QuestionI have been trying to trace a misfire in a 1984 GN250 for months. Engine will very start easily but won't go over 3500RPM. After 10 minutes or so, the engine will suddenly run OK. If I stop the engine for more than a few seconds, the problem will come back and stay for 10 minutes or so. Carb has been cleaned, valves replaced. New leads, plug.
AnswerThe simplest thing to check, if your bike has a vacuum operated petcock, is if the vacuum hose has become cracked or isn't sealing tightly enough on the carb & the petcock.
After that I would check the ignition's timing advance mechanisim. I think it should have a mechanical ATU and they sometimes get gummed up and don't advance the ignition timing enough to let the engine operate at higher RPM. Your shop manual should cover this.
The next thing on the list is the park plug, plug wire & cap and the coil.
Did you check the gap of the new plug before you put it in?
Did you use proper copper core plug wire or crappy automotive "resistor" or "supressor" core stuff that won't last a season because it isn't really wire?
Examine the coil & plug cap carefully for any signs of damage. If either is cracked it should be replaced. Some plug caps have resistors inside them that can fail and give similar symptoms. See if the insides of yours can be removed with a screwdriver (turn the key off before you stick a screwdriver into the cap). If it can, tke it apart and clean the internal parts with contact cleaner and an od toothbrush. If it has a resistor, make sure it is in spec (should be in the shop manual) or just replace it with a short piece of brass rod.
This leaves the carb. Who cleaned the carb and how thorough a job did they do? To properly clean carbs they must be removed from the machine, disassembled and have, at minimum, carb cleaner sprayed through every passage, followed by high pressure air (min. 60 PSI). Any signs of dirt or other deposits should be completely removed and all jets carefuly inspected and cleaned. Your carb is 24 years old and that's a long time for rubber parts that have been in a hot place and in contact with fuel. The rubber parts should all be carefully inspected and if any of them show signs of hardening or deterioration they should all be replaced with the best quality new parts available. Even if they all look good now, you should expect to replace them within the next year or so.
Have a look at the carb cleaning information at www.randakks.com - Randall is one of the top GoldWing restorers and carb specialists so the site shows GoldWing carbs, which will probably be a lot different from yours so you should consult your shop manual, but the same principles for cleaning should apply.
Unfortunately, he doesn't sell the parts you need. I would talk to the parts man at my local Suzuki dealer before I looked for the parts anywhere else - they are usualy pretty close in price to what you would pay online and if anything is wrong with them you can take them back in person instead of dealing through the mail.
If none of the above helps you might have a failing CDI unit, but I am not familiar enough with the GN250 to say. I recommend spending some time on Google looking for a good online usergroup (discussion forum) that specializes in bikes like yours and ask there.
