Topic: Engine noise, help

[72cheyenne]

I have a 1974 Honda 750.  When idling in neutral with the clutch in, the engine is quiet.  If I let the clutch out, it makes a rattling or loose part sound.  I can also hear it while accelerating.  Would the clutch be the likely problem.  The bike has 32000 miles.  I don't mind rebuilding the clutch, just thought I would ask first.

[Lars]

Same with my bike, also a CB 750 K4 and my first - only 10 500 miles. Just had the carbs back on this week and fired it up for the first time. I have the same noise in neutral, clutch in and it disappear. Think this is a normal thing, or what, guys?

Lars

[72cheyenne]

Anybody?

[Markcb750]

This sounds like the primary chain, pulling in the clutch releases some of the tension on the chain, changing/reducing the clatter.


It is not "normal", but I do not believe it is indicative of a immediate failure.

The Cam chain can make a lot of noise also.  Have you done an adjustment?

My cam chain tension mechanism was corroded and would not adjust, I disassembled it from the bike, cleaned & polished the mating surfaces, Then lubed with a good quality grease.

[mystic_1]

I've heard this noise to one degree or another on every 750 I've been around, it's a characteristic noise of the model I believe.  

If the cam chain, primary chains, tensioners, clutch basket, and transmission bearings are all in great shape, there is less noise.  On a worn motor the noise can be quite loud indeed.  Tuning your ignition precisely and syncing your carbs well will also help.

You can measure the primary chain tensioner through the oil pan, there's a spec and procedure in the manuals.  You can remove the cam chain tensioner and make sure it's working properly.

mystic_1


Edit to add:  in my opinion this noise comes from the primary chains "bouncing" against the tensioner, as the crank accelerates and decelerates during normal operation.  With the clutch pulled in, only the primary sprocket and clutch basket are spinning.  When you release the clutch, the clutch center, transmission mainshaft, and associated gears are all brought on-line, increasing drag on the system.  As a result, the crank is slowed more during the "non power stroke" moments, so slack shifts from the lower run of the primary chains to the upper run, then it shifts back again during the next power stroke.

So, the primary tensioner bouncing around, plus any play in the clutch or transmission bearings, all contribute to the noise.   This is all just my wild-ass-guessery, btw, so take it for whatever it's worth.

-m

[Gordon]

Synchronizing the carbs, after a thorough tune-up of course, can help get rid of a lot of the clutch and primary chain noise. 

[72cheyenne]

Thanks guys.  This gives me some places to check.  Since I planned on taking the engine off the bike this winter, that will be a great time to check things out.

Mark

[Johnie]

I hear a lot of guys call this "clutch rattle"...what ever that means.

[ev0lve]

Hmmm, yeah. I was under the impression that my clutch basket was the rattling culprit although the primary, and the reasons stated, makes more sense - I think 

I suppose we need someone who's replaced their primary and subsequently gotten a quiet motor to chime in?

[mystic_1]

I'm installing brand new oem primary chains and primary tensioner into my motor cases later tonight.  New cam chain and transmission/clutch bearings also.  I'll let you know how it sounds when I fire it up, but that won't be for a while yet

mystic_1

[Lars]

Picking up this thread again. First of all - today my CB 750 K4 was approved by the Dep. of Traffic, so finally I got the plates.

Back to the issue: My bike has only 10 400 miles, and I have done all the tuning procedure including the carb synch. Still, there is this noise wich 72cheyenne indicated. I locate this noise to inside the clutch cover, and only in neutral. Clutch in and it almost disappears completely. I do not know this engine that well, but I would not put my 5 cent on the primary chain,  but I could be wrong.

So, my question is: Has anybody had a rattling noise from inside the clutch house, opened it, done something and got rid of the noise? If so, what did you do?

To Mystic_1, had any test yet?

