6Cup - leakdown from both exhaust valves?
10-29-2016, 02:04 PM
#1
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6Cup - leakdown from both exhaust valves?
Okay I'm stumped....
Getting low compression (obviously) and high leakdown out of my Cup engine. It's coming out of the exhaust valves on #2.
Whats weird - is that it's coming out of both exhaust valves at the same time. As if they weren't closing properly. It's only on one cylinder. Why would both valves do this? They don't share a lifter, spring, etc...
Baffled.
Getting low compression (obviously) and high leakdown out of my Cup engine. It's coming out of the exhaust valves on #2.
Whats weird - is that it's coming out of both exhaust valves at the same time. As if they weren't closing properly. It's only on one cylinder. Why would both valves do this? They don't share a lifter, spring, etc...
Baffled.
10-29-2016, 02:13 PM
#2
Two probabilities, carbon build up in cylinder due to lack of use...best scenario but highly unlikely.
More likely, stretched rod or floated valves due to overrev, valves meeting mr piston and slightly bending.
More likely, stretched rod or floated valves due to overrev, valves meeting mr piston and slightly bending.
10-29-2016, 02:15 PM
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It's certainly not from disuse. 
Misuse perhaps. I guess it's not weird that both would be bent, is it?
Have to tear it down no matter what, but I'm at the speculation before disassembly phase.
Misuse perhaps. I guess it's not weird that both would be bent, is it?
Have to tear it down no matter what, but I'm at the speculation before disassembly phase.
10-29-2016, 03:15 PM
#4
There are some ways to check without tearing the engine down.
Remove the spark plug from that cylinder. Remove that primary exhaust pipe from the cylinder. Get one of those really powerful flash lights and shine it into the plug hole. Look up the exhaust port. sometimes you can see the light if the valves are not sealing. If you have access to a borescope, this becomes a lot easier. You can also look with the scope to see if there is carbon buildup on the seats, Turn the engine over until those valves are at their highest lift and with the scope look at the seats and the valve margins. Caution while doing this, do not turn the engine as the scope with destroy its self.
If there are signs of carbon buildup, you can CAREFULLY tap of the follower with a plastic punch or similar while a leak down tester is installed. The % often drops as you bounce the valve back onto the seat. This may not cure the problem but it does give a good indication of the problem. Don't worry about tapping on the follower, as long as you use something soft. You cannot hit the follower as hard as the camshaft does at speed.
Or you can remove the Cam cover and check to see if you have lash between the follower and the camshaft. There should be lash. If there is no lash, the follower can be holding the valve off the seat. Probably not the case as this would have been from the beginning. But you should check.
Or you can figure out the engine has hours of running time and its time to rebuild.
I'd guess the last suggestion is the correct one. We all try to put of the inevitable???
Remove the spark plug from that cylinder. Remove that primary exhaust pipe from the cylinder. Get one of those really powerful flash lights and shine it into the plug hole. Look up the exhaust port. sometimes you can see the light if the valves are not sealing. If you have access to a borescope, this becomes a lot easier. You can also look with the scope to see if there is carbon buildup on the seats, Turn the engine over until those valves are at their highest lift and with the scope look at the seats and the valve margins. Caution while doing this, do not turn the engine as the scope with destroy its self.
If there are signs of carbon buildup, you can CAREFULLY tap of the follower with a plastic punch or similar while a leak down tester is installed. The % often drops as you bounce the valve back onto the seat. This may not cure the problem but it does give a good indication of the problem. Don't worry about tapping on the follower, as long as you use something soft. You cannot hit the follower as hard as the camshaft does at speed.
Or you can remove the Cam cover and check to see if you have lash between the follower and the camshaft. There should be lash. If there is no lash, the follower can be holding the valve off the seat. Probably not the case as this would have been from the beginning. But you should check.
Or you can figure out the engine has hours of running time and its time to rebuild.
I'd guess the last suggestion is the correct one. We all try to put of the inevitable???
