How can I fix an old 1969 Honda CB 250?

Question

I have an old 1969 Honda CB 250. I have been able to start the bike, but it does not run properly. It has ran on one cylinder at tickover even though the twist grip was against the stop. There two sparks and the fuel is reaching in both carburettors. They were rebuilt using the recommended jets, needles and float valves. The cylinder head has been off and there where no sticking of the valves. The timing of both sets of ignition points were checked properly. I don't know what the problem is. Can anyone help?

(posted for JM. Original text and question by JM.)

Answer 1

my guess is that you have the cam chain timing one tooth out. retarded.

Thus your engine is trying to fire but the spark is being produced at the wrong point and your valves are opening and closing slightly out of synch.

Please recheck you have the correct timing marks aligned as the old CB 250's were renowned to have multiple timing marks on both the fly wheel and on the crank case. It is also likely that if you don't have the side cover on you are not using the correct mark.

Recheck and realign the marks and reset the points to the correct gap and use a timing light made of a bulb of the correct voltage and a couple of wires across the points. when the light comes on the points are just opening and the timing marks should line up.

adjust as necessary

Check also float heights and emulsion tubes for damage and to ensure all holes are clear of debris. Check also that needle heights are correct for your engine and your region.

good luck

Answer 2

one of the more common problems on older bikes is a vacuum leak. usually those little rubber boots that serve as intake manifolds distort and although they look good,they don't seal well.and give the symptoms you describe. check to be certain nothing is clogging the exhaust pipes to. mice like to build nests way up in there! I would start over on the tune up,double checking valve timing and adjustment first,then ignition timing and plug and point gap again,sometimes they get thrown off ,or I don't have them tight ,before I turn the motor over.then I feel more confident messing with the carbs. more often then not,when I'm getting an old bike running again,I missed something in the carburetors. a lot of smaller engines will not run without the air box installed. cb 350 and 400's are that way,

Answer 3

Kurt T has it right, I rebuild the old Honda's myself. GET A GOOD MANUAL FOR THAT YEAR MODEL...Always check the following before trying to start the bike...CHECK the tank for rust, water or other, pull the carbs and clean or rebuild. Check the air boxes, mufflers for mice nests or in one case for me a snake. Now usually there was a reason for the bike being stored. It is either tuneup, running poorly, doesn't run, won't run ect....BUT start with all the components that require fuel, air, vacuum, linkage, cables..Old Honda's, most, will generally have carb issues. if you have done all that and it still runs goofy, it may be the Cam Chain timing. check the chain and wheels and make the adjustment. Then there is the valves...this is where a good manual falls into place...good luck....or bring it over to my place and I'll help you......

Answer 4

I used to have a CL 360, which had the same weak, crappy coils that were junk even when new. Don't waste your money on poor performing OEM coils, go with something decent like Dyna or Nology. The cost will the about the same or less and you'll have a lot more voltage. That will allow you to gap the plugs at .032-.035" which will make starting easier, better running, especially at cold temps and better gas mileage. Make sure you get a coil made for points type ignitions, which will have 3.5-5 ohms resistance on the primary winding. Coils meant for electronic ignitions will work, but you'll have to add a ballast resistor. Or, check JC Whitney for a replacement coil. Not sure about the quality though, Dyna and Nology are top notch.

Answer 5

Obviously something is not right, so, go right to the basics. The engine needs 4 things to run 1. AIR, which is pretty much a given, unless the airfilter is plugged up. 2. FUEL, not only to the carburetor, but, able to go through it, you say its been cleaned, but, check it again. 3. COMPRESSION, don't guess, use a compression tester, you got to have at least 90 lbs to run, hopefully you'll have 120+. 4. SPARK, occurring at the correct time, some of those old Honda twins were a little tricky to set, check it again. IF all those things are right it HAS to run.

Answer 6

Lots of new carb parts but no mention of the slides,I'll be honest I can't remember if that one had diaphragms in the top of the slides or not but humour me,take the four screws out of the top covers and remove the slide/diaphragm assembly out and check,by stretching the rubber for any holes,pretty sure you'll find at least one,renew both diaphragms on the carbs.Holes mean it'll start fine but as these carbs a opperated by constant vacuum any holes mean no vacuum..Best bet for these would be Dave Silver,google him.

Answer 7

Did you synchronize the carbs? If they're too far out it'll appear to be idling on one cylinder while in actuality one cylinder is just firing much harder than the other.

Answer 8

i know its obvious but sometimes the simple things are overlooked , try new plug leads and caps before you go any further. i had a similar problem .good luck.

Answer 9

1969? lol. bring it to a scrap yard man!