ok I have a 71 convertable its has a 70 lt1 350 (370hp) motor in it pretty much all stock except the carb carb is just a good old holley 750 vac. secondary. also has a msd 6 al box and msd tach. drive distributor.
Ok here is my problem at idle from a start or just reving the motor slowly it has a bad stumble wether the car is under a load or not pretty bad to the point that it will almost stall sometimes. I have adjusted the air/fuel mixture, I cant find any vaccume leaks. I know this engine is suposed to have a 780 double pumper on it but I wouldnt think that would make a difference. any help will be greatly apriciated.

 

did u put any fuel filter in line ,, if so replace it
and also chek the fuel pump lines

 

Sounds like the accelerator pump is in need of adjustment.

 

good fuel lines and filter I adjusted the accel. pump the way holley wants. with the gap at .015 at full open.

 

i had the same problem with my holley 750.
2 squirter sizes up and the stumble went away

 

that was my next step do you know what size you ended up with mine has a 31 in it now I was going to try a 35 thanks

 

ok tried a number 35 didnt help still has stumble any other ideas now im just getting frustrated.

 

anyone have any ideas??

 

Did the car ever run good? If so, something changed and there is no reason to switch squirters or pump cams or anything else until you KNOW what is wrong. You need to follow a precise troulbeshooting process for both the fuel system and ignition - yes, ignition. (90% of all carb problems are ignition). A good visual inspection of all components (dist cap, rotor, points, plug wires, plugs, vac lines, etc) plus a dwell meter, timing light and vacuum gauge will answer most of your questions.

 

I'm great at spending other people's money but I would put a new Holley on it and be done with it. If that doesn't work you can blame me

 

I have the same carb on mine....79 w/a mild 406,holley low rise intake,headers,etc.
Tried adjusting and no good.Took off the 31 squirter and put in a 42,went from a 6.5 to an 8.5 power valve and have NO more stumble,starts up right away even in 11 degree weather here in S Indiana.Got a full Accel ignition and 14 degrees initial timing.

Make sure your pump cam is in the #1 position and that the diaphram is good.You should be able to just touch the pump arm with your finger and see a squirt start.

 

vertvette71,

I had a 80508S Holley. It had some pitting inside the fuel bowls. Check to see if yours does too.

 

yes it used to run good when I first put the msd system and the carb on. Im not shure what happened it just kinda all of a sudden went down hill and went down fast. Im thinking it is a carb problem. im going to try to switch the carbs and see what happens if that wont solve it I will try putting the stock points distributer in and see what happens thank you for all your help i will let you know what I find out.

 

also I know the 70 lt1 motor came with a 780cfm carb is there a better size carb that would work i have a 750 vac secondary now.

 

The factory carb for your motor was a Holley model 3310-1, (this number is stamped into the airhorn) with vacuum secondary and rated at 780 cfm. Holley sold tons of these back in the '60s and '70s. Your present carb could be a model 3310-2, or maybe even a 3310-3 depending on it's age. In any case your 750 and the factory carb are essentially the same. The difference is the factory carb had a metering block on the secondary side vs a metering plate (3310-2 and up) on the secondary side - they flow the same amount of air and will perform the same on your motor. Holley changed the cfm rating of the 3310 in the early '80s for some reason. I still have a 3310-2 that I bought new back in 1976 and it still works great. If your happy with it's performance keep using it. Back to your problem. Again, if it ran good and now it doesn't - something changed and it won't be that hard to find if you go about in a logical order. The best bet on the carb side is the Holley troubleshooting guide on their website. And, for whatever it's worth, the MSD 6AL provides nothing in the way of a performance gain, and the 2 MSD distributors I've used in the last 7 years both failed and lead to a miserable fight with MSD over warranty. No more for me.