When im cruising along at 55 mph 3500 rpms for my 1983 CRE F450XI my rpms will spike if i give it any gas past 4000 rpms. the rpms spike about 500 and the power drops significantly. it feels exactly like someone reached over and pulled my clutch as i was riding. if i continue to give it gas after the spike the rpms keep going up but i dont go any faster just keep going the same speed. if im going uphill and give it gas it does the same thing and the bike slows down if i slowly roll off the throtlle after it has spiked you can feel the bike almost re-engage and the rpms go down about 500 and you feel the power come back but if you give it any more gas it jumps right back into this high rpm powerless stage. it has more effect in the 4th and 5th gears.
i might also add that the rear cylinder coughs when it idles and i havent looked into its carb yet. i also just put on straight pipes instead of the baffled ones but it didnt affect the front cylinder. the book also speaks of a mixture control valve that may go bad.
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Re: un usual rpm spikes
At a guess....
Your clutch is slipping. Higher gears and increased drag at hiway speeds put the most torque through the clutch and it sounds like that's when the plates start to slip. When you back off the plates grab again until the load over powers the plates again.
But that's just a inter-guess through a computer screen...... ,,
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Some Honda's have slow running (below 3500 rpm) coils and fast running over (3500 rpm) coils. they are behind the engine cover on the side but could be at the front, depending on the bike. Sounds like your slow running coil is breaking or broken.
The model plate on the frame neck will tell you where the motorcycle was made. If the bike is really cheap (under $500) and in great shape it is probably a good deal only because most riders only put 2000 or 3000 miles a year. If it lasts 10,000 miles, your can drive it 5 years with out too much trouble. Keep in mind you can probably buy a brand new 250 cc motorcycle for around $4000 new or With low miles for $2000 to $3500. Example : last winter I purchased 2009 Suzuki DR200 with 100 miles on it for $2400 (new $3899). It gets 80 MPG, will comfortably run at 55 mph, and will go on or off road This fall I purchased a 2009 EX250 with 900 miles on it for $2500 ( new would have been $4199). Stock it will do 100 MPH and get around 50 to 55 MPG, regeared it should get 80 MPG. Look hard in the off season to get good deals. Whether your 94 CB 250 is a good deal or not depends on the price you will pay compared to a different low mile used one or a brand new. If you have $4000 to $5000 to spend there are some great deals on 500cc past year, brand new, with warranty motorcycles on the showroom floors. Check it out.
and i took it down to the shop today just to check something out and in talking to the owner i was told that i was running at too low an rpm most times. i shift from 1st to 2nd at 3k then shift up at 4k for each gear after but he told me i should be getting up to between 5 and 6k as running under 4k could most likely be lugging the engine.
i shift the way i do due to firstly the bike having 40k miles on it when i bought it so i'm trying to put as little wear on the engine as i can in hopes of it lasting me awhile, and also just because past 4k the engine really starts getting noisy, i know this is normal, but i just can't help but think it is hurting the engine.
is there any real harm in running the rpms a bit low like i have, or running them as high as the dealer said, for that matter is anything below the red line fair game? i just want whatever is best for the engine.,4500 to 5000 rpms and up is the area the bike has quickest reponse. But I only use that range for getting up to speed, then I generally shift to a higher gear and lower rpms. It makes a difference in the mpg. and I can easily shift down for passing. I wouldn't suggest going much below 3000 to 3500 in higher gears. That would be lugging it. You don't want to be in 5th and doing 20 miles per hour.,,,