Light acceleration at cruise stumble
 

Hello all,
Holley 750 HP body Dp. Quick lean spike while at a 40 mph cruise at 2000 rpm or so, on light acceleration only. Not sustained. Only when I slightly speed up to keep up with traffic. AFR gauge shows a quick lean spike, as quick stumble, recovers rich quickly, then stabilizes. Happens in about a second. All other conditions are perfect. Wot takeoff from idle is perfect. I have tried everything. Too much to list. This is very annoying. Any help?

What is your vacuum at idle in drive and what is your PV number?

Beautiful car!

Have you played around with the accelerator pump Cams?

Even with a light slow throttle change the pump should work.

What squirter size are you running? Sounds like initial pump shot may be too weak to correct mixture at tip-in of the throttle. I had a lean stumble like that just off idle and at light throttle cruise. I tried a pump cam that delivered an earlier shot and it just moved the lean spot later as you rolled into the throttle. My QF 750 had #33 squirters on the primary side. I upsized to #37 (2 sizes) and both lean-outs went away.

My vacuum at idle in DRIVE is 10 hg
My vacuum at idle in N or P is 12 hg
PV is 10.5.
This AED Holley 750 DP with HP main body came with a 4.5 PV and did not open until after my secondaries opened. It came jetted pig rich with 77 primary jets.

The old Holley PV selection formulas wend out the window because as soon as I touch the gas and am off idle, and low speed cruise, I’m at 16 hg of vacuum. Ended up working my way up to 10.5.

Nothing under 10.5 works. It acts as if I need a 12.5 but that does not exist.

The carb came with a pink cam in hole 1 and a .031 nozzle. I tried a 33, 35, and even a 37. No change.

AFR spikes lean, stumbles, recovers rich, then stabilized.

With the pink cam, it seems to me, that I get a true lean stumble. I tried the orange and I get that lean stumble followed by a rich spike. Maybe a false lean reading. I tried the white and it’s the exact same shape of the orange. Tried the blue and it was way rich.

I just noticed my primary pump nozzle was loose. When I gave it gas, fuel seeped out from the top and bottom gaskets. I tried to lightly tighten it and it started to strip. It’s somewhat tight now but I’m going to need to drill and tap it. I already have hollow bolts.

Could this have caused the nozzle to not hold a prime? I noticed that my squirt patters were more of a cone shape instead of a stream. Could this have been the issue all along?

Bingo!

Ok, I’ll start there. I’ll advise once I fix it. I hope this was the underlying cause and I didn’t strip it changing all the nozzles. I do remember the spray pattern looking weird from the beginning. Which is why I switched to tube style squirters and hollow bolts.

Just curious as to how much timing is being added by the vacuum advance? It is a pretty big player in engine performance at very light engine load........

Hopefully fixing the squirter solves it. I ran an AED 750 DP for years on my old 400. It was the best out of the box carb I ever had, they make excellent carbs. I also ran a Holley HP 750 that was identical but had to go up a couple squirter sizes to get rid of a stumble. Both were great carbs.

I was thinking a long cliffs idea.

You have a fairly high rated power valve in there, obviously chasing a lean tip in. Ive never had to go that high and seems just a little out of the ordinary.

Has me wondering if there is enough timing in the engine in that area, in particular the vacuum advance would help this area.

Yep, VA is a big player at light engine load and without it, or not using the right one you typically have to add a LOT more fuel to effectively burn a lean mixture or you will get hesitation, stumble, bog and even surging in that range. It's not as common with Holley carburetors as they are not emission calibrated and typically have PLENTY of fuel to the mixture screws and transfer slots. In almost all cases tuning at light throttle and light engine load is NOT cured by PV's or accl pump squirters and cams......FWIW....

This is a 383 stroker small block with a 230/226 @50 550 lift 110 lsa comp cam retro roller with 1.6 rockers and AFR 195 heads. 36 locked.

This engine does not like and traditional timing sweep. Anything less than that at any rpm, including idle, and it runs pretty bad. Im locked out at 36. But I still have 12 degrees vacuum advance but it does not apply at idle.

My vacuum is so low at idle but goes high after idle. I have to tune the PV to cruise idle.

Just a general rule of thumb with these things. Anytime you find yourself having to run more than about 10-14 degrees initial timing to get the engine to want to idle well and make good vacuum at idle speed the cam far from the ideal choice for the CID and static compression ratio. Not liking any less than 36 degrees locked-out timing tells me cylinder pressure is pretty low at idle speed due to the overlap bleeding it off and that the build isn't overly happy with the intake closing point.

Not a "big black" cloud over the build but observations made from neary 50 years of building engines, dynoing them, putting the in cars and getting track numbers on them, plus "custom tuning" thousands of them for a living. In any and all cases with engines like that they are going to have considerable "reversion" at lower RPM's which requires MORE timing and fuel to keep them happy......FWIW......

