Honda CB750C Problem (Knock)

I just picked up a 1981 CB750C dirt cheap and Im having a problem with it. (of course)

Im new to bikes, so let me know if this is a stupid question.

While the bike is idling, I can hear an obvious knocking sound coming from somewhere in the bottom end. It is pretty spiratic, with no real pattern to it, so I dont think it could be a rod or anything, but what could it be? Transmission? Thanks for your help.

Also, can anyone point me in the direction towards any sites that focus more specifically on later CB750s like mine? A place to find some parts for it would be great too.

Thanks!

Hard to say without hearing it. I guess you've changed the oil?

Things I would think about...the cam chain tensioner keeps the slack out of the chain so it doesn't rattle and beat itself up...I think on that one it was spring loaded but you still had to release the mechanism and then tighten it back down periodically, or else the cam chain and tensioner blade would rattle, makes a distinct sound like a random rattle or knock. If you look below the carbs in between the center cylinders, you'll see the mechanism as a locknut on top of a rear-projecting feature on the rear of the cylinder block.

Also on older bikes the clutch will make odd sounds (knock, rattle) at idle, because the "fingers" that hold the plates in position in the clutch basket are worn, and that allows them to rattle sharply back and forth in their slots from the firing pulses of the motor when the clutch is disengaged. If you put it in gear and slip the clutch very slightly, and the sound disappears, this is likely what you hear. Loading the plates like that takes the slop out (all torqued into one direction and can't rattle)...this is harmless noise, just ignore it, or put a new clutch in it.

An engine in a poor state of tune will make this noise more, as the crank varies rotational speed due to uneven power strokes, and really rattles the plates.

You can pin the sound down in location with a wooden dowel, just use it like a stethescope, to your ear and against the motor. Start near that cam chain tensioner and the clutch cover, and see if you can find it....

wow, thanks a lot! that was extremely fast, ha. The worn clutch would make some sense, seeing that when I drove it home from where I bought it the clutch did slip when I punched it in a lower gear. also, where can I find the procedure for tightening the chain tensioner you were referring to?
thanks again

looks like the sound is coming from the transmission, though I did what was suggested with the clutch and the sound remained. anythign else I should look for?

new problem now though, while running it leaks gas from the overflow tube like thats its job. what should I look for when I tear off the carbs?

looks like the sound is coming from the transmission, though I did what was suggested with the clutch and the sound remained. anythign else I should look for?

new problem now though, while running it leaks gas from the overflow tube like thats its job. what should I look for when I tear off the carbs?

One of the floats or needle valves is stuck. Try tapping on the bowls on the carbs with a plastic screwdrive handle or a small mallet, something like that...that's an old trick, if it's just hung due to sticky deposits, sometimes that'll make it drop free.

Otherwise if you need to take them apart, you're looking for a small piece of crud holding a needle valve open. Some needle valves have a rubber point on them for seating and stopping fuel flow, some are all metal and use a small rubber donut above the needle to seat and seal...look at whichever real close for deterioration, that'll let them leak.

About your noise...there is just so many things that can rattle in a cycle motor with all the Hy-Vo chains and jackshafts they have, and some of it is normal (or at least not alarming). How long have you been riding? I ask because everybody (including myself) obsesses about engine noises on cycles until they've been around a few for a while....just curious if you might be worrying about something normal, not trying to doubt your judgement...

I did see it said you are new to bikes...

I'd change the oil and filter (or screen, can't remember on yours) and refill with Amsoil motorcycle oil (my favorite) or Honda's brand before doing anything else...people like to put car oil in bikes, and while some tolerate it fine, others will get noisy, run hot or slip the clutch...your bike is a wet clutch (oil bath) design, so the oil's properties make a difference in clutch performance.

Most oil made for cars have friction modifiers that go against the whole idea of the wet clutch, which is to have an oil with a high shear strength working with it...and in case you don't know, any additives with Teflon or similar will ruin the clutch for good...don't try it...

Anyway, an oil change with MC specific oil may improve both the noise and the clutch slip...

