Thread: popping through carb

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Okay, I think I screwed something up. I have a 350 recently rebuilt (1000 miles ago), edelbrock q-jet carb, XE268 cam. I took it out yesterday for a spin and got on it a little bit. All of a sudden it started running really crappy and sputtering every time I touched the gas. It will idle fine but when I touch the gas it sounds like I have no exhaust system from inside the car. I got it home and popped the hood to see what was wrong. When I give it gas, the carb pops like it is backfiring a through the intake. This is at any rpm above idle. Idle is also rough. I checked the distributor and it's adjusted in the right spot (didn't slip). I checked the plugs and they look fine. I pulled the valve covers and all of the springs look fine and everything moved up and down like normal when I started it. What else could cause this? It has a double roller timing chain so I don't think it would have skipped a tooth.

Originally Posted by richmondb

All of a sudden it started running really crappy and sputtering every time I touched the gas.

You are running lean. Popping through the carb means it is not getting enough fuel, provided the timing is set correctly. If you are having cold weather, it could be that the choke needs to be set richer.

It was running fine before I mashed on the gas. I changed out the fuel filter but that didn't fix it.

I can't say for sure if the valves moved up and down exactly the same amount. They all moved up and down and I didn't notice any differance in the distance traveled. The carb was getting plenty of fuel when I changed out the filter. The plug wires are in good shape. I checked the distributor cap and rotor, both still looked new. I have a Mallory unilite distributor. Not sure about the accelerator pump.

I have been doing some searches and reading up on the problem I am having. I am worried that maybe I lost a cylinder or something by bending a push rod or breaking a spring. Is it possible to jump a tooth on a double roller timing chain?

I also removed a spark plug wire while the engine was running and didn't notice any difference so I put it back on and removed the one next to it. Still no difference. I tried holding the wire close to the engine to see if I could see a spark, but I didn't see anything.

Vacuum secondaries, and they're closed.

I wish I could. It sounds like an old tired engine that chugs when you press the gas. Sounds kinda like a lawn mower.

I tried checking that but I didn't see any spark when I removed the plug wire and held it next to the engine while it was running. Should it be strong enough to come out of the boot and hit the engine?

Do you think it could be the coil not able to create enough spark when I give it gas. That would make since. Do they go bad all of a sudden like that?

Another area to check is your balast resistor or resister wire, when cold voltage shouild be 12. 3 volts and when warm voltage from resistor to coil should be 9.4 volts.

I didn't change anything from the way it was set up when we put the engine in it 1000 miles ago. You don't think it has anything to do with the valvetrain? I was messing with the ground on the ballast resister. I took it off and added another wire to it for an oil pressure gauge. I double checked and they are both grounded still.

Originally Posted by DennyW

I had to think a little.

Did it hurt?

Originally Posted by DennyW

Check to make sure the secondaries aren't hanging open. That would cause a lean problem, and rough idle.

Check it again, check the secondary butterflies. There are two sets, the upper and then the lower. I had a butterfly cam move on me and it drove me nuts. It was holding the secondaries open slightly.

Also look for vacuum hoses which might have popped off. If a rocker broke or a spring got weak or a valve bent, it would pop as well.

I have an edelbrock q-jet with the big secondaries. They are closed. I am not sure about what you are refering to as the upper and the lower. The two back valves on top of the carb are closed. All of the rockers looked okay and I couldn't find any breaks in any of the springs. How would I know if I had a bent valve?

Originally Posted by richmondb

How would I know if I had a bent valve?

If you did a compression check and if one cylinder was low in comparrison to the others, this would be an indication. Also, if you have the valve covers off, and as you rotate the engine, if one rocker is loose when the valve is in the closed position.

I'm going to go buy a gauge to do the compression check. I just have to take out a spark plug and screw the gauge into the plug hole and then crank over the engine with the battery disconnected to get a reading right?