Lars

[wrenchmuch]

My engine has new crank and rod bearings as well as new primary and cam chains and new tensioners . I too have "clutch rattle " in neutral untill I pull the clutch in . Valves and tensioner are set properly . Carbs are synced as close as I can get them with a Morgan carbtune and it still rattles a bit . When warm it improves but never completely goes away . I ran into a thread here once about using later model clutch parts to improve the problem but I can't find it now . Found this though .

http://forums.sohc4.net/index.php?topic=42186.0

I don't worry too much about mine . Earplugs help alot .

[MCRider]

I have a 1974 Honda 750.  When idling in neutral with the clutch in, the engine is quiet.  If I let the clutch out, it makes a rattling or loose part sound.  I can also hear it while accelerating.  Would the clutch be the likely problem.  The bike has 32000 miles.  I don't mind rebuilding the clutch, just thought I would ask first.

This is my opinion drawn from 5 years at Honda shops listening to many CB750s, several weekends at drag strips and road race tracks, and reading everything I've ever been able to get my hands on. The CB750s definitely have a "clutch rattle" from day one, on the showroom floor. If it goes away when you pull in the clutch, it is this rattle. It is perfectly normal and I know of no way to get rid of it.

Whether or not its due to the clutch I don't specifically know. It got the name as it goes away when you pull in the clutch. Some have it more than others, I would pay money to hear one that doesn't have it.

Personally, I've never heard it while accelerating, but i may not have been listening for it, or may have simply ignored it.

Honda did some things to the clutch basket in bikes later than when I was involved, (K3+) but I don't know if that was for the rattle or for other reasons.

Its an annoying sound and I would love to be proven wrong by someone who has suppressed it and how they did it. But till then, that's my story and I'm sticking to it.  

[mystic_1]

I have not fired my engine yet, have top end reassembly to do yet.  Last time I did a rebuild reduced but did not eliminate the problem.

Here's an article of The Wrench that presents a possible fix for this problem but I don't know of anyone who has tried it.  Will likely try this myself at some point.

mystic_1

[Lars]

I had some very positive progress this weekend regarding this clutch rattle problem.

First, I have to say that my K4  - with very low miles - rattled a LOT. More than what I would believe is within the "normal" range. So I decided to adjust my clutch according the the book, even though the clutch worked perfectly well. I followed all steps as described in my owners manual, and after job done I fired up. To my big surprice, almost all rattle was gone, only a little noise which disappeared when I pressed the clutch lever.

For those of you who haven't got the manual, here it goes:

1. At the clutch lever on the handlebar, screw the adjuster screw all the way inn (clockwise), it will loosen up the cable.
2. Remove the chrome clutch house cover - 3 bolts.
3. Loosen the locknut in the center and with a flat iron, turn the adjusting screw clockwise until you feel only a little resistance - turn back 1/4 - 1/2 turn.
4. Lock with the locknut.
5. Adjust the clutch bolt on the lower right side outside the clutch housing where the clutch cable comes, until you have about 3/4" freeplay at the tip of the lever. Fasten locknut.
6. Do the final adjustment on the lever adjusting bolt turning it out anti-clockwise.
7. Clutch cover back on.

[the.barrage.pdx]

Synchronizing the carbs, after a thorough tune-up of course, can help get rid of a lot of the clutch and primary chain noise. 

This is an interesting theory to me.  Can you explain the engineering behind how the carbs being synced would effect the primary chain and clutch enough to reduce noise?  I know that everything is connected, that simply seems too far removed from each other to have any major effect.

[Spanner 1]

You could slice a SOHC4 motor vertically into 4 separate motors and run them ( with fuel to each carb )...
So it's like 4 separate motors fixed to one crankshaft..... equal firing pulses from each 'motor' would be necessary to 'smooth' any chain action in the motor ( primary chain noise is not generally a prob. on SOHC4's IMO )... where it shows-up is in the clutch and the plates rattling with a side-to-side motion in the basket...... listen to a U-Tube of any SOHC4 idling and you will hear it every time... even expensive re-builds... it's totally normal IMO and can only be reduced by a really close carb sync.