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Doug E.
Doug E., Kawasaki Master Technician
Category: Motorcycle
Satisfied Customers: 2885
Experience:  Professional motorcycle mechanic since 1978. Vintage motorcycle restorer.
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I have a 1978 Honda CB750F Supersport w/ 17,500 miles. It

Customer Question

I have a 1978 Honda CB750F Supersport w/ 17,500 miles. It runs smooth and has great power. No smoking under power or at idle of 1100-1300.

However I hear a rattle coming from the engine which caused a buyer to back out of the deal.

I don't think the rattle is serious but I can't explain it to my potential buyer.

A local MC shop listened to my story over the phone and advised the noise is typical. It's from the primary chain.

Please provide a second opinion and advise whether the noise can be eliminated
Submitted: 4 years ago.
Category: Motorcycle
Expert:  Doug E. replied 4 years ago.
Hello,

Either the cam chain or the primary chain can produce a rattle in the engine. (Like nuts and bolts in a tin can.) You can try and narrow the noise down with a stethascope. A cam chain will tend to make more noise in the rear of the cylinder and in the head area. A primary chain will make noise behind the cylinder in the crankcase and in the front of the crankcase. Cam chains and tensioner problems tend to make more noise as the motor RPM rises. Primary chains tend to get quieter with a load on them and make the most noise at idle.

Cam chain problems are most often a worn tensioner which is easy to replace. Primary chain noise can be a bike that is running out of sync or poor tune. To check the chain and tensioner you must pull the oil pan.

These motors will make some noise. If you can hear the noise with a helmet on, it's too loud. Otherwise it's normal.

Let me know if you need more information.

Doug
Customer: replied 4 years ago.

I have someone who is intersted in buying the bike but is intimidated by the rattle.

 

Regarding tuneup I checked the point gap and timing. Both were right-on. Carb sync is not checked. I cleaned the carb and orafices in the bowl area with carb cleaner and air compressor, plus installed new goodies in the float bowl and o rings in the air mixture screws. However, I did not touch the sync screws.

The bike runs strong and idles smooth at no less than 1100rpms. Lower rpm idles rough and stalls. I have no idea whether carbs are in sync. It looks like plugs and wires are OEM. I cleaned plugs from 1 and 4. 2 and 3 were too hard to reach.

 

If I drop the pan what am I lloking to find that is a problem? How will I know a problem when I see it?

 

Is the chain tensioner adjustable? What is that thing below the carbs between 2 and 3 cylinders. Is that where the cam or primary chain is adjusted? If yes, which way should I turn it == clockwise or counter . . . .?

 

What do I do to get rid of the chain slack causing the rattle?

 

 

 

 

 

 

 

 

Expert:  Doug E. replied 4 years ago.
That's the cam chain tensioner between 2&3. I'm going to upload some prints showing the tensioner adjustment and how to measure the primary chain tensioner.

I can almost guarantee the noise is from the idle not being smooth. Anytime the motor is running unbalanced, it makes a very loud rattle in the lower end. You may have plugged pilot jets, your air screws may be out of adjustment, or your carb sync is off. If you increase the rpm and the noise goes away, that is the problem.

I'll be right back with some diagrams for you.

Doug
Expert:  Doug E. replied 4 years ago.
Here is the diagram showing how to measure the primary chain after you drop the oil pan. I seriously doubt this is worn unless the bike is trashed and never had the oil changed.

graphic
graphic

Expert:  Doug E. replied 4 years ago.
On the cam chain, if you rotate the motor forward and rearward with a wrench on the end of the crankshaft, you can feel if the chain is loose. There should be no freeplay between going one way and then the other with the wrench.

Rotate the motor a few turns and stop on T. Loosen the locknut on the tensioner and loosen the adjuster bolt. Tighten the bolt and locknut. If it won't adjust, it may be stuck or worn out.

Doug

Edited by Kawasaki Doug on 12/31/2009 at 9:07 PM EST
Expert:  Doug E. replied 4 years ago.
Hello? did you get it checked out?

Doug

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Doug E.
Doug E.
Motorcycle Mechanic
2885 Satisfied Customers
Professional motorcycle mechanic since 1978. Vintage motorcycle restorer.