Need Jetting help on a new build

[Bharris]

Hi guys!

I am not very good with carburetors and I need some advise on which jets that I should start with for my particular situation.

The is a rebuilt '88, BDT 261 Stroker with V3 Reeds and an FMF Fatty Pipe which is set up to run on race fuel - all parts are new, even the carburetor. The carb. is a 38mm AirStriker PWK. I am in central Georgia, where it's very hot and humid. The temperatures will start to drop soon as we go into winter however - so it will go from burning hot, to 50 degree highs. I would guess I'm about 700-900ft above sea level. At the moment the carb. has a 165 main, 52 pilot and DGH @ 3rd clip.

Thank you!

[2ndmoto]

I am assuming you are running with an open airbox. Main seems a bit low. I'd start at a 178, which should be rich. Check the pilot at that point, most bikes are some where around 50-55 pilot. Once the pilot is worked in, move to the needle, most run in the 3rd to 4th from top clip position. Lastly finish off with the main.

261 stroker? Has this been debored and stroked?

Best regards
-Jason

[broken1]

261 stroker? Has this been debored and stroked?

Best regards
-Jason

I doubt it's undersized. Probably a 66mm bore cylinder with a +4 crank which is 260cc.

[Bharris]

Thanks 2ndmoto!  This was a stock cylinder that BDT ported for me.  It has a +4mm crank for sure, but I can't remember the piston size - seems like it was larger than 66mm (pretty sure the previous piston as 67.5mm).  Beyond that, it has a spacer plate between the case and jug.  BDT said that it would be a 261 Stroker.  I'm actually waiting on the engine to be returned within the next 3 days - can't wait!

Please correct me if I'm wrong - if I go from 165 main (as it is now) to a 175 (or so), then I will probably be a little rich.  So as I'm leaning the main jet, how far should the fuel screw turn in before I drop to the next smaller sized jet (within 1 turn out, or can I go to within 1/2 turn out) - or, where should the fuel screw be if I'm running the correct jet (within 1-2 turns out?)?

If anyone knows of a good instructional site which could walk me through the steps of jetting this carb. then that would be awesome.  I have never messed with carbs, am going into this completely blind with hopes of not damaging this brand new engine.

Thank you.

[jwraymond6]

http://trx250r.org/threads/5263-Jetting-Guide-TRX250R-Keihin-Mikuni-Carb this might help

[broken1]

This was a stock cylinder that BDT ported for me.  It has a +4mm crank for sure, but I can't remember the piston size - seems like it was larger than 66mm (pretty sure the previous piston as 67.5mm).  Beyond that, it has a spacer plate between the case and jug.  BDT said that it would be a 261 Stroker.  I'm actually waiting on the engine to be returned within the next 3 days - can't wait!

With a previous bore of 67.5 & the +4 crank it's already at 272cc. Here's a cool displacement calculator compliments of [MENTION=87]havinnoj[/MENTION] so you can get the actual size: http://trx250r.org/TRX250REngineDisplacementCalculator.html

[2ndmoto]

Fuel air screw only adjusts the air going through the pilot jet circuit. The pilot jet circuit is on at all throttle positions, so you need to dial it in first. As mentioned start rich and work down. With it at idle, Crack it to wot. If it sputters, pilot is too rich, if it hesitates like it is going to die, it's too lean.

Best regards
-Jason

[fearlessfred]

Slow circuit only effect idle up to 1/4 throttle . The slow circuit consists of pilot and air mixture screw and when properly adjusted gives you a ratio of say 12 to 1  which doesnt really change through out the rest of the throttle openings ,so it cant have much of an effect . it can cause bog or sputter as already mentioned . the slide cut away can also effect the same throttle opening as slow circuit but most people dont mess with that . As Jason mentioned start with the slow circuit first , go to the needle and then the main jet . You can actually to the main jet out and tune the needle to were it runs good up to 3/4 throttle ,stop with the needle at the point and then install to large of main jet and start leaning it out until it runs clean all the way though the rpms

[fearlessfred]

Slow circuit only effect idle up to 1/4 throttle . The slow circuit consists of pilot and air mixture screw and when properly adjusted gives you a ratio of say 12 to 1  which doesnt really change through out the rest of the throttle openings ,so it cant have much of an effect . it can cause bog or sputter as already mentioned . the slide cut away can also effect the same throttle opening as slow circuit but most people dont mess with that . As Jason mentioned start with the slow circuit first , go to the needle and then the main jet . You can actually take the main jet out and tune the needle to were it runs good up to 3/4 throttle ,stop with the needle at the point and then install to large of main jet and start leaning it out until it runs clean all the way though the rpms