Mississippi-Mud Forums

Chevy 305
Rochester Q-jet 4 barrel
HEI ignition


Started popping back through the carb under acceleration over a monnth ago. Got real bad, until I replaced the fuel filter. It was struggling up highway grades before the filter change. It still popped back if you got on it, though.

It has progressively gotten worse, and now it is hardly drivable.

mileage is 65,000 on a rebuilt longblock.

Here is what I have done so far:
replaced distributor with a rebuilt unit, including all electronics.
new coil
cap and rotor are only a few months old.
fuel cap IS venting properly.
overhauled the carb using a kit.

played around with distributor timing - advancing it helps, but not much.

As soon as you start out, it pops with anyting more than normal throttle. After you drive a couple of miles it feels weak, like the timing is off, or it is starving for fuel. It also begins to pop with very little throttle.

I need some good suggestions that I can list and go through one by one until this is solved. My wife needs her van to get to work. I need your help.

popping through the carb seems like it would be a timing or valve issue. i'd start with a compression test. that should give you a good idea if its a stuck valve or busted spring

I suppose it could be a broken rocker arm. I have seen that before on small blocks. As a matter of fact, I did hear some ticking. It didn't jump out at me before you mentioned it, because I am used to my manual lifters in the Nissan.

I may just pull a valve cover. It depends which is easier. Vans are wierd to access certain things on.

A broken spring would be obvious, and would explain it.

I had a 305 in a truck i had before, it had some similar symptoms, dad and i foud missing teeth on one of the sprockets on the timing set, replaced it with chainless job and fixed my problem. Im sure that will be hell to get to in that van also! Good luck

If it is the timming chain you can throw a timming light on it and watch the timming drift when you rev the engine. This would be the first thing I looked at. If it was in the head somewhere; valve, rocker, etc; you'd expect to hear it. Also if it was in the head it wouldn't change with RPM, it would tend to act the same at all times. Since you didn't mention any noise my next place to look would be the neddle and seat, and especialy the float, in the carb floatbowl. If the fuel pump is overpowering the needle you could be flooding as well. Both of these things fit the symptoms discribed so far.

What year is vehicle? I had a vacuum leak that caused the same problems.

It is a 1979 Chevy G-20 van.

Rogue, the timing is steady as a rock when checked with the timing light. I have not checked it as it is being revved; only at idle with the vac hose off and plugged.

I actually DO hear some tapping under the left valve cover. Sounds like a lifter tap. Not very loud, but should not be there at all on a hydraulic lifter engine.

The float is new and adjusted. I went back into it an verified the float height, and the fuel level in the bowl apeared to be correct.
I wonder if there could be insufficient flow, such as a weak (mechanical) fuel pump. However, I would expect it to bog, but not backfire.

You problem does sound like vacuum issue, I would look at the carb base gasket. I have had several that, even with new gaskets, leaked because the base plate itself was "eroded".

I'll check that, too, Will.

Antichrysler said he had those symptoms and it turned out to be bad wires. These have been on there a while.
If I do wires and plugs, I can check compression while the plugs are out.

good way to check vac leaks is spray ether on the base of the carb or near vac lines... if the engine revs randomly try it again to make sure you just didnt spray the nozzle the wrong way LOL... but that will find your vac problem... also check vac advance... see if its operating... if its not advancing when engine is reving up that could cause it to pop like that...