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Help me jet this 39mm FCR-MX variant (photos) please?

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This isn't the TT FCR-MX, its the so called 'Ebay FCR' from the Polaris outlaw some suppose. I know others have used them with good results, though I'm trying to confirm the correct jettings.

Carb can also be seen here



Bike and conditions are

Sea level, 3x3 mod

opened up exhaust.

94mm JE piston with +4mm stroker crank from Hotrods.

I've had the carb on my bike, it runs well initially (requires choke to start) but then surges and jerks like its lean after warming up. I was running it without the pilot air jet at this point. I think either I need the PAJ in there or I had an air leak. Could the TPS do anything funny to the timing on the bike?

The intake bell is show in the last photo only. The carb to cylinder head is connected with silcone fiber reinforced tube and held with tube clamps.

I've taken it off and added in the PAJ (70) since the intial trial, I'm putting the carb back on soon.

EMN main needle, 3rd clip

160 main jet

45 Pilot jet

70 pilot air jet (last photo)

200 main air jet (last photo)

Mixture screw two turns out to start with.

Not too sure what the story with the coast enricher (2nd pic?). I haven't touched it.






Bell fitted


Edited by Svery
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I tried it without the pilot air jet, configuration otherwise the same and while it started fine and ran well initially, after a few minutes it started surging badly to the point of not being able to ride it. Not to clear on what the story was, I thought perhaps an air leak, and then I thought perhaps too much air through the pilot air jet orifice...

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You referenced my post from last year. My carb was new. Not sure on yours. But used can be a headache. Yours has a different type of adapter, so I am not sure if yours is the exact carb, but should be close enough. I'm not a carb expert by any means, but my "guess" is yours may need even more fuel, being stroked and open exhaust. Maybe higher on the clip or a 165 main may help. From what I've read, TPS is about useless, most people don't use them and the TT version does not even have one. Feel free to disconnect it and test. I would go to clip 4 or 5 just to see if it helps. Wont cost you anything but time. I left my CE alone. No reason to mess with.


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The carb pictured has an integrated coast enrichener (CE). What that means is it has 2 pilot air jets, one controlled by the CE. In normal operation both air jets are open. The CE closes 1 on high vac coast. When you change the air jet in the air intake you only change 1 of the jets.

So you need to determine if the CE is working. And you need to use the air jets sizes that were stock for that carb. From there I would only change pilot fuel jets to jet the low speed operation.

The DRZ OEM FCR had add-on CE that was easy to remove. The integrated CE can be deactivated but not so easily and there is no reason to do so.

Aftermarket FCR MX kits don't have any CE so the pilot air jetting is different.

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Ok, so got the carb on. Seems to be running well in general, though still needs a little tweaking ...

Snapping the throttle open once the bike is warm results in a slight bog - how does one change the accelerator pump (ap) squirt/duration?

Needs choke to start -good

Little to no popping on decel - but the integrated coast enrichner is stopping the popping so I'm not concerned.

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Yeah, I would prefer to wire it, but then I hear people saying you need the flex in the o-ring and the wire might snap something. To be honest I'm not sure what might break. Wire seems like a good fix to me.

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