Horrible Off Idle Stumble

All,

I have bee tinkering with my Charger trying to get some of the long-lived bugs out of it. One thing that has evaded me is getting rid of the horrible off-idle stumble. If I give the car more than half throttle from a stand still, it stumbles....much more and it will jump forward and cut off. Once I am moving I can blip the throttle and she is nice and crisp. This is just an off-idle issue. I swapped out the white cam and (28?) accel nozzle to the red cam and 35 without fixing it. Here are the specs:

- 73 Charger - 500ci engine 10.6:1 compression, mild.483 lift cam, 2500 converter, 727, 3.55 gear, 20* initial timing
- high rise single plane intake + 1/2" spacer
- Holley 4150, 850 cfm vac secondary (single pumper), red cam, 30cc, 35 nozzle

If this is so bad that it cuts off, I feel like I must be a mile away from what it wants. Otherwise runs pretty well. Very crisp at speed and pulls hard - just this annoying bog when you are getting moving.

Any suggestions on what to try next? I just swapped in a 45 nozzle (hollow screw) but it started raining so no testing today. I am hoping somebody here has a similar combo and could give me an idea of where to start.

Thanks in advance!

Does the accelerator pump shoot fuel into the primaries IMMEDIATELY when the throttle is cracked open ?
I'd check that adjustment; sounds like enough timing.
#35 sounds like plenty of nozzle, might also try #31 and see what happens.
Also, you can move the pump cam between #1 & #2 positions.
A long time ago I found a chart online that was a graph of pump cams and their feed rates.

Is there any way you can borrow someone's known good carb for an hour? That will quickly put you on the right track. Just to make sure that the carb IS the problem and not something else.

Do your mixture screws respond ?? If not you might want to take a look at your off idle transition ports with respect to the primary throttle blade position when off choke and against the idle stop screw

What kind of carb ??. I

Yes, if I turn the screws in, I can kill the motor.

I think I checked the transition ports a while back but are you thinking that I have the idle too high and I'm not on the idle circuit anymore?

It's a Holley 4150 with vac secondaries.

I always hate to ask this, but what else could it be besides the carburetor??

First things first. Ditch the red pump cam and use a blue one. Adjust the pump so you use ALL the pump stoke. I never leave .015 at the end of the stroke.

You have a pretty tight converter. It make take a big, short pump shot. That’s what the nozzle controls. The bigger the nozzle (.037 verses .028) the more volume you get and it it’s shorter in duration. The opposite is the smaller nozzle gets less fuel for a longer duration.

You didn’t mention power valve opening, but it’s important. You need to hook a vacuum gauge to manifold vacuum (not ported vacuum like is used for a vacuum advance distributor) and drive the car while monitoring the gauge. See what your vacuum is at a CRUISE and then set your power valve opening by that. I suspect you’ll have 16 or more inches at a cruise. That’s means unless you can find something that opens higher than 10.5 inches, you need a 10.5 power valve.

Do not set your power valve opening by idle vacuum. That’s wrong. Set it by cruise vacuum.

Start with that. As for the accelerator pump...the goal is to use the smallest nozzle that keeps the stumble out of it. If you need say an .045 the that’s what it needs. But, if you can get everything else in shape and you can use say a .033 nozzle without a stumble, that’s what you use. It will be better on fuel mileage and it will drive better.

Thanks for the advice. Currently I am running 8.5 PV's which was what the dyno liked years ago on a similar carb.

FWIW - I just took the Charger out with the 45 nozzle and the dead spot is pretty much gone. She spins tires from a standstill - where I used to have to ease into it or it would cut off.

I agree the 45 nozzle seems big. Maybe with the carb being a single pumper (one nozzle) and such a big motor, it really just needs that much fuel?

If it's running well now - should I skip messing with the cam and try a 42 nozzle and see if it still pulls from a standstill?

I am by no means challenging you but I was taught and have read to do the opposite IE: set the power valve 1" or so below the lowest vacuum reading in gear. The reasoning was that the power valve could / would otherwise open when idling in gear casing an overly rich mixture. So could you explain the theory / reasoning behind your method ?

And to the OP glad you have it on the run

lazy pump cams and big squirters don't work for light to part throttle tip in. driving on the street vs running on the track require different accelerator pump tuning.

