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'89 YZ250 Barely Idles

thedub

thedub

Octane Socks
Joined
Sep 2, 2010
Location
Carson City, NV
Moto(s)
Dirt.
Name
Wiles
BARF perks
AMA #: 2952025
A friend picked up an 89 YZ250, and it's not in half bad shape for $700. Everything, well the important things, seem to be present and functioning. Runs pretty well too, goes like stink. I was almost afraid to find the throttle stop on the street in front of my house with just shorts and t-shirt on.

But

Doesn't idle very well. We took apart the carb and cleaned it. Everything was a-ok. I can't find a service manual, but judging by the part numbers on the fiche, all the jetting is stock. Couldn't find a good number for float height via Google, so left it alone. Set the air screw 2.5 turns out.

The idle is ok, not rough, it's just really low. So low that as soon as you put it in gear the increased drag from the clutch stalls it. It seems to me that the speed just needs to be turned up a little and it would be good to go, but the idle speed adjuster is already turned all the way in.

It did occur to me that this is a 1980s 2-stroke racing motorcycle, and maybe having to blip the throttle when it's stopped is just the nature of the beast.

It has a Mikuni TM carb. Any input?
 
It's out of idle adjustment? Play with the air screw. If you find it at the far end of adjustment, 1 turn or 3 turns, it will tell you to use a different pilot and maybe that will give it a little idle speed.
 
reread post..... idle adjustment all the way in. What Eric said above. Does the carb continue to respond to the airscrew beyond 2.5 turns out, ie increase in RPM? You may need to go leaner on your pilot if so.
 
Last edited:
2strokeYardSale said:
Play with the air screw. If you find it at the far end of adjustment, 1 turn or 3 turns, it will tell you to use a different pilot and maybe that will give it a little idle speed.
Click to expand...

OaklandF4i said:
Does the carb continue to respond to the airscrew beyond 2.5 turns out, ie increase in RPM? You may need to go leaner on your pilot if so.
Click to expand...

Not really. I didn't tell much of a difference as I let the air screw out past about 2.5. 10-4, play with PJ.
 
thedub said:
Not really. I didn't tell much of a difference as I let the air screw out past about 2.5. 10-4, play with PJ.
Click to expand...

While I never owned an 89 YZ, I did own a number of two stroke MX bikes in the 80's and all idled. :laughing

I might also double check the idle screw to make sure its the right one installed. Never know if a PO lost it and just put a short screw in to plug the hole that bottoms but doesnt actually lift the slide. While a really low float could be an issue, I'd be double checking the pilot jet and all passages first. Good luck! :thumbup
 
And it's just old and probably worn. There's no telling how worn are the air passages and non-removable pieces. Mikuni TMs use replaceable nozzles (aka needle jets), right? Those are known to wear out. I would expect a worn nozzle to be leaner at idle and want the air screw turned in.
 
When you cleaned the carbs, did you clean the pilot and main jets. You may want to try replacing them (more the pilot) if you don't have the proper tools to clean them with, as you don't want to remove material when cleaning.
 
Welcome to the dysfunctional world of the Mikuni TM carb.
Those things belong in trash cans. Eric is on the right track with the nozzle being worn. That, along with wear on the slide will keep the bike from idling properly. Even if you get it to idle good just wait a few hours & it will no longer idle.

Replace the Mikuni with a Kehin PWK & you will never think of it again.
 
KazMan said:
When you cleaned the carbs, did you clean the pilot and main jets. You may want to try replacing them (more the pilot) if you don't have the proper tools to clean them with, as you don't want to remove material when cleaning.
Click to expand...

I do not have the proper tools (what are the proper tools?). I use the blast them with brake cleaner, then blast with the air compressor nozzle method. Followed by hold it up to the shop light to check for any visible gunk.

Farmer John said:
Welcome to the dysfunctional world of the Mikuni TM carb.
Those things belong in trash cans. Eric is on the right track with the nozzle being worn. That, along with wear on the slide will keep the bike from idling properly. Even if you get it to idle good just wait a few hours & it will no longer idle.

Replace the Mikuni with a Kehin PWK & you will never think of it again.
Click to expand...

Interesting, thanks for the input.
 
I used to use torch cleaners as opposed to index drills.
 
KazMan said:
When you cleaned the carbs, did you clean the pilot and main jets...
Click to expand...

KazMan said:
I used to use torch cleaners....
Click to expand...

What KazMan said, or a single strand of electrical wire also works well. The important step is to poke something solid through the pilot jet, not just air or fluid. Then check afterwards to be sure you can see daylight through the jet. 99% percent of problems after a bike has been sitting a while are from clogged pilot jets.
 
When you are done messing with the carb, replace the top end and just ride it.
:laughing
 
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