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'06 CRF250R Jetting/backfiring - mostly for reassurance...


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hpc3



Joined: 07 Feb 2006
Posts: 44
Location: Rock Hill SC


PostPosted: Sun Jul 01, 2007 4:45 am    Post subject: '06 CRF250R Jetting/backfiring - mostly for reassurance... Reply with quote

Background: Read the two pages and a good thread with ~ response(s). My bike is 90% MX and 10% Hare/Trail ridden, all stock jetting, at 700ft ASL with 80deg F temps currently, I run 100oct unleaded pump gas in it 99% of the time. Recently went to a TwinAir filter(left the screen/frame in) which I could tell immediately that the bike is getting more air, leaned it out somewhat. Had to adjust the Pilot circuit out to 2 turns to get it to idle again. Noticed on trail/field riding that the bike has decel backfiring in 4th and 5th gears after WOT on long straits, seems pretty crisp everywhere else(small bog still there but I haven't played with the accel pump/circuit as its not critical for me).

Question: At what point do I need to change to a different pilot jet? Going to try another 1/2 turn next ride and see what happens. The backfiring is not noticible on a track, only under high-speed runs/decel(I need to remove a tooth or two on the rear sprocket too... bike top end is a little short). Also are there any other changes to the main jet, needle, that would be recommended from the stock settings? Seems the pilot circuit is likely the one to focus on for this but I want to make sure I'm not missing something. Familiar with auto carbs and tweaks, this one is not too different. Bike starts on 1st or second kick when cold, 1st or 2nd with hot-start/lean-air lever when hot...

Thanks,
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AgentSmith



Joined: 10 Apr 2006
Posts: 14860
Location: In the Matrix...Waiting for an Exit.


PostPosted: Sun Jul 01, 2007 5:53 am    Post subject: Reply with quote

That's funny, I've got a computer named after you...

The 2006 250R was the first year of going to the 40mm carb from the 37mm of previous years.

The stock settings:

NHFS #3, 172 mj, 40 pj, 55 leak, 1 3/4 fs - '06

In 2007 they made some small tweaks:

NKBT #3, 175 mj, 42 pj, 70 leak, 2 1/4 fs - '07

They went up one size in the pilot to a 42 and opened the fuel screw to 2 1/4 turns. They also went up on the main one size. The leak jet was also changed to a smaller gas shot ( too much gas from the AP ).

You might try emulating these changes. The carb was the biggest complaint of 2006 R riders, so much so that many went back to the 37mm carb from the 2005 with great success. Honda stuck with the 40mm and made some tweaks and apparently fixed the issues.
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hpc3



Joined: 07 Feb 2006
Posts: 44
Location: Rock Hill SC


PostPosted: Thu Jul 05, 2007 7:30 pm    Post subject: Changed jetting today, was suprisingly easy heh. Reply with quote

Mines the '06 as you know, so I'm now running all '06 settings minus the PilotJet(now a 42 like the '07 stocker) and the MainJet(now a 175 again like the '07). I had the Pilot screw at 2 turns out and left it alone for now, also left the needle setting(3) the same, thought I may rise it as well but I'll wait to see if the additional flow/gas helps first.

Didn't have my ServiceManual with me but remembered reading and seeing both of these jets in the diagrams... took off the subframe(left mufflers/side panels/airbox/rad panels all attached) and loosened the clamp on the carb side of the intake boot. I initially thought I'd remove the carb but wanted to see if I could get the bowl off w/o having to take all the throttle/vent/shock crap off first. I was able to pull the carb back ~5mm in the boot and twist it clockwise enough to get at the bowl plug(drained gas first of course). To my suprise I was able to use a 6mm socket to remove the mainjet and then a small screwdriver to get the pilotjet out. Put the 42 pilot in first(better access with the main out of the way) and then installed the 175 main and put the bowl plug back in. Took another 15min or so to get the bike back together. Got it started on 3kicks and it seems to run fine at idle, slow circuit speeds. I'll hopefully get to ride it early next week to see if the high speed decel pops are better. Hopefully will have a little more HP as well... was easier than I expected to do this.

Thx for the help.
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