Vintage Dirt Bike Q & A

Need help - when I got the bike it had been sitting for a long while. I gave the bike a thorough cleaning, rebuilt the carb and flushed the tank. Also new air filter, plug, front wheel bearings, plastics, levers, throttle, oil, etc. Compression was 160 lbs so I did not open the top end. The bike starts easily. Shifts fine. When running, it stutters when you open the throttle - all gears. I changed the jets from 160/60 to 180/55. Took it out again and it did the same thing - it winds out slowly, stutters and does not make full power. Its like a high speed cut out. There is some leaking at the exhaust port. What's the next step to debug the problem?

"stuttering" as you describe is usually an indication of too rich fuel mixture. Also since you say that something is dripping from the exhaust, it is most likely the case. Try moving the throttle needle down (clip up) a notch or two and see what happens. Also, look at the reed valve and make sure they close/seal properly.

Did you pressure test the motor? This isn't a compression test, and not a leak-down test either. Rather than measuring the engine wear, a pressure test checks to see whether your crank seals and motor gaskets are air tight. If you have a leaking clutch side seal, you could be passing more oil than you want into the air/fuel mixture - causing your drool and throttle response issues. Get the pressure test done before you fry your motor trying to jet the problem away. Enjoy! Jon

Could be electrical. Fire it up at night. See if any sparks are flying where they shouldn't. Bad plug wire maybe? Good luck....Van

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1980 Honda CR125R

Stock jetting is...

Slide 2.5
Needle 28A
Clip #3
Air 200
Pilot 55
Main 185
Float height 19mm

The bike might not run super-crisp with this jetting but it would run just fine. I would not worry about the exhaust drool until you can run the bike long and hard enough to burn all the pre-mix passing through the engine. If the bike idles fine, your ignition-side crank seal is air-tight. If there is any white smoke in the exhaust and it is running like a dog, transmission oil leaking through from behind the primary drive is a suspect. If it runs at all, I would not suspect an ignition problem just yet...

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

Like Jon said, I'd check for leaks. Both bikes I got had blown base gaskests and caused havoc for me. Fuel is another possibility....are you running 110 octane and a racing plug?
Just throwing some more possibilties out there.

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

I am still trying to fix the bike and thinking its the jetting. I put the jetts lean = 160/55 with a new plug (B9ES). Seems to be better. Winds out faster but still has some stutter. BTW, running pump gas with oil mix @ 20:1. I'd like to try different needles in the carb - then adjust the jets again. The Honda parts manual lists needles, 31A to 31J for the '79. Can anyone tell me the differences in these needles? Suggest which I might try? Thanks.