Damon

It can't. But if you have only 10* vacuum advance and use a more performance-oriented 12-16* intial timing setting, it can work beautifully.

The difference between a performance advance curve and an emissions advance curve is that for performance you set the intial and centrifugal for best WOT scoot. Then you add just enough vacuum advance to get part throttle optimized. 50-52* total is a bit too high on most engines (unless they have low compression and/or run a functional EGR system). For a typical performance small block with adequate compression for the cam used 45* true max total is a better mark to shoot for.

Example of "typical" performance advance curve:

16* initial
20* centrifugal (all-in by about 3000 RPMs)
36* total timing (16* intial + 20* cenrifugal = 36* total)
10* max vacuum advance (RATE of advance vs. vacuum dependent on application)
36* total + 10* vacuum advance = 46* true total maximum advance

skinny z

Presently, I run between 14 -16 degrees of initial with another 18 in the distributor. It appears my heads/cam like about 34 max spark lead for max power. My vacuum can provides another 10 (somewhere north of 10" of vacuum) for a cruise total of 44.
That's providing I can cruise at or above 3000 rpm which is the point where I've dialed in my max centrifugal advance. Problem is, I cruise at 2500 so I'm leaving fuel economy on the table as my cruise total is certainly less than 44. Maybe more like 40 (I have to get my distributor back on the machine and check the curve).
This is why I'd prefer a vacuum can that has more than 10 degress available, has the adjustability to set the amount and rate and run it on ported vacuum. With a more cruise friendly 15 degrees of vacuum advance, running full manifold vacuum will give me more than 30 degrees of idle advance provided I tune the can to be all in below my idle vacuum. All this and staying out of spark knock is a lot of juggling.
I will say that I'm going to experiment with full vacuum this season and make a choice based on how it compares to what I run now.

bestracing

Our 388 SBC that was in our 67 Camaro ran best at 34 degrees total timing and it was all in at 3500 and we hit the traps at 7K. With the red bushing we had ~28 degrees of mechanical timing so our initial timing was set around 6-8 degrees. I've only adjust timing on the race motor by total timing and we ran that motor for 6 years straight

skinny z

Strictly a racing application I'm guessing. I can see how that would work when your rpms are rarely below 3000 or so. By the time your up on the convertor or have the clutch in with the revs up, you're at or close to your max spark advance.
Do you run any vacuum advance in the race car? (Pardon me if you've posted that you did, I haven't re-read the previous posts lately) It's very helpful when idling around the pits and coming down the return road (when connected to full manifold vacuum that is).

bestracing

The race car was strictly mechanical advance. The cam in the 388 had a 106* LSA, 0.506/0.519 lift and duration of 240/245 @ 0.050, flat tappet hydraulic. When we tuned the motor on a Mustang Chassis Dyno we averaged around 480 lb.ft of torque at the rear wheels with a 4.56 rear gear, PG trans and Coan 5500 stall.

oamhmad

iTrader: (1)

Update:

I installed a 1/2" wood spacer along with my carb heat shield and no difference!!. I still get erratic idle, except now the float bowls actually feel slightly cool to the touch when this happens so I doubt its fuel percolation.

Now here is what is interesting. pulling the PCV valve out the valve cover and blocking it with my thumb completely smooths out the idle. The PCV valve is open at idle causing a huge vacuum leak.

Can someone please confirm that the PCV valve should not be letting vacuum escape at idle?

I've tried a new PCV valve from a 96 v6 camaro as someone suggested earlier and it does the same thing. Shouldn't the valve be closed at idle when the vacuum is high?

camarito

PCV doesn't completely shut at idle I've took one appart and the way its made it will always flow.
Do you have it connected to the carb or the manifold?

oamhmad

iTrader: (1)

It's hooked up to a large port at the base of the carb. It gets full manifold vacuum.

skinny z

Where and what are you using for a breather?

oamhmad

iTrader: (1)

Breather is on the passenger side valve cover. It's just a 3" aluminum tube inserted into the valve cover hole with a gromet and a 8" length of rubber hose pushed onto the open end of the tube The hose tucks back towards the firewall and has the plastic filter piece that sits inside a stock air cleaner on the other end.

