QuestionI would like to know what changes have been made, if any, from 1982 to 2007 on the Honda Rebel 250c.c. power plant, including motor, clutch, and transmission gear ratios to the final drive.
Here is why:
I have an 82 and an 83 Honda CM250C with only 7000 miles on it. I have built it into a British looking bike. I have geared it as high as possible to get the revs as low as possible. I don’t care about top speed. I care about being able to cruise comfortably at 50 MPH. As it is now I am getting 117 miles per Imperial gallon. That translates to over 300 miles out of my 14 liter Triumph gas tank. My problem with this engine, which by the way runs perfectly, is that this engine has no low end torque. Once I’m out of first gear it doesn’t matter anymore, but taking off from a stand still requires revving the engine more than I want to, to keep it from stalling. The carb doesn’t have a pilot jet to directly influence the idle. It does have a low speed jet but increasing it’s size, as I have tried, only increases fuel consumption. I mention this because it runs very lean at idle when warm, which is when this problem is most apparent, but takes off much better when I’m on choke. Also the clutch cage spins on its shaft by means of a bushing, as opposed to a needle bearing. I have three of these clutches and they all have a few thousands of what seems to be normal play, causing a slight wobble when the oil is hot and therefore thin. This condition causes the clutch to chatter just before fully engaging. Together with the lean mixture and low end torque makes take off very unpleasant. I have a Honda S65 and a Kawasaki KZ 200 single with way more low end torque than this. This Rebel engine has been produced for a long time now and I can’t help but think that if this was normal that these problems would have been addressed through the years. I would gladly fit another engine into this frame to solve this problem, even if it were a single cylinder of the same size, or a newer Rebel engine if these problems are now history. I am making a big deal out of this because I can’t seem to buy a standard smaller sized, economical bike in today’s market. I am a mechanic and I can fabricate what I need. This bike is beautiful and worth the effort, so I am motivated.
Thank you very much for taking this question, Rob Hilkers, email address: anjerobhilkers@hotmail.com
AnswerRob, you sound like quite a creative guy with your high MPG Honda 250cc twin. Rebels were only made initially from 1985 onwards. The 82-83 CM250Cs were outgrowths of the CM185-200s, based back to the CB125T designs from the late 1970s. I have not studied them in detail, but if you go to www.bikebandit.com you can pull up illustrations of the bikes, through the years and not any changes in the parts/prices/illustrations.
Trying to get more pulling power from a 250cc, single carb twin that is way overgeared is asking a lot. A typical Honda 250 twin doesn't make much power until it is working up over 7k or more. I had a CB125T that would go 80 mph at 12,000 rpm, but you gotta wind them up into their powerband. A 250 single would have more low end torque due to the bore / stroke ratios, camshaft profiles, etc. They are designed to make power at lower rpms than little twins.
The idle jets seem to be around #35, so you could try a #38 and adjust the mixture screw accordingly. If the bike idles lean, then the adjustment screw should be backed out until it idles correctly. Once you get that solved, then you may have to shim up the jet needle with tiny washers to enrichen the mixture off-idle. It looks like the 82-83 engines have a primary and secondary main jet, so perhaps stepping the primary main up one size may help smooth things out, if the hesitation is fuel metering related. The later Rebel and Nighthawk carbs seem to be a different style and calibration than yours.
I would revisit the clutch assembly again, looking for notches on the inner and outer clutch hub/baskets, where the plates move in and out. A little wobble on the shaft shouldn't necessarily create clutch chatter. The plates must all be VERY flat in order to facilitate smooth release. Being that you are slipping the clutch a lot to get it moving, the plates are getting worked pretty hard and probably warping in the process.
Good luck with your fuel miser machine...
Bill Silver
