Topic: idle sound

[evinrude7]

my bike has always sounded like a giant bag of bottle caps being shaken when it's idling.  i've heard other 750s sound the same.  some better, some worse.  am i working towards something that i'll never achieve, quieter idle?  i know it's a noisy engine to begin with.

i can get it to idle down to 1000 rpm when it's good and warm.  it will hold that fairly steady.  she's much happier around 1200-1400. 

reason i bring this up today is i had never had the cam chain tensioner off until today.  wanted to see if anything was stuck or if the ramp had a worn spot from the set bolt.  all looked pretty good so i lubed it and put it back on the bike.  tensioned the chain at 15 atdc.

carbs are synced.  timing and valve tappets look good.   i run 87 non-ethanol pretty much all the time unless i'm low on fuel and no where near ethanol free gas.  rotella t4 oil. 

thanks for the laughs.   

[Johnie]

You are probably hearing what we call clutch rattle. These bikes were born with it. If you pull in the clutch the sound decreases. I believe there is a bulletin regarding this and adding an additional clutch plate to quite it down. I live with it and it does not bother me. Can't hear it when riding, I have the clutch in at stop lights so it's not that loud.

[TwoTired]

The large power transfer gears are always in mesh and turning with the crankshaft via the primary chain.  But, they only transfer power when they are locked by sliders onto a shaft.  The gears have contact faces fore and aft and only one side of each is involved in any power transfer.  The teeth do not have zero clearance fore and aft.  They change contact fore or aft when under power or in deceleration.  This occurs with throttle application and is seldom noticed on the highway.

However, if your idle speed "hunts" at all, the gear contact faces can alternate and make a sound when changing, that I call gear clack, clack, clack.

You have a four cylinder engine and during idle, each cylinder fires in turn to maintain an average idle speed.  But, if each cylinder doesn't fire with equal strength, minute changes in the rpm can transfer to the meshed gears in the trans and cause the gear faces to alternate contact between weak and more strengthened power pulses within the cycle of four pistons firing.  There is a mass harmonic effect that can change in intensity, by adding or subtracting the clutch discs, to the total mass.  This makes you hear a change in pitch or intensity when you add or subtract it's coupled mass to the power train.

The gear train noise you hear is interactive with how balanced each cylinder fires at idle.  This includes, vacuum balance, actual compression of each cylinder, and the equality of air/fuel mixture delivered to each cylinder.

When new, everything was pretty even, and there was little wear or imbalance between the carb's components.  That was 40 plus years ago.  Is cam wear even? Makes a difference to delivered air/fuel charge.  Do the carb slides still have equal clearance in the bores?

Lastly when all tune up items and mechanical clearance are identical among cylinders, the last bit of tuning for equal power pulses are the idle mixture screws or the pilot screws, both of which effect the air/fuel charge being delivered to each cylinder.  These can be varied among them to tailor each cylinder to fire with equal strength.  When new, they could all be set to the very same setting.  Still can today if you wish to put up with gear clack.  But, if reducing this clack interests you, tweak those pilot/IMS screws differentially to gain a smoother idle.

My experience is more prevalent with the Cb550s.  Seems the 750 has similar designs, though.

Well, I'm temporarily out of wind.... 

Cheers,

[evinrude7]

The large power transfer gears are always in mesh and turning with the crankshaft via the primary chain.  But, they only transfer power when they are locked by sliders onto a shaft.  The gears have contact faces fore and aft and only one side of each is involved in any power transfer.  The teeth do not have zero clearance fore and aft.  They change contact fore or aft when under power or in deceleration.  This occurs with throttle application and is seldom noticed on the highway.

However, if your idle speed "hunts" at all, the gear contact faces can alternate and make a sound when changing, that I call gear clack, clack, clack.

You have a four cylinder engine and during idle, each cylinder fires in turn to maintain an average idle speed.  But, if each cylinder doesn't fire with equal strength, minute changes in the rpm can transfer to the meshed gears in the trans and cause the gear faces to alternate contact between weak and more strengthened power pulses within the cycle of four pistons firing.  There is a mass harmonic effect that can change in intensity, by adding or subtracting the clutch discs, to the total mass.  This makes you hear a change in pitch or intensity when you add or subtract it's coupled mass to the power train.