I couldn't find a breather that would fit the gromet in valve cover.

pancherj

What vacuum is your motor producing at idle and what engine was the PCV for? The PCV shouldn't be that active at idle (blow-bye is minimal). it should really kick in at higher engine loads (when the manifold vacuum really drops). My engine idles with 12" of vacuum. I use a valve from a 1967 Corvette L88 (can't remember the NAPA part number off hand). This seems to work real well. If your engine is making over 20" at idle, just about any PCV should work.

oamhmad

iTrader: (1)

The motor pull 21" of vacuum at idle. The pcv is the stock part for an lg4 carbed v8. I think I've just about tried everything to sort this out from cleaning out the carb, to looking for vacuum leaks to insulating everything from heat.

camarito

this carbureted engines don't like to idle any leanner than 13:1 i tried on mine and it does the same thing, i keep mine at 12.5:1 in park and it drops to 13:1 in drive, it runs good like that

skinny z

This sort of thing is obviously difficult to diagnose without actually being there to observe and test.
If it were me I would try two things. One would be to get the breather plumbed into the base of the air filter. I've had excellent results with this sort of closed system. It's also very OEM. The other would be to use ported vacuum for your advance and adjust the initial timing accordingly (even as an experiment as I understand you're convinced that full manifold vacuum is the way to go in your case).
Do you observe a change in idle vacuum with the pcv disconnected and the valve plugged as you've done?

oamhmad

iTrader: (1)

I've tried ported vacuum over an extended period of several weeks. It didn't affect the issue, that I could tell anyway.

As for changes in idle vacuum with the PCV disconnected...it depends. When the idle is surging and doing its unsteady thing, the vacuum fluctuates with it. Blocking off the PCV port on the carb completely smooths out both the idle and the vacuum. When the idle is steady and not acting up then blocking off the PCV raises the vacuum a tiny bit.

oamhmad

iTrader: (1)

Yeah I'm almost beginning to accept this reality. Problem is MPG suffers at 12:5 or so. What kind of mileage are you getting?

skinny z

My idle afrs are in the 14:1 range. I shoot for 14.7 but it will vary. That's at 750 rpm more or less. It depends on the amount of heat soak the engine has.
I try to tune for 16:1 cruise but the old Holley Street Avenger doesn't have enough adjustability. Hi way mileage is 20+ while in-town/ mixed is less at around 15 mpg.
For the record, with the previous combination: 10:1 scr, 16 initial timing, 18 mechanical, 10 vacuum through ported vacuum. Cranking pressure was 185 average. Idle vacuum around 12" .
How much do your afrs change when you have the pcv plugged vs unplugged?

camarito

I get about 17mpg average but I use 66 jets on the primary wich gives me 14:1-16:1 depending on load and a 10.5 power valve to compensate for the small jets so if I drive it easy its not bad and as soon as I get on it a little the power valve takes it to 13.5:1 I kept the secondary stock with 78 jets for a total of 13:1 wot, it works nice like that but I can't figure out why I doesn't like the lean AFR at idle.
I'm beginning to believe that does fluctuations are lean miss fires

skinny z

Discussing this with one of my friends (whose been in the hot rod game for almost 40 years) and the first thing he said was you have the wrong pcv valve. I had mentioned in this thread about changing the valve but was with respect to an engine that has poor idle vacuum. That isn't the case with you. Perhaps a valve that has the ability to work with 21" of vacum is the key here.
You haven't posted the difference between the afrs with the pcv connected and plugged. I doubt that you have a lean misfire condition (unless your plugs are the wrong heat range and they foul up at idle).
Camarito makes an interesting point when he mentions the power valve. What is yours and have you checked it's condition?

