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Steve Wagner

XR100R 1985 no fire

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I got a cheap, not well taken care of 1985 XR100R not running condition. I have good spark and compression is 125psi. Valve lash is set at 0.05mm and points open just after the F mark. Drop gas in cylinder kick it over and no fire. Spark plug is correct with proper gap, coils wire and cap test well. Am I missing something?

Edited by Steve Wagner

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Did you clean the carb and pull the pilot and blow compressed air through it, then hold it up to the light and make sure you can see through before reinstalling it?

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Compression of 125 PSI seems low. The manual I have is for 1998 and newer but besides converting to 12V with CDI I believe the engine is basically unchanged. According to my manual the compression should be 165 psi or greater.

I have a 2002 XR100 and a 2006 CRF100 and both have compression of 170 PSI or greater.

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Ahhh. Thought that 125psi would be ok at least to get it to fire. As for points gap yes it’s within spec and carb is not in the equation- removed just to see if it fires. 

Is this top end rebuild time then?

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psi at new is 138 - so you would think 128 would be enough

thats basing it on 9.4 to 1 @ 14.7 ratio

Edited by S.O.A.N.Z

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psi at new is 138 - so you would think 128 would be enough
thats basing it on 9.4 to 1 @ 14.7 ratio

Did the older models have lower compression? My bikes have the same 9.4 to 1 ratio

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It will run at 125 psi, just not that strong. If it has spark, it'll run.  It won't run if the pilot is clogged or the float valve is not letting enough fuel in.  Spark, fuel, air.  If it's not sparking at the right time (typical point ignition), it'll backfire. If it's not backfiring, you're ok shape.  But, points have a lot of issues, stators go bad, the flywheel, shears keys, and the advancing springs don't advance properly. I'm not sure why you would dismiss the carb. If it's been sitting, the bad gas clogged everything up.  That's usually the number one thing, unless it's backfiring.  a super clogged up air filter can also stop it from starting/running.  Although, disconnecting the air boot will solve that for testing purposes. 

Edited by socalxr

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I’m not dismissing the carb, I just haven’t got to that point. When troubleshooting a bike it’s common to remove the carb and drop bit of gas direct in cylinder to see if it fires. If it doesn’t fire with the carb removed it definitely won’t fire with it on. So the carb is not an issue in my situation.

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I’m not dismissing the carb, I just haven’t got to that point. When troubleshooting a bike it’s common to remove the carb and drop bit of gas direct in cylinder to see if it fires. If it doesn’t fire with the carb removed it definitely won’t fire with it on. So the carb is not an issue in my situation.

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Does anyone have a link to 1985 XR100r service manual. I’m normally able to locate just about any manual for any bike online. Even found the manual for later cdi versions of the xr but not the points version. It’s strange considering they must have built thousands of these bikes.

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Try push starting .. get moving pritty quickly and dump the clutch in 2nd gear !! You should get a pop out of it ,, 

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2 hours ago, Steve Wagner said:

Does anyone have a link to 1985 XR100r service manual. I’m normally able to locate just about any manual for any bike online. Even found the manual for later cdi versions of the xr but not the points version. It’s strange considering they must have built thousands of these bikes.

i couldnt find one - only 98-03

might have to bite the bullet and buy one

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Did a simple leak down test today- no gauges just air in cylinder on listened. Heard just slight leakage at oil filler. Did another compression test and got 135psi. Started thinking about plug and cap. There was some discussion somewhere about plugs and caps, resister type or not. I believe the cap that came with bike was nonresister type but the plug is resister style.I swapped to a resister type plug cap and was able to fire the engine a number of times with carb cleaner in the intake. Yippee the first bit of progress in couple of weeks.

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I messed around with moving the point where the points open but nothing as drastic as that. I will have to give it a go. It got me thinking- this bike was a total mess when I bought it so maybe the flywheel is from another bike that had the timing mark on the case in a different spot; or something outside the box like that. Shoot me any crazy ideas like that if u got them

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Is it backfiring? Does the advancer work smoothly?  How are the springs on the advancer?  Did you pull the flywheel and check the crank for slippage?

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No backfiring. I think advance moves smoothly, nothing to compare it to. I can easily move it by hand and it springs back.

How does one check for crank slippage?

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