I'd have to see which style adjuster you have for the cam chain...if I tell you the procedure for the wrong type, you could break it...

You really should get a good service manual for that bike....

The guy I bought the bike from said he just changed the oil, and the oil does look new, but I dont have the confidence in him to say whether or not he used motorcycle specific oil. I'll change that, and the clutch soon.

This is my first bike, but my father has had them as well as friends, etc... I know that soem noise is normal, but the amount im getting from this transmission doesnt seem right, seeing that it is so loud and evident. But it drives fine, so maybe its not a big deal, or maybe its on its way out. Is there anything with the transmission I could try to narrow it down to anything? Also, where can I by the model specific book for this bike? And I need a good place to buy a clutch, oil, etc... from without getting completely screwed with prices, any suggestions?

Thanks a ton for all of the help.

well I fixed the knocking problem, lol. I took off the left side timing cover and noticed 3 bolts behind the timing system. They were all loose. So I tightened them up, and there is no noise at all. VERY happy about that fix, haha. By the description, can you tell me what their purpose is?

I'll take off the carbs tomorrow, but wow what a load off my back. It made the price of the bike a lot better of a deal.

well I fixed the knocking problem, lol. I took off the left side timing cover and noticed 3 bolts behind the timing system. They were all loose. So I tightened them up, and there is no noise at all. VERY happy about that fix, haha. By the description, can you tell me what their purpose is?

I'll take off the carbs tomorrow, but wow what a load off my back. It made the price of the bike a lot better of a deal.

Wow. That's a new one on me. Were they the bolts for the timing plate, or do you mean behind that assembly? Timing plates usually have "sloppy" holes so the plate can be rotated a couple of degrees to set the ignition timing, so it may need that set if that's the case.

Great deal it was something simple though....:grinyes:

Just for laughs, here's my old CB750, SOHC. I think yours is a DOHC, yes? (crappy scan of a crappy pic, sorry)

http://server5.theimagehosting.com/image.php?img=choppa1.jpg

I think if you go to the Amsoil website, you can find a dealer in your area. The price can be pretty shocking. I think the guy here gets almost $7 a quart...but I figure $30 a year to have excellent oil in the bike is a small price to pay....Last time I bought MC oil from a Honda dealer, it was under $4/qt.

Lotta guys like the Shell Rotella (Fleet diesel oil 15W-40) for bikes as a good cheaper alternative...I've never tried but it's properties are more in like with what a wet clutch bike should need...You can buy it by the gallon at WalMart real cheap...

You can probably order a Haynes manual for it through AutoZone if you have one near you...Clymer manuals are a bit better, I think. it's been a long time since I bought a manual for a bike, so I'm not sure which are more available. Clymers are the ones preferred by serious bike mechanics, I think...

no, it wasnt the timing plate. I had to remove the timing plate and that center mechanism held on by one bolt through the center of it to get to the 3 loose bolts. but wow, what a difference now, lol. Its a completely different bike.

Heres a couple pictures of mine...Im happy with the $400 I payed for it.

http://home.comcast.net/~weirdrichard2000/Storage/DSCF0002.JPG
two mopeds in back, lol. missing a side cover, and I have to reattach one of the pipes.


By the way, is there some secret to puting on these older pipes? I tightened down the clamp thjat squeezes the pipe on to the header, but even when its tightest its still not tight onto the header. Is there some sort of gasket that goes inbetween the header pipe and the exhaust pipe itself?

I have bigger problems now though, I got the carbs off last night, but I broke off one of the bowl mounting screws. Im headed to the machine shop soon to try and get that out.

That's a good looking bike! I looked at a CB900 Custom same year same color last spring, I like that blue.

