Easy alternative for checking compression?

[Reggie08]

I'm nearing completion of a powerhead rebuild for an 85 'Rude 90hp. I just have a few things left to do (install t-stats, rebuild carbs, install cylinder head, etc) but before I tighten everything down and get it all put back together, I'd like to know if my compression is good. If the compression is weak, I know I have a leak somewhere I need to fix. Is there a way to check the compression other than having it hooked up to a battery and turning the key? Would turning the flywheel with a socket produce reasonable/comparable results?

 

[crb478]

Re: Easy alternative for checking compression?

I do not know of a short cut to get accurate compression reading. You really need to spin it over at full speed for 4 or 5 revolutions until you reach the hightest compression number. I would love to hear if some has a way, but I doubt that there is one.

 

[Gary H NC]

Re: Easy alternative for checking compression?

I doubt you will get accurate readings turning it by hand.Although you should be able to tell if the compression is fairly even across the cylinders.I believe they need to spin about 500 rpms for the compression gauge to read correct.
Maybe some of the outboard pros will chime in on this...

 

[Faztbullet]

Re: Easy alternative for checking compression?

Bench testing wont tell you much as crankcase and cylinder walls have good oil films on them and rings have not seated. As long as you were careful and checked clearances( crank seal rings) and pistons installed correctly( oil hole up on rod, eyebrow on piston correct position) you should be allright. Need to up the carb high speeds for the .030 over cylinders and that one .040 over

 

[Reggie08]

Re: Easy alternative for checking compression?

Do I need to up the jet sizes before starting it?

 

[Faztbullet]

Re: Easy alternative for checking compression?

No ,but before running on mid to top end you will,plus its easier to change the bottom carb jets with carbs off than on (more room).

 

[Reggie08]

Re: Easy alternative for checking compression?

How do I determine what size jets it has now and what would be a good size to go with? And are they easily replaceable?

Oh...and thanks for added work by the way. Your mentioning the oil holes on the connecting rods got me to thinking I should break it open yet again and double check. My mind has been unsettled ever since you mentioned that

 

[Faztbullet]

Re: Easy alternative for checking compression?

SORRRRRY!!! .....Just dont want you to mess it up this close to being finished. Remove jet with thin blade screwdriver or there is a post how to make a jet tool. There is a number and letter on jet (example 52c) PM me the number and will look up correct jet for it.Also send me casting number and carb size. Will be stamped below throat of carb(example 333333 and 1 3/8).

 

[HighTrim]

Re: Easy alternative for checking compression?

Ill just post the pic for you , compliments of FR

 

[Reggie08]

Re: Easy alternative for checking compression?

SORRRRRY!!! .....Just dont want you to mess it up this close to being finished. Remove jet with thin blade screwdriver or there is a post how to make a jet tool. There is a number and letter on jet (example 52c) PM me the number and will look up correct jet for it.Also send me casting number and carb size. Will be stamped below throat of carb(example 333333 and 1 3/8).

I'll try to get you those numbers as soon as I can. I just finished up with it for the evening (though I'm dying to go back out and work on it some more). I did however, break it back open and double checked that I had the rods installed correctly and I did! I was fairly certain I had them installed the right way but that little voice in the back of my head kept telling me I needed to check.

And thanks for the pic High Trim!!