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Big Bore XR80 carb question


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Building a new motor for the KXR80 conversion bike. Doing a 2mm+ overbore to the engine and running a CB400F piston. This increases the cc's to 95, requiring an XR100 head gasket & carb. So I am looking at a (Jap) Mikuni 22mm. Is this the right carb to be running for this size engine?

 

We may step up to a hot rod cam in the future as well. We tried the China Keihin carb (eBay $25) and had poor luck with it.

 

Rider on this bike is 120# and still learning.

 

Jim

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Building a new motor for the KXR80 conversion bike. Doing a 2mm+ overbore to the engine and running a CB400F piston. This increases the cc's to 95, requiring an XR100 head gasket & carb. So I am looking at a (Jap) Mikuni 22mm. Is this the right carb to be running for this size engine?

 

We may step up to a hot rod cam in the future as well. We tried the China Keihin carb (eBay $25) and had poor luck with it.

 

Rider on this bike is 120# and still learning.

 

Jim

 

Jim,

 

Yes, 22mm is the stock size for an XR100R.  If you're looking OEM, try to find one that's from a '98 or older bike, as they had the adjustable a/f screw.  You also might be interested in an XR100R cam as they had a touch more lift/duration than the XR80R - just be sure to use the XR80R sprocket on the 100R cam.

Edited by theraymondguy
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  • 4 months later...

OK, I'm officially stumped. Have the motor built since February, butcan't get it to fire up. Have fresh gas, fresh plug, verified the timing 3x, set the point gap to .016, Verified TDC on the piston vs. the "T" mark on the flywheel. Cam is set with the lobes in the down position (8 & 4 o'clock lobe positions)

 

Carb  is a 2003 XR100 unit.

Plug is NGK CH7HSA

 

Have tried starting fluid as well, nothing.

 

Best I have gotten is a pop out the carb or pipe.

 

Jim

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OK, I'm officially stumped. Have the motor built since February, butcan't get it to fire up. Have fresh gas, fresh plug, verified the timing 3x, set the point gap to .016, Verified TDC on the piston vs. the "T" mark on the flywheel. Cam is set with the lobes in the down position (8 & 4 o'clock lobe positions)

 

Carb  is a 2003 XR100 unit.

Plug is NGK CH7HSA

 

Have tried starting fluid as well, nothing.

 

Best I have gotten is a pop out the carb or pipe.

 

Jim

 

Take apart the carb and clean all the jets especially the pilot jet. Double check your timing.

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Take apart the carb and clean all the jets especially the pilot jet. Double check your timing.

Have cleaned & soaked the carb 2x now. Motor has been out on the bench 3x to verify mechanical timing/cam & piston position. Last night I pulled the flywheel & reset the timing to .016.

 

Last time on the carb I sprayed carb cleaner through the carb jets & all passages. Compression is over 100lbs.

 

My only change on the bottom end is the flywheel. Correct p/n for the engine. Makes tons of spark.

 

I resealed the intake @ the head & carb to eliminate a vacuum leak. I also tried starting fluid to eliminate the carb as an issue. Still nothing. Tried to bump start on the hill, no-go there as well.

Edited by GSjimmy
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Have cleaned & soaked the carb 2x now. Motor has been out on the bench 3x to verify mechanical timing/cam & piston position. Last night I pulled the flywheel & reset the timing to .016.

 

Last time on the carb I sprayed carb cleaner through the carb jets & all passages. Compression is over 100lbs.

 

My only change on the bottom end is the flywheel. Correct p/n for the engine. Makes tons of spark.

 

I resealed the intake @ the head & carb to eliminate a vacuum leak. I also tried starting fluid to eliminate the carb as an issue. Still nothing. Tried to bump start on the hill, no-go there as well.

 

Hum double check the pilot jet many times soaking them doesn't work. You need to carefully clean them with something going through or replace with a new one. It's usually gas or timing.

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  • 1 year later...

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