The gear train noise you hear is interactive with how balanced each cylinder fires at idle.  This includes, vacuum balance, actual compression of each cylinder, and the equality of air/fuel mixture delivered to each cylinder.

When new, everything was pretty even, and there was little wear or imbalance between the carb's components.  That was 40 plus years ago.  Is cam wear even? Makes a difference to delivered air/fuel charge.  Do the carb slides still have equal clearance in the bores?

Lastly when all tune up items and mechanical clearance are identical among cylinders, the last bit of tuning for equal power pulses are the idle mixture screws or the pilot screws, both of which effect the air/fuel charge being delivered to each cylinder.  These can be varied among them to tailor each cylinder to fire with equal strength.  When new, they could all be set to the very same setting.  Still can today if you wish to put up with gear clack.  But, if reducing this clack interests you, tweak those pilot/IMS screws differentially to gain a smoother idle.

My experience is more prevalent with the Cb550s.  Seems the 750 has similar designs, though.

Well, I'm temporarily out of wind.... 

Cheers,

You are probably hearing what we call clutch rattle. These bikes were born with it. If you pull in the clutch the sound decreases. I believe there is a bulletin regarding this and adding an additional clutch plate to quite it down. I live with it and it does not bother me. Can't hear it when riding, I have the clutch in at stop lights so it's not that loud.

thanks for the feedback TT and johnie.  i'm sure at 39k miles, on a bike that wasn't very well taken care of, i have some worn engine parts.  perhaps the carb tuning could be a bit better.  maybe i'll revisit that soon.  i'm not completely averse to the noise however it does make me think something is not exactly right.  other than idle the bike pulls hard and at the higher rpms she sings.   

[Paintedseat]

Ride a Ducati with a dry clutch and you won't hear the cb750 any more.

[evinrude7]

yeah painted seat the 2nd bike will be a modern one although not a ducati.

btw: clutch disengaged or engaged sounds the same.     

[jaytee-nz]

My 1974 CB750 has done about 32,000 miles and is original other than a new set of rings. It rattles just as you describe and the noise reduces by 50% when the clutch is pulled in. My local factory trained Honda mechanic puts the remaining noise down to the original and now stretched primary chain.
It used to bug me a bit but like Johnnie says - you can't hear it at stop lights so I'm happy to ignore it.

[PeWe]

Using points?
Both points must be set equally. Only way to do that is with a dwell meter. Set both to same numbers of degrees and the idle will most likely improve.
1200 rpm will ensure oil pressure and easier to ride when decelerate by the throttles, not that sudden stop.

[evinrude7]

Using points?
Both points must be set equally. Only way to do that is with a dwell meter. Set both to same numbers of degrees and the idle will most likely improve.
1200 rpm will ensure oil pressure and easier to ride when decelerate by the throttles, not that sudden stop.

yes i need to get a dwell meter. 

[PeWe]

I use a rather cheap automotive multimeter with dwell function. Useful for measure volt, resistance and buzzer when checking the wiring, battery etc
http://images.biltema.com/PAXToImageService.svc/byfilename/large/15-292_l_1.jpg

[Erny]

I just recently bought strobe light with built in dwell meter and rev counter. Did not tested yet. But I have cheap multimeter with these functions too (not teste yet too).
Waiting for carbs to come after cleaning and then full tune-up

https://m.ebay.com/itm/AccuSpark-BLACK-Professional-Timing-Light-With-Digital-Advance-and-Rev-Counter/111945334787?epid=1753653085&hash=item1a10764003:g:YhcAAOSwMqBaaxGu

[dave500]

I got two strobe lights,saves having to switch back and forth to check even on twins.

[strynboen]

make a good vaccum cyncro of the carbs..it vill silence the engine..and clutch rattel..

[dave500]

don't try and idle too low,the stock tach is unreliable just set idle so its nice n smooth,harleys must have a contest to see who can idle the lowest?check compressions if way uneven syncing can be almost impossible.

[PeWe]

Another method is to use your ears
Set both points with feeler gauge, 0.35mm (.0138")
Adjust only one of the points while engine is idling until you here the bike will run much better.

You will learn that it will change when tighten the screws. Do it again with correction that will end-up perfectly when tightened.
It's a huge difference when both are equal.

This difference must been why some guys gave up points and installed Dyna or similar.

