71 Honda CB350 Engine slapping noise

M

msch345

New Member
So I just picked up a CB350. From what I was told, it ran great when parked last summer. Fluids drained, and parked for approximately 3/4 of a year. Put a new battery in it, new fluids and fired it up. And of course, with my sitcom style life of everything being an issue, the engine is slapping something fierce! Re-adjusted the cam chain, and turned over the motor with the valve covers off to see if anything was stuck. All valves are moving fine.Still makes the slapping noise (very loud, not a tic). I'm not the best mechanic, so I'm now kinda stuck... Any ideas? I was hoping to just get on this sucker and enjoy the last part of the season, not have yet another project... Does it sound to you guys like I will now have to take the top end apart?

Thanks in advance for any help!!!
 
From what you've written, piston slap comes to mind.It's when the wrist pin is worn, and the piston rocks laterally in the bore. Cylinder boring and refurbished rods are the usual cure.
 
Try adjusting the valves first. Mine was really loud until I adjusted mine, and good thing I did as they were way out of spec.
 
Double check the timing also. it may simply be preignition.

I guess start with the easy stuff before you take everything apart. While Corsair is probably right, i hope for your sake it's something simple.
 
cam chain tensioners are known to wear out. even if you set the tension there is a good chance that the tensioner is shot. Black bits of rubbery plastic in the oil are the main indicator. By fluids drained, do you mean oil as well? you shouldn't ever drain the oil just to store a bike.

Piston slap is unlikely. My bet is the cam chain is smacking against the side of the chain galley. If its not fixed it can break a hole through the galley wall in-between the two cylinders.

As mentioned before, check rocker (valve) gap (and double check! they easily come out of spec at the last second) first. if the noise continues you need to remove the head and do a visual inspection.
 
hillsy said:
Also check the starter clutch / chain arrangement.
Click to expand...

good call. I forgot about that as I took mine off ;)

In case you don't know, it's located behind the left side cover.
 
Hi guys,

I was just reading this because I recently rebuilt the top end of my '71 cb350 twin and I'm getting a slapping/knocking noise. I've narrowed it down around the left side rocker arms. Already adjusted the cam chain tensioner and adjusted the idle as it was a little slow. I noticed that took out some of the knocking but still present. It's most prominent at idle but I do hear it occasionally on the few test rides I've been on.

I'm curious about the starter clutch/chain arrangement. I did remove my starter motor as well as the chain when removing the engine. Just curious about the arrangement and how it could cause this noise?

Thanks and sorry about the late reply.. didn't have a cb until recently!
 
camshaft endplay check it,it will make noise if too great a value. you can use a dial indicator to check it.the starter clutch wont make noise when running its not the starter vclutch for anyone to suggest it is ,is a sign they dont understand how it works
also check for timing chain slackness by the crank rocking method
also getting the tappet clearance correct is tricky ,you are likely running too high a tappet clearance
 
gingerbanzai said:
I'm curious about the starter clutch/chain arrangement. I did remove my starter motor as well as the chain when removing the engine. Just curious about the arrangement and how it could cause this noise?
Click to expand...


If you have removed the starter motor and chain then obviously it wont be making a noise. The comments were for engines with the starter motor and chain still installed.
 
Thanks for the suggestions. I'll look into that stuff. Sorry, I do have the starter motor installed. Just removed it to get the engine out easier but it's definitely running on the bike. How do I perform the crank rocking method? I'll check the rest and keep you guys posted about it.

Cheers!
 
"crank rocking" to actually observe if the cam chain system has little to no slack
remove points and stator cover, remove spark plugs
put a 14mm box wrench on stator end bolt rotate crank ccw until it gets to the point where it wants to rotate on its own(this is caused by the valve springs pushing the cam) go a bit past thaT point and then gently rock the crankshnaft back and forth a few degrees..if the timing chain is in good shape and being tensined properly there will be pretty much zero lag in movement of the cam(up at the points) as you move the crankshaft to and fro
 
Update: just did a crank rocking test and as I was I noticed that the washer and the little cylinder that the Phillips is pointing at were slipping just a tiny bit. I feel like this has a hand in the problem.

f3f56b6cd5159adf23ac459e594f1eff.jpg
 
all that washer and bold do is keep the advancer from coming off the cam the washer shoild not be loose though the bolt may have come loose or is a wrong one and too long DO NOT overtighten it
 
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the piece you are pointing to is the mechanical advance unit . . it will not make a klanking noise unless one of the advance springs is broken . . if you grab it and can move it up and down very much, it is worn and will cause slightly erratic timing.

you can grab it and rotate it . . it should have spring tension on it . . it it has any freeplay the springs are loose.

your points gap looks way too big to me but it might just be the photo
 
I'll have to recheck the points gaps tomorrow. Stay tuned on that one.

The washer and bolt are the same ones I took out when removing the top end so they shouldn't be too long. I made sure it was tight but not overtightened.

So, took it for a quick ride and noticed a few, possibly helpful things. When warming the bike up, the knocking sound was present while the choke was closed but as soon as I opened the choke it went away and sounded as it should. However, after I got a few blocks away, there was a serious lack of power to the wheel and the engine felt like it was just chugging like it was in a 3rd or 4th gear trying to get moving from a stop. Really felt like it wanted to just die.

When I got back to my place I started it up again and there was a noticeable difference in the back pressure between the right and left exhausts. Left was noticeably lighter. Originally I bought the bike with the left cylinder having low compression but have since replaced the pistons, rings and all gaskets so I don't think that is the issue. I'm still unsure what the issue is though so don't wanna rule out that I may have messed that one up.

Any ideas?

Cheers.
 
I should note as well that I had the cylinder and head bored and honed and the valves checked by a local Toronto professional which lots of experience. I'd be hesitant to think that the valves weren't reseated properly. Though that would explain a knocking sound, the one that I hear on my bike isn't as rhythmical as it would be if it was a piston hitting a valve I think.
 
.
sounds like your spark plug loaded up . . i would di the following.

pull both plugs

do a compression test.

plug the one from the dead cylinder into the plug wire and lay it on the head and turn the engine over and see if it sparks

clean the plug from the dead cylinder.

plug it into the plug wire and lay it on the head and see if it still sparks

put it into the cylinder that was good.

put the plug from the cylinder that was good into the cylinder that was bad.

post results.

the points might have caused this or a flooded carb etc.
.
 
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