.

 

Sorry, I have to agree with everyone who disagreed with you for the reasons they gave.

 

Who are you referring to, and what reasons are you agreeing with? There are a few discussions and topics is play right now between timing, definitions, and
carburetor ports.

 

I agree with the others who disagree with you. The clarification is in the explanations they gave.

There is a tendancy (of which I'm probably guilty from time to time, too) of equating ping and spark knock with detonation.

 

You would be correct to equate ping and detonation.

The 'ping' reference received its name from the sound of detonation which
always occurs after ignition has begun (whether from pre-ignition, or a spark).

Pre-ignition occurs before the ignition system fires the plug. This sounds
more like a thud as opposed to pinging.

If you think about the advance of the spark whether it be 30 degrees, 36 degrees,
or more...pre-igntion would happen before this point. The piston is moving upward
on the compression stroke and the gas is expanding quickly. This sort of reaction
can bend connecting rods and seriously damage the motor.

Detonation occurs mostly within the chamber under higher pressure and
also creates a quick, sudden pressure spike.

Any variation of the definition "ping" comes from late model technology such
as the" knock sensor" hence the related term "knock retard", or "spark knock".