It sounds like you are now running the RED cam and the .045 nozzle. If this fixes it when it is hot outside, then once the temps cool down you will be looking for more of a pump shot. Keep in mind that you need the hollow squirter screw when using above apx the .040 size squirter.

I would test the bigger/longer cam that is ORANGE with your .045 nozzle. If you see black smoke, then go back down in squirter size to around the .035. Then if that .035 works in the hotter air, then you can just put the .045 back in when the temps drop down.
If the ORANGE .045 works good, then you can test the largest of the 30cc cams (BLUE) with the .045's. Work your way up gradually until you get the feel that you are after.

I would also take the advice from above and check your cruise vacuum reading. If is in the mid teens or higher, then I would suggest the 10.5 standard flow PV. If that helps, then there is also the 10.5 High Flow too.

""I am by no means challenging you but I was taught and have read to do the opposite IE: set the power valve 1" or so below the lowest vacuum reading in gear. The reasoning was that the power valve could / would otherwise open when idling in gear casing an overly rich mixture. So could you explain the theory / reasoning behind your method ?""

I will give it a shot.
Can the PV be open at idle? Yes, IF the throttle blades are open enough at idle to get the main circuit started. But the OP said that his metering screws will shut down the engine if they are turned all the way in. So this means that he is still on the idle circuit and not high enough on rpms to get the carb transitioning into the mains. So his PV selection should not be affecting at idle.

Might be time to drill the butterflies and close them more at idle.

Please refer to [video:youtube]https://www.youtube.com/watch?time_continue=180&v=mE_xTUXQJQ8&feature=emb_title[/video]

Again not disagreeing nor wanting to start a peeing contest. But per the above holley video is it possible his PV is opening to late causing the stumbling that he is able to overcome with a much larger pump shot ?
And BTW over the years I have read two conflicting methods, # 1. Is as the video states, 1/2 the manifold vacuum at idle in gear. #2 is to select a power valve 1" or so below the lowest vacuum reading in gear. That is what i was taught and have used for the last 45 or so years.

This subject has been beat to death on Moparts many times before; the idle vacuum shouldn't have any impact on the selection of the power valve because the power circuit should not see any vacuum at idle. If the power circuit is seeing vacuum at idle then something is wrong and needs addressed. At least in most cases.

I agree with the position that the cruise vacuum is much more important when choosing the opening point for the power valve. I often use 9.5 or 10.5 power valves. Our engines are not near as picky about an early opening PV as they are about a late opening one. Late opening PVs can cause all kinds of driving problems, while early opening ones rarely, if ever, cause any.

And while I am sure that all the suggestions here are well intentioned, many of them are frankly not something that the OP probably should be attempting until he has more experience modifying carbs.

Sorry that was the wrong video link
[video:youtube]https://www.youtube.com/watch?reload=9&time_continue=180&v=mE_xTUXQJQ8&feature=emb_title[/video]

And I'll just say that the subject is similar to do I run ported or manifold vacuum for the vacuum advance ?

I'll start by saying that this opinion is aimed at primarily street oriented cars. Not necessarily stock, but not particularly radical either. What it seems the OP has.

Personally, I have not seen a situation where dialing in the proper amount of initial advance in the distributor would not work and using manifold vacuum would. I will never say never on this kind of stuff, but I have sure re-curved a bunch of distributors over the years and then moved the vacuum back to being ported. Every one of those situations has been that the one who put the vacuum source to the manifold did not attempt (or maybe know how) to properly curve the distributor.

There is nothing wrong with using manifold vacuum, but it is usually (not always but usually) a band-aid covering up an improper distributor curve. Same as throwing more or an earlier accel pump shot to cover up a poorly chosen, late opening power valve.

In that context, I agree that there are similarities in the topics.

I have a similar motor to yours. A little bit bigger cam, 470ci and a Holley 850 street HP.

I had the same issues as you when I put it all together. I eventually had to go up to 50cc accelerator pumps diaphragms, a 37 pump squirter in the front and a 39 in the rear. Everyone told me that I shouldn't have that big of squirters in it but, that's what it liked and that's what it took to get it happy. After that, the fuel map was right everywhere.

