hawk
New Recruit
Posts: 4
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Post by hawk on Oct 30, 2022 9:55:55 GMT
bought this bike as a none runner, found the problem l/h carb pilot jet damaged replaced the carb, i them had both carbs ultrasonic cleaned rebuilt them with all new jets and seals etc with good quality parts from japan. bike now starts and runs well will rev then idle ok in the garage, but go for a ride ( it goes well) then stop and the revs hang up at about 3000 rpm for about a minute, switch it off then restart its ok, its only a problem when its ridden. have replaced the rubbers between carbs and engine and air box, everything else is new or set up as per the book. its driving me mad, great bike goes well just cant ride it in town. thoughts. its a 1979 250 by the way and its only done 9000 miles!
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Post by johnjones on Oct 30, 2022 10:59:35 GMT
See cb250t cdi thread a few posts down, last posted a day or two ago....
Long shot, rag under saddle ?
Revs hang, by that you mean idles at 3 k revs when "hanging" ?
How are throttle cables and fuel flow to carbs ?
You clean carbs by starting at the fuel filler cap.
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Post by buster on Oct 30, 2022 11:10:18 GMT
which switch are you knocking the engine off with? I'd try unplugging the black and white wire next to the cdi, you wont have an engine kill at this point so deliberately stall it in first to stop it.
if that makes no difference then I'm wondering if it might be stator related (windings can start to break down, connects ok at cold but gets a bit warmer and loses it a bit?) really tricky to test when its an intermittent problem.
also worth saying the 250 carbs can be very problematic to get cleaned through, I'v heard of people running them through an ultrasonic cleaner 3 or 4 times before they get them clean.
did you do the pilot screws in the rebuild? the old orings can be a pig to fish out and the assembly order of screw, spring, washer and oring is critical
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Post by johnjones on Oct 30, 2022 13:10:10 GMT
Really tricky to test when intermittent...
Yes, but those red neons are great for this, no HT no flash and can be seen when riding, particularly at night, so a cheap real world diagnostic aid.
AC voltage checks are also good, increase revs, increase rate of change, increase AC volts output. An increase in revs but a decrease in ac v output, coil problems in stator.
Yes, the handle bar kill switch, and the ign switch can "leak" voltage as the voltage can get quite high...a leaky tap if you will.
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hawk
New Recruit
Posts: 4
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Post by hawk on Oct 30, 2022 17:50:00 GMT
thanks for the help. ok from the top, no rag under seat, fuel flow to both carbs good, tank clean, new throttle and choke cables. when revs hang up i just switch off the ignition, as i say it only does this when its ridden, warm it up in the garage its fine. looks like another carb clean! i'll let you know
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