Holley bog on take off
 

HELP! I've got a holley 650 vac secondary manual choke carb on a stock 350 vortec with a dual plane intake and headers tucked into a 4200lb Cadillac. Every time I mash the pedal on take off or while cruising the engine will sometimes die, sometimes pop through the carb. While cruising if you somewhat gradually give it gas it will surge like its starving itself. Here's the setup right now where it seems to run best..

shooter was 28, up to a 37 now
62 main jets
6.5 power valve
5.5 psi fuel pressure
1 1/4 turns out on the idle air screws
pump set to .015 plus a little extra helped a little
timing set at 14 degrees base (dont have an advance timing light)

what am I overlooking here?

Re: Holley bog on take off
 

Re: Holley bog on take off
 

Read the post above and install a wide band O2 sensor and AFR gauge.
You'll be glad you did.

Re: Holley bog on take off
 

Ok I'm a little confused with the transition slot deal. I followed the steps in the link but my idle ended up around 2000rpm. I can obviously turn it down with the screw but doesn't that defeat the purpose of setting up the transition slot? I did turn it down anyways, and it has made a difference with the cars take off. It still seems like a slug out of the hole tho and when I'm running around 30 mph and stomp on it it still falls on its face. Right now my distibutor advance hose is hooked to a ported source, but I keep reading conflicting stories of being hooked to manifold vacuum. What's the deal? Lol

Re: Holley bog on take off
 

Without actually being there to see it, that sounds like your accelerator pump is not working. Easy to check, engine off, work the linkage as if you were stomping on it while looking down the carb. Did fuel squirt down the bores? No - bad accelerator pump.
The additives in today's gasoline shrink the pump's cup. Get one compatible with today's fuel.

Re: Holley bog on take off
 

Accelerator pump is definitely working, 2 good streams of gas and the carb is only 4 years old, bought new.

Re: Holley bog on take off
 

Wow, you are really up north, huh? Still pretty cold there I'd assume. Are you running the choke with this set-up via manual pull cable or is it set to full open all the time?

Manifold vs. Ported.... I have seen heated debates over this. As you likely know ported was an "emissions thing". Give this a quick read for some more details: http://chevellestuff.net/tech/articl...r_manifold.htm Ported may have some advantages sometimes with certain cams, or so some say. I prefer manifold myself, it just seems to make life easier.

Have you verified that the distributor vacuum advance is actually working?

I will assume you have set your float level properly, and have good gaskets, decent tune-up parts in place. If not, tend to these maintenance points.

I am not a carb guru but that squirter size seems kinda large. One thing that has helped me in the past when I seemed to be chasing my tail with a carb was to take it apart, clean the living daylights out of it, new gaskets, and return the carb to factory specs for jets/pumps/cams/squirters. Also verify your floats are not leaking and the pumps are in good shape. You can look all that up via the list number. I suspect doing that will help you. It may not be a perfect set up, but it ought to run better than what you are currently experiencing.

Idle screws shouldn't matter a ton for WOT hits but they seem pretty far out to me. Go to Princess Auto and get a cheap vacuum gauge ($15?) and hook it up. Engine off verify both screws are set the same amount out, 1-1/4 is a good starting point but be sure they are both the actual same # of turns. With the engine fully warmed up turn the screws in 1/4 turn at a time until you get the best vacuum reading. If it stumbles you likely went too far. You may need to readjust your idle RPM and re-verify. Once you get there, turn them back out a bit. 1/8-1/4 turn each -richer idle, which will help cold idle a little.

After this you should have a good vacuum reading. divide that number by 2 and that should approximately be the number for the power valve.

That is all going off memory, hopefully I haven't contradicted any proper procedures.
Good luck.

Re: Holley bog on take off
 

Haha starting to warm up a bit here but taking its sweet ol time up here in the great white north lol. My choke is cable operated and everything on the carb is set up as per instruction. Went over it 3 times so I'm pretty confident it's really closely set up. Only issue I seem to still have is the bog when you mash the pedal while doing 30mph, it'll still cough and want to stall. It will give a hesitation while snapping the throttle from idle but what I have noticed is if I give it just a touch of accelerator pump shot with my finger as I snap the throttle it won't hesitate. I tried giving a little more adjustment on the pump but doesn't help. Could i just be needing to change the pump cam from the green one it has now to something else? Thanks for the help so far! at least I'm seeing a little bit of progress here.

