compression 50 hp yamaha 4 stroke

gerryjn

Recruit
Joined
Jan 24, 2012
Messages
2
I have a 50 HP yamaha 4 stroke OB, measured the following compression Cyl 1/100 psi, 2/140 psi, 3/100 psi, 4/100 psi.
understand that 90-100 psi is normal. Why would cyl 2 read 140 PSI. and what can be done?

Did the leak-down test with 100 PSI the air loss was zero
 

robert graham

Admiral
Joined
Apr 16, 2009
Messages
6,908
Re: compression 50 hp yamaha 4 stroke

This is a 4 cylinder 50HP motor?....Regardless, if any of your compression readings are on the high side, where's the problem?:confused:
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: compression 50 hp yamaha 4 stroke

It could be that way because #2 is not at fault and the other three are. Why are you doing a compression test? It may also be because that cylinder is using oil. A leakdown test would prove which cylinders are actually at fault. An engine is considered to have normal compression if the numbers are over 100 and within 10% of each other. Yours are not.
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
Re: compression 50 hp yamaha 4 stroke

I always thought 4 strokes had a lot higher compression than 2 strokes. Yours looks like a bad reading on a 2 stroke.
I would try to find someone to get a reading on a different 4 st 50, just to make sure.
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: compression 50 hp yamaha 4 stroke

As long as an engine has compression over 100 psi it will likely run very well. Maybe not making 100% of available power but nevertheless a good running engine.
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
Re: compression 50 hp yamaha 4 stroke

he said he did a leak down test and got zero leakage.
So I wonder if his tester or method may be the problem
 

gerryjn

Recruit
Joined
Jan 24, 2012
Messages
2
Re: compression 50 hp yamaha 4 stroke

Gentlemen, thank you for your response
Engine problem was rough running, unable to get to max speed and difficult to start.
Took the carburetors apart and cleaned the float chamber and nozzles, engine runs better.
Did the following tests:
Leak-down test 0 %
Compr. test carbs removed open trottle closed trottle
cyl 1 210 200 100
cyl 2 210 200 110
cyl 3 210 200 130
cyl 4 210 200 135

It seems obvious that the problem is related to the carburetors is there anyone who can direct me which part of the carburetor can cause this problem?

Again many thanks gerry
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
Re: compression 50 hp yamaha 4 stroke

Your carbs have not been synced properly.
Need a 4 tube manometer and follow directions in manual.
Seem you did not take compression 1st time with carb throttle plates cracked open some and some of them are closed too far.
All need to be open the same for motor to run properly
 

WIMUSKY

Moderator
Staff member
Joined
Sep 26, 2009
Messages
19,790
When you do a compression test, it should be done at WOT. Whomever told you 90-100psi is normal was completely wrong... 90-100psi on a 2 stroke you're thinking about rebuild time. 4 strokes have higher compression...... Your compression is very good at 200 WOT.
 
Top