[pontiac1981]

Does a burnt valve fill like a bad miss?

[ponchonlefty]

yes, check the compression it should check low in the cylinder if it is possibly burned. but could be anything causing a bad miss. first check the compression on all cylinders,look at the spark plugs,check the wires,distributor cap,this will get you closer to finding the problem.

[25stevem]

It's a very rare thing that a Pontiac with stock iron heads burns a valve!

Pontiacs have very wide seats from the factory in terms of both Intake and Exh valves, and with 99% of the Intake valves in Pontiacs having a 30 degree seat I must say that I don't think I have every seen a Pontiac with a burnt Intake valve unless there was a lot of rust pitting taking place already due to a water leak.

I would sooner say that you have a blown head gasket, then a burnt valve if not a ignition problem.

[hurryinhoosier62]

Originally Posted by 25stevem

It's a very rare thing that a Pontiac with stock iron heads burns a valve!

Pontiacs have very wide seats from the factory in terms of both Intake and Exh valves, and with 99% of the Intake valves in Pontiacs having a 30 degree seat I must say that I don't think I have every seen a Pontiac with a burnt Intake valve unless there was a lot of rust pitting taking place already due to a water leak.

I would sooner say that you have a blown head gasket, then a burnt valve if not a ignition problem.

Had two different Pontiac engines suffer burnt exhaust valves. It isn’t common but neither is it “rare”. It can and does occur.

[NeighborsComplaint]

Originally Posted by pontiac1981

Does a burnt valve fill like a bad miss?

Which heads do you have?

[geeteeohguy]

Originally Posted by hurryinhoosier62

Had two different Pontiac engines suffer burnt exhaust valves. It isn’t common but neither is it “rare”. It can and does occur.

THIS^^^. I've seen it more than once over my career. Last time was two burnt exhaust valves on my 173,000 mile '67 GTO 400 back in the '80's.

[hgerhardt]

Our old family wagon ('66 Tempest with OHC-6) burned exhaust valves several times in the 100k miles my parents drove it. Of course, the puny 6 cyl driving an overloaded wagon with 5 people and camping gear, in the CA central valley in 100+F heat, looking back on that abuse, I suppose it was the expected outcome.

My '66 GTO used to have a 455 with #96 heads. It ran well until the #6 exhaust valve recessed into the head to the point where the lifter bottomed out which caused misfire. But I digress, that's not valve burning.

[25stevem]

Thinking back on my machine shop days most of the burnt exh valves I have seen where in the years of low compression and cat’s restricting the exh system.

The valve seat area would crack first and the miss was driven with at times for weeks until the crack spreads out to undermine the head gasket and then starts the accompanying coolant leak.

The factory seat induction hardening process to deal with unleaded fuel was a very large factor in cracks starting in these heads.

This seems to be prevalent even more so in motors that have over heated.
This conclusion really came to light when my Uncle overheated his 74 GP due to a slow leak in a hose.

Since he lived local I was able to get out to where he broke down to help him, but it was too late.

By the time I got to him he had gotten some jugs of water from a house near by .
He put two gallons of 55 degree water into the 230 degree motor and started it.

The next thing he new just before I got there was that the motor now had a miss, where as before it was running normal but just overheating.
The result of that thermal shock of 55 degree water passing thru 230 degree heads was a nice big crack in the passenger side head.

Also on a few of the earlier type heads I have seen the oil burning issue and the resulting carbon deposits where what started to not let a exh valve seat all the way, and once this starts taking place a burnt valve and pitted seat will be yours in short order!

There is always a reason for these kind of failures and it can never be traced to preventive maintenance!!!!

If your Pontiac is stock and has over 90,000 miles on it and all you have done is to replace the timing chain and gears and not the valve stem O-ring seals then your much more likely to be faced with these types of engine failures.

[mgarblik]

Agree with Steve that a head gasket failure is far more likely on a Pontiac V-8 than a burned valve. I have seen some burned exhaust valves typically in the center cylinders. Just like a SBC but to a much, much lesser extent. I can't honestly say I have ever seen a Pontiac "Burn" an intake valve. I have seen a few "coke up" with so much carbon/oil deposits, that the valve wouldn't seat, causing the same symptoms as a burned valve. Running a top engine cleaner mixed with water would sometimes clean that up. Exhaust seat recession after 50+ years on the older heads without induction hardened exhaust seats would be more likely to burn exhaust valves.

[Tom Vaught]

Originally Posted by hgerhardt

Our old family wagon ('66 Tempest with OHC-6) burned exhaust valves several times in the 100k miles my parents drove it. Of course, the puny 6 cyl driving an overloaded wagon with 5 people and camping gear, in the CA central valley in 100+F heat, looking back on that abuse, I suppose it was the expected outcome.

