Stuck valve?--340 V8---Compression check RESULTS

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blano86

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I am a total newb when it comes to fixing stuff but I no enough to know there is a problem. I have a 340 in the 1967 cuda notch.

I may have a stuck valve. I was driving when all of a sudden the cuda started to lurch and back fire thru carb and tail pipes. It ran okay if I accelerated but when I let go of throttle it back fired and it would backfire when I tried to accelerate from idle.

I checked each cylinder, by removing spark plug wires at the block one at a time to check to see if it would miss when I removed them. Every cylinder seemed to miss when I removed wire. My thinking was that if a cylinder had a stuck valve it would not make a difference when I removed wire. I drove it again and about a half a mile down the road it started to act up again.

I removed the valve covers and could not visibly see any problems.
From the pics below could anyone tell me which heads I have? I thought that rockers were adjustable? A compression test would probably help a lot too, but I do not have one on hand. Is there anything else I can check to see if a valve stuck?

Here are some pics of the heads and valve train:

100_3103.jpg


100_3107.jpg


100_3114.jpg
 
Few things;

1. "A" stuck valve would be either and intake or an exhaust so you wouldn't get back firing through both intake and exahust.
2. Can't tell from sure but in the pictures all the valves look closed, you should have some that are in various stages of being open. Disconnect the coil wire and crank the engine over while observing the valves opening and closing, they all should move about the same amount. The picture makes me think of a wiped cam.
3. Buy, beg, borrow a compression gauge, it's the quickest way to tell if there is a problem.
4. from the looks of your wires you could have some leakage or cross firing going on.
5. get a small mirror and take a close look at your valve springs and make sure you don't have a broken one. The inner damper will hold it closed so a quick inspection may miss it.
 
Put a vacuum gauge on it. Check for a steady reading. If it bounces up and down would show a mechanical problem
 
Most likely the timing chain jumped a tooth, so check for low manifold vacuum at idle.
 
Looks like a good place and sounds like a good place for new plug wires. Mechanical problems like timing chain and stuck valves wont come and go. Plug wire failure will come and go.
 
Well, I did a compression check and here are my results. I did it twice to make sure. They suck.

Newb Question: On the manifold it says cylinders 2,4,6,8 are on the left side of the engine if you are looking at it from the radiator and 1,3,5,7 on the right. Is that the right numbers for the cylinders?

Anyways her are my results:

Left side of engine- (I think 2,4,6,8)

2---95/95
4---105/110
6---95/100
8---105/110

Right side of engine- (I think 1,3,5,7)

1---60/50/60
3---115/115
5---100/100
7---85/85

I noticed that when I got the first reading of 60 from cylinder 1 I heard a squirt sound after I cranked it over. Could this be due to a bad valve seal?

Let me know what you guys think. Rebuild in the future? ;)
 
driver side of engine is odd bank....1, 3, 5, 7
passender side of engine is even bank...

Do you have access to a leak down gauge?
 
The 60/50/60 number is low but the rest are within a reasonable range of each other. A simple test would be to squirt some oil in each cylinder and repeat the test. If they all come up then you know you have worn rings. If they do not change appreciably then I would say that the 60/50/60 cylinder has a valve leaking.

The rest of the cylinders being low could be a case of a cam with a lot of over lap bleeding off cylinder pressure. A leak down test will tell you if the low readings are do to the cam bleeding off pressure or if the engine is worn and its leaking past the rings or valves.

In anycase those readings would not be a cause for your back firing and rough running.

A timing chain jumping "a" tooth on the cam sprocket would be like retarding the cam 9 degrees, it would have no low end power but would run pretty good on the top end and it wouldn't account for your back firing. If the chain was to jump a tooth on the crank sproket that would be 18 degrees and you would likely not be able to start the engine and if you did get it running it would barely run.

I would start looking at fuel and ignition. As I mentioned before those wires look pretty suspect around the boots.
 
All of the problems started on the day I got it running great! I put a new cap and rotor and adjusted the timing to 11 degrees. Where it should be right? I drove it about 40 miles home, and that night I took it to work and about a half mile down the road it started lurching and backfiring when I gave it gas to pass a car. I pulled over to see if anything looked wrong on the outside, and there wasn't. I revved the engine by the carb pushing the throttle and it started to back fire out of carb and tailpipes. The carb sounded like a pop sound and the tailpipes sounded like a rumble sound. Should I check some other stuff before I rule it a rebuild?

Oh yeah, it idles okay too though, maybe a little rough.
 
You are a long ways away from deciding if the engine needs to be rebuilt. You have to do the leak down test to know where the compression readings are low from the cam you are using or low from the engine being worn.

It's been mentioned three times now and this is the forth the ignition wires do not look good in the picture and can easily cause the problems you have. Is see some grey discoloration on the boots that could be an indication of leakage and arcing.

Also, now that you are mentioning you just replaced the cap and rotor. Is it seated right? Did you get the rotor down on to the shaft all the way? if you didn't you may have torn up the spring tab that contacts the center of the cap. Are there any signs of tracking on the inside of the cap?

What do the plugs look like?\

Any possibility you put some bad gas in it?
 
Those compression numbers are Very low for a 340, even a late model one like a 72 or 73. I would suspect a timing chain has jumped, like previously mentioned. You are lucky if runs at all when that happens.
 
I just ordered some new wires. I will check the cap. Tracking signs on cap?

Sorry guys for my ignorance I am a newb, but hopefully have a steep learning curve.
 
spark plugs look white at business end , but the #6 cylinder looks a little brown and oily.

The cap and rotor look good. The grey on the wires looks just like dust.
 
HemiEd said:
Those compression numbers are Very low for a 340, even a late model one like a 72 or 73. I would suspect a timing chain has jumped, like previously mentioned. You are lucky if runs at all when that happens.
Click to expand...

i agree.i would suspect timing chain as well.i have a 340 with some hard miles on it and its got a 284 duration cam. it cranks around 165-170 psi.the readings you posted are way low imo.
 
Here's my stupid question.
Does it still have a points ignition, or has it been converted to electronic.
A bad condenser will do funny things.
 
If you set timing make sure the distributor was tight and didn't turn a little, check rotor inside, (I had one arch and backfire) and change plug wires. Those are the easy things first. Just a matter of "process of elimination". Good Luck.
 
A quick way to check the condition of your timing chain is to pull the distributor cap off and check the movement of the distributor rotor as you turn the motor over by hand going one way then the other.You can do this with a socket and breaker bar (or ratchet) on the front crank bolt or just by turning it with the fan (if it's a direct driven one) while pressing on the fan belt to keep tension.

The rotor should start to turn as soon as the crank moves but if there's a lag then your timing chain is stretched and is likely the cause.

I'm thinking it's an ignition problem,though,possibly a weak plug wire that failed when removed from the old distributor cap.If a new set of wires does'nt solve the problem you could try re-installing your old cap and rotor,if you still have them,and see if that could be causing the misfire.
 
not sure-- but if this car was running good b-4 tune-up-- can you return it to that state?
you need to phone a friend before you waste $$$ or ruin a running car---- as you can tell by the varied responses on here that a correct diagnosis is in order-- it could be that the new parts just installed are faulty-- good luck, lawrence
 
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