Two wildly different compression readings, why? On in around 60 and on around 200?

[danielpl]

Other than a faulty gauge or bad technique, what could cause two opposite compression readings on an 2006 Yamaha F50 with one reading at around 60 and the other around 210? What should I look after first, second, and so on...

 

[Lou C]

Well the low one could be a valve hanging open or bad rings….the high one could be water in a cyl….

 

[jimmbo]

Other than a faulty gauge or bad technique, what could cause two opposite compression readings on an 2006 Yamaha F50 with one reading at around 60 and the other around 210? What should I look after first, second, and so on...

Welcome to iboats
Are you saying, the compression in one cylinder is 60psi in one test, then upon a retest, it is 210?

 

[racerone]

How did it run for you last season ?------Check valve clearance and valve train.----Did you hit something with motor ?-

 

[99yam40]

run a leak down test to see where the air goes

 

[mike_i]

I was wondering the same, did he test one cylinder twice with two differant gauges?

Welcome to iboats
Are you saying, the compression in one cylinder is 60psi in one test, then upon a retest, it is 210?

 

[saltchuckmatt]

Well the low one could be a valve hanging open or bad rings….the high one could be water in a cyl….

I've never heard of a reading so high, can water actually do that?

 

[danielpl]

run a leak down test to see where the air goes

Thank you all for replies. All tests were done on all cylinders, with + or - 5 lbs variances, done at 2 different shops. First shop said motor was done, 2nd noticed some wear in timing belt. Based on the symptoms I relayed to him, he suspected that might be the culprit, he will change and retest as well as check if symptoms still present(unusual rpms, rough running, difficulty starting, ticking sound)

 

[danielpl]

How did it run for you last season ?------Check valve clearance and valve train.----Did you hit something with motor ?-

Didn't hit anything, except my wallet I suspect will take a hit!

 

[danielpl]

Based on Dr. Google Mechanic, normal readings in 4-stroke are 180-210 lbs. I evidently prefer the reading the 200 range rather than the 60s, some chance of saving my motor, fingers crossed...

 

[danielpl]

Other than a faulty gauge or bad technique, what could cause two opposite compression readings on an 2006 Yamaha F50 with one reading at around + or - 60 on all cylinders and the other at + or - 210? What should I look after first, second, and so on...

Update on situation: timing has been changed and they noticed the flywheel key was sheared and will also be replaced. Could these two items be the source of the varying compression tests since both will throw timing off and cause unusual engine malfunctions. I still have a rough running engine, unusual rpm readings, difficulty starting when hot. Hoping these symptoms disappear once timing is reset. With theses added bits of information, any further thoughts to explain these wildly different compression test results?

 

[99yam40]

I would want to know if this motor is an interference motor,
(if timing is off will the pistons hit the valves and bend / break things)

 

[danielpl]

I would want to know if this motor is an interference motor,
(if timing is off will the pistons hit the valves and bend / break things)

Try as I may to make that determination, Dr. Google mechanic failed me. I can only say it is a Yamaha 50 hp 4stroke year 2000 motor.

 

[racerone]

I would say you have a bent valve.