I have done at least 10 valve jobs back in the early 1970s on OHC-6 engines.
If that engine had a known issue, it was the lack of durability as related to the OHC cylinder head.

I also did a lot of timing gear/chain repairs, V8 engines, for the plastic large timing gear failing and getting plastic into the oil pan and oil pump screen.

Tom V.

[Half-Inch Stud]

never seen a pontiac burn valves, seats, nor seat crack, nor head damaged by heat.

Did have a head gasket breach across the 2-4 CYLs which "torched" a ~1/4" wide metal removal from the block surface. High compression on a hot highway stint after a good 4- years of Street/Strip use. Heads were fine.

Did pull heads which showed the 8 EXH valve heads were white. Ground them and reused.

[25stevem]

To rightfully get back to the OPs original question, a burnt valve which will always not allow normal compression feels like I worse miss then if you simply had a plug that was not firing in a good sealing cylinder.

If you have the rarity of a burnt intake valve or the occasion of carbon build up holding a intake valve open then as mentioned in a above post you would be hearing popping coming back thru the carb when the cylinder somewhat fires and that compression pulse goes back into the intake manifold

One of the easiest and fastest ways to tell if you have a cylinder that is very low on compression is to just kill the spark and then crank the motor.
Listen closely and you hear the cranking speed go up when the motor passes over the low cylinder.
In 5 to 8 seconds you will know if you have a very low cylinder, then comes the search as to why!

[Formulas]

since condition of OP's engine has not been determined if i was him i would do a compression check to learn overall condition then know at least what bank a problem may exist or if it is mechanical or spark related

consistant compression reveals a possible ignition problem
a cylinder that is down on compression can be a few things blown head gasket, burnt valve, and also a bent pushrod which i believe happens more often than a valve issue

but cheap engine diagnostics will lead OP to the real problem and he hasnt even started apparently

[NeighborsComplaint]

Originally Posted by 25stevem

Thinking back on my machine shop days most of the burnt exh valves I have seen where in the years of low compression and cat’s restricting the exh system.

The valve seat area would crack first and the miss was driven with at times for weeks until the crack spreads out to undermine the head gasket and then starts the accompanying coolant leak.

The factory seat induction hardening process to deal with unleaded fuel was a very large factor in cracks starting in these heads.

This seems to be prevalent even more so in motors that have over heated.
This conclusion really came to light when my Uncle overheated his 74 GP due to a slow leak in a hose.

Since he lived local I was able to get out to where he broke down to help him, but it was too late.

By the time I got to him he had gotten some jugs of water from a house near by .
He put two gallons of 55 degree water into the 230 degree motor and started it.

The next thing he new just before I got there was that the motor now had a miss, where as before it was running normal but just overheating.
The result of that thermal shock of 55 degree water passing thru 230 degree heads was a nice big crack in the passenger side head.

Also on a few of the earlier type heads I have seen the oil burning issue and the resulting carbon deposits where what started to not let a exh valve seat all the way, and once this starts taking place a burnt valve and pitted seat will be yours in short order!

There is always a reason for these kind of failures and it can never be traced to preventive maintenance!!!!

If your Pontiac is stock and has over 90,000 miles on it and all you have done is to replace the timing chain and gears and not the valve stem O-ring seals then your much more likely to be faced with these types of engine failures.

This is why I asked which heads are being used. Lots of 5C and 4X castings developed cracks between the 4/6 and 3/5 between the exhaust seats. No coolant leakage occurred but a persistant miss was noticeable through the exhaust.

[Cliff R]

Every 7M5 heads I've ever seen had cracked exhaust seats. Have seen a few 4X heads cracked, and one 5C.

Early the factory wasn't that good at the induction hardening process plus they put more material around the exhaust seats in later years. 1973 and later heads will have induction hardened exhaust seats from the factory. I've seen 1972 SBC heads with them, but don't believe Pontiac made the change until the 1973 model year.......

[pontiac1981]

There is no water in the oil or oil in the water, there is no popping through the carb or tail pipes. when i get it up to 35 or 40mph and hold it there it seems to level out until i lightly touch the gas then it bucks like crazy if i push the gas harder to a higher rpm it levels off again but i can still feel a miss. There is spark at all plugs.

[25stevem]

Sounds like you more have clogged passage ways in your carb or your timing is unstable and bouncing around then a head or short block issue.

Unhook and plug the vacuum advance if your using it and Put a timing light on the motor and see that when you bring the motor rather fast up to 4500 rpm that the rate of added advantage is smooth, then just let the motor drop right back to idle.
In doing this the timing should drop right back with the rpm and not be seen to bounce around.

If your dizzy is set up with light advance Springs you need to know that they go south fast, and many times with no warning.

What do you have on the motor for a carb?