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Discussion Starter #1
2005 TRX450R, I’m at 5,600’ with a 170 main and a 48 pilot, air screw 2 1/2 out.

Has to be full throttle to start and it will not run unless full throttle and/or high rpm. Dies if not full and backfires if not high rpm. It will even shoot a flame or two out of the exhaust sometimes!

I’ve cleaned the carb multiple times and still no luck.
 

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The 450R carb does not have an air screw, but a fuel screw - the farther it is turned out the more fuel it gets at idle.

First question - why a 170 main jet? Initial setting of the fuel screw is 1 3/4 from lightly seated - ride for 10 to 15 minutes and set the screw for the smoothest idle - std pilot jet is a #48 - is the jet new or has it been drilled out.

Second question - since you worked on the card, are you certain the enrichener valve (by starter) is closed?

It's pretty obvious the carburation is over rich at low speed - that is controlled by the pilot jet, fuel screw and float level. It can also be affected by valve lift, cam timing and spark timing. Spark timing is set by the flywheel so it's unlikely the spark timing is too far advanced, so figure out why it's so rich at low speed.

Try the carb off a good running engine and see how your engine runs with it - process of elimination.
 

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Discussion Starter #3
So the screw on the bottom of the carb is a fuel screw?

This is my brothers bike he bought for $900 and he rode it all this season but he kept complaining about it backfiring when on going down hill in a lower gear and losing power up top. I thought this was a lean condition and went from the 165 to 170. Put it all back together and it worked perfect for about 5 minutes and then had major issues.

Never touched the pilot and i'm not sure if it was drilled out.

Is the enrichener valve the same as the choke?

I pulled the plug out and it was pretty black but still sparking, i'm going to get a new one and see what happens.
 

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Yes - the enrichener functions like a choke but in reverse - instead of 'choking' off air to enrich the mixture, it adds fuel to the air through a separate jet and air passage which bypasses the throttle butterfly or slide - thus the term by-starter

Backfiring on deceleration is a lean condition with the throttle closed (pilot circuit) - may indicate the air cut valve is not functioning or is indicative of an exhaust modification - if the exhaust has been changed to a low restriction system or if the OEM muffler is modified or rusted out, it will induce backfire - does not hurt anything, but is annoying. Install an exhaust which provides the right amount of back pressure and the backfire goes away.

You might try a 50 pilot jet and reset the fuel screw for the best idle after 15 minutes of riding (engine has to be hot when setting the fuel screw)

The carb also has a hot start circuit - if the hot start circuit is open, it will be hard to start and will not idle either.
 
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