360 has a low cyl

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BWDart

BWDart

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Just took comp test, cold, # 5 only has 90 rest at 160 where do I start? Rebuilt approx 500 miles on motor. Could this be where the pulsating vibration I thought I fixed is coming from? Gonna buy adapter n do leak down after I cool down pretty pissed right now every time I fix one thing another pops up
 
If it has adjustable rockers, try loosening those 2 off, and hope one was tight. Then check the comp in that cyl again.
 
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Remove the plug on the low cylinder. Remove the valve cover on that side. Rotate the engine until both valves on that cylinder are closed. Apply air pressure to that cylinder via the plug hole.

Air coming up through the carburetor indicates a burned or leaking intake valve. Air coming out of the tail pipe indicates a burned or leaking exhaust valve. Air coming up into the valve cover area indicates a ring issue or a crack or hole in a piston.

No need to fiddle fart around with the wet and dry compression test. That's a total complete waste of time. The air test will tell you everything you need to know.
 
I read on FABO not long ago that the proper way to check comp was to warm up the engine first then check. Some people agreed and some did not.
 
RustyRatRod said:
Remove the plug on the low cylinder. Remove the valve cover on that side. Rotate the engine until both valves on that cylinder are closed. Apply air pressure to that cylinder via the plug hole.

Air coming up through the carburetor indicates a burned or leaking intake valve. Air coming out of the tail pipe indicates a burned or leaking exhaust valve. Air coming up into the valve cover area indicates a ring issue or a crack or hole in a piston.

No need to fiddle fart around with the wet and dry compression test. That's a total complete waste of time. The air test will tell you everything you need to know.
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Exactly, Spot on!!!!!! If this is your first compression test since the build, and you had the valves done by a professional... could very well be a valve not seated. Had a set of 383 heads done by a geru. He installed hardened seats, new exhaust valves. All 4 exhaust valves on one head let alcohol through, light through, and it wasn't close to sealing. I checked Before I installed. It took me 4 hours with a lapper to get them to seal. Just throwing out stuff, cause we don't know much about your build or what was done to the heads
 
Thanks guys gonna do those tests tomorrow need to go get adapter
heads were done by mach shop many yrs ago been sittin since 05 luckly exhaust is off makes tests a little easier
 
leblanc said:
I read on FABO not long ago that the proper way to check comp was to warm up the engine first then check. Some people agreed and some did not.
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If the engine does not start, then the only way to do it is cold...

It's best if warm, but you can still learn from the numbers on a cold test...
 
RustyRatRod said:
Remove the plug on the low cylinder. Remove the valve cover on that side. Rotate the engine until both valves on that cylinder are closed. Apply air pressure to that cylinder via the plug hole.

Air coming up through the carburetor indicates a burned or leaking intake valve. Air coming out of the tail pipe indicates a burned or leaking exhaust valve. Air coming up into the valve cover area indicates a ring issue or a crack or hole in a piston.

No need to fiddle fart around with the wet and dry compression test. That's a total complete waste of time. The air test will tell you everything you need to know.
Click to expand...

If he doesn't have a leak down tester, a wet-dry compression test can rule out if it's in the rings or not...

The leak down test will be able to isolate any of the problems more accurately if you have one available or a way to pressurize the cylinder...

If you have a valve problem and know which cylinder it's in, you can always pull the head, flip it upside down and fill the combustion chamber with liquid (water) and see if it leaks. A quick simple "bench test"...
 
ateam said:
And if #5 intake lobe on cam, went really flat, might be a cause for 90 psi ? ateam
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Yes, a worn lobe will lower compression pressure...

I would first take off a valve cover and crank the engine over and look to see that all the valves were opening fully... If you see one that is not opening all the way, then pull the intake and inspect the lobe...
 
krazykuda said:
Yes, a worn lobe will lower compression pressure...

I would first take off a valve cover and crank the engine over and look to see that all the valves were opening fully... If you see one that is not opening all the way, then pull the intake and inspect the lobe...
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Just curious how does a flat lobe affect compression?
 
BWDart said:
Just curious how does a flat lobe affect compression?
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The valve does not open so the cylinder can fill/empty so there can be no new "charge" coming in to compress...

Try sucking on a straw filled with liquid while your finger is on the other end.... You can't suck the liquid out until you take your finger off the other end....
 
krazykuda said:
The valve does not open so the cylinder can fill/empty so there can be no new "charge" coming in to compress...

Try sucking on a straw filled with liquid while your finger is on the other end.... You can't suck the liquid out until you take your finger off the other end....
Click to expand...
That makes sense did leak down test intake valve leakin guess thats better than piston or rings not lookin forward too raslin that iron head off think I'll remove mstr and booster
 
BWDart said:
That makes sense did leak down test intake valve leakin guess thats better than piston or rings not lookin forward too raslin that iron head off think I'll remove mstr and booster
Click to expand...

You should be able to remove the head without taking the master and booster out... It will save you from having to bleed the brakes...
 
Have you applied air to the cylinder?
 
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