I know this is an ATV forum but I really need some help. I recently bought a 99 YZ125 and am having trouble with it running well. I'm having trouble starting it...when I try to start it, it seems like it is flooded...I pull the plug out and kick it over a couple of times and dry the plug off and put it back in. It starts fine after I do that. Then when it is running it smokes ALOT, I'm running the mix at 36:1 with Yamalube. It runs REAL sluggish on the bottom end and then wakes up at higher RPM's. When I let off the gas it dies and won't start back up until I clean the plug and do the whole thing over. I tried taking off the filter to see how it runs...but it doesn't run much better. Any suggestions will help. Also the bike is bone stock and the guy I bought it from said that it has brand new factory pistons in it.

99 Banshee, FMF Fatty's, Power Core 2's, K&N filter with precharger, yellow maier racing fenders

99 YZ125

 

Try looking at the jetting. I have a RM125 that had the same problem, and the pilot jet had shaken loose and was hanging out in the bowl.

 

skyman, thanks for the reply. It was running at 32:1 and was smoking more so I figured If I leaned it out a little it would run better, but that wasn't the case.

 

32/1 is 4 oz per gallon. WHat I am reading is that the engine has just been assembled to sell the bike. First thing I would check is compression...I would be worried that the owner ran the bike till the piston broke (this is a FUBR situation), cleaned the case, did'nt bother to have the cylinder bored, and slapped the new piston in for a quick sale.

 

I would check the jetting, you said it is stock, but make sure the guy you bought it from did not have a pipe and jetting done, then take the pipe off to sell it and leave the jetting. Also make sure he did not put a RAD valve in it. That will also richen it.

What kind of plug are you running? What heat range?

 

If what you stated is true...what is the best way to go about fixing it or checking it? I contacted the Yamaha dealership and they say that these cylinders are not boreable????? The guy that I bought it from I don't think he put new rings or gaskets on it. I think he just replaced the pistons. On the other hand it may be true that he put pipes on it and removed them before he sold it. The factory pipe that is on it looks brand new like it didn't have much riding time on it. What would the first thing that I should take off and check???

99 Banshee, FMF Fatty's, Power Core 2's, K&N filter with precharger

99 YZ125

 

Not positive about the cylinder being boreable. If you don't have access to a leak down tester, you'll have to have someone do the test for you. It would'nt hurt to check the float height too. Can the original owner help you?

 

I have a compression tester that I will try and see if I can get any info from that. It is smoking alot so thats why I'm leaning more to the jetting. I'm going to take the carb. off and check the jetting and needle setting, then I'm going to check the compression in the cylinder to see if it needs a new top end. How would you check a leak down test?

 

skyman, how did yours run when the pilot was not seated right? Did yours run exactly how I described?

 

You would need to know what recommended cranking compression is first (call dealer if you don't have a manual). Then you could insert your leakdown tester (of course making sure than the end does'nt go farther into the cylinder than the spark plug end does) and crank it over a couple of times. Like you said, the carb could be the problem. You will need standard jetting specs...I believe fmfracing.com's web site has that info.