Heys guys im running a holley 4160 vacuum secondary 750 cfm carb on my Mild 350. Dual Plane intake, small cam, headers.

Fuel pressure is set to 6 psi, and fuel bowls are set.
Base timing is 14 btdc and i get 36 btdc at around 2500 rpm with vac advance disconeccted.
Idle mixture is set to get the highest vacuum reading at idle.

First problem is that i can not get the idle any lower than 1100 with the Idle speed adjusting screw. I have it turned out all way and 1100 is the lowest i can get it. It seems my secondaries are open a little is there an adjusting screw i can close the secondaries more?\

Second problem is this hesitation i have, if i floor it from a stop. the engine almost dies for a second then it picks up. Ive tried this with the secondary linkage disconected, and i get the same same stumble so it cant be the secondaries opening too fast.

Ive played with different squirters, from 31 to 41. i just cant get this stumble to go away.

 

What is your timing at idle with the vacuum advance hooked up ?

If it is an automatic, if you floor it from idle with an automatic, the carb would hve to tuned perfect for no stumble, mine would probably stumble a bit if I floored it from 1000RPM. Fix the first problem first and then worry aboit the off idle stumble.

 

Secondary adjustment screw is on underside (tiny screw).

Do you have a vacume leak?

 

Had a similar problem.same carb & basically the same engine set up....the rpms stayed around 1100...had a vacuum leak...found it by spraying carb cleaner around the base of the carb and then between the intake and the block....found a small leak on the gasket between the intake and block....fixed it and now it idles purrrrffffectly

 

I would agree with the others as far as checking for a vacuum leak first.
Next I would verify and make sure you know the timing is 100% correct.
Next I would look at the plugs and see what they look like.
From the results of looking at your plugs what about your jetting?
Next usually an of idle stumble is a squirter issue. Smaller number squirter = less of an initial shot of gas but the gas is delivered over a longer period of time, larger number squirter = larger initial shot of gas but delivered over a shorter period of time. Try a larger squirter first if you still have an off line stumble. On the secondary side get a secondary spring kit. You want to pull the secondaries open as soon as possible without causing any kind of bog after you have left the line. Start messing around with these things and document your combinations till you find the right combination.

 

If your secondaries are cracked open too far, just lower the secondary idle speed by backing off the secondary idle speed screw. It's located under the carb on the passenger side. Backing it off 1/2 turn should get your idle down where it belongs.

 

On a mild 350, I wouldn't run any larger than a 600 cfm. Just my .02

 

For the hesitation problem, you need to make sure the accelerator pump is adjusted properly. If you still have the problem after this a more aggressive pump cam will fix it.

It would be a good idea to get a Holley manual also.

BigBlockk

Later.....

 

 

Why is this Wayne?
Mine is an auto and has a stumble too, gets a bit better as it warms, I just figured it's something I need to tune out with the accel. pump. Willthere always be some, just cuz it's an auto?

 

650cfm holley and it will give you a very crisp throttle response, 750 on a mild 350=too much, you will kill yourself adjusting that off idle stumble out of it.

 

 

When you pull your carb to fix the vacuum leak, flip it upside down and set the primary side throttle plates such that 0.025" of the transfer slots are exposed below the bottom edge of the throttle plates. Leave it set there forever. Set the idle speed with the secondaries (1/4 to 3/4 turn off fully seated position), and with the PCV valve selection if necessary.

Also, that carb is huge. The 570cfm Street Avenger would have been a much better choice. Just think, some dirt track series engines make 550hp with essentially just the primary side throttle bores of your carb. Late Model Stock engines make almost 400hp with a third of the flow of your carb and mediocre heads.

 

No it's got nothing to do with the auto really. Just with a stock auto you would be starting at such a low RPM ( as compared to stick or a stall converter ) that the carb would have to be perfect or near perfect not to hesitate or bog if you punch it from < 1000RPM's, I know I have problems getting rid of a hesitation punching it a 2300RPM sometimes.

The reason I asked about what the timing was with the vacuum advance hooked up was that the vacuum advance might be advancing the timing too much ar idle and this could cause run on.

Or there is another scenerio where you don't have enough advance at idle and you bump up the idle to compesate and this also causes run on.

 

 

First place, stumble off idle is a tuning problem, and has nothing to do with the size of the carb (not to step on anyones toes). The 750vs carb is a very good carb especially for a mild 350 if it is tuned right.
The best fix for your problem will be to take the carb off the engine and flip it over, on the throttle plate on what would be the passenger side of the carb there is a set screw for the secondary butterfies. Pull this out and put it in backwards so you can get to it from the top (this only makes it easier to adjust later without pulling carb). Now set the butterflies till they are fully closed and then back it out 1/2 to 1 turn as an initial adjustment. If your idle is still high at this point, try turning the secondary IN up to 1/2 to 3/4 turn only to bring the idle back down. This should be done AFTER the float levels and curb idle adjust screw are adjusted. Then go about adjusting the carb as usual, keeping in mind that there still may be a hesitation if the spring in the secondary is opening the secondaries too early for the demand of the engine.
BTW, Holley recommends a between 650 and 700 cfm carb for the 350 with a stock converter in a car heavier than a camaro. They also recommend a LARGER carb for lighter cars, those with higher stall converters, and those with a higher rear gear or manual trans using a higher rpm launch speed. There is much more to the choice of carb size than just a theoretical value based solely on the cubic inches of the engine without taking into account all the other variables involved with the application such as those mentioned above. I am not trying to say anyone else is wrong, just that all factors cannot be taken into consideration when these recommendations are made based on the limited info in the thread description.
(FLAME SUIT ON)