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Mikuni TM40 Pumper Carburetor

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I bought a Mikuni TM40 flatslide pumper carburetor kit from Pro Cycles for my 2014 XR650L which I upgraded from a Dave's Carb Mod. I liked it, it had a lot of gain but it also came with a backfire issue I did not like. It sounded like there was a leak from my header exhaust, it was not nice having this on my daily commute. Every time I wouild shift through my gears it would pop (backfire). I called Pro Cycles and addressed this issue with them and was told this is how this carburetor works. I was also told how to roll-off the throttle when shifting to avoid this backfiring from happening. Why would I have to ride a certain way to rid the backfire issue, I don't get. "Please Help? With my frustration building, I finally called Pro Cycles back and asked for a refund which I got credit for a Keihin FCR Pumper Carburetor Kit. Iv'e been running this Keihin for about six months now but I would really like to go back to the Mikuni TM40. My bike is de-snorkeled, de-smogged, Big Gun Evo-R Exhaust with a Uni filter. Any advise on how to get rid of the dreaded backfire issue while using the Mikuni TM40 would be greatly appreciated.  

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Did you try any adjustments on the fuel screw? 

Edited by ossagp
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You don't. The FCR has an anti backfire valve that momentarily richens the mixture when you let off the throttle. The TM40 does not have this, it is much simpler. The best thing to do is just set your idle mixture correctly and roll off the throttle smoothly rather than chop it. If you try to make the idle fat enough to stop the popping it will be terribly rich and get lousy gas mileage. If you are running an aftermarket muffler you will of course hear it much louder than if you have a stock one.

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What are your jets? I have the same carb but the jets aren't correct. Also if you know your elevation. Thanks!

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Sorry I wish I could help but all my work gets done by mechanic, that of which I don't know because I just ride and if something goes bad or it doesn't feel right I take it right back and he does his magic until I'm satisfied. My Keihin FCR came as is from Pro Cycles and without playing around with the jets, I was told, it works amazing. No backfire issues, fires up with ease, and pulls my tall gears with ease. My bike has been upgraded so my mechanic has again changed my carburetor to run with an XR600R carburetor. May I mention I live in Hawaii on the Island of Maui so I'm between sea level and 300 feet. 

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11 minutes ago, MauiWowee said:

Sorry I wish I could help but all my work gets done by mechanic, that of which I don't know because I just ride and if something goes bad or it doesn't feel right I take it right back and he does his magic until I'm satisfied. My Keihin FCR came as is from Pro Cycles and without playing around with the jets, I was told, it works amazing. No backfire issues, fires up with ease, and pulls my tall gears with ease. My bike has been upgraded so my mechanic has again changed my carburetor to run with an XR600R carburetor. May I mention I live in Hawaii on the Island of Maui so I'm between sea level and 300 feet. 

It's fine I'll figure it out I'm just tired of looking everyday. Post some cool pictures of your bike!

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