popping through carb

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.

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Originally Posted by richmondb

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

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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.

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Originally Posted by DennyW

I had to think a little.

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Did it hurt?

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Originally Posted by DennyW

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

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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?

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Originally Posted by richmondb

How would I know if I had a bent valve?

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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?

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Originally Posted by richmondb

...and then crank over the engine with the battery disconnected to get a reading right?

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umm, I don't know how you would crank over the engine fast enough without the battery connected. All the power to you but your wretchet arm better be real quick, maybe you have experince in this department..

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Originally Posted by richmondb

.. and then crank over the engine with the battery disconnected to get a reading right?

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Ohhh, sorry, you meant the battery disconnected to the ignition!!:LOL: :LOL: Yes, remove the coil wire but if you can, have the throttle open when you crank it. When the test is done, make sure you close the throttle if you have it propped open.

How close have you looked at the rocker arms? I bet dollars to donuts, that you have a broken rocker or a bent pushrod.............or, a collapsed lifter.

I think Poncho62 is right it sure does sound like your not getting an exhaust valve open somewhere, which is why it's popping out the carb.

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Originally Posted by poncho62

I bet dollars to donuts.

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OK, 100 donuts. Oh, and if I lose, the donuts might get a little stale before they arrive via snail mail especially over the holiday season:LOL: .

What is a collapsed lifter? I will do the compression check tomorrow and let you know the results. I think I would rather it be a bad coil than have to mess with replacing a spring or a lifter.

Yeah, no loud clicking or smoke out the exhaust. Oil pressure is fine. That's why I am leaning towards the coil going bad.

I had this problem on a 6 cyl engine once we did every thing and checked everything. Finely we took off ALL vac. lines and caped them problem went away. Found the PCV was stuck open.

I hope this problem turns out to be something that cheap. I checked the PCV, it rattles when I shake it.

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Originally Posted by richmondb

...it rattles when I shake it.

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This is good. If ever you shake it and it does not rattle, you know you got a problem.
I've heard of guys who shake their valve and it does not rattle, do you have any idea what they have to go through?

Count your blessings! It is not as cheap as you may think.:CRY:

Some poeple have gone through deep pockets to rectify this situation. A faulty valve can cause, leaks, misfire, and even situations where you do not have enough braking power. In other words, you can not stop.

Why?? Because of loss of suction.

lol, heres a vid of our Coupes zz4 "popping the top"

http://videos.streetfire.net/video/2...9a00ffe963.htm

hahahaha

Note: This is a old video, from mid September. There have been no backfires since the carb got tuned and the timing set.

You never said what the dist is-if its points, a condenser will do that and if not, a bad module can do it also. You might want to check and make sure your advance is working too. I realy think your dist is at fault and a switch out of the dist might tell you something. And that part is easy!

Well, I did a compression check and sure enough I have a cylinder with 0 psi :( I pulled both rocker arms and they looked fine. Both push rods looked fine also. The springs don't seem to be broken, but the inner ones are hard to see so one of them might be. Now what do I do? Should I just pull the head to see if the piston has a hole in it or what?

Spin the motor by hand first and see if your pushrods go up and down. Could be your cam has dropped a lobe or a collapseed lifter. If not then pull the head. Then you might find a blown head gasket, hole in piston or most very likely a burnt intake valve or seat. Then take an asperin and see the engine rebuiloder to reseat or replace your valve(s) in the morning.

I checked the rest of the cylinders. I actually have two bad ones. Both middle ones on the passenger side of the engine. Could that be a blown head gasket? That would probably be the best case senario. Worst case senario, I have a hole in the engine block between the cylinders.

Its a head gasket.....if it was a hole in the block, smoke would be everywhere.....not to mention, all kinds of weird noises.

Head gasket probably blowen between them That does happen used to happen a lot "BACK IN THE DAY":LOL:

Check to make sure the block isn't eaten away or the head in that area if that is the problem. If you cought it soon enough you'll be OK

This is starting to sound more loke the valve clearances were set to tight. If you had a hole in the piston the engine would be pumping smoke out the exhaust and breathers, but you have not mentioned that.
Can you get your hands on a cylinder leakdown tester ? Rotate the engine untill you close both valves on the bad cylinder, than hook up the leakdown tester to the plug hole, pressure it up and listen, If it hisses at the carb = bad intake valve, if it hisses out the exhaust = bad exhaust valve, if it hisses out the breather = bad rings or hole in piston.

BLAST ! :eek: Went back and read the query properly, two middle cylinders on pass side of block, I agree with everybody else, probably a blown head gasket.

Hey Rich, If you have an aluminum coated exhaust system it could have a partly blocked exhaust. If it's not flowing good,it will run fine at idle and fall on it's face as you open up the gas. Tommy

I just got the passenger side head off and sure enough, you guys were right. The head gasket is blown right between the two middle cylinders! Man you guys are good. That is a cheap fix. Lots of labor, but a cheap fix.

Check your block real good at the leak.

The block looks good. I only drove it about 1 mile after it happened and that was at 30 mph. What type of head gasket should I get to stop this from happening again. I have a 4" cylinder, should I get a 4" gasket or should I get one a little bigger? How much difference does the thikness make?

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Originally Posted by richmondb

What type of head gasket should I get to stop this from happening again?

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Cometic. That is the name brand and they can be reused and you can order any thickness you like. They will cost you $200. Edelbrock and Felpro make regular ones and if your heads are true, you can use them. Cometic can handle nitrous, blowers and turbos.

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Originally Posted by richmondb

How much difference does the thickness make?

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I would reccommend .040. Thickness matters in the area of compression, but not durability, hopefully that helps.:)

I have a 350 bored 30 over. The cylinders measure 4" exactly. World products S/R Torquer 67 cc heads, Comp cams XE268, edelbrock performer intake, I just ordered some Thorley tri Y headers. The head gasket that blew out was part of a $30 summit engine rebuild gasket set. I guess I need a more durable one. What other info is needed to determine size and thinkness of head gasket needed? Any recommendations?