Stihl 661 , black crank

[Jza]

Bought a bulk lot of part saws the other day . This 661 looks good , good compression but wouldn't go full throttle ( assuming bad solenoid) as it passed vac/pressure .

Anyway I pulled the cyljnder off and there's so much carbon everywhere , especially on the crank , transfers etc.. The bottom ring around the locating pin looked coked up too .

I've talked with a guy I know and he suspects bad oil with a combination of heat .

What are your guys thoughts , I'm interested to know

 

[Tobystihl]

Worn rings causing mega blowby, I'm surprised it still ran at all!!!

 

[Gord404]

got a 661 that suposed to have been streigt gased and the cylinder and piston are in great shape but the crank is black and big end bearings are crunchy and ready to fall out- in my experiance the piston should have taken a real beating- crank maybe

 

[Patrick62]

I am going to vote that it was flogged hard doing milling. They thought that they could compensate by running more 2C oil in the fuel (cheap oil too?) anyway, the throttle was pinned hard for long, long cuts. Until it started to make funny noises.
Now you get to split the case, new bearings, new seals, and a new piston?
what does the cylinder look like?

 

[Gord404]

cylider is really good and piston seems good but do have a new piston or just rings whatever I think wheni take it off- have a good oem crank and bearings to go in- gaskets arived today and have new seals I guess it's good the crank seiezed before all the neadles went everwhere.

 

[Jza]

Worn rings causing mega blowby, I'm surprised it still ran at all!!!

I was wondering this too . I don't think the oil he used was very good either

 

[jltrent]

Looks like too much 2-cycle oil. It passed a pressure test and if no noise in the crank bearings I would put a new piston in. A bad fuel solenoid can cause them to run rich so that could be a problem or the problem.

 

[stihltech]

That piston does not look good to me. Lower side of skirt worn. Same idea as a bad valve in a 4 stroke. If cylinder is good, scuff it up and put in a new piston. Vacuum and pressure test a must.

 

[Jza]

That piston does not look good to me. Lower side of skirt worn. Same idea as a bad valve in a 4 stroke. If cylinder is good, scuff it up and put in a new piston. Vacuum and pressure test a must.

Yeah mate , will order a meteor piston kit and a new solenoid. Should be back in business after that

 

[cookies]

Run too rich using heavy oil mix, the rings can get stuck in their grooves from the carbon. I'd bet you can move the rings up and down pretty good in their lands from the piston being worn. Looks like it ate some sand or lots of carbon from the pictures I thought I could see the sealing edges of the ring chipped, with low compression the fuel mix will not burn properly causing excessive carbon buildup. The carbon breaks loose damaging things as it passes through the saw.