I eventually put a better set of heads on it and the problem returned and I could not tune the stumble out of it with pump nozzles and cams. I sent the carb to a well know carb tuner and he too said that I should not have that big of squirters and that 30cc diaphragms and smaller squirters would suffice. When I got the carb back from him, it ran worst than it's ever run. I called him and began doing what I'd done before but, I eventually had to put the 50cc diaphragms back on it and bigger pump squirters. He did other emulsion work on the carb, which was the saving grace. I messed with the idle bleeds, high speed bleeds and a bunch of other stuff but, I still have not gotten the carb to do exactly what I want it to, it needs some more work. WOT is great, idle could be better but, my light cruise sucks.

Ultimately, what I'm trying to convey to you is don't be afraid to throw all types of pump shot at it. I don't care what people think should be on anyone's car. If you're running it on a dyno, with a wide-band or know how to read plugs really well and the AFR's are what they should be, than that's what the car wants. If than means 49 pump nozzles front and back and 50cc accelerator pumps, than so be it. I'm not saying it's right but, there is more than one way to skin a cat. If your fuel map looks awesome, then who cares what it took to get there. The car doesn't know the difference.

I missed this question until just now. So..................The first thing I always do is throw a lot more lead in the timing. Our big blocks respond to early advance like no other engine I have ever worked on. With 500", 3.55 gears and 2,500 converter, I would put in as much initial as the starter will pull. 25* or more as long as it will start hot. And then, get the rest in by 2,000 +/-. This is more effective the bigger the cam, but is always a good idea even on nearly stock motors. Even if that turns out to not be a big part of your problem, it will still pull the lower RPMs much better. And might just do the trick for your bog, too.

The stock curve is a very conservative one to allow for less talented operators to be less likely to hurt something. Much of these kind of things were designed with warranty and engine safety in mind, more so than peak performance. The stock curves are ALWAYS too conservative. Just like most cruise ratios are too fat. Safety first.

Only after getting the ignition curve dialed in, should you start opening up the carb. And just an FYI, the transitions in the carb are the absolute most difficult area to get right. Mainly because you have as many as 3 circuits coming and going at the same time. Even if they were all in the right range of mixture, their timing to each other could be off enough to cause a problem. Usually, if a carb has real problems in the transitions, it is tough for the typical guy to get right. Which is one of the main reasons so many guys pin their hopes on plug and pray EFI. They expect it to tune itself. And sometimes it does.

The key to picking the right carb out of the many, many choices out there is to get one with a tune in it that is as close as possible to being right for the particular combo it's going on. That way, a guy doesn't have as far to go to get it right. The farther away a carb is from being good with the combo it's going on, the more changes and circuits that will have to be experimented with. Just look at the varied suggestions in this thread already.

Ideally, a guy needs to determine his CFM desires and them try several carbs with different calibrations in them to see what his combo likes. Street calibration, race calibration, in between, vacuum sec., mech. sec., what boosters does it like, and on and on...................... Which is part of the reason I suggested borrowing another known good carb. If it does better, you have a better handle on the tune that the combo likes. And buying another carb (as long as it works) can be a bunch better than trying to re-calibrate a carb that has the wrong tune to start with.

I guess other than suggesting starting with the timing curve, the other point I would like to make is it is much, much better for most guys to find the carb closest to what his combo likes, rather than try to re-calibrate a carb with the wrong tune.

That's what I think anyway.

So are you saying the red cam + 45 nozzle isn't a good choice for a street car? If that's true what would you suggest? A more aggressive cam and smaller nozzle?

Some good advice and some I don't agree with but MOST IMPORTANT thing is do you know how to adjust a pump-shot? I am surprised by the amount of people that have no idea.

If you know this and have done this then my apologizes: With a homemade tool or another person helping engine off have the carb. at full open, now push the pump arm down fully and check with a feeler gauge between pump arm and the bottom of adjustment screw for (Holley says .015) I like .006 clearance. Just make sure you are not over stressing the diaphragm and ripping it. You could have .000 if you want just not past that. I can't tell you how many cars I've seen with stumbles because of this step not being followed. Also everytime you change or reposition a pump cam you have to check and or adjust this.
Ron

There are several charts out there showing pump cam lift curves. AFAIK, the most aggressive cam is the BLUE cam and I think it’s got the number 427 on it, but I’d have to go out and look to say for sure. But it’s definitely blue.

