750SF cam chain tensioner question

TeddyG

TeddyG

Hero member
Location
Portland Oregon
Greetings,

Ever since I received this bike, I've heard a slight repeatative chain-like sound inside the motor, and noticed that the lock nut on the plunger set bolt was loose, and loosening the bolt itself didn't free up the plunger. I removed the tensioner and see the nylon wheel is chewed up some, and there's absolutely no "spring" to the tensioner when I press the housing together. Shouldn't there be some "spring" so that the tensioner is self adjusting to the cam chain? And is the nylon wheel available from another source?

Thanks!
 

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Not sure what the 'chain-like' sound you describe might be - is it rhythmic? Can you be more specific?

That wheel is fine, Ted. They wear naturally from chain contact and will look a lot 'worse' after another 20,000 miles! Fitting a new one just means your motor will receive another dose of nylon shards as it beds in.

The spring isn't strong, but it's definitely more than 'none' - and you don't want to put excessive tension on the chain, or that wheel will quickly disintegrate. My suggestion is remove the pinch bolt and extract the shaft completely. Then, with no spring in place you should be able to freely move the shaft fore and aft without any more friction that that created by the sealing o-ring inside the tensioner body - if there's any sign of oil leakage now's the time to fit a new o-ring.

With the spring fitted and the pinch bolt loose you should have to use strong palm pressure to compress the spring completely. Once compressed, pinch up the bolt lightly and refit the tensioner. Proper tension is obviously achieved when the front part of the cam chain between cam and crank is under its highest tension. My method is to fit the tensioner, release the pinch bolt, and then rotate the motor fwd (plugs removed) while feeling the plunger move; when it's depressed inward (trying to avoid oo-er, fnaar fnaar comments here :cool: ) as far as it will go - without pushing on it - pinch up the bolt and locknut - job done.

Some new cam chains are very tight causing the tensioner to be under excessive pressure when fitted - to be avoided! If the head is fully torqued down even a new cam chain should allow the tensioner to be fitted and pressed against the barrel face without much hand pressure. With a new chain the plunger may not move at all when the pinch bolt is released. The rubber block that the plunger head contacts also wears/spreads with time. If you force the assembly in with the four bolts it will damage this rubber as well as everything else.

It's a super-simple mechanism.
 
The rubber dampener part tends to harden over time, if it is rock hard order a new one and replace it when you receive it, don't need the bike off the road waiting for it.
 
Thanks guys,

I now see how the spring loaded plunger works after releasing the locking bolt and the rubber damper is fairly hard. The plunger may have been frozen in place and not fully pressing against the cam chain which I've now lubed up for freer travel.

The internal engine noise has been a rythmic slight "loose chain" noise, coming from the upper center section of the motor- which I suspect could be from a loose cam chain. I've checked the valve clearances twice which are in spec.
Tippie, I tried your finger on the spark plug hole while advancing motor to TDC, and feel pressure as motor advances to TDC, but don't feel much vacuum pressure as motor passes TDC.
Due to the lack of engine's power I've outlined in another posting, I probably will remove cam box cover to check the cam timing.
What is proper compression- I'm seeing 130 lbs each cylinder-
 
Your noise from the top end could well be pitted/dimpled valve tops- you can set the valve clearance with feelers but the two contact patches are way out and seems quite common on these motors. Noise seems to manifest itself around the 3500/4000rpm region. You can check by whipping out the tappet screw adjuster and checking valve tops.
 
Exmoor beast said:
Your noise from the top end could well be pitted/dimpled valve tops- you can set the valve clearance with feelers but the two contact patches are way out and seems quite common on these motors. Noise seems to manifest itself around the 3500/4000rpm region. You can check by whipping out the tappet screw adjuster and checking valve tops.
Click to expand...
Very common. I've not had to suffer that annoying trait for decades after I fitted the Rotax tappet adjusters. I've built probably 5 motors with this mod. Been discussed on here a number of times - beware of imitations.
 
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