Re: Holley bog on take off
 

Sounds like the vacuum secondary flap is opening too quickly. Try a stiffer spring.

Re: Holley bog on take off
 

Reset the transition slot. It should appear as small "square" when set correctly. Don't mess with this adjustment even if the result is a high idle. Also be sure that the secondaries are not opened excessively. This will contribute to a high idle. Your comment about an improvement with the slot adjusted is encouraging.

Return the carb to the out of the box settings. Bring the accelerator pump to where it was originally. The pump shot is rarely, if ever, an issue on a near stock spec engine.

Get away from the accelerator pump. I will bet you that it is not the issue. Bring it to where it was from the beginning.

If you have followed the tuning sticky then you may want to check and/or proceed with the following:
Get a vacuum gauge and determine what it is at idle.
Verify your timing. Don't concern yourself with where the vacuum advance is connected at this point. The tuning is easier with the advance plugged into the ported vacuum however once you've got a handle on the tune and have better tuning equipment, full manifold vacuum will give you better results.
If the idle is still high after you have verified, once, twice, three times, that the transition slot on the primary is good and that the secondary blades have not been opened too far, your next step is to look for vacuum leaks.
Mentioned earlier was the need for a vacuum gauge. If you don't have one, get one. Or give up on carb tuning. It's vitally important.
Give it more timing. However to do that you'll have to determine the distributor specs. You'll be amazed at what a few additional degrees of timing will do to an engine that "falls on it's face" when you mash it. For what it's worth, where the vacuum advance is connected has no effect on the sudden opening of the throttle blades.

If you decide to follow up on this, report back. There are additional tips that you may find useful. But first, you have to baseline the carb. You said you bought it new? Restore it to that condition. Otherwise you'll be way out in left field and may never return.

Re: Holley bog on take off
 

Try what skinny suggests as a starter. Especially the parts about LEAVE THE ACCEL PUMP ALONE and PUT IT BACK STOCK.

The "Holley bog" has 2 root causes, both of which result in the proximate cause of a lean condition as the throttle is moved toward the open direction, and especially off idle.

1. The transition slot being too far exposed. This causes the idle system to draw fuel out of the transition passage at idle, which means that then when you open the throttle, it's already drained. It goes SOOOOPER lean until the main system reaches full flow.
2. The power valve being too low a "number". The PV is normally closed by manifold vacuum, the idea being, when vac is high the driver is not demanding power from the engine (throttle is only moderately open, e.g. steady cruise); but then as the driver demands more power (throttle opens further) and vacuum begins to drop, additional enrichment is needed. The "number" on the PV is the vac below which it is open. If it's too low, then a WHOLE BUNCH of throttle opening has to occur, before it opens. So if you have a 65 PV (opens when vac drops to 6.5") and you're cruising along at a typical 15 - 20" of vac, and you decide to open the throttle a bit, the vac has to drop all the way from 18" or whatever, to 6.5", before the carb switches (yes it really is much more like a "switch" than a steady transition) into "power" mode.

Note that Holley's ancient "PV should be half of manifold vacuum at idle" is WRONG. There is no other word for it: it is WRONG. (I don't ordinarily make such a claim about mfr's instructions, but it must be done in this case) In reality, it should be set to within about 2 - 3" below CRUISE vac. This allows the jets to be made smaller (better gas mileage, lower engine running temp, less stinky exhaust) for cruise, but for the carb to IMMEDIATELY go to "power" mode as the throttle is opened.

In general the PV has negligible effect on idle. That is, even if it is set to a vacuum ABOVE what exists at idle, it will NOT affect idle mixture significantly. Therefore idle vacuum IS NOT a factor in choosing the PV.

