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RASPING VIBRATION; CAMBELT PROBLEM? - Engine/Ancilliaries - The Lotus Forums Jump to content
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RASPING VIBRATION; CAMBELT PROBLEM?

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Grrr; if it's not one thing its another!

Having sorted out my carb problem I now find that as the engine passes through about 2500 revs (either revving up or down) there is a really nasty rasping vibration.

More by luck (?) than judgement I noticed that the rasping coincides with the cambelt also "vibrating"/ very noticeably moving "up and down" at high frequency on the run between the auxillary and inlet cam sprockets.

I set the tension using a Krikit Gauge to 55 lbs cold (which is correct) and when I had it apart, the cambelt tensioner bearing was fine.

... any ideas?

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I set the tension using a Krikit Gauge to 55 lbs cold (which is correct) and when I had it apart, the cambelt tensioner bearing was fine.

When you set the tension. Was it a brand new belt? Who was the manufacturer?

Andy

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As a rule of thumb a 'whining' cambelt is too tight and an overun vibrating cambelt is too slack - I dont know anything about a Krikit, but why is 55lb correct, thats much lower than Lotus spec approx 95lb IIRC?

I checked the calibration of a Burroughs gauge by cutting an old cambelt, attaching it to the ceiling and suspending the appropriate weight from the other end. No idea if thats a sound principal, but it worked for me.

cheers Steve

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Bob

I had exactly the same (and I do mean exactly) on mine a while back, and it was a slack timing belt. You may very well have set it up correctly, but what if the tensioner has become damaged or the lock nut slack since?

Recheck it, you don't want it jumping a tooth on the cam gear.......


Regards

Mat

post-1-0302470001278592957.jpg

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Not that I am an expert but I have experienced what you are seeing. On the early G car the tensioner uses a spring while the later style is mechanical (solid ). In the Lotus book there is two mentions on how to adjust belt tension with the spring style. 1.) Screw in the adjuster nut with abut two threads showing and tighten the lock nut. 2.) be able to put a .016 feeler gauge between tensioner and belt (how? I don't know). According to my engine builder the HOT engine tension is what you need to look at. As the engine expands belt tension is going to increase. Too tight and the belt wears out , too loose and it jumps around. At a cold temp I adjusted the tensioner to where a little was left on the plunger to go into the housing. Once warm I checked it again to see if it still had some play. Too tight and the plunger is going to ride on the adjuster. So far it has worked fine and the loose vibration in the belt is gone.

Also I the past with other cam belt cars I have used the 90 degree method (COLD). Once you set the belt tension, on the longest length of belt run see if you can twist the belt 90 degrees. If too tight won't do it, too loose goes further. Never had a failure doing it this way.

Jeff


www.espritturbo.com

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as above... if it flaps its too loose if it whines its too tight... and change the belt every 12 months...

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Many thanks for your advice guys.

It's a brand new belt supplied by SJ; can't recall the belt's manufacturer.

I read from an earlier post last year that referred me to an even earlier post from a chap called Tim Engel/s (I think) that a reading of 55 lbs on a Krikit KR1 gauge is equivalent to the appropriate reading on a Burroughs gauge.

I used my Krikit at 55 lbs on my last Esprit (the one that decided to incinerate itself a couple of months ago - the insurance company are starting to be a pain in the arse about my claim!) and that seemed to be fine; no vibes at all.

Either way, I'll check it again.

For the sake of good order however, when measuring the tension COLD,what should the tension be, .... in lbs, rather [burroughs units]?

Ta very much.

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Bob,

As far as I know there has never been a belt tension reading in lbs. for the Lotus 9XX engine. The readings for the Burroughs and Kriket are in units and 95 Burroughs units equals aproximately 52 Kriket units.

The timing should be adjusted/checked on a cold engine and is stated as such in the Service Notes. If it was adjusted hot then it will be too loose when cold. Basically you want it as tight as it can be but yet doesn't whine when hot. Whining = too tight, fluttering=too loose (your problem).

The belt should be adjusted to 52-55 Kriket units on a cold engine with the engine at Top Dead Center (TDC).

52 units on a used belt and 55 on a new belt to allow for some stretch.

Cheers,

Jim

Edited by lotus4s

1995 S4s

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Now the only problem is that abelt changes as soon as it's used, so standard tension settings applied to a (very slightly) used belt will result in it being too tight, so you may want to measure the current setting when cold, and slightly increase it, but keeping below the setting for a brand new belt.

Andy

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i had the same problem on a belt change a number of years ago, turned out that the belt was too loose.

I now have a Burroughs gauge and the correct reading is too tight i.e belt whines when engine cold, so i now set it with the Burroughs gauge then adjust so that the belt just whines when engine at operational temp.

three belts under this procedure and no failures or wear on the belt or pulleys.

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Succes at last; tightened the belt and we're back on the road again!

Many thanks for everyones' help; much appreciated.

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