XR carb setup?

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Sep 23, 2000
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#1
I have a question regarding setting the carb on an 85 XR100R (Keihin 20mm piston slide). The manual says: set pilot screw at 2 5/8 turns out, adjust idle, turn pilot srew in until it stalls, and then back out pilot screw one turn.

My trouble is the bike doesn't stall. In fact the RPMs increase. The plug is black and wet. It will run and idle, but throttle response from idle is very poor.

The valves are adjusted, new plug gapped at spec (0.28"), new air filter soaked in gear oil and wrung out. Condition of points not yet known. The carb was disassembled and passages blown out with carb cleaner. Float height set to spec.

Any ideas? Worn needle? Air filter too restricted?
 
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#3
Where's the needle at? You should put it in the thrid clip (from the bottom). I find that mine runs a tad bit rich. Also, check the point gap. I still need to check the plug and see how it is after the rebuild. Good Luck.
 
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#4
I don't think the "gear oil" is helping you any. When you say gear oil, must of us think of 90 wt. If that is the case, there are oils made just for air filters and they do a great job.
Back in the 70's we used motor oil, most the time I selected a light wt oil. The heavier wt. oils do not pass as much air thus you could run VERY rich. If the jets are all clean and all passages in the carb. are not blocked, you should get reasonable running conditions with stock settings.
Those little bikes would run at just about any altitude and only minor adjustments were needed.
Points..I think it has electronic ignition. Needle worn...maybe, but I've NEVER seen anyone wear out a needle on the XR 100's. A friend of mine had the exact problem. It took about 3 times to get the pilot jet, (some call it idle jet) clear. The passage is VERY small. Don't cram a drill bit through it. It will end up too big and not run right.
Has it ever run properly since you have owned it?
Also...I may be wrong,(someone out there help me) but on those bikes turning in the screw leans the mixture, just the oppisite of a 2-stroke. It is a fuel mixture screw where-as on 2-srokes it is an air mixture screw. So turning in the screw reduces the amount of fuel not air..that would explian why it runs faster.
The running condition you describe sounds like the choke is on! It most likely starts easy if that is the case.
Hope I have helped.
 
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Sep 23, 2000
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#5
Thanks for your feedback. Here's what I've done so far.
1.Removed the air filter. No change in performance. So I thought fuel must be coming up through the main or slow jet.
2. Squeezed out all of the excess oil from the air filter. Dropped the needle all the way down (clip in top position). It will run and idle ok, and now it will stall as I turn in the pilot screw.
3. It still idles eratically (it is very sensitive to the idle screw adjustment) but does transition well from idle to 1/4 throttle.
4. The bike was a neglected basket case when I bought it. The needle hold-down piece was missing and the air filter was packed with dirt. The needle appears to be slightly bent and "scuffed". The float adjustment was set much lower than spec.

Should I replace the needle and main jet? I'm thinking I should probably replace the float needle while I'm at it.
 
- a d v e r t i s e m e n t -

- a d v e r t i s e m e n t -

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#7
The mixture screw and pilot jet controls the way the bike runs at idle to about 1/8 throttle. Standard screw setting on almost all bikes is 1.5 turns out. If you have to make huge adjustments with the mixture screw, you need a richer or leaner pilot jet.
If the needle is bent, yes it must be replaced and maybe the needle jet to be on the safe side. The main most likely isn't hurt. The needle jet is the one that the needle travles through. If the needle was bent badly, it might rub on the needle jet itself. When you say "float needle" are you refering to the valve that shuts off the fuel to the float bowl, it is loacted when you remove the float assy. If you suspect it is not cutting off the fuel supply then replace it. It could be that the float is incorrectly set as you said. It is normally an item that doesn't get replaced...I have original carb. and all parts to it in a '77 Triumph 750 and the bike runs perfect. Has the exhaust system been tampered with??? Hang in there.......
 
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#8
Did you ever solve the probelm with this bike? I have the same issue with a 1990...a little hard to start, funky rolling idle with an occasional miss/ stall but it runs fine further up. The plug is black and I can hear an occasional fart in the exhaust. Everything has been clean, timed and adjusted multiple times but it still has the same consistent issue. Let me know what you found out with yours.