In virtually EVERY carb I have ever tuned for street use, the PV has ended up MUCH higher than the carb's original one. Usually a 105 or 125 produces the best results. Similarly, in nearly EVERY such carb, the jets have ended up several sizes smaller than original. Holley sets up their carbs so that they'll "run" on almost anything you slap them onto; but they will generally only run "right" if they're too big of a carb, on too small of a motor, with too little gear, in too heavy of a car, with too little torque converter. They won't run RIGHT, but they'll at least RUN. Note that this is THE CLASSIC hot-rodder carb choice and car setup mistake: think, bright shiny new 750 cfm carb on an otherwise bone-stock 283 in front of a 3.08 and a Powerglide in a 64 Impala. Yes THE SAME THING has been going on for THAT LONG.

Follow the tuning sticky, in order, from start to finish, step 0 to step however many it has, skipping none, starting at the beginning, doing every step in order, from the first step to the last, in order. Did I mention, skip no steps, start at the start, do it strictly in the order given? In case I didn't, let me emphasize, start at the first step, do each step in the order given, don't skip over any, don't jump around, don't start in the middle, do it IN ORDER. It is CRITICAL that you do it this way: the process and order given, allows the effect of each system of the carb to be ISOLATED from each other, in the order that they interact; if you DON'T do it IN ORDER, then whenever you do something OUT of order, you'll have to go back and re-do something else, because that part's effect wasn't isolated from the one you messed with.

Re: Holley bog on take off
 

If you have followed the above advice and still have problem:

I had similar stumble with same carb I chased for a long time (years ago so forgive me if I am not accurate) right after carb rebuild as I purchased it used. Under the accelerator pump (bottom side of carb) there is a check ball with a small bar keeping it in place. Mine had too much clearance. I tapped it in slightly. My problem went away.

Re: Holley bog on take off
 

Accelerator pump curve adjustment. First loosen acel pump screw in till all the slop is out between the screw bottom and pump lever arm. Also hopefully you are not using one of those midget air cleaners?

Re: Holley bog on take off
 

I've got a couple power valves ordered and hopefully will be in tomorrow. When I get them i will give you guys a follow up. Thanks again so far! I'm definitely learning a sh@t ton about these carbs from everyone.

Re: Holley bog on take off
 

Don't forget to reset the transition slot or the power valve won't solve the problem. Do you have a vacuum gauge? If not, I strongly suggest you get one.
What PV sizes did you order. On a stock engine with high vacuum I'd say 12" might be a good starting point. That's where the vacuum gauge comes in. You check your vacuum at cruise and also as you lean into it.

Re: Holley bog on take off
 

Ya gonna reset the carb while its out. I got a vacuum gauge and I got a 10.5 pv for now, if it needs more than that it's only a phone call away.

Re: Holley bog on take off
 

So, partial update.... I put in the 10.5 PV and set the carb exactly the way it should be, went for a rip up the street and my alternator blew out on me lol. So while I wait for one tomorrow, it seems like it has some more pull, definitely a noticeable difference from before. But it still has that bog when you hit it real fast from idle. And the idle is still way high. I set the transition slot and accel pump properly and there are absolutely no vacuum leaks anywhere. I've blown propane into every conceivable spot that could leak vacuum. Ive even isolated all vac accessories and the only thing that makes a small difference in rpm is the pcv valve. I found a video on YouTube that has you put an o2 sensor into the exhaust stream and measure the voltage it produces, and adjust idle air screws as needed to get .5v which they say .5v is ideal. Is there any merit to this method? Because I cant get it down below 1v. It definitely still seems to run rich tho. Could this carb need smaller jets than the 62s I've got in there?

Re: Holley bog on take off
 

The PV makes the difference accelerating from a cruise condition.
It may have been asked earlier, but have you addressed your ignition timing? What is the curve like?

Re: Holley bog on take off
 

As for the O2 sensor, I use with combined an AFR gauge mounted permanently in the dash. You can monitor the air/fuel ratio full time and see when it's lean or rich (no need for the voltage conversion). If nothing else. You can dial in your highway cruise and get the fuel economy up. My cammed 350 is knocking down 24 mpg.

Re: Holley bog on take off
 

So this would work then for me reading an o2 sensor voltage? Just wondering why I cant get it any leaner. And as for timing, my light junked out on me and havent found a new one yet. Joys of a small town lol. So I'm not entirely sure right now where it is all in. Base is set to around 18 deg.

