GS250T running on one cylinder.

[81gs250]

I have an ’81 GS250T that will start and run on only one cylinder. It’s got 17,000 miles and has sat for roughly four years. I read all of the great advice here and went through the basics – replaced gas, charged battery, etc. Still the same, so I dived in to it all: pulled the carbs, soaked/cleaned/rebuilt them, and did the valves.

Here’s what I know:
1. Carbs. When I pulled the carbs off (Mikuni BS30SS), they weren’t quite like all of the descriptions. There were no rubber plugs over the pilot jet, and no o-rings on the air screw (adjustment on the outside of the carb, formerly covered with a metal plug) and air jet (in the carb throat). The only o-ring was on the choke controls, which looked good but I replaced. I also put a plug over the pilot jet.
2. The rest of the carbs looked good – no tears or holes in the CV diaphragms, clean inside, etc. They looked even better after a couple hours in dip.
3. Checked the valves – on the good side, measurements were .06mm exhaust/.08mm intake. Bad side, .10mm/.10mm
4. Checked spark and compression, both look good. Nice blue spark, but I changed the plugs and tried again with the same results.
5. I couldn’t find number for the jets and needles – they were all different than anything I found online, not even close. I stupidly didn’t write the numbers down. Since I am pretty sure nobody has ever torn the bike apart, I figured that they were all stock. Plugs still on the air screw, for example.
6. I checked the airbox for leaks, looked good. Cleaned/lightly soaked the air filter, looked for air leaks. I did not replace the o-rings in the intake/carb boots, which I should probably do.
7. Petcock works fine, checked it all different ways.

After doing this, I get the same results – one side runs, one doesn’t. So, I tried fiddling with the air screw on the bad side. At 2 turns out, it will run when it’s on partial choke. If I keep it on full or at about 1/2 or 1/3 choke, it will run fine on both sides. Didn’t try to ride it, but it sounds good. If I let the choke off, it dies right away. I let it run for about 5 minutes, still doing it. When it’s running on just the good side, it will backfire every few strokes, so it seems like it’s getting gas to the bad side.

So, what’s wrong? My thoughts:
1. .10/.10 valves need to be shimmed, but it doesn’t seem to be off that much.
2. Since it runs on choke, it must be too lean, right? Could it be that the needles/jets are completely wrong? Does that contradict the adjustment for the air screw on the bad side?
3. Do I need the o-rings on the air screw and air jet, like in BikeCliff’s walkthrough? Did they just vanish, or did someone leave them off, or something?

I think I’m thinking about this too much, and should probably put the o-rings in the air screw and jet, shim the valves, and replace the o-rings in the intake/carb flanges. OK, enough from me. Your suggestions? Am I missing anything?

[pete]

I can see one check that's missing which is compression, although to be honest I'm not experienced enough to know if that could be your issue. Definitely would be worth while to check though!

If I was you, I'd cover off the valve adjustments first, and are there pic's around somewhere of your carbs? Depends on what they are but I suspect you're probably right about requiring O rings. If there's an air or mixture screw I'd be surprised if it didn't have an O ring to stop external air getting sucked in at that point in the circuit... however again I'm no expert...

[tom203]

81gs250 said...."I think I’m thinking about this too much, and should probably put the o-rings in the air screw and jet, shim the valves, and replace the o-rings in the intake/carb flanges. OK, enough from me. Your suggestions? Am I missing anything? "

Yes! The fact that the "choke" system (actually an enricher) is enabling it to sorta run suggests that your carb/carbs still aren't clean- extra fuel from "choke" is compensating for clogged idle sytem- not enough fuel creeping up those nasty little holes.

[Briars]

Tom, I would have to look at a exploded view again but I do remember the 250 carbs do not have all the o-rings the other bikes do.

I went through mine last year around this time. I was surprised how nasty they were, the bike had been running pretty well considering.

The parts fich actually shows what you will need. It sounds like you got it right, but you need to look at the diagram again and verify it.

If you are leaking around the boot o-rings you can spray some carb cleaner around there while it is running and you can tell if it is sucking air.

But from the way you describe it I would concentrate on the carb for the bad cylinder. Make sure those little fuel passages are open and check your bowl float setting height.

Thats about all I got. Hope you figure it out soon.