[dave500]

or boyer ignitons?kick those points to the curb!get the most out of your engine.

[evanphi]

Its a feature, not a bug.

Slightly stretched primary chain, combined with a rattly clutch, ever-so-slightly-out-of-sync carbs, and probably not properly adjusted points will all have a wonderful effect on this rattle sound. It is pretty rare you're just leaving it set in Neutral anyway (stay in first at a light!), so I wouldn't concern myself.

[dave500]

nice avatar evanphi!

[evanphi]

here's the full size for those who can't see so small.

[Erny]

Concerning points

Nowadays it is much easier to make proper settings. I can tell you that in mid 90 I had 2-stroke Jawa 350 type 634, that had points and 6V power system. No strobes for 6V available at the time, and Jawa never had any marks on shaft to show proper point for ignition fire...
So I had to use measureing gauge inserted into spark plug hole to set proper position and then to set point. And having 2 cylinder, mechanical tolerance everywhere... At the end, cylinders were never set equally. Carb was just 1 common for both.


Now on CB750K, having 12V, dwell meter, strobe light, marks on shaft. It is so much easier even with points.
But honestly I'm thinking to replace point by Dyna-S or something simular, just set once and forget.
On my other Jawa I did replacement of stock point system by electronic, combined with 12V generator. Set once and run superb w/o any futher tuning. Pity it was not available 20years ago..

Attached Jawa points (mounted on generator)

[PeWe]

It is a nice challenge to get it to run fine with almost stock ignition (Dyna coils).    Thanks to this forum, I know how to...

[tlbranth]

When my CB750 K0 was new it purred like a kitten at idle. Now, with 60k or so miles on it, the idle is exactly what you describe Evinrude. Floats are set, timing is nuts-on, valves adjusted, carbs synched, Hondaman ignition .........but it runs like a top. I get 55+mpg and it runs great and never misses a beat. So I've learned to accept a noisy, nasty idle where I have to give it throttle to keep it from dying. It's the run that's important - not the idle.

[evinrude7]

thanks for all the feedback.  i did go in and file dress my points yesterday because the gaps were getting a little small.  sounds a bit better.  i'm not terribly worried about it.  more like, am i missing something as far as tune.     

[HondaMan]

Is this a K6 with the double-spring clutch plate in the stack? Many of the K6 (and some K5) got the same clutch as the F0 bike. This has one double-steel plate in the stack and the top fiber plate of the bunch has slant-cut (if OEM) faces on the corks, with slightly wider outer fingers on that one plate. This is the "slipper" plate.

If so....this slipper plate often wears the rivets that holds the two plates together. This makes the whole clutch rattle at low engine speeds in Neutral. The simplest fix is to replace the slipper plate on the top of the stack with one that has the square-cut corks (PartsNmore) and replace the double-spring steel plate with the bottom plate from the Gold Wing GL1000 or GL1100 clutch pack (Honda part), which is nearly as thick as those 2 plates added together. Put this one at the bottom of the stack when removing the double plate, which will be either right behind the slipper plate, or 2nd steel form the bottom of the stack, if still OEM.

Another rattler, if high miles, can be from the primary chains. This one can be diagnosed: if it doesn't do it when cold, but does do it when good and hot, it may be the primary chains have worn more than 50% of their total life span. The one near the clutch gets longer than the other one, and they make each other mad at idle, so they argue...

[PeWe]

One more detail that make the ignition adjustment almost impossible is when the ign plate has a play sideways when seated.
If you adjust points perfectly, set ignition perfectly and recheck points again you will find that points need to be adjusted again and when that is done ignition has changed, points again and ....and.... and....

Adjust ign 2:3 will also move point in-out and gap is now changed   Set dwell (gap) again

I noticed that on my bike. I loosened all 3 bolts holding the plate and hammered with a small metal hammer on top of the ridges sticking out on the cases holding the plate. When hammer correctly these will swell and cause a tight fit of the plate that will no longer go sideways when adjusting the plate (1:4 ignition).
Best if all 3 ridges can get same treatment that will ensure plate mounted perfectly centered. If not the points distance to cam will change when rotate the plate. 

An electronic ignition will most likely not bother if plate change sideways a little, not perfectly centered, only points that get different gaps and horrible idle as a result.
But, points are possible to adjust