(R question) Problem Solved..I think!

[DixiePlowboy]

I have found that someone has apparently put a MIKUNI #140 main jet in my KEHIN carb! The Mikuni jet is both shorter(not reaching as far into the bowl), and the inside diameter noticeably smaller than the #140 Kehin jet. No wonder the power died on top! It was starving itself to death. :evil:

The Kehin and Mikuni jets appear to be just slightly different in the threaded section, but the parts man at the local Honda dealer says that they could share the same hole with minimum effort. Anyone have experience with crossed up brand jets that could point out potential problems created by having the wrong jet screwed into the hole?









After cleaning the air filter on my '85 R and leaving off the airbox lid, I made a high speed run. At the top of 5th and after 3/4 throttle in 6th gear, the machine bogged down as though someone had shut off the fuel flow or the switch. Let off the throttle a little and it would scream, but from 3/4 to full throttle wasn't happening.
I put the airbox lid back on and repeated the run. Now, I still have power after 3/4 throttle, but it's kinda flat. It's not bogging down like it was with the lid off though.
My guess is the main jet's too small and with the lid off it's way too lean. I'm running the stock carb(stock exhaust), but I don't know what size main is in it. I do know that the needle clip is on the 3rd groove from the top.

Am I headed in the right direction, or is there something else more likely to be the cause of the symptoms I described above?[/b]

[85200xer]

maby you sucked dirt in and it ate your rings? I had a yz 80 that after cleaning the carb I forgot to put the airfilter box tube to the carb back on and rode it up my dusty driveway and it developed a flatspot in the topend.
hope this isn't your problem though.

[Jeroboam7]

Your probrably lucky you didn't burn it up, with out the air box lid you leaned the fuel air mixture by dumping more air and less fuel per volume into the cylinder. Put a larger main jet in if your going to run without the lid.

[DixiePlowboy]

I pulled the airbox lid because it seemed to be rich on bottom end and part throttle. It did improve part throttle operation, but the flat topend became worse, shedding light on the problem that hadn't been that noticeable.
I have found that the carb has a #140 main jet, which I believe to be too small with the lid on, amplifying my problem with the lid off.

When you buy someone else's toy, you don't know what you're getting until you get it home and open it up.

[Jeroboam7]

Ya a #140 seems pretty small, I don't know what stock was for the 250R but It would be my guess that you are about 4 sizes to small!
I think I would try maybe a 155 or 160 and then try to work out the rest of the carb. Get the main jet set first and then you can get the rest of it working.

[waterfowler]

I just checked the book and it says the following:
If you have a PE37A carb (stock) you should have a 142. Although most of the guys I've talked to recommend a 145. The books says the limit is a 148 for an '85.

The '86 uses a PJ03A carb and a stock jet of 145 going upto 150.
Hope this helps,

[DixiePlowboy]

Thanks.
That confirms what I thought. The main's too small. I figured that with the problem worsening with the lid off that it had something to do with the mixture being lean to start with.
Funny, it's only noticeable in 5th and 6th gear. Reckon why? Could be that it's more noticeable there because that's when the motor is under the greatest load.?.?
I thought at first that I had an ignition problem or a grounded out wire the way it bogged down like it was dead almost. Let off the gas a little, and it woke up and started screaming.
Put the lid back on, duplicated my previous run. This time the power went flat at full throttle, but not as bad(bogging) as before. Eased off the throttle a little, and it sunk my eyeballs back. :twisted:
To me, it's got to be the main.

[djcheez]

did you happen to check your plug after that run? that will tell you very easily whether or not you are lean or rich. i really cant see it bogging from being too lean. it doesnt sound like it would be that lean anyhow after removing the airbox. do a run again and just before it starts to bog, pull the clutch in and kill the motor. coast to a stop and then check the plug

[DixiePlowboy]

I did a couple of plug checks tonight. I cleaned the plug with a wire brush first. Due to the fact that my headlight bulb is shot, I didn't run it wide open in 5th and 6th gear, but I did run in 4th gear full throttle/under load/out of the power band, and kill it out with the throttle open.
The first run the plug was actually wet around the base of the threaded part. The second run, the insulator was still brown, but the ground electrode was light on the end. It almost appeared to have no color, as though it were still showing the metal through.

The reason that I deemed this to be a jetting issue(other than now knowing that my R has a #140 main jet when #142 was stock) is that the problem was worse with the airbox lid removed and with a clean air filter, thus allowing more air into the engine and leaning out the mixture.
When the airbox lid was reinstalled, I still had flat high RPM power, but not the "flip-off-the-switch" kind of power interruption that it had with the lid off. If I backed off the throttle a little, the power got stronger.
What else could it be?

[4cylinders]

hey, just a thought, if you still have the stock exhaust, could the spark arrester be plugged? 2 cycles can be really weird sometimes. take the insert out and try it.

[DixiePlowboy]

The baffle in the stock silencer has already been removed.

[DixiePlowboy]

To the top due to new info!!