Lotus Elan

cam timing marks - where should they be?

PostPost by: Johnfm » Fri Oct 04, 2013 6:21 pm

I have this type of sprocket on my standard cams - I assume a previous owner replaced the stock sprockets.

Where should the markings be for me to confirm cams are correctly timed?

I assume the same place as for the stock sprockets as noted in the manual?

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PostPost by: billwill » Fri Oct 04, 2013 10:22 pm

I can't see where that differs from the standard sprocket?

Image

http://qedmotorsport.co.uk/qed-shop/lot ... iming-case

Although the QED picture does not seen to have the centre (of 3) hole in line with the centre of the ring support spoke, whereas yours does look central.

Be very careful, you could be miles off place; that could break a valve followed by a broken head and broken piston. (I did that in my early naive days). Did you forget to take a digital photo of the sprocket wheel marks before you took them off the engine?

If you are not sure, in your place I would be inclined to go through a proper valve timing exercise with a micrometer dial gauge etc (it's detailed somewhere on this site) and then make sure I make new timing marks on the sprocket wheels to cater for future occasions.
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PostPost by: billwill » Fri Oct 04, 2013 10:46 pm

I've got the 'vernier' sprockets on mine and they were set up by my 'engine man' not me, but in this photo you can just see the three holes and see that the mark is on the opposite rim to the centre hole (where the cam dowel goes) which matches where those pre engraved marks are on yours and the QED wheel, so perhaps the markings are already correct, I see they give two marks one for the inlet cam and one for the exhaust cam.

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PostPost by: Johnfm » Fri Oct 04, 2013 11:41 pm

I haven't taken anything apart - so have not upset the timing.

I just wanted to check the timing while I have the cam cover off.
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PostPost by: billwill » Sat Oct 05, 2013 1:18 am

Photograph it Now at TDC cyl1 compression stroke to establish a base line.

To get valve timing more accurate, you first work out what is needed using micrometer gauges & angle disk etc etc. them you adjust the position of the camshaft relative to the timing chain. Whose position stays exactly where it is; (fixed by the fact that it is a taut timing CHAIN, wrapped around a CHAINWHEEL on the CRANKSHAFT).

You move the camshaft by either using vernier cogs like in my picture above or with offset dowels, which move the peg (dowel) in the camshaft, a small amount relative to the hole in the sprocket wheel. Dowel are available with various amounts of shift.

http://qedmotorsport.co.uk/qed-shop/lot ... us-offsets
Image

Because the position of a taut timing chain is fixed at TDC, it follows that timing marks on the standard sprocket setup can only be out of place by one whole tooth position (10.59 camshaft degrees 1/34 of a circle), which is a lot more than you would normally want it to be.
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PostPost by: Johnfm » Sat Oct 05, 2013 9:37 am

Is the timing mark on the sprockets the short straight line in front of the 'EX'?

And why do both sprockets have IN and EX on them?

I just got #1 at TDC with distributor rotor point at 11 o'clock etc so top of compression stroke.

I would expect to see some sort of timing marks on the sprockets at horizontal lining up with the top line of the head (as shown in the manual).

Mine are not quite. The inlet is about a sprocket (10.6 degrees) more clockwise than horizontal. The exhaust is pretty close. Should I leave it as it is?

(I think I should for the time being as the car runs ok - and wait until I can afford to get a clever bloke to do it.)
Last edited by Johnfm on Sat Oct 05, 2013 9:42 am, edited 1 time in total.
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PostPost by: MickG » Sat Oct 05, 2013 9:39 am

Hi Jon
Top tip
If you haven't already got a one get yourself a copy of Lotus twin cam engine by Miles Wilkins.

Regards Mick G
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PostPost by: Johnfm » Sat Oct 05, 2013 9:45 am

Cheers

Is a 10 degree 'error' a big issue. I expect it would be.
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PostPost by: rgh0 » Sat Oct 05, 2013 9:51 am

It sounds close enough for a road engine like you say. Those last coupe of degrees may be worth chasing when you do an engine rebuild using offset dowels and vernier sprockets.

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PostPost by: Johnfm » Sat Oct 05, 2013 9:59 am

Cheers Rohan

I expect the guy who set it up didn't allow for the slack in the chain with the tensioner backed off.
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PostPost by: rgh0 » Sat Oct 05, 2013 10:07 am

If your a whole tooth out you need fix that!

Generally most motors are a few degrees off perfect when you have the chain and sprocket teeth set correctly. Machining of the head and block and the stacking up of tolerances from the crank up to the cams affecting the distance and thus the timing a little. If your building a race engine where you want everything spot on to get that last horsepower then offset dowels or vernier sprockets enable you to get that last couple of degrees right

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PostPost by: Johnfm » Sat Oct 05, 2013 10:13 am

Simplest way to correct?

I would assume it is to mark up the sprockets and chain with something like tippex, then loosen of the cam chain tensioner.

Then if there is enough slack in the chain, move it one sprocket across and re-tension then check it all again.
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PostPost by: Craven » Sat Oct 05, 2013 10:19 am

Hi
Standard Ford Sprocket is used of Exhaust (I think ) and a special used for Inlet ( I Think ) the sprocket you have can be used as the Exhaust or Inlet as originals are one tooth different in timing mark position . Sorry I can’t remember which way round it is but it could explain the 10 deg error.
For what it’s worth Ron.
Edit especially for billwill
Last edited by Craven on Sat Oct 05, 2013 11:57 pm, edited 1 time in total.
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PostPost by: promotor » Sat Oct 05, 2013 10:45 am

With those cam wheels the inlet cam uses the dot of the cam wheel on the level of the cylinder head, and on the exhaust side you align the line on the level of the cylinder head.
Original cam wheels just have lines on them but the line is either where the dot or line is depending if it's an exhaust wheel or an inlet/jackshaft wheel.

It is unlikely you will ever get the marks to line up exactly on the level with the cylinder head so it always feels like they are slightly wrong whatever you do! The trick is to check everything with a degree wheel and dial indicator gauge, and do piston to valve checks with plasticene if not totally sure you've got enough room.

Setting the wheels to their exact positions with vernier wheels or offset dowels does make a big difference to the responsiveness of the engine but isn't a necessity if just pootling around - it's likely you'll only be 2-3 degrees out at most.

If you have a photo of how everything looks then we can advise more accurately.
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PostPost by: Johnfm » Sat Oct 05, 2013 11:32 am

Good stuff. If it is for on inlet and line on exhaust, all is ok.

One less thing to worry about!

Now off to install dynamator and figure out how to wire it in and lose the regulator.
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