Vintage Dirt Bike Q & A

My '82 CR250 runs ragged for about the first 1/3 throttle, and great the rest of the way. Assuming it was rich, I've gone all the way to a 42 pilot, the needle all the way down, and the air screw 2 turns out. No help. Next I bought a used carb. No change. Next new reeds. No change. And it's consistent, warm or cold. I'm using the plug the manual calls for.

I'd appreciate any help. Someone suggested that it might be electrical, but without any details.

Thanks,
Bob

By "ragged", do you mean smoking/four-stroking (rich) or surging/bogging (lean)?

The range you're having trouble with is dominated by the throttle slide cutaway and the needle's straight diameter/needle jet I.D., although the pilot jet/air screw will have a big effect at idle and just above. If the slide/needle/needle jet circuit is way out of the normal range due to wear or modification, the other carb adjustments won't have much effect on that problem.

As your stock pilot jet is apparently a #72, I would think a #42 would be super mega lean. Does your air screw have any effect over 1 turn open?

If it were my bike, I would:

1) Inspect both carbs' needles for wear, writing down any numbers you find
2) Inspect both needle jets for wear (this isn't easy, but sometimes they get an oval shape)
3) Check the throttle valves for number (stock should be a 3.5, stamped on the bottom)

You could try combining your best needle jet with your best needle (be very careful removing the needle jet; it might be a good idea to just leave it in and use that entire carb) and see if that helps. Alternatively, you could try to find a new needle jet (16164-KA5-004), needle (16012-KA4-004), or 4.0 throttle valve (16111-KA4-823) on eBay.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Perform a leak down test

Thanks, guys. Looks like it's back to work.

I'm glad you don't think it's electrical - those parts are expensive to experiment with.

Bob

What am I looking for in the leakdown test? I assume that if the ignition side is leaking, I'll get a lean, running-on. (I'm not.) If it's the other side, I'd assume I'd be getting oil in the cylinder. (I don't think I am.)

What else am I looking for?

Thanks,
Bob

Actually in a leak down test, you are looking for air leaks which can wreck havoc
with a two sroke engine. Seal up the engine by blocking off the exhaust port,
install the tester in the intake and pump it up to 6 psi and time how long it
holds pressure. Any loss in less than two minutes is not good. If it holds all
pressure longer than two minutes than I would look elsewhere for the problem.

If you do have an air leak, the next step is to determine where. Use your ears,
soapy water etc. to pinpoint the leak. it could be anything from a crank seal
to a crack in the intake boot.

dogger

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1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

Check your fuel line as well. If there is air being sucked in to your carb along with the gas, it can cause irratic behavior.

de carbon the exhaust they realy build up with shit

Theres a thread on here somewhere about how to do a leakdown test and make the intake cover out of PVC.....something a 2 stroke guy should not be without.

Search Leakdown test

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Garry Mainer
#33m (retired)
Honda Cr250r 1980
Honda Cr250r 1980
Yamaha TY250C 1976
Arctic Cat 650 H1 2007

Float level? Check it - sounds like it could be too high. I'd also consider chipped reeds (not broken just chipped).

I know this is an older thread, just curious if you ever found the problem though?

Peace...
Mikeee P

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The Race Isn't Over Until YOU Cross The Finish Line!

Thanks, Mikee.

An old thread, but sadly still current.

I have looked at the float level, but not really set it carefully. I did try a different carb, without any change in the symptoms. I'll try to reset the float.

I put in new reeds, as the old ones seemed worn.

I just got it back from having a pro set the timing by the book - timing light at 6000 rpm. It was right as it was.

Next step - take out the spacer between the carb and reed block, that, I think, was supposed to improve low end.

Again, thanks,
Bob

Hi Bob... yeah I saw it was back in... May maybe that you asked the question? I just never saw a "Hey got it figured out!" so I thought I'd throw that out there just in case you were still looking for a cure.

Hope you find the problem.

I didn't realize how long I've been looking for a left hand side case for my 78 YZ until I saw that I posted it on here the day after Christmas in 2006. There were 500 looks and no responses... figured I'd become the site loud mouth and help where I can hahaha.

Mike

_________________
The Race Isn't Over Until YOU Cross The Finish Line!