bestracing

I started to think back about the timing ported vs manifold vac advance and I remembered that my 72 Camaro, when I first got it, had the vacuum advanced hooked up to manifold vacuum because I had to disconnect the vacuum line in order to get the base timing set otherwise it would be way too high. (this is back in 1985 so I wasn't really thinking about it much when this discussion came up) Just an interesting observation.....

oamhmad

iTrader: (1)

When the idle is acting up after sitting in traffic for a while for example, the wide band shows the AFR going up and down in sync with the engine surging. So it may go back and forth between 13 to 15, spiking at 16. Now disconnecting the PCV at this point smooths out the idle and the AFR settles back down to 13-13.6.

The power valve is a 9.5, and yes I know its working from looking at the vacuum gauge. The engine really likes a 10.5 better but driving around with a vacuum gauge I realized I was dipping into the power valve too often so I went down to 9.5 in the interest of MPG. Not that it made too much of a difference, I'm only averaging 9-10 in the city. I envy you guys getting 15-17.

I'm beginning to wonder of if the inefficient low compression heads and tiny stock cam shaft have something to do with it. Where could I find a pcv valve designed for 21" of idle vacuum?

camarito

I FIXED MINE...LOL
It was not the engine that didn't like to idle lean, it wasn't the PCV valve nor vacuum problem.
One of my friends mechanic/engine builder came over today and took a look at it and said: "This carburetor come set up to idle at 12.5:1 and the only way to lean out the idle without drive-ability problems is to drill the idle air bleeds"
Sure enough we drilled all 4 with a 5/64 drill bit and now she idles nice at 14:1+ no stumble or fluctuation.
I'll drive it to work tomorrow and report

oamhmad

iTrader: (1)

Is that so? Which Holley carb do you have? I'm almost tempted to come see it since I'm in North Jersey as well.

pancherj

I had to do something similar on my Mighty Demon to help lean the idle out. However, 5/64" is pretty big!! I ended up at .072" _.070" was stock) for the IAB's and I leaned the IFR's down to .030" (.032" was stock). My carb has annular boosters, which I am sure has an impact on what the final numbers will be as compared to a downleg booster.

skinny z

Idle air bleeds....5/64ths...?
Do you mean throttle blades?

pancherj

5/64" (.078) seems large to me. I went from .070 to .072 which represents an 11% increase in area. That was certainly enough (when combined with the decrease in the IFR) to lean out my idle. I guess the danger is that you will "desensitize" the idle circuit to the point that it does almost nothing. But, Camarito had success, so who am I to argue.

I have never been a fan of drilling the throttle blades. I tried it twice with no success. The Demon carbs have the idle bypass which is nice. You can do a similar mod on standard Holley's. I saw a write-up on it once.

camarito

i have a Barry Grant Speed Demon 650 VS, and yes it did fix it, the idle mixture screws only control the output amount of mixture the mixture itself is done internaly and the air bleads limit the amount of air in the mixture

camarito

5/64 is only 0.075 on my digital caliper

camarito

i did get a small off idle stumble in the 14's area but at high 13's runs great
i got this from another forum online, it will explain everything about idle mixture.

You have an idle jet that controls how much fuel gets into the idle circuit. And one or two (depending on carb) idle air bleed jets for the idle circuit. These both add air to emulsify the fuel mixture and bleed off some vacuum signal. It is this bleed off affect that changes the timing and mixture with bigger air bleeds leaning and smaller air bleeds richening the mixture.

Now the confusion you have regards the fact that the idle circuit has two different outputs: the curb idle port and the transfer slot. At the lowest idle position the transfer slot is basically open to the high pressure side and not to the manifold vacuum. Thus it works as yet another air bleed and all fuel mixture must pass through the curb idle port. As the throttle blades are opened the transfer slots progressively move from the high pressure side to the manifold vacuum side. As they do this they change from air bleeds to fuel mixture draws providing more fuel to match the increasing air flowing around/past the throttle blades.

