Light throttle stumble, Holley carb.

Dont think they would get much out of it Johnnie. The bolt-on brigade has arrived.

 

With that big cam and small carb, with the engine off, look down carb throats and check the position of your throttle blades. If they are passed the idle circuits with the engine off, your idle circuit isn't working with the large cam. You can drill holes in the front half of your throttle blades to give the engine more air. This will allow you to turn down the idle so your engine can use to the idle circuit. Best book ever, "How to Hot Rod the small block Chevy".

 

While I agree that the 1/2 the idle vac (in gear for an auto) is acceptable, I always had better luck with dropping a number or 2 when the cam is big. I think the half reading is better for a clutch car. JMO.

He has a heavy car with a big cam and an automatic. He admits the 2500 stall converter doesn't flash right but is closer to 1900. He needs to stay with a vac sec carb on a street car. Yes, people make double pumpers work well but vac sec are designed to only open when needed by the motor while a double pumper forces the issue. Especially important with an automatic in a heavy car, look at the whole picture.

As for AED's method of adjusting an accelerator pump, that is not what Holley recommends doing. The .015 clearance at full throttle is a safety built into the carb to save the pump arm and diaphram damage. Maybe an AED modified carb should be adjusted differently but he has a Holley, not an AED modified carb.

I've done it this way for 35 years and it works, I know that much.

SPark

 

A few of us are looking at the whole picture. I suggested a DP because of the cam he is using. You can tune any carb to work. A StreetAvenger carb (and most vac secondary carbs) are not set up for a big cam idle vacuum and the way it responds off idle. It would be closer to tune from a DP, but he has requirements for tuning what he has.
Getting the timing worked out is first anyway. Not enough initial for that cam/combo. More initial will give more vacuum and allow the throttle to be closed more at idle. After that is done, I suspect it will still need a bit more carb tuning. Gonna end up similar to a DP.
Holley or AED, either pump adjustment method works. One is faster. Holley's recommendation just ensures no bottoming out.
Much faster this way. Loosen it up until ya got some slop, tighten until ya don't. Check that ya have a shot as soon as the throttle moves. Ain't gonna bottom out unless ya got a cam for a 50cc pump on a 30cc set up. JMO.

 

thanks for all the input guys. i think i'll try a few different things with the tuning before i buy any parts, just to see where i can get it.
to reiterate, the stumble isn't a real problem, just an annoyance. she pulls hard above 3500 and will accelerate from a 3rd gear pull nicely, she just doesn't have a lot down low. not ALL bad, but i would like to be able to fry the hides with this motor that clearly has the capability. i'm pretty sure the convertor needs to go either way, as it will move the car if i take my foot off the brake at idle.

 

did you try putting in bigger squirters in the primary? I didn't read everything, but I lef the accel pump alone and just upped the squirters and fixed my stumble. stumble is caused by A lot of air when you open up the blades and not enough gas to go with that air till the carb catches up

 

i didn't. the stumble is while moving. transition from idle to primary when i leave from a stop is fine. it happens while cruising at light throttle.

 

exactly - when you are rolling and open it up - it'll stumble then take off

with the car off. look in the carb and actuate the throttle by hand to full throttle. It's the two squirters that shoot the jet of gas from the accel pump. I was thinking they were on primary side - maybe the secondary side, it's been a while since I fixed mine. Each one has a screw in the top that holds it in, you can see then when you look in the top of the carb. Go up two sizes and see what happens. You can remove the squirters with the carb installed, obviously, just be carefull and don't drop the screw etc. Can not fall into intake if throttle is closed anyway.

 

adjust the accelerator pump as lm14 suggests. don't do anything else. that's probaly your whole problem. also, a white or brown accelerator cam are not as aggressive as the red ones. i have race mods on circle tracks for years and this adjustment has cured many accelerator woes in the past. steve

 

If the camshaft was installed straight up, how about advancing it a couple degrees? A little more low end torque, a bit higher vacuum at idle, throttle blades maybe slightly more closed at idle? Might make it more responsive with the existing converter and axle ratio.

 

not exactly. if i hit the throttle, it's fine. it happens when i'm rolling along at light throttle, staying at a certain rpm. it just cuts out for a half a second. sometimes a couple seconds between hiccups, sometimes a few minutes. if i mat it froma light or hit it while rolling, it's fine.

 

Sounds like a vacuum leak or lean primary to me. Course a vac leak will at certain speeds cause it to be lean acting only at certain times. Esp with a non stock cam. JMO Lippy

 

so at a constant throttle/rpm situation it's cutting out?

When you said stumble, I interpretted that you were giving more gas to accelerate at that instance it was stumbling.

Then that's weird, seems like it's not keeping up with the gas flow or the gas to the venturis is intermitten

 

that's exactly what it's doing, but only at light throttle. like it's sitting somewhere just between the idle and primary circuits, "confusing" the carb. it pulls hard and smooth, it's the light throttle cruising that's giving me the problem.

 

Lower your float level a bit, may be drawing over the top.

SPark

 

i've got the float set so fuel is just trickling out of the sight plug hole.

 

Bad choice. That will move the intake closing point earlier, and increase cylinder pressure, which will mean he will have to pull even more initial out, and the stumble will get worse.

 

Again, this will make it worse, not better.

 

The issue he is having is not a launch issue, its an issue with the car transitioning onto the main circuit at part throttle, yet with the exception of Johnnie and I, virtually every "solution" offered is a solution I would be looking at if the car was bogging on the starting line. You really do have to read the question he is posting carefully.

 

I'm tricky like that!!

Seriously, I really appreciate everyone's input, even if it doesn't apply. See, now I know what to do if I get a stumble off idle!!

 

Bring it my house and I'll fix it..

 

LOL! Yea, good point...

 

Sorry Dueces, I have dibs. Just drop it off, put the keys under the mat. Come back in oh, say October, and I'll have it running great...

 

hahahahaha, i'll leave a case of beer in the trunk for payment.

if either of you were close, i'd bring it by!!

 

Update? Did you fix it? I'm still chasing mine down.

 

I'll bring this one back from the dead to update and respond (albeit, terribly late ) to the above.

Yes, I somehow fixed it. Not sure what I did, but it's gone. I removed the choke plate and assembly from the carb, lowered the float level a hair, added a K&N Stub Stack, added a taller air filter to clear the stack, changed the intake over to a RPM air gap, disconnected the vacuum advance (for the time being until i can find or fab an advance limiting plate for the MSD distributor), and the stumble is gone. The car pulls noticeably harder at WOT, so I'm not sure if there was some sort of issue before, or if the new parts and changes really made that much of a difference. It does want to light the tires up from a dead stop much easier than before as well. Either way, I'm pretty happy with it.

 

So much for changing one thing at a time.

 

The only real change to the carb was to lower the float level, and maybe a few added CFM from removing the choke assembly. If one of the other changes cured it, it obviously wasn't in the carb.