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1986 CR250 Jetting success!

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So originally this bike is meant to run 20:1 mix. So says the manual. Stock carb comes with a 175 main jet, and a 60 slow/pilot. Needle on the 3rd grove (middle of the 5).

 

Bike was junk when i got her so i rebuilt everything and went for 32:1 from the get go. Needless to say that 32:1 made the bike run absurdly rich (more fuel to oil that goes past jet). I was running with the air screw out till it almost fell out and still it was bogging under a good bit f throttle. So finally I bought 170 and 165 main jets (for safe measure i thought) and a 58 pilot.

 

Switched in the 58 pilot and the 170main. Turned the air screw to 1.5turns out from closed (cw = richer, ccw = leaner). Tuned a bit, still had to turn the air screw out some and that ran way better but still bogged and smokked (signs of running rich) a bit near/at WOT. Response was greatly improved.

 

Kept the 58 (idled well still and good response in the 0-30ish % throttle). Switched down to a 165. Reset air screw to 1.5 turns out from closed and Bazzzing! I'll ride it this weekend and tune it a bit. I have a butt feeling i'll drop the needle one grove (2nd from the top, leans out top end) and its all air screw from that point on.

 

Anyways, if you have this old bike i hope this helps... Even as a really good starting point: Bike has probably 6hours on a fresh engine replated and all.

 

Atmospheric conditions here are 300ft and 72deg, low humidity but not dry cause i'm close to the ocean.

165 Main, 58 Pilot, 1.5turns air screw, 2nd grove from top on needle.

I'm sure this jet sizes will fit a wide range of height and temperature.. If anything you'll need to go leaner for altitude and warmer weather. With winter coming soon i think the 165 would still do a good job, maybe need the air screw turned in a bit and the needle raised. Anyways the stock carb settings just don't play with 32:1, much less 40:1 if you do plan to run that. Sharing some stoke good night!

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ive got an 87 does it use the PJ 26 A?  mines been a bit wonky lately ive got a 175 main 58 pilot and stock needle in 3rd (4th is stock) position....4 th clip is too rich 3rd is ok but still a tad rich but did a few plug chops the other day and the 175 was pretty lean...im mixing 40-1 0-400 ft above SL v force reeds and PC pipe ....i just threw a 178 main in today and dropped it to the 2nd clip position and see what happens lol

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yes, you're right PJ 26 A. So we have the same bike/engine, ~ same altitude temp conditions hmmm.. what is wonky? I'm all stock. I wonder if your Reeds and PC Pipe make you move that much more air than me because not only are you running more fuel on your mix (richer), but you also put a bigger jet (richer) and are raising the needle (richer even).

 

Let me know how that turns out. My plug chop test was on the money, a bit damp even.. last i checked compression i was 192psi so about as fresh as it can get.

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I had a 1986 CR250 and the carb was just plain worn out. I just could not get the bike jetting consistently and it leaked fuel from the float bowl. I ended up replacing the stock Keihen with a 38mm Mikuni flat slide carb and I was able to get the jetting set quite easily.

 

I ran Torco race fuel with Power Plus synthetic oil mixed at 50 to 1.

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interesting... what's involved with hooking up the throttle cable to that carb?

 

...and do you remember what jets you have for 50:1? i'm afraid to run that a oil mix that low... then again i'm new to the dirt bike world. (but not engines).

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yes, you're right PJ 26 A. So we have the same bike/engine, ~ same altitude temp conditions hmmm.. what is wonky? I'm all stock. I wonder if your Reeds and PC Pipe make you move that much more air than me because not only are you running more fuel on your mix (richer), but you also put a bigger jet (richer) and are raising the needle (richer even).

 

Let me know how that turns out. My plug chop test was on the money, a bit damp even.. last i checked compression i was 192psi so about as fresh as it can get.

wonky like its always in between on jetting.... alot better yesterday though with the 178 and needle on second clip was a lil bit humid but gettin close.... PO put in a PR1369 needle in gonna order the stock needle and see what happens probably where the issues are comin from lol did a compression check two weeks ago still have 190 psi ( did the top end last winter) do u know how to read the keihen PJ needles? the stock for the 87 was 1366....whats in there now is a 1369 is that one leaner or richer needle?

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My manual is 86-87. Says 1368. Middle grove for 86. 2nd from the top for 87.

 

What does your spark plug look like man? i mean you're going in the totally opposite direction as me with the jetting. I wonder what gives, or if your pipe makes this much difference. My plug is beautiful brown to ashy black (a minisule rich even) and my jets are significantly smaller.. same atmospheric conditions it seems. dunno

 

Rode last night... i've never heard this bike rev that high before hehe it was beautiful : )

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ill take a pic tomorrow and show ya gettin a nice tan brown now after the last change but with the stock settings in my manual (its a clymer) 175 main needle on 4th clip ( 5th being the very bottom) its really rich on the needle and very lean on the main... on monday had those settings in was pretty cold out and did a bunch of road runnin so had her pinned a few times in 5th and the plug looked pretty white and when i came to a stop i had a hanging idle like i was overheating  thats why i  tried the 178 yesterday ran ALOT better on WOT ...does seem weird your setting are WAY leaner on the main than mine i wouldnt go lower than a 172 the way my carb is workin lol i mostly do trail riding with some short wide open stuff in between but ive never had any good luck running a BR9 which is stock so ive been using a BR8 .... the 9s would always foul after an hour or so 

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interesting... what's involved with hooking up the throttle cable to that carb?

...and do you remember what jets you have for 50:1? i'm afraid to run that a oil mix that low... then again i'm new to the dirt bike world. (but not engines).

Throttle cable just hooked right up.

Main 320

Pilot 40

Jet needle 6fm46 middle position

Needle jet Q8 series 389

38mm Mikuni flatslide

torco race fuel

Power plus synthetic oil 50 to 1

Hope that helps

Edited by bigpaul

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