B&S 6.75 with zero compression

[iwannadie]

Ok first post

I have a craftmans mower with B&S(125K02-0578-E1) engine that has given me no trouble since the day I bought it. I've done yearly tune ups and run fresh gas and it has always started first or second pull until recently.

One day it suddenly took 5 or more pulls to start then wanted to bog down and stall on "normal" cutting. I checked the oil after I was done and it was pretty low. No sign lf a leak anywhere so I blame myself for not checking the oil right away but it showed no sign of a leak. Following week checked the oil and again took a bunch of pulls to start. It started to bog down much sooner and worse than before so I stopped mowing to hopefully not cause more harm.

Got it in the garage and checked things over. Air filter looks as it should no sign of oil or blocked. Exhaust looked fine again no sign lf oil infact it looked almost new. Spark plug just fine also. Did a compression test, 0psi. After that I hooked up the leak down gauge and gave it maybe 15psi, wasn't sure how much pressure it could take. I herd air hissing out of the intake no matter the position of the valves.

Pulled the head and everything was coated with oil, cleaned it all out and lookd almost new inside now, piston looks great as does the cylinder wall no signs of a problem. The head gasket was pretty wet along the bottom so I put a new on in hoping maybe that would help, no change.

My plan, replace the rings, clean the valves and inspect them better and maybe new valves / valve job. Anything else I should look at? To me the valves seemed to seat just fine but air from the intake could only mean intake valve? Also oil getting past the piston could only be rings?

I'm new to small engines but a weekend mechanic and do all my own work on cars, jeeps and motorcycles just never engine internals so I figure this is good / cheap practice.

I really want to work on and get some experience with small engines vs throwing it away and replacing it.

 

[ILENGINE]

Over time the valve face as well as the seat will wear, causing it to float the valves, leading to low compression. Low compression will cause incomplete combustion of the fuel air mix, and wash the cylinder walls down with gas causing thinning of the oil and excessive oil usage. If the cylinder walls look good and you don't have a ring ridge at the top of the cylinder, I would just work on the valves and see what happens.

 

[bertsmobile1]

the inlet cam has a bump on it to act as a compression release at low speeds.
Thus when you crank the engine slowly you will always get a leak past the inlet valve.
When I do a leak down test on these engines I always back off the tappets to ensure the decompressors are circumvented.
I use metholated spirits to test valve seating by filling the head and checking for any leakage

 

[iwannadie]

Thanks for the info guys.

I'll focus on the valves for sure and see what happens.

 

[iwannadie]

I finally got around to working on the mower.

Did a basic valve job, when I pulled the valves out they didn't look bad at all. No pitting or anything unusual but i did a round of course and then fine lapping and got them looking brand new. Also wire wheeled the valves to clean them all up nicely.

Having never messed with valves I was a bit gun shy grinding them down to find the correct clearance after lapping. I may pull them back out and take a little more off, I was on the high side of spec and maybe just a hair over.

After it was back together it started up a little easier than before, still several pulls. Could be valve clearance or still the rings. I plan to do rings maybe next weekend and double check the valves and see where that gets me.

Having fun working on the little motor though.

 

[riseandrun]

Found this thread in a search and wondered if you ever determined the problem. My engine has 75lb but has "z-start 30% easier to pull" so I'm guessing some kind of compression release. But I can't get mine running either.
R&R

 

[iwannadie]

I totally forgot about the thread.
It was valves. I got the clearance within spec and it ran like a champ no issues at all. Then after I got it running I moved to a house with zero grass. The irony there.