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Discussion Starter #1 (Edited)
This started right after i changed oil to 20W-50, i dunno why. It runs flawlessly above 2000rpm, but below that it misfires occasionally and almost dies if you don"t help it with a throttle on lights example.
I took carbs apart yesterday, and there was nothing expect a big waterdrop on the right carburetor bowl. Rubber plug covered slow jet is weird because you can not remove it. Pilot screws were out over two and half turns, so i corrected those (CB250N). Two days ago i got in bit of trouble with it because it stopped running under 2000rmp at all with both cylinders, just tried to stall. Hard to ride like that. Now after the carb clean-up it runs again, but not well. There"s misfifre like in every three seconds, and revs try to drop. Plugs looked pretty good, insulators were totally white, no deposits found at all.

Stator has been replaced recently by the owner before me, i don"t think it"s the stator, it charges well, starter motor sings high. CDI or the coil? Performance actually got clearly better afer the carb clean-up and adjustment. It accelerates better now. One weird phenomeon is that it accelerates best when you slowly unwind the throttle while the revs grow. I have had this phenomeon with some other bikes too in the past. Wait when it hits around 6000-7000rpm, and then start to unwind the throttle slowly so that you end up with around half throttle when it"s time to change up. You get extra kick out of it this way.

It needs new exhaust rocker arms and camshaft fix pretty soon, head gets bit noisy pretty soon after the valve adjustment always. Exhaust valves make bit of noise around 5000rpm. Noise almost goes way after the valve adjustment, but comes back pretty soon. Exhaust valve clearance grows from 0.15 to 0.25 in 300km again. I think exhaust rocker arms and cam are bit worn. Long time ago i had VF750F, which ate rockers arms for breakfast. This has all the same symptoms. CDI boxes are not expensive, but original rocker arms are.
 

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The problems occurring after switching to 20-50 are coincidental. Not sure why you would be running 20-50 unless your ambient temp is 85*F plus while riding.
The "pilot" screws are the idle mixture and since these carbs are cumulative circuit type that will affect the mixture at higher rpm. The book setting, turns out, is a starting point for setting the mixture. It's kind of close but not always.
The rubber plug does come out but it's usually quite stuck with age.
I would check the timing and timing advance with a strobe light to verify that timing does advance and that you are getting full advance.
You mention CDI's are cheap, where are you finding them? The stator is of 2 sections, one for charging that rarely fails and the other for powering the CDI and timing which does fail at times. Just because it charges doesn't mean the other half is good.
These engine aren't known for wearing rockers/cams etc. out. You'll need to inspect everything from the cam to the valve stems to see where the issue is.
www.cmsnl.com may be a source for the parts
 

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Discussion Starter #3 (Edited)
20W-50 is basically been just a habit with older bikes. Company in UK sells stators at 75 euros, so that"s not too bad but i am suspicious because owner before me just replaced the stator. I think it might be the CDI or the coil.
Top end noise is definitely there at certain rpm"s, and comes from the exhaust valves or exhaust valve rockers. Camshaft does not look bad. Below 5000rpm it sounds sweet as a kitten though it does not idle well, particularly when cold. There"s zero mechanical noise below 5000rpm. Cold starts need half a throttle. During the winter i think i take the top end apart and replace the CDI and coil, possibly piston rings too.

One thing with is, that this is a thirsty beast for it"s capacity. If you ride really sweet, you can reach 4,5l/100km. Usually it burns 5,5l/100km. I bought this because it has a museum registration, so i will get my money back pretty easily. If you have museum registration, insurance for a whole year is 37e. Otherwise it is around 330e for a year. I do like to tinker with bikes too though. If i keep this over the winter, i will replace lot of things.
 

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When you have it apart, it may be a good idea to examine the oil pump closely for wear or damage.
 

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Discussion Starter #5
I made a short video of it. It"s bit hard to catch, because it comes and goes but sometimes it starts to run like on that video on one pot all the time under the 2000rpm, misfiring occasionally too.
It"s not a major problem, just annoyning. You have to keep revs above 2000rmp on lights. Otherwise it ran like a champ today, head noise was gone! I think it might be somehow related to those ignition problems, dunno.
What happened was that two days ago a car over the bike when it was parked. Clutch handle broke and handlebar looked funny, but that was it. No other damages.
So i straightened the handlebar out yesterday, removing all handlebar switches in the process of course. Now it ran better. Fair enough. I"ll keep you posted if it goes to worse again.

 
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