Re: Holley bog on take off
 

Yes. Buy it as a kit similar to this one.
https://www.summitracing.com/parts/avm-30-0334
The O2 sensor has to je of the wide band variety rather than a narrow band as some OEM applications are.
As for timing, a light is critical as you can imagine. A base of 18 is pretty good to start. It would be interesting to know how quickly the advance comes on.

Re: Holley bog on take off
 

Ya I know. Light is critical. I want to get it running as good as I can so I can bring it to the city if I have to. got line on an old school guy with a dyno. Apparently he works wonders.

Re: Holley bog on take off
 

If you have the carb set to original spec then the next step is timing. Has anyone messed with the secondary throttle opening?
Did you say what your idle vacuum is? I don't recall.
Where is your vacuum advance connected? As a baseline for tuning I tend to lean towards ported vacuum. There aren't as many variables. Once things are sorted out, I've found that manifold vacuum adds a lot of drivability.

Re: Holley bog on take off
 

The secondary stop is just touching the adjustment screw. Idle vacuum in gear is about 15.5 and right now the advance is hooked up to ported source .

Re: Holley bog on take off
 

Do the idle mixture screws have much effect on the idle speed /idle vacuum?
Can you reduce the idle speed by backing out the idle speed screw? If not, despite your testing, there could be a vacuum leak. The lifter valley to intake port isn't uncommon.

Re: Holley bog on take off
 

Hmmm. Never thought about an internal intake leak. It is difficult to get the screws to change the vacuum substantially. It will change a bit when turning screws but not crazy. It will stall tho if I close the screws. My alternator will be here for tonight so I'll be back in the garage this evening. I'll know more once I get it charging again.

Re: Holley bog on take off
 

A couple of things you can try when you're up and running.
Back the idle speed screw out until it no longer touches the throttle shaft. Keep track of the number of turns ao you can reset the transfer slot. Note any change in the functionality of the idle mixture screws.
Another is with the idle speed screw backed out close the choke and cover the choke horn with a rag. That should stall the engine. If it doesn't, you have some investigating to do.

Re: Holley bog on take off
 

K so this car has a night and day difference in performance now. Now all I'm left with is a slight bog on hard accel from idle, and it still seems to be running rich. Sure smells like it too at idle. No vac leaks anywhere on this engine. What am I missing here? Btw thanks for the help so far everyone! I'm happy to have my car running way better than it was!

Re: Holley bog on take off
 

As a carb tuner my best advice is to get a wide band 02 sensor and AFR gauge. Combine that with an in dash vacuum gauge and you'll have what you need to dial it in.
You still have to address the timing. One of the greatest seat of the pants improvements I've experienced is to get the timing to the maximum the engine will accept before detonation.

EDIT: Tools needed; Vacuum gauge, AFR gauge, dial back timing light, and in addition to that, a timing curve kit for whatever distributor you have in use. When and if you want to make the step to full manifold vacuum for the vacuum advance, there are a few other bits you will need.

Re: Holley bog on take off
 

Any news?

Re: Holley bog on take off
 

Still running a little rich at idle and on hard take off it'll leave a cloud of smoke behind me lol. Not black smoke tho. Decreased fuel to 4psi and still seems rich. But the bog is almost gone. Havent had a lot of time the past couple days. Hoping to get back into it next week

Re: Holley bog on take off
 

You can eliminate that "seems" part with the addition of the AFR gauge discussed earlier. And also as mentioned, the performance part notwithstanding, it'll pay dividends in the form of better fuel economy.

Re: Holley bog on take off
 

Ya I'm definitely going to have to look into an afr gauge. Get the car running that much better for sure

Re: Holley bog on take off
 

Fuel pressure WON'T affect "richness...Should be 7.5-8 psi, period. Anything that makes it rich will cause off-idle bog. too little info, still.

(49 yrs Holley experience)

Re: Holley bog on take off
 

That's not STRICTLY true. Especially if it's up into that 8 psi range.

That's pretty much the ABSOLUTE MAX that a Holley can tolerate on the street. It's usually TOO MUCH if the particular carb has the larger needle valve (more sq in at any given psi = more lbs) The pressure on the needle valve affects the float level, by acting against it. In effect, it raises the bowl level. That DEFINITELY affects the mixture.

