Motorcycle Repair: Sprockets, path of least resistance, rear sprocket

Question
Bill,

You did nail that one! The reason for the change is a Vibration ("Buzzing")Issue that is pretty noticable around 5500 rpm. I bought this bike new in 1981, although I don't recall that issue when it was new, It has been a issue for many many years. I really cant tell where it comes from but it seems like the front of the Motor or somewhere around the Front end. Sometime in the mid 80s I put 6" over forks on and I've also had to replace the Factory Exhaust over time. I replaced the The Exhaust with Factory OEM.

I have seen on some Internet posts that guys have had simular problems but never a solution. Could it be the Fork Springs rattling in the Tubes?

I thought that Changing the sprocket would be the path of least resistance.

Maybe need to start removing parts off the bike one by one and try that.

In your opinion are these types of viberation issues typically something other than the Motor?

         Thank Bill,
         -Mark







Followup To
Question -
Hi Bill,

I have a 1981 cb750c. I would like to replace the rear sprocket (43 tooth) with a 38 tooth to
lower the rpm. It runs around 5800 rpm @ 70 mph and I would like to lower that. Besides Chain length are there any things that I will need to take into consideration?
         Thank you,
         Mark
Answer -
Mark, how did you arrive at the 5 tooth change number? If you gear it too high, then you will have more problems launching it from a stop and you may be shifting it more, as you are losing the "leverage" advantage. These engines don't make huge amounts of torque, so going overboard on the gearing will have you stuck in 2nd or 3rd around town, to keep it from bogging down at low revs. Redline is up around 9k or so and the peak torque is generally around the halfway mark. Hondas love to rev, so don't be too put off by the rpm vs mph issues, unless you are experiencing a vibration issue at your normal cruising speed that could be affected by changing the gearing.

It depends on how you drive and where you drive. If you live in the middle of Kansas, then getting the revs down for long flat stretches is okay. If you are in a hilly, windy location, then you will be shifting the transmission more often to keep the engine up on the torque/power band.

The bike should have an 18t front sprocket from the factory to drive your 43t rear = 2.38 final gearing ratio.
Going to a 38t rear will bring it down to 2.11.

I checked the online microfiche services at PowersportsPro.com and see that they used various wheel sizes, sprockets front and rear for the various DOHC 750 models in the early 1980s. Looks like a new 38t rear sprocket is $100 from Honda. Check with http://www.sprocketspecialists.com/ for cheaper options. They can make just about anything, but perhaps they can give you some guidelines about what people usually buy for that model. A quick look showed that they had 41 t rear sprockets in steel. They also offer aluminum sprockets.

If it was my bike, I wouldn't change the gearing more than about 3 teeth at the back. Going from larger to smaller sprockets won't affect the chain guard clearances, but check to see that the chain isn't hitting the frame/swing arm when it gets slack. Don't want it sawing through critical parts of your machine!

Bill Silver
www.vintagehonda.com


Answer
Mark, engine harmonic vibrations can radiate and affect parts all over the bike. Many of the 750-900s had "floating" exhaust systems to help reduce the resonant vibration into the bike. Be sure that all of the mounts and fittings are correct and in good condition.

A good way to isolate some exhaust buzzing is to loosen the exhaust system all the mounts and connections, tighten the exhaust header flanges first, then move backwards along the system. Check for binding of mounting bolts and shim out any gaps in the muffler to chassis connections mounts with flat washers/spacers rather than just tightening them down, causing a bind. This can reduce harmonically induced vibrations.

I have noticed that some models have a vibration at the fuel tank, which is related to hardened fuel tank mounting rubber parts or a wire/clutch cable/component lying up against the underside of the tank. Check all the little mounts for brackets, electrical components and accessories.

Your fork mods could be a source of spring vibration, especially if the springs are not the correct OD. They would be longer than stock and more apt to rattle due to the increased length.

Some models have vibrations from front fenders and/or the speedometer cables or brake hoses rattling in the stays or mounting. Many of the soft "rubber" mounts have turned into hard plastic now, which reduces their ability to absorb vibrations.

If the bike still has a center stand, jack the front wheel off the ground and run the motor up to 5 grand briefly and see if you can locate the noise source. Tach and speedo cables running up around the headlight shell and wiring connections inside the headlight can be a cause. Large wiring connectors can vibrate around inside the headlight shell or under the fuel tank, along the frame. Check for loose battery mountings, fender mountings, head and tail light mounts, turn signal mounts and even the glass inside the mirrors or the mirror heads.

If you can't isolate noise from the front end, pull the seat and tank and look carefully along the frame for components that are loose and at the frame paint where the paint has been worn through from some kind of continuous contact.

Bill Silver