The idle mixture screws only control how much mixture can pass through the curb idle port. Note the use of the term "mixture" and not fuel as this is an important distinction.

The other affect is that the more vacuum signal is bled off by the idle air bleed the more vacuum is required to pull fuel through the circuit. This has two important affects both based on manifold vacuum dropping below this threshold. The first is on the low end with an off idle stumble and the second is on the high end when the main circuit takes over. So this means that if you lower the vacuum threshold with smaller idle air bleeds, you also prolong the upper end of idle circuit operation too.

Now let's apply some of this theory... say your off idle, transition, and cruise are perfect but idle is off. You would adjust the idle using the idle mixture screws. But should your idle be perfect and your transition and cruise be off then you would have to use a combination of the idle fuel jet and the idle air bleed to adjust the mixture.

So in your case I might first open up the idle jet a bit to richen cruise since you appear to have no other adverse affects working. You will have to also close the idle mixture screws a bit after this.

oamhmad

iTrader: (1)

Interesting but sort of confusing. I'll have to research this in more detail. What affect did drilling out the air bleeds have? And also as someone mentioned earlier 5/64 is indeed 0.078. Your calipers might need calibration.

pancherj

In Camrito's defense, a 5/64 drill bit from a standard set may measure slightly less. I use numbered bits and a pin vise to do all of my carb work. Those bits are very precise in diameter.

My understanding is this (and I am by no way a carb expert!): The bleeds help emulsify the mixture and also control when that particular circuit becomes active. The larger the hole in the bleed, the higher air speed required (moving over the opening) to activate that particular circuit. So, making the IAB's larger should delay the activation of that circuit to a higher RPM and it should lean the mixture a little. There is also the IAF that restricts the amount of fuel mixture entering the idle ciruit. Decreasing it should have a much more dramatic affect on leaning the mixture. You can go too far and lean this circuit out so much that there is a lean stumble when transitioining from the idle circuit to the main circuit (as the throttle is opened).

Lastly, don't believe what your AFR tells you at idle if you have a cam with lots of overlap. If you have a very stock cam, then the reading should be more believable.

camarito

yes 5/64 is 0.078 but the bit is only 0.075 the caliper is dead on i checked

The engine runs great even the sound is better, i can even "see" the fuel economy on the fuel gauge, I've been trying for a long time to lean out the idle but could never do it without stumbling and stalling and poor throttle response, a few more tweaks (it kinda change the a bit the entire rpm range)
to go and it will be almost like fuel injection

skinny z

In an effort to make my BG carb tunable, I've drilled and tapped the high speed air bleeds and idle air bleeds. This season will see a lot of tuning for fuel economy via my data logger.
With stock bleeds of .070" (IAB) and .039" (HSB) I see that camarito isn't too far off although I'm pretty sure stock Holley air bleeds are smaller than what was originally in my Speed Demon 750V.
My response to .078" was more of a knee-jerk reaction on my part.
Good work camarito.

oamhmad

iTrader: (1)

what do you guys recommend i drill out my air bleeds to? I have a 600 cfm 4160 and a base low compression good wrench 350 with headers and an RPM intake manifold.

skinny z

That's the thing. With your combination I can't see why there should be a need to modify that carb in any way.
As I posted before, this sort of thing is difficlut to diagnose in a forum setting. I can't help but think there's something fundamentally wrong. That your pcv valve makes such a difference seems odd to me. I can only think that there's a leak somewhere and with your high manifold vacuum, it doesn't become a problem until you compound it by removing and plugging the pcv hose.
Did you say the carb was new? I also thought you mentioned that you had to swap out the carb base because of a crack.

oamhmad

iTrader: (1)

Yes I bought the carb new. It's about a year or so old now. The original base plate cracked because I gorilla-ed the carb mounting bolts. I replaced it with a used base plate off ebay. I've sprayed carb cleaner all around the carb and can't find any leaks.

I do notice noticeable wetness from oil on the carb gasket directly under the pcv port. Not sure if that's normal or not.