A carb doesn't work off of fuel pressure. It works off of VOLUME. If you could keep the fuel bowls full at .001 psi of pressure, then .001 psi is all you need. Any more is just a waste. Of course in the RW nothing is that easy; but the point remains: as long as the bowls are kept full, then you have "enough" pressure. You gain nothing by increasing it from there and instead only create more opportunities for trouble.

Also, with extreme pressure like that, it's possible under some circumstances, that when the needle valve needs to close suddenly, it won't be able to. Which will overfill the bowl. Going from high-RPM WOT to idle suddenly (round track, end of the straight) is just such a circumstance.

I personally prefer 5 psi or thereabouts for street use. Now for max drag racing purposes, esp with a big motor (high fuel demand), that's a different matter.

Low fuel level will help create the bog by requiring the main system to lift the fuel too far to begin flowing. Setting the level right is fairly critical to the tuning process. When I talk about doing the tuning process IN ORDER, START TO FINISH, SKIPPING NO STEPS, this is one of the reasons why... the level will affect EVERY other system (adjustment) in the entire carb, therefore if you change it, you have to GO BACK TO THE BEGINNING OF THE PROCESS AND START OVER. Similarly, if you change the jet size, you have to go back and do everything that comes after it in the process, including the PV.

This is very true, although it is not the usual case in an out-of-the-box Holley. But yes, a too-rich condition will drown the motor in fuel at tip-in and cause a stumble. Messing with the accel pump before properly tuning the carb, then tuning it right but not backing the pump back off, is the usual way people get to this point, which is one of the reasons I tell people to LEAVE THE PUMP ALONE until the very end of tuning. A stock Holley pump is usually bigger than it really needs to be anyway, if you find that enlarging it (change the cam) makes the motor run better, it's a sure sign that something ELSE is what's REALLY wrong with it. Usually you'll see a puff of black smoke at tip-in if you've gotten it into this condition.

Re: Holley bog on take off
 

Well, we better tell the engineers at Holley (you know the ones that DESIGNED the carburator). Again fuel pressure HAS NO EFFECT on rich/lean, just serves to keep the float bowl full. Atmospheric pressure in the bowl "pushes" the gas through the jets to the lower pressure area in the venturies. If you can't run 7-8 lbs. there's a problem of another kind, like a bad needle and seat (no matter the size).

Re: Holley bog on take off
 

WRONG.

Not sure how this is can't be OBVIOUS.

Fuel pressure pushes on the needle valve. The needle presents a certain area to the pressure, which is to say, some # of sq in. The fuel exerts a force on the valve which can be readily calculated; force = psi × sq in. This force opposes the float; that is, it tries to open the needle, whereas the float is trying to close it. The net effect being, higher fuel pressure raises the bowl level, as compared to lower pressure. And higher level in the bowl means that it's easier for the carb's venturi "signal" to cause fuel to flow, because the fuel doesn't have to be lifted as high (pushed down in the bowl by atmospheric pressure in response to the lowering of pressure in the venturi) to "spill over" into the passages leading to the discharges.

It's really not that complicated. Not sure how this CAN'T be obvious.

While we're telling Holley engineers things, let's get them to abolish the "half of idle vacuum" crap about PV selection.

Let's also tell all the MILLIONS of owners of vehicles that came with Holley carbs from the factory back in the day, and typical stock fuel pumps that put out 3 - 4 psi, that they needed to go out and buy $$special racing pumps$$ for all those years, because their cars & trucks were inadequately fueled.

Bottom line remains, high FP does NO GOOD WHATSOEVER that lower fuel pressure doesn't, IF the lower pressure can keep the bowls full. All that matters is keeping the bowls full, PERIOD. Keeping the bowls full is the operative concept and the main goal, NOT hitting some number with the pump. Now... IF some lower pressure CAN'T keep the bowls full, for whatever reason, which THE ONLY reason would be a restrictive needle valve (compared to the engine's fuel demands), the YES, the pressure needs to be raised. Otherwise, high pressure just causes more problems than it solves.