Recently bought an 85 FXR, changed the exhaust from drag pipes to thunder header. Runs good but craps out in the higher rpm range. Put a CV performance needle in and bigger main jet. went down on the slow jet cuz thunder header has back pressure unlike the drag pipes. any ideas??? I was gonna check timing but didn't know if that was a likely suspect???

 

You did all the carb changes at once?
the new needle plus the bigger main is probably to rich. Change one at a time.

 

Do a plug chop and see what colour its running.

 

Yep you might be too rich on the top end and the engine is "laying" down.
John

 

Checked the color on plugs. The electrode has a bit of white on it, but there is black on the base of the plug where the threads start????? any ideas??

 

Did you check it after an idle or after a wot and shut off with new plugs.

 

The black is why it is lying down WOT w(Wide Open Throttle), the white is where it is cleaning itself, just too rich WOT. The timing has not been moved so it does not need changed. Agree with Harleycruiser, put the original main jet back in and try with just the needle change which should provide a richer across the board AFR. If she seems to run out of gas at WOT the main is too small to feed the needle change and you will need to bump up the main jet 1 size at a time. What was the original main jet size and what did you put in?

 

So when it "craps out" are you getting misfiring? A sudden stall? Sputtering?

You don't say where it craps out rpm wise. The stock Ignition Module has a limiter, you may notice it crap out when it hits the limit. Any chance you have a tach? If so where does it start to crap out. Stock limiter is about 4,700 rpms.

You didn't mention your jet sizes, which is important to know, to determine if you have a usable combination.

Also, with all the carb work be absolutely positive your float adjustment is correct. If it's off the float bowl can dry out at faster speeds. Back off the gas and it allows the bowl to fill, and off you go again. This would be a starvation stall when accelerating hard.

It may sound simple but did you check plug gap? Do you have the right plugs?

You may even have a timing issue. Often you can have a good idle but when running fast the slightest amount off can hold back the motor from performing correctly.

 

You also have to remember that drag pipes work best at hight revs, and will most likely out do any other pipe configuration. I have drag pipes on my 99 EVO, and they really come on at about 75 on up. At 90 it is really starting to run with lots of throttle left. I am running the stock needle with a 45 pilot jet, and a 180 main.

 

A competent dyno tune will sort it out in very short time. If you are not sure what to do find an expert.