Cb750f engine rattle

[TexasCb]

Hey y'all, I bought a 1975 Honda cb750f back in May of this year and there has always been some sort of rattle coming from the engine. Occasionally it will overheat and shut off too, not sure if that is related to the rattle or just the hot Texas summer. If anyone would happen to know what the cause of this sound is, I would really appreciate the help.

Here is a short video of the sound

 

[madmtnmotors]

A good carb sync will often quiet things down. The uneven pulses from unbalanced carbs tends to cause the clutch to rattle. As for overheating and shutting down, I suggest trying to correct his behavior in earnest. Do a full tune up, carb sync, and check for any blockage/obstruction in the cam cover crankcase vent.

 

[brettp]

I'm not sure where that's coming from. Take a long handle screwdriver and put the tip on the crankcase and put your ear to the handle and do that to different parts of the bike and you'll you'll find out where the sound is coming from. Try crankcase, along the cam chain, spots on the valve cover, etc.

 

[1975 750]

A good carb sync will often quiet things down. The uneven pulses from unbalanced carbs tends to cause the clutch to rattle. As for overheating and shutting down, I suggest trying to correct his behavior in earnest. Do a full tune up, carb sync, and check for any blockage/obstruction in the cam cover crankcase vent.

ive owned several of these .when the day time ave is 90+ they just shut down ?
vapor lock?engine so hot the fuel isnt atomizing?fiik,but she will just stop
…..a stethoscope works wonders!
how about the intake and exhaust valve clearance ?0.002 on the intake and 0.003on the exhaust.

 

[TexasCb]

A good carb sync will often quiet things down. The uneven pulses from unbalanced carbs tends to cause the clutch to rattle. As for overheating and shutting down, I suggest trying to correct his behavior in earnest. Do a full tune up, carb sync, and check for any blockage/obstruction in the cam cover crankcase vent.

Should I go ahead an rebuild the carburetors while I am at it, or is that unnecessary?

 

[TexasCb]

ive owned several of these .when the day time ave is 90+ they just shut down ?
vapor lock?engine so hot the fuel isnt atomizing?fiik,but she will just stop
i listened to your video …..a stethoscope works wonders,not a screwdriver against your ear!!!
how about the intake and exhaust valve clearance ?0.002 on the intake and 0.003on the exhaust.

I have not check that yet, but when I get back from school, a valve adjustment along with carb sync, points adjustment, and cam chain tensioning are on the to do list. For the over heating problem, I have seen some people put after market oil coolers on their bikes to help, do you have any thoughts on them?

 

[brettp]

Used a screwdriver many times to pinpoint stuff like that. The vibrations run through metal really because the metal's molecules are tightly bonded. The vibrations carry through the metal screwdriver. Not unlike a stethoscope picking up vibrations, but through a diaphragm.

 

[madmtnmotors]

Should I go ahead an rebuild the carburetors while I am at it, or is that unnecessary?

Check the spark plugs first. If you do not have any evidence of a lean or rich condition you could very well cause more problems. Carb rebuilding is as much an art as it is a science. I absolutely detest rebuilding carbs, but I have gotten quite good at it. Lots of small parts that are easily lost, bent, or broken, small screws that are easily stripped, numerous figgity/fussy adjustments, and lots of small passages that are easily plugged. Not to mention the old stiff/brittle rubber carb manifold boots that may start leaking if cracked/damaged by removing the carbs. Try not to change too many things at once. Do one thing and see if conditions improve then go from there. I live in central Florida and have never had mine shut down from extreme heat and I've been riding my CB750 around here since 1986.

EZPZ

 

[madmtnmotors]

I have seen some people put after market oil coolers on their bikes to help, do you have any thoughts on them?

Overkill. If it's running right it will not overheat. Oil coolers usually just create more opportunities for oil leaks. Get it running right first.

 

[1975 750]

Overkill. If it's running right it will not overheat. Oil coolers usually just create more opportunities for oil leaks. Get it running right first.