mastiff

I've slowly been getting all the kinks out of the new 383 I put in my 72. Now I've hit a problem I can't figure out. I'll start the car and it will idle fine. Its a little hesitant when I begin to drive and might backfire some, but runs fine as soon as it actually gets moving. Then, as I slow down for stop signs, turns, etc, the engine will sputter and die. When it starts to sputter I'll give it gas but the car will usually die anyway.

Does anybody have any ideas? I think its something to do with the carb- its a Holley 750. I was told to check and see if the power valve was alright, but I swapped that out with no improvements. The cars at a point where it dies every time you slow down and its driving me crazy. Any help is appreciated. Thanks.

The Money Pit

Start with the basics. Check the timing and firing order. I once thaught the engine felt flat at 110mph,and popped the hood to find one plug wire hanging loose. You'd be surprised how forgiving a V-8 with some power can be.

cardo0

Knowing the Holley model is a must. Vacuum secoundaries? Spreadbore?Holley is known for thier "modular" construction and ease to trouble shoot. Verify your choke opens once warmed up. There is a power valve that has to be selected for your engine needs - vacuum and altitude. Some models are not protected from engine backfire and the power valves can be ruined. Vacuum secoundaries need the correct spring to prevent lean bog. Mechanical secoundaries are adusted by the linkage arm. Six psi is what I remember for fuel pressure. You'll really need a book for jetting changes but I'd look here last. You can look for air/vacuum leaks with carb cleaner sprayed on the outside while looking for RPM changes (increasing).
Good Luck :chevy

mastiff

Its a Holley model 4150- mechanical secondaries and electric choke. The carb is brand new and was working well. It has only been in the last few weeks that the car has had problems.

hudman

If all else fails, you may want to drain the fuel tank.

My car was performing exactly as you describe, and after spending a lot of money at the local Corvette shop going through all the obvious stuff (carb, timing, advance, etc.), they decided to drain the tank. Store the gas in some big 5 gallon buckets and let it settle. The water will separate to the top of the fuel.

I ended up having about a quarter of a gallon of water in my tank from either rain water somehow getting in the tank over the years or by getting some bad gas somewhere.

They drained off the water and she was good as new.

cardo0

Try the easy tests first. Open choke when warmed up. Fuel pressure. Spray check for leaks.
My guess is the power valve and you really should have a "How To" Holley book to test for and select the right one.
Its just good experience to do it yourself. Master your carb now or pay the dyno. shop a lot more. :thumbs:

mastiff

I already replaced the power valve and that didn't seem to change anything. I guess my next test will be to check for vacuum leaks.