[oonighttrain]

hi, i saw your past posts and it lookslike you really know your yamahas.. any help from anyone would be greatly appreciated!!!
i have a 1997 yamaha 50 hp 4 stroke that runs ok but, has very low compression on cylinder #1 (like 30 p.s.i)and slightly low compression on cylinder #4. (around 90) 2 and 3 are over 120 psi....
i removed the head to take a peek and do not see any damage whatsoever on the cylinder walls. i took the head to a machine shop so they can do
a vacuum test on each valve. im hopingthey find something wrong so i dont have to get any deeper into the engine.



thanks!!!

[drewmitch44]

Shame there is not like a "compression adjustment" screw. You could adjust that! LOL Its like, awww man my 4th cylinder compression is low like 20 psi. You could just turn it up! LOL Anyways man i hope everything works out for you and you dont have to rebuild.

[oonighttrain]

Originally Posted by drewmitch44

Shame there is not like a "compression adjustment" screw. You could adjust that! LOL Its like, awww man my 4th cylinder compression is low like 20 psi. You could just turn it up! LOL Anyways man i hope everything works out for you and you dont have to rebuild.

i think your on to something there.... you could make a fortune!!!


by the way.. if someone can tell me where my compression adjustment screw is, id appreciate it..

[noelm]

I hope Rodbolt has a simple explaination for you, but I don't like the sound of it so far, sounds like some stuck rings maybe, but you never know, you may be lucky with a leaking valve or something, a leakdown test would have painted a clearer picture for diagnosis, but the head is off now so just wait and see what they find.

[rodbolt]

vacum test on each valve?
this I really really gotta see.
tell me all the Yamaha 4 stroke certifications hanging on your techs wall.

beyond that, what dummy would ever take a 4 stroke apart without a leakdown test when the reason for loss of compression wasnt known ?

[reeldutch]

the numbers are extremely low for a 4 stroke.

these compression numbers are more 2 stroke ish

4strk are depending on compression ratio from model to model, but generaly from 150 al the way up to 220 psi.

i would defenetly perform a leak down as rodbolt sugested.

good luck

[rodbolt]

about all that can be done without reassembly is to remove the cam, place the head upside down and fill each combustion chamber with solvent and see if the valve leaks and inspect the old head gasket.
my reccomendation would be to reinstall the head with a new gasket, do a leakdown.
test can be run without the cam installed.

[oonighttrain]

after reading all the posts which by the way, was after i removed the head.. i realize i should have had a leakdown test done..

can they perform the leakdown check if i slap the head back on without hooking everything else up?? thanks!

[rodbolt]

as the head gasket can only be crushed once you will have to use a new head gasket, IF the headbolts use an angle torque method for final tightness its reccomended to replace the head bolts at final assy.