8HP 2 stroke no start - Low compression

[viper1216]

I have a 2004 8HP Yamaha 2 stroke. Started last week, and was idling for about 5 minutes, then died. Good water flow. Haven't been able to start it since. New plugs, new fuel filter, carb was rebuilt beginning of the year. Checked reed plate and all looks well. Has spark and fuel. Drained the carb bowl and no water. Compression is reading 80psi in both cylinders, which seems really low to me, so I am going to gander it's that. Is this thing toast, or something I may be able to rebuild? It did overheat to the point of shutdown in May. My 15 year old son was using and didn't catch the water indicator wasn't pissing. Needed an impeller and has been running great ever since, but not sure if something got cooked. Any thoughts or directions form here would be greatly appreciated!!! Thanks!!!

 

[QBhoy]

Take the carb off it again. I’d bet money on it running again after a tinker with the carb and fuel system. Alternatively, perhaps check any other electrical components that might have suffered with the overheating perhaps.

 

[racerone]

Just proof that motors need a new impeller every 5 years or so.-----But check the flywheel key.----A sheared key puts timing way out.----Not a big deal to take the head off for inspection.----The head will be warped and needs refinishing.-----Glass and emery paper does that repair.

 

[viper1216]

Carb was off so I could inspect the reed plates. Didn't help when I put it back together. I'll check the flywheel key. Does that compression seem really low? From what I am seeing, it should be about 120.

 

[QBhoy]

Carb was off so I could inspect the reed plates. Didn't help when I put it back together. I'll check the flywheel key. Does that compression seem really low? From what I am seeing, it should be about 120.

It’s a little low if the gauge is known to be calibrated..but most gauges I’ve known that aren’t of a certain worth and are calibrated…can be as much as 20 psi out very easily. Could also be down to conditions in temperature and remaining oil film too. I’d be quite confident that even if it is both around 80…she should still run for sure. Also if they are both around that figure too…you’d be a hell of unlucky for them both to drop compression all of a sudden, to an equal result ! The head could be warped or have lost seal…but knowing these particular engines as I do…they are hardy things and suffer worse than this, and still run. It’s likely something simple

 

[viper1216]

That would be nice if it was simple lol, but those are the ones that kill ya lol. I tried spraying brake clean right into carb and it still won't run. Should have fired right up given it has spark. So popping head off is my next step I take it?

 

[flyingscott]

Brake clean was the worst possible thing to spray in there. How did you check spark?

 

[viper1216]

Holding plug against block. Nice bright spark on both plugs.

 

[flyingscott]

get a proper tester and make sure it jumps a good 7/16" gap

 

[QBhoy]

That would be nice if it was simple lol, but those are the ones that kill ya lol. I tried spraying brake clean right into carb and it still won't run. Should have fired right up given it has spark. So popping head off is my next step I take it?

Probably not a great idea to be fair. At the very least…it might make a mess of the paint around things…if not…attack any vulnerable components within the carb. Take the carb off and clean it properly. Honestly. Do this before going any deeper. It should run, even if she is 80 psi. Spraying stuff in there doesn’t always mean it will start…especially so, on an engine that’s pull started. Your float could be stuck or anything tiny sticking in a jet. Trust me. Simple things first.

 

[viper1216]

Probably not a great idea to be fair. At the very least…it might make a mess of the paint around things…if not…attack any vulnerable components within the carb. Take the carb off and clean it properly. Honestly. Do this before going any deeper. It should run, even if she is 80 psi. Spraying stuff in there doesn’t always mean it will start…especially so, on an engine that’s pull started. Your float could be stuck or anything tiny sticking in a jet. Trust me. Simple things first.

Do I need to get anther rebuild kit, or because I just had it apart a few months ago, I can just disassemble, clean and put back together?

 

[flyingscott]

Probably not a great idea to be fair. At the very least…it might make a mess of the paint around things…if not…attack any vulnerable components within the carb. Take the carb off and clean it properly. Honestly. Do this before going any deeper. It should run, even if she is 80 psi. Spraying stuff in there doesn’t always mean it will start…especially so, on an engine that’s pull started. Your float could be stuck or anything tiny sticking in a jet. Trust me. Simple things first.

Brake cleaner is a horrible thing to do!!!! It strips the oil off of the the metal parts. What do shiny clean metal parts do when the protective coating is stripped off? Hint: it rusts 2nd thing strips oil off of the cylinder what happens when that is not lubed?

 

[flyingscott]

Do I need to get anther rebuild kit, or because I just had it apart a few months ago, I can just disassemble, clean and put back together?

Did you clean it properly?

 

[viper1216]

Did you clean it properly?

Yes. Soaked and rebuilt.

 

[QBhoy]

Do I need to get anther rebuild kit, or because I just had it apart a few months ago, I can just disassemble, clean and put back together?

Not usually. Most carb cleans don’t really need any parts or further items. Not sure I’ve ever needed a kit for any of the dozens of carb cleans carried out on my old Yamaha’s around this size. That’s over a couple of decades at least. Ultra sonic bath is the ticket for sure. But if you’re thorough enough, a good clean will do.

 

[QBhoy]

Yes. Soaked and rebuilt.

Takes more than soaking and putting back together. You need to go through every item and completely stripped apart. Particularly the float needle valve and assembly. Each one of the jets too. Doesn’t take much with these wee carbs.
Did you re set the mixture to about 1 1/4- 1 1/2 turns out ? Making sure there was nothing stopping it seating well when screwed in…to give a false indication of where things are ?

 

[viper1216]

Not usually. Most carb cleans don’t really need any parts or further items. Not sure I’ve ever needed a kit for any of the dozens of carb cleans carried out on my old Yamaha’s around this size. That’s over a couple of decades at least. Ultra sonic bath is the ticket for sure. But if you’re thorough enough, a good clean will do.

Great thanks. I had soaked it in Chem-sip originally. Then sprayed down with carb cleaner. Maybe I will go grab an ultrasonic cleaner.

 

[viper1216]

Oh I had it all apart, didn't just toss it in the cleaner lol. It was down to bare carb before soaking.

 

[QBhoy]

Great thanks. I had soaked it in Chem-sip originally. Then sprayed down with carb cleaner. Maybe I will go grab an ultrasonic cleaner.

Ok. Yeah…just make sure you take it all to bits. I’ve messed with these wee things enough to know that my lazy efforts at it in the early days…usually just mean to need to take it off again and do it properly

 

[QBhoy]

Oh I had it all apart, didn't just toss it in the cleaner lol. It was down to bare carb before soaking.

Mixture screw set up right ?