Pops lightly through carb... bent valve?
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Senior User
Joined: Feb 2003
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From: Houghton, MI
Pops lightly through carb... bent valve?
My engine jumped time, so I replaced the timing chain and gears. Now it pops back lightly through the carb, and won't rev past about 2000 rpm, so I thought maybe when the timing jumped it bent a valve. I did a compression test, and all the cylinders were at about 145, execpt for the #1 cylinder, which read 110. I was wondering if this still sounds like a bent valve or not. If there is a bent valve, could the cylinder still hold 110psi?
if not, any other ideas?
Thanks
if not, any other ideas?
Thanks
Senior User
Joined: Dec 2001
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110 is still enough psi for working that piston, but just barely. You most likely have a timing problem, especially since you mentioned it wont rev. Are you totally sure the cam marks were set up properly? Most cases, "dot to dot" on the crank and cam, meaning they should line up together at thier closest point. Hows the distributor gear? they can wear out too. If the timing marks are on the money, it could be the balancer slipped and your actually timing a false referance mark postion. Check your firing order too while your at it.
Regards, Stephen
Regards, Stephen
Freshman User
Joined: Apr 2004
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From: Las vegas
Check your vacuum advance...pull the dist cap...then pull the hose ands suck on it or use a vacuum pump...if the plate does not more it means your not gettin any vacuum advance which will bog the engine and not allow high revs. most of the low end timing advance is from the vacuum pull on the breaker plate....it would be my first place to look
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From: Louisville,Ky.
Unless the timing is way too slow to start with. But if all else is well, I don't think that it would cause the misfiring either.
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Senior User
Joined: Feb 2003
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From: Houghton, MI
Put in new plugs, found one that was broken in the ceramic part. Checked the firing order, separated the wires so I was sure there isn't any cross-firing. Tried advancing the timing, and checked vacuum. Even with the timing advanced an extra like 6 degrees it still would pop through the carb jsut the same.. Also, the engine just in general sounds sorta funny even at idle... Only thing I can think of is maybe the timing gears and chain were installed incorrectly.
Would I still have 110 psi if a valve isn't closing all the way?
By the way, when I replaced the plugs and the one that was broken, it has more power but still the exact same problem with the popping thru the carb.
Dan
Would I still have 110 psi if a valve isn't closing all the way?
By the way, when I replaced the plugs and the one that was broken, it has more power but still the exact same problem with the popping thru the carb.
Dan
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Joined: Dec 2001
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From: Louisville,Ky.
If a valve isn't closing, it wouldn't build compression. It sounds like the chain is in wrong and the intake valves are opening too soon causing it to fire compression into the intake ports.
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Senior User
Joined: Feb 2003
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From: Houghton, MI
Okay-- I tried something else.
I pulled every plug wire off one at a time, and each time I did the truck would stall, except for the #1 cylinder. When I pulled the #1 plug wire off, it had no effect on how the engine ran, even when revved. I held this plug wire against the block, and it was arching, so I know its getting spark.
I have heard engines run before with one dead cylinder or plug wire off, and this one still sounds wierd... not very loud, kinda like there's no definition in the exhaust note, like all the "pistons firing sound" runs together, and this one doesn't even have a cat or muffler on it.
Dan
I pulled every plug wire off one at a time, and each time I did the truck would stall, except for the #1 cylinder. When I pulled the #1 plug wire off, it had no effect on how the engine ran, even when revved. I held this plug wire against the block, and it was arching, so I know its getting spark.
I have heard engines run before with one dead cylinder or plug wire off, and this one still sounds wierd... not very loud, kinda like there's no definition in the exhaust note, like all the "pistons firing sound" runs together, and this one doesn't even have a cat or muffler on it.
Dan
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Joined: Mar 2003
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From: ma
Get yourself a leak down tester kit. borrow or rent. It fills the cylinder with compressed air. If you hear air through the carb, its a bent intake, if you hear air through the exhaust its a bent exhaust, if you hear air through the positive crankcase vent then its rings....best of luck, hard to diagnose online...C
Lead Driver
Joined: Nov 2000
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From: Albuquerque
You didn't mention the motor type. I think most 400's are non interference engines. It would be hard to kiss a valve. Jumping a tooth could cause the balancer to seperate so the timing marks are off. It could also shear the dizzy drive pin and throw the timing off.
Oops. I just noticed this thread is getting old. You still out there Motorhead?
Oops. I just noticed this thread is getting old. You still out there Motorhead?
Thread Starter
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Senior User
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From: Houghton, MI
Yup, still here.
the engine is a 302. Do you mean the harmonic damper on the front of the engine? I have jsut been adjusting timing by ear. I think we are going to take the water pump and timing cover off and see if they were installed correctly. Also how do i check if the dist. drive pin was sheared? I know I cant turn the rotor when the engine is off, so would that mean the drive pin is okay?
Thanks
the engine is a 302. Do you mean the harmonic damper on the front of the engine? I have jsut been adjusting timing by ear. I think we are going to take the water pump and timing cover off and see if they were installed correctly. Also how do i check if the dist. drive pin was sheared? I know I cant turn the rotor when the engine is off, so would that mean the drive pin is okay?
Thanks
Lead Driver
Joined: Nov 2000
Posts: 6,628
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From: Albuquerque
If the pin shears it tends to slip then hang up again but usually will slip again causing you to advance the dizzy untill you cannot turn it anymore due to interference. You would have to take it out to check it for sure. Yes the damper.
Post Fiend
Joined: Jun 2003
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From: NE Texas
Are you sure you installed the distributor correctly? Sounds like you are one slot too far one way are the other. How did you time it? With a light or did you bump up the first cylinder and go from there?