I’m not saying your combo doesn’t need a 45 squirter. It very well may. I tuned a front engine dragster a long time ago and even though it was light, it didn’t have the correct converter so it took a squirter in the high 50’s. It was a foot brake car, and you couldn’t get a big enough nozzle on it.

I your case, I’m still suggesting to get a vacuum gauge on it, and see where you are for manifold vacuum at a cruise. Your 8.5 power valve is better than what most use, but there are several things I’ve learned lately.

One is it’s a bummer you can’t buy a 12.5 power valve any more. Most guys could use it. I know I could. Another is when you test the power valve, the vast majority of them open LATER than what is stamped on them. I tested an 8.5 that didn’t start opening until 6 inches of vacuum. That can definitely affect your tune.

As for the correct method of setting power valve opening, Holley, most authors and the vast majority of end users, because of the aforementioned people, have been incorrectly taught and have over the decades been doing it incorrectly. In fact, the vast majority of end users (and evidently Holley because they still teach it) have no real idea how the power valve (which is really an economizer valve, but that doesn’t market well) circuit functions.

When you are at a cruise, you are running on the primary main jet. Holley (and most very other company producing Holley clone carbs) sends the carb out with very rich jetting. So at a cruise, they are just slobbery rich, they get horrible fuel mileage, kill plugs and are just generally miserable to drive. That’s why many guys won’t use a Holley. Horrible fuel mileage, hideous rich idle and all that. That’s where the power valve comes in.

At your CRUISE, the power valve is shut (or should be) and all the fuel is coming from the primary main jet. And it’s most likely rich. Then, you accelerate and the power valve OPENS and ADDS fuel, and you are now pig rich, wasting fuel and most certainly giving up driveability. The amount of fuel added by the power valve circuit depends on the Power Valve Channel Restriction holes, which are the small holes you see when you remover the power valve. I forgot to mention that as you accelerate, you also add pump shot too, which is more fuel added to an already rich condition.

So, the correct way to tune the carb is to keep reducing the primary main jet until, at a cruise you get a SLIGHT lean stumble or surge. At that point you add a couple of sizes to the primary main jet and move to tuning the power valve, keeping in mind you’ll never touch the primary main jets again, unless you are tuning cruise fuel issues.

The two tuning options you have for the power valve circuit are WHEN it opens, and how much fuel it flows when open. That’s why cruise vacuum is how you set the opening. The later you open it, the later the engine gets the added fuel it needs because of the added demand by the engine. So you need to get the opening time as close to cruise vacuum as you can. Certainly you don’t want the power valve open at a cruise, but it needs to open quicker than what Holley has guys doing. My engine has about 9 inches of idle vacuum so by the de facto setting, I’d need a 4.5 power valve. I have about 15 inches at a cruise. So I use a 10.5 and would use a 12.5 if I could get one.

That later opening requires a bigger pump shot to cover up the lack of fuel because the power valve is opening so late. Running more nozzle than you need just wastes fuel. If you get the power valve/main jet set correctly they run cleaner and make more power.

So now you have the power valve opening at the right time, you have the primary main jet set for best clean cruise but now at WOT you are lean. The first thing guys want to do is grab a screw driver and ADD main jet to the primary side and maybe add some main jet to the secondary side and that is DEAD WRONG. You have now made your cruise air/fuel ratio much richer than it needs to be.

When tuning for WOT after you have the primary main jet correct for cruise and you have the power valve opening as correct as you can get it, you adjust WOT air/fuel ratio by changing the size of the power valve channel restriction (from here on I’ll type out PVCR so I save some strokes). On the “better” carbs, the PVCR’s are threaded holes and either brass set screws or little 6/32 threaded jets are installed in the PVCR holes so you can easily adjust them.

Of your carb doesn’t have the holes threaded for brass, you can get some 6/32 x 3/16 brass set screws from McMaster/Carr. A box of 50 is about 5 bucks. You'll need the correct 6/32 tap and drill, a pin vise and a set of 80-61 number drills and 60-1 number drills. Any decent hardware store will have the 80-61 drill set and pin vise and Harbor Freight has the 60-1 drill set for about 20 bucks or less IIRC.

