i know this is a repeated question.. but was never able to find a solution for it..
sometimes while driving slow and when i want to accelerate the engine starts to hesitate (as its going on and off), so i have to take my foot off the gas.. and for this not to happen i need to accelerate very slowly.. but if i hit the gas harder the engine stalls (as if it goes off)..
i have a 454 engine on my truck with all standard specs and a q-jet carb on top..the carb has recently been overhauled and spark plugs changed..and a new gas pump has been installed.
what could the problem be from?
distributer not giving electricity?, spark plug cable? carb?? gasoline pump? air issue?
why does this only happen when the gear is on "drive" never happens when the engine is on "N" and giving it high RPMs..
help please.
If it will spin high RPM's in neutral -- i.e. the engine is unloaded -- I am thinking that the ignition system is okay. Generally if plugs, wires or the distributor/coil setup are the problem, low RPM's will be okay but at high RPM the problems get worse.
*Might* be timing though... unhook the vacuum advance and check base timing; should be something like 8-10* BTDC, if I remember right.
On the fuel side, you would also think that a bad fuel pump would peter out at high RPM, but you can get a fuel pressure gauge and test it. Carb trucks are fairly low (4 or 5 psi maybe? I forget) but look for it to stay pretty stable and not vary wildly.
If the carb was rebuilt recently, we'll assume it was done by a competent person who knows how. After fiddling with mine for a weekend, I refuse to go inside a carb any more, and paid a local shop to do mine.
However, this means the vacuum lines were removed, and I am wondering if the vacuum advance isn't hooked up to the right place. Many folks say it should be on the ported side, which IIRC is higher on the carb; other folks like it at manifold vacuum, the lower side. In your case I would trace the vacuum like from the distributor, noting the location on the carb where it ends, and post up what that is. You'll get varying opinions, but it'll give you something easy to play with. (I seem to recall mine going to the bottom right of the carb, looking at it from the front, i.e. driver's side, but it's been a while and I'm too lazy to run out and pull the air cleaner and ducting off mine to look =)) You can also check the vacuum at idle and at WFO with a vacuum gauge, but it depends on whether you do ported or manifold.
Past that, as mentioned, vacuum leaks would throw the advance off the same way. Make sure that all the extra vacuum ports on the carb are covered well with the little rubber covers, and that none are cracked or loose. Also check the vacuum line off the back to the vacuum booster (or if hydroboost, make sure that the vacuum line on the back is plugged appropriately.)
If the vacuum is stable, it's possible that the distributor is at fault. The vacuum modulator (the cone-shaped thing the line goes into) can leak, or the diaphragm may be cracked. The lever assembly from it into the dizzy and/or the centrifugal weights may be gummed up too. Prolly worth pulling the cap and rotor off and looking inside, maybe spray some WD40 on & around the weights and make sure you can move them by hand, i.e. they're not frozen in place. This is a good time to make sure the wires to the ignition module are connected well -- assuming it's an HEI dizzy -- and that its screws are tight. (Again, I don't think it's an ignition problem, but these are general Good Things for the dizzy =))
That's my .02 -- and possibly worth at least three cents less, as I am in no way a professional mechanic nor an expert in diagnosing driveability issues
-- A
