Low Compression on 4hp. What am I missing?

[Sea Rider]

What is 35# is it PSI. The only way to know if cylinder is perfect round or if inside specs is with a tool and removed piston if not will have compression loss. That small OB is cheap to repair, if both cylinders are out of specs will need 0.5 mm piston & ring set, head gasket, spark plug + machine shop work on cylinder which are the minimum required parts to bring that OB back to life.

What's the purpose of squirting vd-40 through plug hole along 50:1 fuel/oil mixture. Timing is fixed on that model along bullet proof key. Assume you have spark, right ?

Happy Boating

 

[Wee Hooker]

Revisiting this old thread as I just pulled the OB out of the basement for another go at it. To be clear. good spark and fuel. 50:1 mx sprayed down the carb or directly into the cyl produces nothing. I'm going to pull the head again for reinspection/measurement.. Any way to dye penitrant test for cracks?

Otherwise, I may just drop it off for repair. I've never replaced a piston/rings before and wouldn't know where to start. Any idea what such a job should cost?

 

[Wee Hooker]

Ok, just went out and retested. Got 35 PSI initially. Added 80SAE gear oil to the cyl, swished it around and got 75-80 psi. The cyl walls look unscathed ( if not a bit discolored) and the head showing some corrosion) does not appear to be cracked or uneven. I checked uniformity of the cyl walls for roundness and it seems good (although my method with basic vernier was not perfect.)
I'm going to reinstall the head , retorque and try it but I suspect rings at this point. :-(
Attached are pics of what I have.

 

[pvanv]

Cound have a cracked ring, or just carboned-up. Using start fluid can crack a ring. Dunno, but a ring for the M5B may fit. It probably has 2 rings.

 

[Sea Rider]

If PSI went higher when oil was squirted on cylinder it's a clear indication that OB is way low on compression. A new piston/ring kit will fix it, will need to know when cylnder is measured properly at a machine shop if will accept a standard or next size piston/ring kit ?

Happy Boating

 

[Texas Prowler]

Hello. I just rebuilt the same block. Mine is a 98, but looks to be the same as yours. I took mine apart home the cylinder and replaced the piston rings. That was easy as was putting the piston back into the cylinder. I used the standard sized rings. No special tools used as the bottom of the cylinder has a concaved shape. I also replaced the reed and reed block. Chris Carson has great reeds and cheap. Upgraded to the 5hp reed block. Lastly I replaced the carb with a to the 5hp carb. Your compression is very low. Mine had 90psi before the rebuild. I'll have to test it soon to get the new comp psi. Very easy rebuild.

 

[Texas Prowler]

Looking at the cylinder the dark areas are hard tell if they are embedded in the cylinder or a reflection of the piston top. If in they are in the cylinder they might just be the culprit.