Lotus Elan

T/c Timing Chain

PostPost by: Gra-ham » Thu Oct 23, 2003 3:23 pm

I have an issue with a rebuilt Twin Cam in regard to the camshaft timing sprockets and the chain tensioning sprocket .
The new chain I installed roars quite loudly and can not be tightened to the normal tension with out excessive noise. The old chain did not make any noise but did have some wear, it was about 25,000 miles old. The new chain is standard and measures the same as the old.
The problem is that the tensioner sprocket is running about 40 thou plus in front of the camshaft sprocket and as such the chain is wearing the front of the tensioner sprocket and the rear of the inlet cam sprocket. There was some wear with the old chain but it is now quite obvious.
This is compounded by the fact that the rear of the tensioner sprocket carrier is hard up against the head.
I checked the carrier before installing it and the sprocket was 90 degrees to the pivot shaft.
If I remove metal from the rear face of the carrier, then if the sprockets are in line the chain will be running against the lower edge of the head.
Any suggestions?
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PostPost by: gjz30075 » Thu Oct 23, 2003 4:31 pm

Any wear on the plunger? Any wear on the sprocket carrier? The carrier is brass and if the plunger has any wear on its tip, it could skew the carrier. I've also seen carriers bent up from abuse and its not obvious until its in the head and on the motor and the chain then will show the mis-alignment.

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PostPost by: elan_fan » Thu Oct 23, 2003 6:45 pm

Who did the rebuild? you :D or someone else :( as i'm wondering if the small spacer that goes between the jackshaft and it's sproket is fitted? or is the tank gear on the tensioner the wrong way around? my head is off at the moment if you need measurements.

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PostPost by: types26/36 » Fri Oct 24, 2003 9:14 pm

Graham,
Sorry but I don't quite understand what you are saying, you say the tensioner sprocket is in front of the cam sprocket but is hard up against the head on the back of the sprocket, maybe I'm reading it wong but is the tensioner sprocket too far forward or backward?
Brian.
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PostPost by: type26owner » Fri Oct 24, 2003 9:53 pm

The tensioner should be free to slide laterally on it's pivot pin and accomodate the chain position as defined by a flat plane comprising the jackshaft and intake camshaft gears. If the tensioner is shifted forward of the engine and is bearing against the housing. You might have had a major failure of the thrust plate which secures the jackshaft axially in it's bearings. You might be able to remove the dizzy or oil pump and gently pry on the drivegear teeth of the jackshaft and look for any motion before tearing it down for an inspection. Another dead give away is if the dizzy has a huge amount of scatter. I've had this type of failure before on a race engine but it was probably caused by us purposefully running excessive oil pressure. Good Luck.
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PostPost by: JACKJABBA » Sat Oct 25, 2003 7:30 pm

Keith (type26owner), what exactly do you mean by scatter, what are the symptoms?

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PostPost by: type26owner » Mon Oct 27, 2003 12:50 am

Scatter is a term which describes a timing problem were the timing is departing from the correct curve in an erratic way. Mechanical wear is the usual cause and the diagnosis is observing the timing marks with a timing light and looking for the telltale 'scatter' of the timing. If the dizzy and jackshaft are in good condition the timing mark will not vary by more then +/- 1/2 a degree at any steady rpm.
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PostPost by: Gra-ham » Tue Oct 28, 2003 5:46 am

Keith, Brian
The problem is the tensioner slides on the pivot pin but when it is hard to the rear, ie against No.1 cylinder, the sprocket is running about 40 thou in front of the jackshaft and camshaft sprockets plane. It cant move back to get the chain running in the one plane.
I reassembled the motor myself and all the parts are in the right place regards spacers etc. The tensioner was not pulled apart.
I had heard that they could bend but checked as best I could before putting it back in. There is some wear but not enough to allow that amount of movement.

Regarding scatter the jackshaft tries to throw the distributor driveshaft away from itself and thus the dist. shaft bearings wear a lot. I have this problem and it causes an uneven idle. If only the dist. drive was end on like a Mini or my 260Z!

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PostPost by: type26owner » Tue Oct 28, 2003 3:31 pm

Gee, could it be something really obvious like the timing cover cork gasket slid out of place and is bearing against the tensioner pivot arm. Cork doesn't cold flow and the contact area would about an inch long at that location. The crush strenght of it I'm guessing would be at least five pounds or more. The chain would not have the mechanical advantage to crush it either. Now that I think about it I've had this very thing happen to me too way back. :angry:
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PostPost by: types26/36 » Wed Oct 29, 2003 2:02 pm

Graham,
The misalignment by 40 thouth does not sound a great amount in the great scheme of things (and certainly not desirable) but are you sure the noise is from the chain? and not due to somthing else?
Is there any marks on the head/cover where the chain is catching? the screws holding the sliper are not coming loose?
I have seen a 1600 block used where the chain was fitted around the pin in the front main bearing cap (its used for the chain tensioner in an Xflow) and it didn'nt make that much noise! ........NOT DONE BY ME!
I have also seen bent brass carriers but visible just by looking at them, maybe yours
is slightly distorted, try to borrow one to test. Its not to difficult to fit and then you
should know.
Good Luck, Brian.
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