351w off idle stumble maby ????

hi guys i'm having trouble with my 351w it has what i think is an off idle stumble. its a stock motor with 70000km with a holley model 4160 carb i have read posts about changing the booster pump cams and nozzle's witch made no difference at all i tried disconecting egr valve changing from ported to manifold vac advance changed plugs and wires dis cap and rotor button, nothing i did made anny diff. i'll try to explain what the car is doing when i step on the gas it starts to go then stumbles then takes off the lower the rpm the worse the stumble it will even do this while i'm doing 140 km hr on the highway if i back off on the gas and then back on it although recovery time from the stumble is quite noticably shorter. i've also set the timing according to spec. any help would be greatly apreciated as i have no idea what to try next


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Is you accelerator pump arm set tight as it should be? There should be zero slack between the arm and the pump actuating lever. Even the slightest movement of the throttle should squirt gas into the primaries. Also, what secondary diaphragm spring are you using? Might try stiffer springs to delay the opening of the secondaries. If they are opening too soon, air flow velocity goes to zero, killing the signal to the booster venturies and killing gasoline flow.

my 351w did the same thing and i took my holley off and put an edelbrock intake and 850 on it and it straightened right out now i made my 351 into a 426 stroker with dual holley 450s it pushes about 650 hp

the pump arm is set proper and i went from a purple secondary spring to a plain steel one anny more ideas ??

It is only a problem in the acclerator pump circuit if it is immediately out of idle. If it occurs above that it is probably an incorrect powervalve or a timing issue.

Check your float levels again.

Set your mixture using a vacuum guage. Take the highest reading from the guage while the car is in park if auto, or in neutral if manual. Divide this number by 2 and add .5. 16" divided by 2 is 8 + .5 = and 8.5 powervalve.

Get an advance analyzing timing light and figure out exactly what your timing curve is. Without knowing that it is pointless to try and tune the carb or anything for that matter.

thanks 77 i will try the timing i changed the powervalve to the correct size and set the mixture screws with a vacuum gauge. when i put the carb on the motor i checked the timing it was 9 deg btc and i set it to the factory recomended 12 deg btc and come to think of it thats when i started having the trouble. i'm glad someone mentioned something other than the carb i was getting ready to throw it away i've been over it several times and everything about the carb seems as it should. i'll play with the timing and post a reply i hope it does the trick

fixed it put a new edelbrock model 1405 on it runs like new

Well...You narrowed it down to a carburetor problem...



That is after determing there is a .015" clearance between the actuating lever and accelerator pump lever at WOT, correct?

Yep .