86 YZ 125 throttle screw / carb

[Vermonter]

Hey guys, first post. I bought a YZ 125 as a project, previous owner says that it was running last summer. The engine is solid, body is pretty good. I think it had a recent top end rebuild, you can see the stampings on the piston if you look through the top of the cylinder.

My friend and I have been attempting to start it, with little luck. When we first picked it up, the compression was really low. I installed a gasket on the exhaust, and that helped. Installed a new spark plug (B9EG) as well. We get a spark, so it's not that. Took the carb apart and ran carb cleaner through it, it was pretty clean anyway. It was missing the airbox, so I've been collecting the parts for it.

Managed to get it to cough twice on us, but won't start. Mind you, it's still cold outside, so that may be a factor. However, we noticed that the throttle screw is missing. Could this be a factor in why in won't start? We even tried injecting a little bit of fuel mix into the cylinder, no luck.

Any help is appreciated.

~Josh

 

[BigRedAF]

Throttle screw? Are you talking about the idle adjustment?

No air box will cause a lean condition making cold starting difficult. Pouring fuel into the cylinder will cause flooding making starting difficult.

Finish gathering your parts and complete the bike. Once that's done you can start and tune your machine.

 

[Vermonter]

BikeBandit's microfiche lists the part as a "throttle screw".

It's seated above an overflow tube. I now have the airbox, but I still need a few parts, and mainly the air filter and mounting hardware.

I attached the .jpg of the microfiche with the part in question circled.

 

[BigRedAF]

That's for the idle adjustment. If it's missing I suppose it will cause an air leak and a slight lean condition but I don't believe it would prevent your bike from starting.

Make sure your pilot circuit is clean and set your air screw to 1.5 turns out for a ball park setting. Have you checked under your left side ignition cover? Make sure it's clean and dry. Make sure that the stator is mounted correctly and the dots, lines or triangles are lined up. Manufactures use different markings but they do the same thing. Make sure you have fresh gas and spark.

With gas, spark and compression it should light up. You can adjust and tune from there. Good luck...

 

[rmc_olderthandirt]

More specifically that is the idle speed adjustment. The idle mixture is item 23.

Without the idle speed screw in place I would expect the throttle slide to completely shut off the carb at idle. I also have an '86 YZ-125 and I find the easiest way to start is to leave the throttle at idle. Fuel enrichment knob pulled for cold starts but left down for warm starts.

I suspect that without the idle stop being present and adjusted at least close it would be hard to find a throttle setting that would allow of easy kick starting. I would recommend "Bump" starting. A few friends pushing, tow it behind a quad or truck or carry it to the top of a steep hill.

If it simply won't run then a complete cleaning of the carburetor would be my next approach. When an engine fails to run after it has sat for a while it is usually becayse the gas in the carburetor sat for a year and turned to gunk. This can be a much worse problem on a dirt bike if the gas was left on because you can end up distilling an entire tank of gas down to a really nasty gunk that clogs all the hard to get to passages inside the carb.

The only solution is a proper cleaning. This involves removing all the jets, soaking over night in a good carburetor cleaner, rinsing in water and then blowing the passages out with compressed air. Just cleaning the outside of the carburetor does nothing and if you don't remove the jets and float valve and get the gunk out of the tiny passages it won't really be "clean". You should buy a complete set of gaskets. I recommend buying a new float valve assembly. It will set you back $20 to $30 but it will solve any float valve issues.

Rod

 

[Vermonter]

Hey guys, got a chance to futz around with the bike today. Got a compression tester from AutoZone, max compression was right around 120 psi. Tomorrow my friend and I are going to inspect the gaskets and piston rings to check for wear. In the meantime, we've been working on getting the brakes operational, got the pads installed, just working on bleeding the system now.

Also trying to get the rear axle off, it's pretty stuck. It's almost off. Tomorrow that sucker will be mine. :-D This coming weekend will be the big bump start test, once I get everything ready.

 

[BigRedAF]

120psi is quite low. If you take it apart put a fresh piston, rings, wrist pin and bearing in it. You will see it pump around 170+ after it breaks in.

 

[Vermonter]

It's also about 34F outside when I work on the bike. I don't have access to a garage (especially a heated one!) so I'm working with outdoor temperatures.

Anyhoo, got the axle off. It was filthy. Gonna have the local MC repair place clean the wheel off and put some grease in it when they replace a bent spoke.

Took the head off the cylinder. Piston is very new, has some detonation marks on it. Likely from the previous owner, caused by junk gas and 2-stroke oil. The two o-rings look pretty new as well. I think at one point there was a piston seizure, and quite possibly an o-ring failure. Guessing the piston was replaced along with the rings. Anyhoo, it moved perfectly when I turned the kickstarter. Going to get some new o-rings for later. I believe the piston is fine.

Once it gets warm out and we have a chance to bumpstart it I'm pretty sure the compression will go up to 150 or so.

 

[rmc_olderthandirt]

If you have the head off then it is just a tiny bit more work to replace the ring. The ring should be replaced on a regular basis as well as the piston, slightly less often basis. I recommend putting a new ring, cylinder base gasket and o-ring head gasket. At least then you will know where you are at.

Rod

 

[Coop]

I agree, rebuild the top end. You can't really inspect an old piston just by looking at it. You need to measure it. If you were anywhere near Pittsburgh I would gladly help you out.