Help in diagnosing compression issue

[jailhousebob]

So I recently looked at an 8cm merc flathead that was for sale.The seller thankfully left the engine in the old truck it had been transplanted into so prospective buyers could hear it run.
Checking compression showed about 120 lb.s in each cylinder.When I got to #8,it pumped right up to 120 but drained back down instantly.I'm guessing the problem is a hung valve but concerned it may be more serious.The motor starts right up and runs good,of course the # 8 cyl is not firing.No water in the oil and no bubbling in the radiator so I don't think it's a crack.
I'd like to make an offer on the motor but without knowing what the issue is I don't want to overpay.
Any other ideas on how to narrow down the problem?
I have him soaking the offending cyl in fuel oil and marvel mystery in hopes it would free up the valve if that is the problem.
Thanks for any help,Bob

[Karl Wolf]

Bob, that was the gauge that went back down- There's a valve in the compression gauge to release the pressure, right?

Karl

[Mike51Merc]

I'm with Karl. A compression test isn't a leakdown test. Any cylinder will eventually blow by the rings.

I've found that leaky valves or rings will not allow you to get up to 120 in the first place, so something good is going on there.

[Ronnie]

A leak down test is your alternative. To confirm any troubles.

R

[jailhousebob]

Thanks for the response guys but maybe i'm not being clear.The guage spiked to 120 and immediately dropped to zero.I'ts not like it took a few seconds to get there,it dropped right away.So the piston is initially creating compression but the combustion chamber will not hold it at all.I don't think the guage is at fault because it worked fine on the other cylinders.I don't think a leak down test would show me anything I don't already know.
Again,i don't know what would cause the instant loss of compression other than a stuck valve.My thought is that even a cracked piston or scored cylinder wall would take a few seconds to leak down,i don't think it would be instant and a blown head gasket ,if severe enough to cause the instant loss of compression,would bubble into the radiator.Am I missing something or is my thinking flawed? Thanks,bob

[Bulligen]

Sponsored Links (Register now to hide all advertisements)

I am thinking if it is a hung up valve it would show almost no compression right off the bat. As it is trying to compress the air it is blowing right past the valve and valve seat. Is the plug good? has good spark? switch it with another cylinder. If you get up to that 120 mark, I would think it would fire

[Bob C]

Stop and think, what are the odds of cranking an engine over and having
it stop with both valves closed and on the compression stroke which would
have to happen to keep pressure in the cylinder for a few seconds. Also
like others have said the gauge should hold the pressure until you release it.

Bob

[40cpe]

[Edit: Deleted duplicate post. The guys before this post beat me to my comments.

[jailhousebob]

Originally Posted by Bob C

Stop and think, what are the odds of cranking an engine over and having
it stop with both valves closed and on the compression stroke which would
have to happen to keep pressure in the cylinder for a few seconds. Also
like others have said the gauge should hold the pressure until you release it.

Bob

Just to be clear,it's not holding pressure even for a few seconds.The guage goes down as fast as it went up so not holding any pressure at all.I would think (maybe incorrectly)that the volume of the cyclinder being so much greater than any partially open valve ports would allow for initial compression build until the piston reaches the top of it's stroke when,of course all compression would stop being produced and leak out of the partially open ports.But I agree it shouldn't read as high as it does under those circumstances.I see your point about the guage not holding whatever pressure it reached and honestly can't remember if we retested a prior cyl. known to be good after we got the weird reading on# 8 (shame on us if we didn't).Still,that cylinder was not firing (we did switch plugs) and with all the cranking and running of the engine the plug never seemed to be wet with an unburnt fuel charge.I think I will go back with another guage and investigate further.Bob

[JSeery]

It will not build up pressure and then drop suddenly like you are describing, that is not how it works. A compression gauge should have a release valve and the pressure should increase with each engine cycle until it reaches a maximum and will not increase any more. This takes several revolutions of the engine. The gauge will hold the final pressure until you release it. You have a bad gauge or are not testing correctly or something.

[chap52]

If you are using the old style rubber tipped press in place gauge I don't understand how it goes down at all if it is not manually reset. The one I have takes the reading, locks it in and only resets after I press the little valve stem to release the internal pressure. Maybe the one you are using is different???

[tubman]

The fact that you are experiencing the problem only on #8 suggests there may be an access/alinement problem with the gauge.

[jailhousebob]

Again,i overlooked that there may be a guage problem based on the fact that it worked fine on the prior 7 cylinders.Obviously the guage is not holding pressure and I guess just by coincidence it became faulty between testing the 7th and 8 th cylinder.Sometimes the obvious isn't so obvious (at least to me)so a big DUH! to me.I still need to find out why # 8 won't fire and that ,of course,could be anything.I'm going back with a new guage and renewed enthusiasm.Thanks all,Bob

[George/Maine]

I would switch sparkplug wires and plug and check dist. cap.

[Mike51Merc]

Anytime I've found even slightly leaky valves I also found serious lack of compression---- like 60 or 45 lbs when the rest of the engine was at 100+lbs. I've never heard of such a high pump-up/dropoff.

On the other hand, my buddy has a compression gauge that behaves like you are describing-- it won't hold the reading and his release button does nothing.

[George/Maine]

I think the exhaust valve the 2 keeps fell off or broken valve spring.
The cylinder next to it blowing exhaust valve up. Just a guess.
Pull the head and intake.

[Ronnie]

Originally Posted by jailhousebob

Again,i overlooked that there may be a guage problem based on the fact that it worked fine on the prior 7 cylinders.Obviously the guage is not holding pressure and I guess just by coincidence it became faulty between testing the 7th and 8 th cylinder.Sometimes the obvious isn't so obvious (at least to me)so a big DUH! to me.I still need to find out why # 8 won't fire and that ,of course,could be anything.I'm going back with a new guage and renewed enthusiasm.Thanks all,Bob

Do the test over to repeat readings that will confirm the gauge reliability.


R