Thanks for the advice. I appreciate any advice. Especially from experienced people. I’m still learning. After waiting decades, this is my first modded engine. They have always been stock before this.

That sucks because I spoke to comp cams and my engine builder before buying this cam and told them I wanted something mild. Something that still sounded nice and was fast but not considered a big cam.

Any advice on what I should do? It runs even better locked at 42 degrees, but that seems crazy. I confirmed my balancer probably 5 times with a piston stop. I confirmed my timing with two timing guns.

Engines with well thought out compression and cam events for the CID will not want, like, or respond well to a lot of timing anyplace, especially at idle and low RPM's.

Most of the engines I build here are fine with about 10-12 initial timing, another 10 from the mechanical advance (20 at the crank), and 10-14 from the VA at light engine load. I seldom use manifold vacuuum to the advance but apply it with ported vacuum for all the benefits otherwise.

I don't know all the details of your build but have built a LOT of SBC engines over the years but never a 383. I do mostly 331's, 3.55's and an occassional 406. I much prefer the shorter stroke engines with higher compression and good flowing heads on them.

SBC's absolutely LOVE super-tight "squish", target is .035" and NEVER over .040" for any reason. Those builds turn into "turds" quicky with a lot of squish area combined with a butt-load of overlap in the cam, will require more timing, and lack the quick revving instant throttle response and hitting the redline instantly when you whack the throttle a bit. I've had so many SBC "builds" brought up here to custom tune that never had a chance of working well I could retire if you gave me $5 for each one!

Almost all of the "turds" I've seen in here had pistons too far down in the hole combined with thick head gaskets, plus tight LSA cams with too much duration and overlap for the true static compression ratio. Most of the owners/builders of these engines thought they were at least a full point higher in compression than what they actually were. That's where troubles begin. They also put WAY too much cam in them. SBC's are super efficient and the worst heads sold for them have more port flow and better chambers than some of the best Pontiac offerings. It's an engine you love to hate and they can make great power for the CID without a lot of duration and seat timing.

Even SBC's "top of the pile" 327/350hp and 350/350hp engines had LESS than 224 degrees duration @ .050". I've used the "old" 327/350hp cam in 355 builds and cranked out over 400hp with 10.6 compression and aftermarket 200cc intake runner heads on them. Plus they idle nearly stock, with good vacuum for power brakes and excellent street manners. Just add high ratio rockers and/or go to a similar HR cam and you can bust 425hp without much effort. Even so nearly EVERYONE thinks you have to stroke those engines with a 3.75" arm crank to make good power with them. I'd rather reap the benefits of improved bore to stroke and rod length to stroke ratios and spin the engine a few hundred more RPM's to make the same power......FWIW......

Agree, doing a 327 right now with AFR 195's, pump gas deal with a nice little hydraulic roller (227 @ .050). Paul had just finished another 327 ironically with the same heads, same cam, same compression that did 494 hp and 430 tq on his dyno. Pretty stout little package that's very mild.

I'm assuming you have a CD box with some sort of start retard? Locked out at 36 seems excessive to be needed at idle.

Usually what I do when I have to tune an engine like that is run 16-18 initial and then dial in some extra vacuum advance at idle. Usually another 10-12 degrees so it idles with about 28 degrees give or take but doesn't have to start on it. If I have an engine that still doesn't want to respond and idle cleanly with that then there is something very unhappy going on inside that engine. Cam and compression choice or maybe moving the cam to a different ICL is needed.

Hopefully your loose/stripped squirter will help to solve your issue but I still think there is another underlying issue going on as to why the carb is so hard to dial in.

Great info. Here is my set up. Let me know what you think. It’s a 1972 350 small block 4 bolt main. Virgin block. .030 over and decked .010. Skip White 383 stroker kit. Scat 3.75 forged steel crank and 6 inch forged steel Scat rods. Weisco 10cc D cup dished pistons. Pistons are zero decked in the hole. AFR 195 72 cc heads. I don’t remember the thickness of the head gasket, but my builder said the RA factor of his decking matching allows for stainless steel gaskets. I want to say no more that .030 thick. Plus that cam I mentioned. 230/236 @50 550 lift 1.6 rockers comp cams retro roller. Edelbrock performer rpm air gap dual plain. Headers, X pipe, and straight through mufflers. Plus the 750 AED Holley HP BODY.
9.5 inch 3200 stall. On the tight side with a #0 pump. Great multiplication. 082 stator.
3 speed auto, 3.73 gears, 28 inch tires.

I am running a progression ignition electronic unit. All the settings are set electronically. It has a MAP sensor for vacuum advance.