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Tuning a Thunderheader??

3.6K views 8 replies 7 participants last post by  compressor#v  
#1 ·
Last year, I spent quite a bit of time, attempting to get the carb tuned correctly. In the lower RPMs, I never got it right. Still a bit, stuttery? and some mild backfiring on low rpm deceleration.

Am thinking now, about two things I've heard of that may or may not help, and I want to check in with the experts.

I read a long time ago, in an unremembered place, that old timers would use a hammer and screw driver to fold over (up to, if I remember correctly) 3 sides of the square baffle, to increase back pressure, and if I understand correctly, help with low end torque.

Someone else mentioned some 'cones' that could be installed inside the headers, to help create some more backpressure.

What say the experts? Would either of those options be my next logical step in getting her tuned up right?
 
#2 ·
None of these are good solutions and your tuning solution is not specific to the FXR platform. A better title could have included the type and size of the engine, maybe the cam, etc. FXR tells us nothing.

Back your tuning issue. There are gazillions of bikes running around with that pipe that do not stutter. At all. At any RPM. Your engine has other issues that need to be dialed in. I would look at the accelerator pump.
 
#5 ·
Folding the baffle is to help with the off idle dip or flat spot those pipes have. I have no idea what carb you are using, so I can't say anything as far as the specific tune. But I can say that 2-1's on an Evo are a whole nother animal when it comes to jetting. You have to get the transition above the flat spot in the pipe. They are probably why S&S put a jet in the air bleed on the Super carbs.
 
#6 ·
Thx, all.

CV carb for now...

EV 3 cam

Tried many jet combinations last year, while learning to tune. Runs great in mid & upper rpms.

Pipes are good.

I know it's not the accelerator pump.

I checked for an intake leak last year. All seems fine there.

How could an exhaust leak really affect such a free-flowing pipe as a TH?
 
#7 ·
I have an EVO with a EV3 cam, ThunderHeader and CV40 carb with a 185 main and 45 slow jet, lightened slide spring and it runs perfect. Your stumbling could be the slide. Has it been "worked"? Drilled, or the spring changed? Maybe the rubber diaphragm has a pinhole.
 
#8 ·
No. I made sure to put a stock
spring in the CV when I rebuilt it. No hole in the slide. No hole in the diaphragm.

Just milking it thru this season. Planning a drive train upgrade over the coming winter. Gonna ride this year though.

An working in the Duluth, MN / Superior, WI area. Recommendations on the best place to have it tuned?? Working six days a week, I wanna ride the seventh,... Not play with the carb.