Then you just drill the PVCR holes to the correct number drill size (I forget what it is off the top of my head) and then thread the holes just enough so the set screw is flush or just below flush and you’re done with that. Now it’s a simple task to drill some brass and tune the PV circuit.

Now at WOT on the primary side you either make the PVCR larger to add fuel or smaller to reduce fuel, never touching the primary main jet. That way, you still have a clean, crisp cruise and the correct amount of fuel at WOT.

The next question is how do you know when to add WOT fuel to the primary side and when to add it to the secondary side. The best answer I have is to disconnect the link from the primary to secondary throttle shaft, wire the secondary throttle shaft SHUT and only tune in the primary side until you get it nailed. Then hook the secondaries back up and tune for WOT only using the secondary main jets. You don’t touch the primary main jet or the PVCR’s, because you already have it nailed.

All this takes time, but the payoff in the end is well worth it. Of course, like everything else Holley, once you get that dialed in, you can start tuning the idle circuit to clean that up. Tuning the idle circuit requires some of the same work...you drill out the idle feed restriction and tap it for a 6/32 set screw,but that’s a discussion for another thread.


Hope this helps clear up some of the bad information out there on how to tune the main jet/power valve circuit on a holley carb. It’s harder to type out than it is to do.

yes. for OP's carb i'd try a .028" squirter with tubes and a green pump cam to start out with. those lazy cams are ok for the secondaries but work poorly for light tip-in which most street driving is. if all you do is race and your just stabbing the throttle and go then big and lazy can work. I want my cars to drive nice with no spits, hiccups, or flat spots and I don't do the big squirter slow action accelerator pump stuff. big squirters and lazy pump cams don't provide enough pressure in the accelerator pump circuit to push fuel into the air stream at light throttle. generally fuel will dribble down the side of the venturi and not mix in the air stream creating a flat spot. the old 950 3bbls (which have the same throttle bore and venturi on the primary side as an 850) used a .028" squirter with tubes with the blue 427 cam and drove great.

Wow so much information but I would expect nothing less!

I went ahead and swapped to the green cam (#1 position), adjusted the pump arm (.015), and dropped to the #28 nozzle (no tubes). I took her out and part throttle tip in is good as is nailing it while already moving. However, if you nail it from a standstill, it does have a slight bog. However, this is much less than it was before I started all this tinkering.

The bog is slight enough that I think going up a nozzle size should fix it. When the car cools down, I will swap to a #30 nozzle and take it back out. Once I find the right nozzle, is it really worth it to order one with the tubes?

Thanks as always for the input everyone. This fall, I will likely take the car to the dyno and get it tuned which will address and issues with PV or jetting (which I think are already close).

too much fuel can cause a bog as much as too little. try the .030" if you want but you'll be stepping back towards the tip-in problem. try a white 218 cam first. green and blue cams are pretty aggressive and sometimes they can be too aggressive. the white 218 cam was always the first step in pump cams back when holley was holley. float levels being too high or too low can create off idle issues. trying to run the carb too lean, especially on the idle circuit, can create a low speed stumble.

I had the red cam on it before and it bogged horribly with a #35 so badly it would cut off if you nailed it. I had to go to a #45 to get that bog gone. The white cam seems to be very close to the red cam from the charts I see so I don't see how dialing it back down won't take me backwards.

I don't see how going to a mild-ish cam and a small nozzle won't make this motor bog like heck. I just installed the #31 nozzle (green cam) and I will let everyone know if it helps (or hurts). At least I know one winning combo that worked well (red cam + #45 nozzle)

If the GREEN/031 combo gets you closer but you need more, test the ORANGE/031 combo next 👍

The Holley cam charts seem to make the orange cam look slightly lazier than the green one. The only more aggressive cam I see is the blue one.

The blue cam, number 427 is the most aggressive cam. If you have .015 between the pump arm and the lever that’s too much. I set them so I can just wiggle the pump arm a skosh and run it. More than that delays the pump shot.

""The Holley cam charts seem to make the orange cam look slightly lazier than the green one.""

If you look at the attached Holley chart, reference the throttle percentages between the GREEN and the ORANGE. The green has a shorter duration which is completely used up by 50% throttle. It does have more cc's but only at 40-50% throttle. The ORANGE actually provides more cc's from 10% up to 40% and the ORANGE has a longer duration up to 70% throttle.