1981 CB750 Engine ticking

WhyteRaevin · 04-03-2009, 5:59 PM

I recently acquired a 1981 CB750C. The bike starts fine, but won't idle without the throttle cracked a bit. But that isn't the part I'm worried about. The part that I need help with is the noise coming from the engine. I used a steel dowel to listen to specific parts of the engine and it seems to me that the severe ticking sound I am hearing is coming from the top of the engine where the cam shafts reside

Does anyone out there have an idea as to what is wrong with this bike? I'm just wondering if there is something I could do myself, or if top end work is better left to the professionals. Any help would be much appreciated

Twincam · 04-03-2009, 6:40 PM

Maybe the tappets need adjusting?

MTXMUGEN · 04-07-2009, 5:39 PM

I would remve the valve cover and inspect the cams and valve buckets. If you have a feeler guage i would also map out the tolerances on a piece of paper and see how the valvetrain sits as far as lash goes. make sure the cam lobe is pointing away from the valve bucket when you take your reading witht he feeler guage. I would go by the largest blade that will fit since the cheap sets susally dont have enough steps to do a really accurate job. Also, turn the engine over by hand to get the lobes in the right position. I usually remove the riderside right cover on my '82 750 nighthawk and turn the engine over via the bolt holding the charging rotor in.

I would crank the engine over with the starter motor while the cover is still off(make sure the killswitch is set to off or the spark plug wires are pulled, if the engine starts, oil will fly), just look for anything unusual.

MTXMUGEN · 04-07-2009, 5:48 PM

Quote:

Originally Posted by Twincam

Maybe the tappets need adjusting?

the 78-82 750-4 is a DOHC motor. there are no lifters. did the knock show up suddenly or was it gradual? does it tic with it being turned over by the starter motor?

WhyteRaevin · 04-07-2009, 9:28 PM

I don't know when the ticking sound first started. I just bought this bike. I pulled it out of a snowbank rolled in onto my truck and brought it home to my garage. Once it warmed up enough I tried to start it and I noticed that it was ticking. It doesn't tick so bad at higher rpm's but that might just be because I can't hear it over exhaust lol. Thanks for the pointers. Now that I have an idea of what to look for I'll tear into it and see what i can find.

MTXMUGEN · 04-07-2009, 10:19 PM

Quote:

Originally Posted by WhyteRaevin

I don't know when the ticking sound first started. I just bought this bike. I pulled it out of a snowbank rolled in onto my truck and brought it home to my garage. Once it warmed up enough I tried to start it and I noticed that it was ticking. It doesn't tick so bad at higher rpm's but that might just be because I can't hear it over exhaust lol. Thanks for the pointers. Now that I have an idea of what to look for I'll tear into it and see what i can find.

Just to be thorough you might want to check the cam chain tension. theres 3 bolts on the cylinder block. theirs one on the front of the block near the top center, then 2 in the back of the bock. the upper one on the rear is a pain to get to but its nessicary for proper adjustment.

have someone help you with this. loosen the lock nut on the outside of the front bolt (14mm)then loosen the inner(10?). theres no outer nut on the rear bolts so you just need to loosen them(10mm wrench). DO NOT GO MORE THEN 1/2 TURN AFTER IT LOOSENS. if the bolts come all the way out the tensioner will fall into the block and you will have to tear apart the engine to get it back in place. once all the bolts are loose, turn the engine forward by hand and have seomeone retighten all 3 bolts.

since the tensioners automatically adjust after the bolts are loose, you want tension on the cam chains to ensure there tight. you spin it by hand to maintain that tension.