Hey guys, i own a 1987 corvette converted to carb and mech/vac ignition. I know this is a third gen forum, but i figure theres a lot more carb knowledge on here.

I got the car running 99 percent, perfect idle, cruise is also great,so is full throttle 2500 and up. I just have a stumble, if i floor it under like 2500 or so rpms. If im at higher rpms and floor it i have no hesitation or stumble.

Hers the basics of the car. 87 corvette coupe manual trans

stock Roller block, refreshed stock heads
Mild cam ( previous owner Dont know specs.)
Weiand xlerator intake manifold Imca legal version
holley 750 4160 Vacuum secondaries
hooker super comp headers 1 3/4 primaries dual 3 inch exhaust
Summit brand hei with adjustable vaccuum advance.
Running vaccum advance full manifold vacuum
recurved dist to 36btdc by 2500rpm vac advance disconected..
fuel floats on carb set

The motor is running great everywear except when, giving it throttle quickly at low rpms. At higher rpms i dont get this stumble no matter how fast i open the throttle .

my carb came with a .031 acclerator pump nozzle, i recently steped up to a .040 nozzle, this seemed to help but still the car almost stalls for a sec if i go wot at low rpms, floor it high rpm no hesitation at all.

would the idle mixture affect this? Should i try vacuum secondary different springs?

 

When I had a vacuum secondary carb, mine would often do that. I switched to a mechanical secondary Holley DP and its been running great with no tuning.

 

I have an engine with a similar setup to yours and running the same carb. i had the same problem with mine stumbling. I even took my carb back and got another one that did the same thing.

I finally found out that my accelerator pump just needed to be adjusted. Once i did that the problem was solved.

The carb you have should be more than adequate in stock form to run well on your engine, ive found that you just have to do a lot of fine tuning on holley's even when you buy one new.

Hope that helps. Good luck

 

1)hardly at all.

2) yes, but after you get enough fuel from the accelerator pump. the travel of the arm and the plastic cam profile have a lot to do with it.

one way I check is to watch the secondaries as you use your hand to rev the motor (hold it wide open for a second or 2, if the secondaries start to open, use a heavier spring(slower opening).

you'll know right away when you drive it if you have gone the right direction.

 

yea.. I have a 4160 650cm carb.... it is the accel pump adjustment... one of two possibilities.. either to much fuel or not enough fuel.... supposedly, when you adjust it you should have .015 in worth of gap between the pump arm and the part that pushes down on it. This is when the pump arm is fully peressed, and with the throttle in WOT position.. At least that what my book says..

I have not done that fine tune on mine. and i get that off idle stumble as well, with everything being nice and strong after 2500..

 

My carb came with the orange pump cam, With a screw in the number one slot. How do i adjust the position to 2? Someone told me position 2 gives more fuel?

 

there is a single screw that holds the pump came onto the throttle lever.. you remove that and ten move the pump cam up i believe. and put the screw into the second hole.

 

Change the shooter size back to .031" . Install a Blue Cam on the #2 hole and your problem should be almost gone. If you have an Auto and\or tallish gears ( Less than 3.42 ) you might have to go up to a #35 shooter..but no more. Pump cams control the timing of the Fuel shot. SBC's in general like a sharp, hard shot early in the movement of the throttle. The Blue cam in the #2 hole will give you that. Adjsut pump lever to pump cam clearance to zero at idle, then add 1\2 to 3'4 turn preloda. Open throttle to WOT...and check that the pump diaphram lever still has a minimum .015" of travel left. Minimum travel at WOT can be more of course. The .015" of pump lever travel is just the minimum spec to avoid damaging the diaphram. Usually only a problem with really big pump cams.

BTW, your timings coming in a bit too fast IMHO. 36 btdc by 2,500 is too quick. Shouldn't be in before 3,200. What is your static timing ? ( no Vacuum advance ). You should be looking at a minimum of 12 deg BTDC. Depending on Cam specs and fuel quality, you might even be able to run more. I can run 16 degrees lead timing ( on the street ) with an additional 15 degres of vacuum advance. 10.3 to 1 compression, Edelbrock Performer RPM heads with 70cc chambers.

SBC's like a lot of initial timing. It really helps their throttle reponse at low RPM's and builds torque. Thing is...you cannot run that much initial timing with your timing curve coming in so fast. All in by 3,500 is just fine. 3,200 bare minimum. Curve should not start till 1,000 to 1,100 rpm.

Can you supply heads specs ( Aluminium, Cast iron CC size ), Camshaft specs, Comp\Ratio and what base timing you currently have? Regards.