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Help with engine noise

shaunbe

CB750 Enthusiast
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del rio
HI all, I've been working on getting this 76 cb750k going, it runs pretty decent but I have what sounds like cam chain noise, can someone please help me on this one

https://youtu.be/qiu59Dffsrg
 
That is a YouTube link of it at idle, I adjusted the valves yesterday before I started it and let it run a while, bike hasn't run in about a year but hasn't been on the road since probably 2010, I don't really wanna pull the motor to replace cam chain and rollers if I don't have to, I haven't tried adjusting the chain yet, is there a way to look at the rollers to see if they are still intact without opening the engine? I also reset the valves today again before I started it to see if things changed, the motor itself runs great, doesn't smoke or anything like that, and no major oil leaks. I don't really wanna pull the motor right now if I don't have to, if I have to I'm going to install the other engine I have that is supposed to be an 836 from years ago.
 
The engine runs and idles great it sounds like too end noise, I'm not sure what you mean as far as all of them having an 836, they are cb750 so most would be close to 750cc.... or am I confused with what your saying.
 
These engines will have a certain amount of mechanical noise. Adjust cam chain and sync carbs...two biggest things that make top end noise. Primary chains make a lot of noise in these engines and make the whole thing sound like its coming apart. You gotta pull at least the head to see the rollers, you can watch to see if the cam chain gets tight then loose then tight...indicating timing chain rollers have chunks missing.
 
I think adjusting the cam chain will help, I tried putting in gear and stuff and it never changed sound I'll try adjusting cam chain and see if it helps much. Thanks for the help.
 
I just listened to the video...its kind of hard to tell in videos online but the noise I heard sounded kind of deep. See what the timing chain adjust does to the noise but what I heard sounds more major then valve train.
 
It's hard to hear it for sure in the video, the bike doesn't have any type of air filter on it and has an aftermarket header and straight pipe I know that doesn't help much, the noise is defiantly top end noise, doesn't sound like a rod knock and doesn't sound like an old v8 with mechanical lifters, I was thinking about it and the cam didn't seem to jump when coming off lobe when I set the valve train, is there anything else that could possibly be lose like make flywheel or anything?
 
If cam chain worn enough they can set up a buzz that vibrates through the tank and frame at some resonant frequency higher rpm, say 5000 or higher. You can often hear that buzz on a harder rack of the throttle once engine is warm. You can feel it too through the tank.

Try carb sync again, often it must be just so, I hear at least some primary chain erratic rattle at low tickover. If engine old enough, syncing dead even to shut the chain noise up drops way off, a cylinder not pulling quite as hard as the others from ring wear or valve leaks will then have chain rattle that you often have to slightly mis-sync to quieten. Don't ask how, there is no instruction manual on doing it but I have before. Basically you kill the stronger cylinders slightly to match the slacker one to be more even, using individual carb idle speed and mixture screws. I call the cylinder with lowest compression the 'slacker'.

If worn enough it could be piston slap if it gets quieter as engine warms up.

Grab the clutch lever and pull it in and see if any noise goes away at idle. If trans old enough could even be gear slack rollover.

They are engine noisy even when right. Ain't no hydraulic liftered or timing tensioned cushmobile there.
 
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Well, I adjusted cam chain and it helped, the noise is I fact now the transmission, if I pull the clutch it gets quieter, but if I put in gear and hold rear brake as it's on center stand the noise pretty much goes away, I need to read through the repair manual and see if there is any type of primary adjustment, makes me feel better that it isn't cam chain anymore, oh the adjuster moved in about 1/4 an inch when I loosened the adjuster. I also gave it an oil change and thag helped, hopefully it's nothing major I think a carb sinc would help cause it's most notable at idle and runs like it has a small cam not choppy but not real smooth.
 
I just reread your reply amc49 and gear roll is more what it sounds like now that you mention it, my 94 f350 does the same thing at idle.
 
Well.... my dad just stopped by, he's 61 and has owned about 7 or 8 750 Honda back in the 70s he said it sounds like cam chain noise, I feel bumbed out as I was hoping it wasn't anything serious.
 
1/4" is a lot of movement for the adjuster....unless the previous owners never adjusted the chain then I would guess one of the wheels is coming apart.


There is no adjustment for the primary chains, its a spring loaded tensioner wheel inside.

The 750 will run pretty darn smooth at idle if it has a good tuneup and engine is sound, if the idle is choppy something is not to spec as far as engine condition or tuneup
 
Let me say this, I know this bike hasn't been run on the street in at least 5 years I got it running for the previous owner who had no idea about points and electrical, that was about 5 years ago, the carbs have yet to have been synced I made another video that is longer and I put a rag in the exhaust to tone it out, I sat the phone on the ground and put the bike in gear and held the rear brake, the bike is also idling around 800 rpm and not real smooth, it's hitting on all cylinders but not very smooth at that idle. I can say it's been at least 5 years if not longer that the chain hasn't been adjusted. Here is the link to the video I just made.

https://youtu.be/JDeQIpWWsBk
 
I know the idle is low the engine was fairly cold once fully warm it will idle around 1000 to 1200 according to tach, but the lower the idle the more noticeable it is.
 
About the only thing I can hear in the video is the rattling transmission gears and some primary chain noise. Also sounds like an exhaust leak in certain spots but its hard to tell. Dont know what to tell you, your going to have to make a call for yourself on the noise it you are going to go after it or risk it and run it.
 
I would do a compression test see if its even worth going any farther, if the engine is tired no point going after noises.
 
The rattling is the noise I'm talking about, but it sounds like it's from up top and in the front so I was thinking it's from cam chain, I have Nother motor but I would like to go through it and upgrade that motor and one day swap it out, I'm looking at doing this within about a year, the engine seems to have great compression on all cylinders, I can put it in gear and drag the engine down to about 500 rpm and it runs decent once all the way warmed up.
 
I owe you so much digger, I should have done this first, but I just watched about 10 YouTube videos of cb750 and they all seem to have the same rattle as mine does, some not as bad and some seem worse, must just be a design of the engine, makes me feel a lot better, and I might have been exaggerating about the cam adjuster movement it might have been about an 1/16 to an 1/8 inch, if what you hear doesn't sound too bad as all the bikes I watched on YouTube made the same noise isn't bad I think I will run it, I live in South Texas so riding is about an everyday thing, sorry for all the newb questions, the last old bike I had was a 73 Suzuki gt750, I wish I still had that bike was run and sounded like a big chainsaw running down the road. A cb750 has always been the bike I've wanted as my dad has had a few and always talked about how good they were and that he never had any issues with them. So I'm hoping to have mine road ready in a couple of months, this will never be restored it will always be a toy, I have a 71 cb750 that will possibly get restored some day. But I like to go fast id love to have a cb750 that makes about 90 rwhp but I know that isn't the easiest thing to do with a naturally aspirated cb750 but maybe one day.
 
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