Yeah there should be a split collar that goes between the pipe and the head...

yeah, those collars are there, but it seems liek there isnt enough or something...anythign I can do to make it a tight seal?

also, heres another fun problem....

sometimes the bike just doesnt want to start. as in, I press the start button and nothign happens. sometimes if I keep holding it, the starter will kick in and start the bike. sometimes if I just wait a little bit and press it again, it will start as well. other times, it just starts with no problems. what could this be? starter relay? starter itself? the lights also dim regardless, I dont know if that will help or not.

yeah, those collars are there, but it seems liek there isnt enough or something...anythign I can do to make it a tight seal?

also, heres another fun problem....

sometimes the bike just doesnt want to start. as in, I press the start button and nothign happens. sometimes if I keep holding it, the starter will kick in and start the bike. sometimes if I just wait a little bit and press it again, it will start as well. other times, it just starts with no problems. what could this be? starter relay? starter itself? the lights also dim regardless, I dont know if that will help or not.

I dunno, unless you can just stack some washers before you put the nuts on the studs and make it fit tighter....

I'd just start cleaning connections, starting at the battery and working out to the engine ground and solenoid, and then the smaller primary connections related to the solenoid, sometime when it doesn't start keep holding the button down and see if turning the bars or bouncing up and down on the seat makes it come to life...sometimes that will help you find a bad connector or pinched wire, if you can discover some logic to it when it fails...try working the clutch too, sometimes the clutch/starter interlock switches go bad and cause problems like that, but only when starting it in gear...the switch prevents starter operation if the bike is in gear and the clutch lever is not pulled in....sometimes they go bad and won't let you start or work intermittanly.

if you have a voltmeter, check the volts at the battery with the motor running at about 2000 RPM, should read over 13.5v...most of those didn't charge very good at idle and you might only see 12v or so then.

If you can't get up in thirteens or better regardless of engine speed, it may have a bad stator or regulator/rectifier. Reg/recs were common failures in Jap bikes that age...easy enough to take care of though.

I dunno, unless you can just stack some washers before you put the nuts on the studs and make it fit tighter....

you misunderstood me, happens all the time, lol. Im talkign about where the 2nd half of the exhaust, so the muffler, clamps on to the header. Not where the header bolts to the head. Ther are those sleeves inside the exhaust piece, but there is still too much of a gap from the muffler to the header pipe for a tight fit. I need soem sort of gasket to fit inbetween them.


try working the clutch too, sometimes the clutch/starter interlock switches go bad and cause problems like that, but only when starting it in gear...the switch prevents starter operation if the bike is in gear and the clutch lever is not pulled in....sometimes they go bad and won't let you start or work intermittanly.

where is the neutral switch located?

you misunderstood me, happens all the time, lol. Im talkign about where the 2nd half of the exhaust, so the muffler, clamps on to the header. Not where the header bolts to the head. Ther are those sleeves inside the exhaust piece, but there is still too much of a gap from the muffler to the header pipe for a tight fit. I need soem sort of gasket to fit inbetween them.



where is the neutral switch located?

Oh ok. Hmmm. Seems you might be dealing with a worn out clamp. You could cut a band of metal to go under the clamp, that might tighten things up, or if the gap is real small, you might even get away with using some high-temp gold silicone sealer, I've used it on bikes and car's exhaust before, it won't burn up unless you use it real close to the head. Hell I've even used strips of soda cans, wadded up aluminum foil, etc to make a seal before.

The switch is under the gearbox portion of the motor, close to the clutch on most I can remember...you may or may not have one...some have a sidestand switch that kills the motor if you let the clutch out in gear with the stand down, but that one shouldn't keep the starter from working...i doubt it's related.

Oh ok. Hmmm. Seems you might be dealing with a worn out clamp. You could cut a band of metal to go under the clamp, that might tighten things up, or if the gap is real small, you might even get away with using some high-temp gold silicone sealer, I've used it on bikes and car's exhaust before, it won't burn up unless you use it real close to the head. Hell I've even used strips of soda cans, wadded up aluminum foil, etc to make a seal before.

try 4 worn out clamps, lol. they are all loos. one was so bad that i just pulled it right off without using any tools, thankfully it didnt fall off on the ride home. some aluminum cans would probably do the job, i didnt think of that. i had some left over ceramic gasket material, but it was too thick. i'll use a combination of ultra copper sealer and pop cans, haha.


as for the starter, im just going to start looking around for loose wires. the neutral switch does seem likely though, becuase it is doing the same thing as if you were to try to start the bike in gear.