Lotus Elan

Camshaft Positioning

PostPost by: david.g.chapman » Mon Nov 19, 2018 7:15 pm

I am looking at my camshafts after taking the cover off to check the valve clearances. I have Burton power camshaft sprockets fitted which have both IN and EX engraved on them, so they are interchangeable. I set the cam position last time up to get the mid point of the letters to indicate the timing mark.
Now looking at them again I can see a faint line next to the EX and a faint dot next to the IN. I then looked at the Burton power website and these marks are quite clear on their photo of the sprocket.
So I have been running with the inlet camshaft too far clockwise (as viewed from the front) by nearly a tooth.

A couple of questions:
1. The rear inlet and exhaust cam lobes are not quite in mirror postions at TDC - probably because of the timing mark confusion. Should they normally look exactly the same when the timing is reasonably right?
2. What effect would the inlet cam being a tooth out have on engine running?

Dave Chapman.
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PostPost by: nmauduit » Mon Nov 19, 2018 9:47 pm

regarding 2) a tooth is 1/34*360=10.5? at the camshaft, so about 21? at the crankshaft : regardless of other timing inaccuracies (including true TDC vs. the pulley mark, which may or may not have partially compensated that) this is quite far off (more advance on the inlet timing) and would not let the engine running as one would expect (esp. at high rev I would guess, since inlet would close too early preventing a good cylinder filling).
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PostPost by: Chrispy » Tue Nov 20, 2018 1:32 am

Best off getting a dial indicator and dialling in the cams properly. Cam opening and closing times are in Miles' book and I think the workshop manual. You can also check the mark on your crank pulley for accuracy.
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PostPost by: david.g.chapman » Tue Nov 20, 2018 8:17 am

Thanks to both of you for responding.
Dialling in the cams is a good idea, but involves lots of bending over - not good at the moment. I will try the inlet cam back one tooth and see how my discovered marks line up at tdc.
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PostPost by: alan.barker » Tue Nov 20, 2018 9:09 am

I have always just used the engraved lines on the Sprockets to set Valve Timing and never had a problem.
Of course with the Sprocket marked "EX" on the Exhaust side.
I'm talking about Standard Cams and not reprofiled ones or High Performance :wink:
Alan
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PostPost by: david.g.chapman » Tue Nov 20, 2018 9:39 am

Yes, the standard sprockets just have a line mark, and a hand engraved ex somewhere on the exhaust sprocket. I changed mine a few years ago. The Burton Power ones have all the engraving around the rim.
I now have the inlet cam out, and will be looking in my stock for a thinner shim as inlet 1 clearance is down to 3 thou.
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PostPost by: alan.barker » Tue Nov 20, 2018 11:02 am

Hi Dave,
good job you saw it was only .003" . Strange for an Inlet which run a lot cooler than Exhaust.
The Exhaust tend to change much more. I try to set the Exhaust towards the top of the limit to try to reduce the chance of the Seats burning.
Alan
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