Hot Rods sbc stumble that won't go away

ok sbc gurus i got a tuning issue. i posted a while back on cyl head suggestions for my car. i got some good feed back. now i have the car running and I'm not happy with the way it runs off idle. here are the specs. the engine is a 327 with a six inch rod. it is mechanically sound. it was rebuilt last spring. carb is a new 750 dbl pumper stock jetting and squirters 6.5 power valves 1.5 turns out on all four idle screws, air gap rpm manifold, holley alum heads that are brand new 202s with 180cc ports straight plugs, roller rockers stock ratio. solid lifter cam 520 lift duration at .050 intake is 244 exhaust 252 compression is 10.25. headers are ****** super comp. msd ignition 6al with a pro billet dist. timing is 12 initial 24 advance 36 total. the timing is all in at 3000 rpm. i have the red bushing and two blue springs in it. the dist was set up on a dist machine. plugs are gapped at .045 with msd wires. all quality parts in the build. warms up to 185 degrees. trans is a four speed. gears are 4:11 car is a 66 chevelle about 3200 lbs now the problem. the car starts right up and idles great at 900 rmp. under full power it hauls ***. 900 rpm to 1800rpm is smooth. its at 1800-2200 rpm that it gets a stumble. this is under slow acceleration like leaving a intersection. once the car hits 2200 rpm the engine is happy again. the problem is repeatable. i have verified timing, fuel pressure, accelerator pumps working, valve lash, firing order, rotor and cap for cracks, no va***n leaks. i have dis***embled the carb and blew it out with no help. i have fresh gas in the car. short of installing a air fuel gauge i have run out of things to try anyone have any ideas?

 

Four idle mixture screws? Mine all had two. Maybe try a vacuum gauge and see if the problem kicks in at power valve 6.5? Play with float levels any?

 

i checked the float level. some holleys have four idle screws. i have not used the va***n gauge yet. that is a good idea.

 

post pic of carb with four idle screws

 

Carb is a dp It has a rear metering plate just like the front. Your problem sounds like you have the wrong power valve. It is to low. Get a vacuum gauge on it and read what the vacuum is at idle. Then get a power valve that is .5 higher than the vacuum reading at idle.

 

vacuum gauge will reveal your problem.

 

I was told to go two numbers below the vac reading for a stick car. Can you explain more.

 

My Holley 750HP has four idle mixture screws and the secondary has a block instead of just a plate.
Your problem is probably a combination of wrong power valve and the float level is too low.
Go to the Holly website and watch their videos on tuning, free and good info.

 

Are the throttle shafts tight ?

 

brand new carb. going to the shop now with a vac gauge.

 

You are running a race setup not a street setup. The timing should be fully advanced between 1800 and 2100 rpm. Also for coming off idle and partial throttle you need a vacuum advance on the dizzy hooked to manifold vacuum. This also allows the initial advance to be lower for ease of starting and makes the engine very responsive coming off idle. This is a tried and true formula. Of course you want as much mechanical advance as is available. Your currrent setup is for idle and wide open throttle, not really a street setup. have a good Easter

 

I changed the power valve to a 4.5 it had 9-10 vac. This helped some. I changed the stock jets going up two sizes. This helped a little more. The car is now driveable at 2000 rpm. There is still a slight stumble but way better. I will talk to my guy at the speed shop about a vac advance on tuesday.

 

If you have 9-10 vac you should be running a 8.5 power valve. When you are driving the vac is much higher. You want to stay on the idle circuits when you are at idle, not trip the power valve. The lower the power valve the more you have to open the throttle to get into the power circuit. If the power valve is to low you will creat a lean condition when you have to open the throttles to far with out getting into the power circuit.

 

For sure you need to change the timing to total out under 2400 and hopefully lower. Adding vacuum will probably require a distributor change. Totaling at 3000 is not a performance tune. If you want to test it throw in an old chevy dizzy with these mods before spending more money. I think you will be surprised with the results. The car is not that heavy and you have gearing that will help and with the described compression ratio you are running good fuel already.

 

A lot of times a stumble can be cured by going up one or two sizes on the squirters. Cured mine.

 

the holley web site and chevy hi performance site say take the vac reading at idle divide that by two. i had 9 so thats why i put in the 4.5. i have lighter springs for the dist. I'm going to try them that should bring in the timing earlier. i thought about the squirters also. the problem is also there holding a steady 2000 rpm. the squirter is not doing anything holding steady rpms.

 

Are the transition slots squared on the primary and squared or less on the 2ndary?

 

Pull the carb, flip it over and look to see how much of the idle transfer slots are exposed. You are having a problem in the transition between idle and full power.

Might have the idle screw too are open, exposing too much transfer slot and drawing excess fuel in and it has to clean up before it runs right.

I could also be the squirters in the same circuit. Have somebody follow you, stomp on the gas and see if the person behind you sees black smoke when you stomp it. If they see smoke, the squirters are too big. If they don't see any black smoke, the squirters are too small. Go 2 sizes at a time.

Your carb is a touch too big for a 327. It could also probably stand a smaller jet and be sure the accelerator pump is adjusted correctly.

SPark

 

slots are ok. on other engines i had to drill holes in the ****erflies so i get that idea. i ran a bigger jet due to our reformulated gas around here. it contains up to 15 % alch. i just tried the lighter springs and things got a little better. throttle response improved a bunch. i can be in first gear going 15 mph stomp it and destroy the tires. timing should all be in by 2500 now.

 

Do you have good response to turning the idle mixture crews in and out? Can you tell a big difference as you try to adjust them?

SPark

 

yes if i screw one in all the way the rpms drop from 900 to about 700. when the screw comes back out the rpm goes back up. with the power valve change and the faster advance the car has really picked up. it now destroys the tires in the first two gears. there is just the slightest stumble now. I'm getting closer.

 

With a cam that radical think about advancing your initial timing to 16-20* , you'll have to limit your mech. adv. to stay at 36 or so total , but the extra initial should help to clean things up a bit....then maybe your carb will need fewer changes..... do not under any circunsances drill the ****erflies , that's pretty much a hack job way of doing things...there's always a better way...
dave

 

i don't plan on drilling this carb it idles fine. i will try advancing the dist on tuesday. with 12 degrees it starts fine hot so maybe it can use some more. i may have to change from the red to the blue bushing.

 

2OLD2FAST, I've had several very high dollar, big name, custom built circle track carbs with drilled plates. It's still pretty common when you get into the really radical roller motors where you drop a lot of RPM in the corners and need to pull off again lap after lap. I wouldn't consider it a hack job, just some are too quick to go that direction when tuning could have fixed their problems.

SPark

 

A 31 squirter in the front acc. pump will cover up a lot of problems.

 

the problem is also at a steady cruise around 2000 rpm. i dont know how a squirter will help when its not being used. the throttle shaft has to rotate to make it work. i did change pump cams from pink to white when i first started to go after the stumble. it did help some. now im after the steady cruise stumble.

 

I'll throw this out there...when I was chasing drivability problems w/ my 2x4 tunnel ram , I went back to what I was taught years ago...write down anything & everything you do , time consuming but needed... also w/that cam , you may not be able to cruise in 4th gear at 2K rpm , I don't have as much cam & can't pull steady under 1500 rpm in 4th.. wants to stumble/buck.. might just be the nature of the beast ??
dave

 

switch to a 190 thermostat. I built a 355 with an Air-Gap intake and it kind of stumbles sometimes until it gets good and hot under the hood. Cheap to try.

 

Having a vacuum advance will fix that.

 

Are your heads Holley's 20 degree valve angle ones??

pdq67