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Discussion Starter #1
Just took the jets off of my 88 trx250r, she was bogging in 1st and 2nd, then every other gear pulled flawlessly, recently bored the jug and went from a 66.5mm piston to a 68mm piston, the clip is on the 2nd position (for my elevation) I have a 152 main and a 42 pilot, I believe it's my pilot jet, my mind tells me go up a size, because it's a bigger motor, am I right? My elevation is 100ft from sea level and I run 40:1 castor 927 with non ethanol gas, the carb is a 36mm kiehin
 

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If everything remained equal except the over bore I suspect the over bore would not be enough of a change to go from running great to running so lean it “boggs” now. (That’s not to say you don’t need a bigger pilot but I don’t think an over bore all by itself would have that significant of an effect on pilot size) I suspect your pilot jet is plugged. The simplest way to eliminate things As problems is to start with a brand new pilot, same size as current set up, and see what happens. I have had countless pilot jets that look good but are plugged in some fashion.

This will give you a baseline to start adjusting things. While you are at the store buy the next 5 jets in size.

If the new 42 does the exact same thing as the old, jump a size or two and reevaluate did the bigger pilot make it better?

Also, make sure your airscrew starts at a base line position usually 1.5-2 turns out. Don’t mess with the air screw until you have it “close” with the correct pilot.

For me the most important thing is start from a known position and only change 1 variable at a time. It’s the only way I can deduce what helped and what didn’t. If moving up in pilot does not change the problem, then you Likely have a different problem.
 

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Discussion Starter #3
If everything remained equal except the over bore I suspect the over bore would not be enough of a change to go from running great to running so lean it “boggs” now. (That’s not to say you don’t need a bigger pilot but I don’t think an over bore all by itself would have that significant of an effect on pilot size) I suspect your pilot jet is plugged. The simplest way to eliminate things As problems is to start with a brand new pilot, same size as current set up, and see what happens. I have had countless pilot jets that look good but are plugged in some fashion.

This will give you a baseline to start adjusting things. While you are at the store buy the next 5 jets in size.

If the new 42 does the exact same thing as the old, jump a size or two and reevaluate did the bigger pilot make it better?

Also, make sure your airscrew starts at a base line position usually 1.5-2 turns out. Don’t mess with the air screw until you have it “close” with the correct pilot.

For me the most important thing is start from a known position and only change 1 variable at a time. It’s the only way I can deduce what helped and what didn’t. If moving up in pilot does not change the problem, then you Likely have a different problem.
Okay, I'll try a new pilot jet, I just got my jets in today, and I have 10 pilots and 10 mains just in case, there all genuine kiehin jets so fingers crossed it's just a plugged jet, the air screw is 1.5 turns out and I believe the idle is 2 1/4
 

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Just took the jets off of my 88 trx250r, she was bogging in 1st and 2nd, then every other gear pulled flawlessly, recently bored the jug and went from a 66.5mm piston to a 68mm piston, the clip is on the 2nd position (for my elevation) I have a 152 main and a 42 pilot, I believe it's my pilot jet, my mind tells me go up a size, because it's a bigger motor, am I right? My elevation is 100ft from sea level and I run 40:1 castor 927 with non ethanol gas, the carb is a 36mm kiehin
IMO; way to lean on idle to 1/2 throttle...will be most noticeable in the circuits you detailed. Below is closer jetting per your description at basicly Sea Level, Try:

158-166 Main (I would start at 160)
46-50 Pilot (I would start at 48)
3rd Clip on Needle (DGH)
#6 Slide (Important to be a #5 or #6 for a 250R)
1 1/2 at AS
2 1/4 at BPS

Carlos
 

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Discussion Starter #5
IMO; way to lean on idle to 1/2 throttle...will be most noticeable in the circuits you detailed. Below is closer jetting per your description at basicly Sea Level, Try:

158-166 Main (I would start at 160)
46-50 Pilot (I would start at 48)
3rd Clip on Needle (DGH)
#6 Slide (Important to be a #5 or #6 for a 250R)
1 1/2 at AS
2 1/4 at BPS

Carlos
Okay, thanks carlos, I'm going to try putting in a new 42 and 152 jet in, and then if that doesnt fix it, im going to try a 160 and a 48
 

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Discussion Starter #12
Could pressure test coolant system. My oil used to turn grey, it was from my wore out clutch and basket. After a full bdt rebuild it’s clean and get serviced every couple rides. I’m using maxima mtl. 🍻
How would I do that? I have a feeling it's the ceramic seal, so just to be on the safe side In going to buy a water pump rebuild kit, and I'm going to buy the BDT collar set for the kickstart spindle so I can use my chrome clutch cover that doesnt have JB weld covering a hole
 

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How would I do that? I have a feeling it's the ceramic seal, so just to be on the safe side In going to buy a water pump rebuild kit, and I'm going to buy the BDT collar set for the kickstart spindle so I can use my chrome clutch cover that doesnt have JB weld covering a hole
Posted this bellow on another thread:

"Perform a full leak down of all three, this is how we do it at BDT; it insures no leaks.

1). Cylinder Water Jackets, WP Assy, Engine Hose; Pressure test from Inlet at WP/Clutch Cover through Outlet at top of head.

2). Cylinder; Exhaust Manifold, Head, o-rings, Base gasket(s), Intake reeds, Boot, Crankshaft Pocket & Main Seals; Plug up spark plug hole, plug up exhaust flange, pressure test with piston at BDC through Intake boot.

3). Transmission Pocket area, center case gasket, seals; plug up breather tube, pressure test through Oil Fill on Clutch cover

All three is the 'Best' practice, for all 3 use 4-6 PSI for 6-8 minutes with an acceptable loss of <1 psi. You do all three and pass, and you will have a 'sweet' running & jetting engine.

Carlos"


Carlos
 
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