Motor is popping back through the carb... Help!

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dustermaniac

dustermaniac

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So I have recently changed out my cam, timing chain and gears, lifters, pushrods, oil pump, pickup tube, oil pump shaft drive gear, spark plugs, cylinder heads. When I changed out everything I made sure that both timing marks lined up dot to dot, car started right up at initial fire up and ran well. Took it out tonight and on the way home from a short run (about 5 miles) the car started popping back through the exhaust and carb. Throttle response is not good. Got the car home and popped hood and checked for any signs of spark that may be jumping. All wires look good, no sparks jumping, cap looks good. wires and cap are less than 1 yr old. no visible intake leaks. tried to adjust distributor, does not matter where I set it at, it still pops back. Wondering what the issue may be. Carb is an edelbrock 600 and is fairly new.
 
Tom, you're not going to like it. Something is holding an exhaust valve shut. This will not cause a skip, but reroutes the exhaust back up through the intake port and out the carb. Although it does not skip, it certainly does disrupt an otherwise good running engine, doesn't it? Things to look for on the good side: Broken rocker arm, bent or broken pushrod. Things on the bad side: Wiped camshaft lobe. I said you wouldn't like it.
 
I can't win. I am ready to pull this motor and look for another one. Pushrods are new and I do not think they are bent, car has only been run maybe 20 miles.... Maybe the camshaft lobe??? Rocker arms, I got another set. Man I do not want to tear this damn engine down again. I will remove valve covers and inspect tomorrow.
 
I know you said the marks are aligned, but this would be my first guess.

The other distinct possibility is that you might have a couple of the plug wires mixed up.

Popping through the exhaust happens when the plug fires when the exhaust valve is open so something is definately wrong there.
 
Yup. Could be an intake HUNG OPEN, though as well as an exhaust NOT opening

So, something wrong--lifter/ bent/ broke pushrod, could be broken valve spring

Run a compression or leakdown check, and or pull valve covers and inspect
 
Hmmm, just noticed the title of your thread says its popping through the carb but in the body it says popping through the exhaust.

Both???
 
dustermaniac said:
Checked orientation of wires, all are in proper sequence.
Click to expand...


Hmmm, just noticed the title of your thread says its popping through the carb but in the body it says popping through the exhaust.

Both???

Yeah seems like i can hear it mostly through the carb but sometimes once or twice i can hear it pop through exhaust.
 
flynbyu2 said:
I know you said the marks are aligned, but this would be my first guess.

The other distinct possibility is that you might have a couple of the plug wires mixed up.

Popping through the exhaust happens when the plug fires when the exhaust valve is open so something is definately wrong there.
Click to expand...


Timing marks were dead on and I checked this twice. The first time when I installed cam and the second time when i pulled engine apart to change oil pump and shaft and check for anything on the engine affecting oil pressure. Timing marks are dead on.
 
If the intake is hung open, it will not pop. The pop is the cylinder actually firing. The intake valve being hung open would bleed off cylinder pressure and result in a miss and there would be no pop. Let me ask you this Tom, is the pop very repititious when you increase the throttle? As in a cylinder firing every revolution? If so, dollars to doughnuts, there's an exhaust valve staying closed for whatever reason. Why don't you give me a call tomorrow and we'll talk about it?
 
Sounds like a broken rocker arm if its adjustable type check all the tips,could just be misadjustment whats the valve lash set at??..unless it's non adjustable valvetrain..
 
If you can't find any bent pudhrods I would bet that you lined up the wrong mark on the crank timing gear. There are a few marks on the gear and it can be confusing on which is the correct one, especially in low light. If you did use the wrong mark and the engine runs good until you start to open up the throttle, the engine then starts to pop through the carb. Take your breather off, start the car, then stomp the throttle. If you see a mist blow out of the carb and it does it over and over, I would bet that the timing mark used was the wrong one.

Jack
 
To rule out the cam timing and valves being held open, do a simple a compression test. If they all show equal levels and over 120psi, it's not hanging anything open and the cam timing is probably close enough to work. Sounds to me like 5 and 7 wires might be crossed. Is the distributor sitting flush on the block?
 
StrokerScamp said:
If the intake is hung open, it will not pop.
Click to expand...

Disagree, I've seen it happen. I believe what happens is the open valve sucks out of the intake, then blows it back into the intake, and if the thing is open enough, when the plug "goes off" it fires by chain reaction up through the intake.


THIS IS NOT CAM TIMING I don't believe.

I believe you've either

1 wiped a cam lobe

2 have one or more valve adjusted too tight

3 damaged something in the valve train.
 
He just called a little while ago. We're all wrong on the mechanical stuff. Thank goodness!!!! It was a plug wire boogerin up somehow. I'm sure tom will be along and paint the details up. lol
 
This is a perfect example of going to the basics, first. I'm sure he is relieved.
 
NO doubt about it. I assumed he was meaning a repetitive pop, but he didn't. What is it they say about assumin? lol
 
Yeah the plug wire (Accel) on #1 cylinder was broken inside the boot where it connects to the brass end. I took off the wires and found the wire had been broke and re-spliced it and the issue is no longer an issue, thanks for all the help!!
 
Nice to get good news ain't it?
 
I'm late to the thread but he said the engine was running good for a while, even driving it. So why worry about the cam timing?

Glad to hear things are all well
 
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