Can Bad Tappet Adjustment = Bad Compression?

[54Illinois]

As I mentioned a while ago, the number 4 cylinder on the 52 Dodge has a compression reading of 30.  When I had the access plates off, the intake valve was dirty.  The car had not been street driven in over 20 years, so the deposits did not happen over night.

 

I was thinking about checking tappet clearances before pulling the head.

 

 

[Dave72dt]

Yes it can.  Too tight a clearance setting can hold a valve open.  What you may find instead is a large gap setting.  With a dirty valve, there may be a piece of carbon between the valve and seat, holding it open enough to reduce compression.

Edited July 12, 2013 by Dave72dt

[squirebill]

Maybe do the compression test again but squirt a bit of engine oil into the spark plug hole. Try to angle it so the oil ends up in the piston cylinder rather than squirting it straight down onto the valves.  Do the compression test.  If value comes up, probably have a problem with the rings and the oil sealed it off a bit.  If no improvement on the compression value, probably a valve problem.

[greg g]

Valves especially those that have been on a high point of a cam lobe for years will have a tendency to stick in the open position when you turn the engine over, you can sometimes free them up by sliding a thick screwdriver between the coils of the springs.  The best way is to pull the head and play whack a mole with a rubber hammer on the ones that are sticking up. Since you suspect that its on a certain already identified cylinder, I would work it from the spring end to see if you can get it to close.

[54Illinois]

Thanks for the input.  I did try the oil trick, and compression stayed the same.  I will get some feeler gauges and some tappet wrenches and check it out...

[Don Coatney]

You may or may not have seen this. Leak-down test may be helpful.

 

[squirebill]

Have you heard this one.  I do some work on small engines with pull starts from time to time.  It seems lately a lot of these engines have a compression release built in to make them easier to pull start but this also makes it difficult to do a compression test.  What they do now is set the engine at TDC on compression stroke and pump air thru the spark plug hole.  I did this on a 6.5 Briggs that wouldn't run.  WAs getting 30 psi with regular compression test.  My compression tester has a hose that screws into the spark plug hole andt the gage plugs into the hose with a standard air hose quick disconnect set up.  Popped the gage off and plugged in the hose from my small pancake compressor.  Brought the pressure up slowly to about 20 psi.  By listening at the carb, muffler and crankcase oil fill can determine if you have an intake valve leak, an exhaust valve leak, or a piston ring/ cylinder problem. Worked good. Fixed rings and now running like a champ. Since I had the head off I lapped the valves anyway but it sure identified the real problem quick. Read where others modify a spark plug by breaking out the porcelin and threading in a male air hose fitting so they could connect their air hose. 

[squirebill]

Guess while I was typing my post, Don posted his. Note another leak point past the head gasket and into the cooling system. Guess should also look for leaks past head gasket at block/head surfaces.

[Don Coatney]

 It seems lately a lot of these engines have a compression release built in to make them easier to pull start but this also makes it difficult to do a compression test. 

 

How does this work?

[greg g]

Usually a ramped cam lobe on one valve, and some cam runout,  it moves the cam as rpms go to idle speed and above,  kinda like honda V tech in reverse.........

Edited July 13, 2013 by greg g

[squirebill]

Don....if I remember correctly the 6.5 hp Briggs I was working on I don't think had a compression release...but I did the leakdown test as suggested for a compression release engine. A 12 hp Kohler I worked on awhile ago had the compression release. It's been awhile but I know there was a notch on the exhaust cam lobe at the low point and a tab of a centrifugal weight fit into this notch. The weight was spring loaded and was out of the notch and locked there by another centrifugal weight when the engine was stopped or at low rpm. So this tab would hold the exhaust valve open or partially open during the compression stroke. Once started and rpms come up, the centrifugal weights shift and the tab on the one is pulled/pushed into the notch on the cam lobe. It's been a while so I'm a little foggy on this. Now that I think about this I think you could still do a compression test on this engine but you had to spin it in the opposite direction.

[Don Coatney]

Compression release is an interesting concept. User frendliness is a selling point. I remember the old days, maunal choke, manual throttle, rope start, and the metal tab to ground out the spark plug for shut down.