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Carb Help!!!!!

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mrpesas

mrpesas

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My father, cousin, and I are rebuilding a 1982 Yamaha XT250 (for my cousing to ride as his first bike). But all is not going well. The XT250 is a single cyclinder, air-cooled, 4-stroke.

What we have done:
so we have rebuilt the carb with a commericial rebuild kit. We replaced the stock filter box unit thing with a clamp on filter. The carb has an "enrichener circuit" instead of a choke. Same purpose, but I think it adds extra gas instead of limiting the air. We also changed the oil & filter and replaced the battery. Gas tank is in good condition too.

Status:
The bike will kick over and idle with the choke (easier than saying enrichener every time). And it will idle all day long without and real issue. We let it warm up for a reasonable amount of time. Then, if you try to turn off the choke or give some throttle it dies. We can sometimes get just right to get it past the stumble and get it to throttle up high, but it very difficult. It basically have a very bad stumble just off idle that kills it.

So we basically think that we are not getting enough gas at idle. Which I think is controlled by the pilot? We know which piece it is and my dad could tell you the name, but I have forgotten for the moment. Also the pilot does not seem adjustable, but I think it should be.

Can anybody give me some pointers, or even possible be willing to look at it?

Also, if we can't get this carb sorted, could I use any carb off of a 250?
 
Something is plugged in the idle circuit if all of the settings are correct. Did you clean all the brass jets and orifices with carb cleaner, wire and compressed air?
 
yeah, we cleaned everything in a parts bin, with carb cleaner, wire, compressed air, general scraping with fingernails, etc.

We'll check the idle mixture screw. I have the Clymer for it, but it has been a little less than helpful on "adjusting" everything.
 
There is still varnish in the carb somewhere most likely. Not familar with this particular model but some of these came with Hitachi carbs. What carb brand do you have? Is it a CV design? If so, make sure there is no compomise of vacuum to the diaphragm. Also, inspect very carefully for leaks or a small pin hole in the diaphragm...it takes very little to upset the mixture on these small disp engine carbs. Check your work and check it again. If you find a hole, repair it with rubber cement and some silk or dacron.

On some model carbs, there is a vent circuit in the float bowl. Also, there is a tiny ball check valve in that circuit that fouls easily. You will need special skill and persistance to get this operating properly. I would first inspect the diaphragm and next, remove the float bowl and apply cleaner and air repeatedly to the float bowl vent check valve until it is clear. I recommend a overnight soak in Gunk Hydro Seal. This is about the only stuff that removes all deposits. Wash with Castrol Super Clean and reassemble first spraying everything with WD-40.

After the bike is running again, add Techron to the fuel tank and change or clean the filter/strainer.
 
it's either a kiehen or mikuni, and I don't think it is a CV. There is only a small diaphram for the accelerator pump, and it was replaced with a new one from the carb kit we purchased.
 
A CV carb is identified by the chamber on top with a rubber diaphram and spring inside that moves the slide up and down.

I would start over...soak the whole mess overnight and wash and blow with copmpressed air.
 
There may be some help here:
http://www.ttr250.com/

The XT was/is basically the TTR with road gear on it.
That same engine and carb has been in that bike for a very long time.

Those bikes are notorious for being cold blooded. As you're guessing, I'd strongly suspect the pilot circuit.
Take the pilot jet right out of the carb and inspect very carefully.
It's possible that it's got some gunk.
Soaking the jets out of the carb should also remove any varnish in them.
If there is varnish in there you really can't get rid of, put new jets in the carb which is a pretty cheap solution.
 
You're putting too much air into the engine. Put the stock air box back on and a properly oiled air filter. Small displacement bikes are very touchy about getting too much air since they get very little gas to begin with.
 
when I first got my bike, it was not quite as bad as you describe. I ran Sea Foam through it and it pretty much cleared up by the second tank. However, I think Irondawg has a valid point. If you aren't running stock breather, you'll have to rejet and open things up.
 
It's definately stumbling because of the aftermarket air filter. You need to find the idle mixture screw and tweak it.


BTW, did you drill out an aluminum cover and remove the idle mix screw before you dipped the carb? If not, you fubar'd the tiny O-ring in there, and that's compounding your problem.

Check this out for a great tutorial on rebuilding CV carbs...

http://www.thegsresources.com/gs_carbrebuild.htm

The idle mix screw looks like this,
026_Air%20screw%20removed.jpg


BTW, this reference calls it an air screw but that is incorrect. It actually meters fuel.

Another great resource is this,
http://www.factorypro.com/tech/carbtune,CV,high_rpm_engines.html

Good luck!
 
I agree with to much air. And I agree that if you soaked the carb with the o-rings in it that may have messed them up and you'll need to replace them.
 
I know this is an old forum but any help would be appreciated. I just picked up a 1980 XT 250g and having a hard time getting it to idle correctly. I just put in a rebuild kit so it is working much better after that, but for some reason I can't get it to idle, nor can I find an idle screw.
 
cdagne said:
I know this is an old forum but any help would be appreciated. I just picked up a 1980 XT 250g and having a hard time getting it to idle correctly. I just put in a rebuild kit so it is working much better after that, but for some reason I can't get it to idle, nor can I find an idle screw.
Click to expand...
Idle may be adjusted only by the throttle cable.
Have you checked the valve clearances?

Could you provide a picture of the carb?


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Here is a picture of the carb. I have not check valve clearances not sure how to do that.
 

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cdagne said:
Here is a picture of the carb. I have not check valve clearances not sure how to do that.
Click to expand...
It appears that 39 may be an idle screw

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cdagne said:
I have screwed in 39 all the way in and it didn't make a difference.
Click to expand...
It appears as though it should hold the throttle cam at a certain position. Maybe it needs to be backed out instead of in. A picture of the left side of the carb on the motorcycle with the throttle linkage would help

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