Strange noise....

Have just got the bike running again after a year with gearbox rebuild and new rgm primary belt which was adjusted to the 90 degrees twist and 30mm socket test. After probably 50 miles the bike has developed a very strange noise that sounds like stones hitting my helmet hard, The chain tension is correct. It happens in any gear but particular when accelerating hard and demanding torque. It only last about a second but make a right racket that sounds like it could be causing some damage. Could this be the primary belt slapping against the lower chainsaw, it is really strange? I won't be able to pull the cover off now until the weekend but the belt looks OK through the inspection window.

Pop primary cover and see if a nut off or belt slapping but belt slap is rather mild to notice over plain engine combustion and ya see rubber dust collection to verify. Could be clutch rubbing inner case. I have had gear teeth pop off and do this but doubt applys to you. Check chain run. Could be something fouling between isolated frame/stem/tank and whole power unit, maybe headsteady tank tunnel area. Best wishes finding it by easy access.

Fred,

After running a belt primary, and still do, for the past 17 years....

my personal opinion is that too much belt looseness is called for in the installation directions

I set mine up at about 75 degrees twist cold and even that is probably a tad too loose

yes, the belt baskets expand along with the engine to gearbox plates as the motor heats up

and the entire primary heats up and so looseness is taken out of the cold belt setting

you can easily do your own test as I have when I rode over 100 miles at close to 100 degrees here in the desert SW USA with my primary totally enclosed, inspection covers left in place

when I got home I pulled the outer primary cover and checked the belt tension, there still was no sign of being "too tight", the belt still had some lag, and this at 75 degrees twist measured on the top belt rung middle when cold, with the primary fully hot and maximum expansion

you check this by pushing the kick start down with your hand and this will cause the belt slack to be taken out in the upper rung and you can see how much slop you have in the lower rung

so yes you could be hearing the belt slap the bottom of the primary and even if that is not the noise you hear you can still tighten your belt a bit more without any fear of being too tight

Don't be affraid to give the RGM units a little more slack then discribed.
Also, the cover is optional and I say this only in regards to watching to see what is going on in there and NOT as a good idea to run all the time.

I slack my belt so loose that it would jump the crank hub teeth when kicking it over, then tightened it just so that it wouldnt. Been great ever since. After a few years now, I show little to no wear on neither the hub nor the belt. These things are totally forgiving except when too tight.

Fascinating conflicts of good belt tension reports so nothing for it but check enough hot to know for sure then knife edge mark setting to reproduce forever more. Would suck if something in tranny or head.

Get that thing good and hot. Quickly get that primary cover off and check to see how tight the belt is. Simple enough!

Thanks all. I won't get to this till the weekend but I have a feeling it's a nut in the primary that's loose but everything was torqued up properly. The only other thing is I did fit a Dave Taylor head steady at the weekend and a new front isolastic which have made the bike much smoother but......? Could I be anything wrong there?

pete.v said:

Don't be affraid to give the RGM units a little more slack then discribed.
Also, the cover is optional and I say this only in regards to watching to see what is going on in there and NOT as a good idea to run all the time.

I slack my belt so loose that it would jump the crank hub teeth when kicking it over, then tightened it just so that it wouldnt. Been great ever since. After a few years now, I show little to no wear on neither the hub nor the belt. These things are totally forgiving except when too tight.

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+1
A little too lose and no harm is done. I doubt that you are hearing belt slap.

A little too tight is instant disaster when everything gets hot. Jim

Keep in mind only takes one over tight event to bend mainshaft some so may not be catastrophic as comnoz example but everything will tend to wear out a lot faster and never quite right to seal or keep clutch support bushes stable long. Creep up on safe tightness not down from risky tension. I've spent years investigating proper belt tension with experts and all said 90's twist in longest run cold is spot on. Well supported and robust shafts can run tighter initial tension so nothing fixed in stone as long as shafts/cases not put in over bind. Only one way to know for sure.

FastFred said:

Thanks all. I won't get to this till the weekend but I have a feeling it's a nut in the primary that's loose but everything was torqued up properly. The only other thing is I did fit a Dave Taylor head steady at the weekend and a new front isolastic which have made the bike much smoother but......? Could I be anything wrong there?

Click to expand...

New head steady with a LOOSE bolt will rattle a bit.. Might be worth a revisit!

I'd look at the head steady. You may have to move the frame clamp slightly to avoid hitting the head (which jumps around quite a bit). This will make the link rod not quite the ideal 90 degrees to the frame but it really doesn't matter that much.

It was the belt way too slack. How it got like that after so little distance I don't know but there was no debris in there. All is well now but it must have been the noise of it jumping teeth or slapping the casing.

FastFred said:

It was the belt way too slack. How it got like that after so little distance I don't know but there was no debris in there. All is well now but it must have been the noise of it jumping teeth or slapping the casing.

Click to expand...

I'm not so sure about running the primary belt so slack as others suggest. No way I can twist upper rung on belt 90 degrees. Less. Enclosed primary cover no ventings. New sleeve gear bushings too. Perfect and lovely after a good 1000 kms. Off to Monteal Jazz fest with Wife on back n' minimal gear feel confident. See ya in Cookstown just south of after. :|

Link rod of head steady maybe moving laterally enough to be striking side of tank tunnel.
If so tank should show some abrasion.

His belt is slopped out.