Lotus Elan

Still pulling hair- Twin Cam timing marks. How accurate?

PostPost by: M.J.S » Sat Jun 14, 2008 7:50 pm

Thanks for all the suggestions and advice. Decided to check cam timing.

Workshop manual shows cam pulley timing marks set exactly opposite each other and dead in line with top of front cover.

Have tried moving a tooth on both cams to get everything in line as per the book, but careful and precise though I am, they will not line up like the illustration. The closest they get with bottom pulley on tdc, is about 5mm lower than the top of the front cover, when lined up with a straight edge.

Move a tooth so they line up straight at their outer markings and are also level with the top of the front cover, and the bottom pulley is about 5 degrees out; which suggests a tooth out.

I'm trying to do it without taking the bonnet off

Getting late and tired so have knocked off for now.

Is that illustration wholly accurate and should they be bang in line with the top of the front cover as well as each other, or is it right they do actually sit about 5mm below it.

Perhaps this is why it might have been done wrong originally.

The chain doesn't seem too stretched for the tensioner is less than half way in.

Thanks everyone,

Mark.
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PostPost by: john.p.clegg » Sat Jun 14, 2008 8:35 pm

Mark

Where are you in Lancashire,to do the job properly you need a stick down the plug hole and timimg disk to find top dead accurately and then a dial gauge to set the max lift peak...

John :wink:
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PostPost by: miked » Sat Jun 14, 2008 8:48 pm

Mark,

I have the bits if you want to use them. At Culcheth.

DTI
Disc
Mag base
Also plate to straddle head for mag base.

Mike
:)
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PostPost by: CBUEB1771 » Sun Jun 15, 2008 3:18 am

john.p.clegg wrote:Where are you in Lancashire,to do the job properly you need a stick down the plug hole and timimg disk to find top dead accurately and then a dial gauge to set the max lift peak...


I think that anyone in the area would do well to help Mark set cam timing by aligning hash marks as a first measure. They never line up exactly and any metal taken off the head or block will amplify the misalignment. I do agree that ultimately you want to find crankshaft TDC and cam lobe centers with a degree wheel and a dial indicator. However this information is pedantic unless you have a range of offset dowels, offset bushes or vernier-adjusting cogs to make the fine adjustments. Let's give a new owner some basic help to start and then get more esoteric as he wishes to proceed.
Russ Newton
Elan +2S (1971)
Elite S2 (1962)
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PostPost by: john.p.clegg » Sun Jun 15, 2008 6:23 am

It's early in the morning and not fully awake yet but what if the inlet and exhaust cams were swapped during the rebuild,how much would that put the timing marks out.....would have to go out to the shed to play to find out exact....any offers?

John :wink:
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PostPost by: tdafforn » Sun Jun 15, 2008 7:50 am

I Don't think it would be the first time such a mistake had happened
Tim
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PostPost by: M.J.S » Sun Jun 15, 2008 10:31 am

Thank you to everyone. The response from the forum for a guy with a problem is overwhelming.

Please remember I am just an enthusiastic amateur and specialist tools and timing disks are beyond my experience. I have to rely on factory marks and handbooks.

I have looked at it this morning with fresh eyes. Here is a photo, I hope it comes out clearly.

The bottom pulley is dead on TDC.

You can see the two factory timing marks are facing each other but are not 100% level, as can be seen my the temporary scratch marks I have put above them, which shows the inlet cam mark is approx 4mm below the level of the front cover. The exhuast mark is approx 6mm below the line of the front cover. The illustration in the factory manual shows them both in line with the top of the front cover.

By moving teeth I did manage to get them far closer to how they look in the book, facing each other and much closer to the level of the top of the front cover.

I thought I'd cracked it until I tightened the tensioner and settled everything back in, to find that when the top lined up it threw the bottom out by approx 5 degrees.

Urgggh! :evil:

If I could have moved it a tooth on the bottom pulley it would have been right, but then that would mean it a tooth out on BOTH cam pulleys, and surely that cannot be right.

Perhaps it is timed right and tolerances or wear mean the marks are not where the manual shows them. Even if the head or block were skimmed it would not alter the marks in relation to the top of the cover. Maybe it is a tooth out at the bottom, therefore two at the top.

Perhaps my eyes and brain are deceiving me!
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front timing marks.JPG and
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PostPost by: rgh0 » Sun Jun 15, 2008 10:58 am

Assuming the crank and pulley marks are reasonably accurate (and they normally are to within a couple of degrees) then I would say your cam timing is near enough and not the source of your performance issues.

Unless the cam sprockets have been swapped. The cams are identical but the sprockets are not and should have IN and EX marking under the washer at the front.

I would next get a leak down test done on the engine to determine if any identifiable problems with valves or rings sealing.

If thats OK its got be either fuel or ignition problems and you need to dig deeper into these to find where it is

cheers
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PostPost by: M.J.S » Sun Jun 15, 2008 11:22 am

Hi Rohan,

The old carbs have been replaced by a brand new pair, with new fuel lines and fuel filter, and new air filters.

Dizzy cap, leads, plugs and rotor arm all new.

Dizzy has electronic ignition. Age and model unknown, but it does start first time and appears to advance with the revs as it should. I would hate to spend ?250 on a new electronic dizzy and coil just to fault find if its not the problem.

Think I will have to pull off the head to take a look inside.

Mark.
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PostPost by: rgh0 » Sun Jun 15, 2008 12:06 pm

I would not pull the head off just yet.


Get the leak down test done first - if it shows no problem no point pulling the head

Carbs may be Ok but have you checked the fuel pressure at the point the problem occurs under load. Not unknow for a rust / crud blockage in the inlet pipe in the tank .

I presume you have also checked the distributor advance and are satisfied you are getting around 25 degrees total advance at around 4000 rpm.
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PostPost by: john.p.clegg » Sun Jun 15, 2008 12:10 pm

Mark

Whereabouts in Lancs are you?

John :wink:
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PostPost by: david.g.chapman » Sun Jun 15, 2008 6:34 pm

Just a thought on the compression readings - if they are 120 to 130 psi on all cylinders then either your engine builder missed out one of the piston rings on every piston (unlikely!), or you need to recheck the compressions with another gauge, or do a leak-down test.

This is because the readings are so even. The change of a random valve leak or piston ring problem affecting all cylinders equally seems remote on a low mileage engine from a rebuild.

Don't take the head off until those additional checks are done.

Good luck...

Dave Chapman.
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PostPost by: ppnelan » Sun Jun 15, 2008 7:54 pm

john.p.clegg wrote:...what if the inlet and exhaust cams were swapped during the rebuild,how much would that put the timing marks out...

Camshafts themselves should be the same but the sprockets are dowelled differently - is this what you mean?

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PostPost by: john.p.clegg » Sun Jun 15, 2008 8:07 pm

Matthew

What I was wondering about was the relationship between the lobes and the teeth on the sprockets,I would not think a rebuild would encompass removing the sprockets but the camshafts will have been removed and if not marked up,replaced IN for EX and EX for IN...?

John :wink:
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PostPost by: bill308 » Mon Jun 16, 2008 12:50 am

If this were my engine, I'd go back to basics.

1. Set chain tension to specs. It's best to use a new chain without any stretch wear.

2. Install a degree wheel on the nose of the crank and extablish true TDC with a dial indicator reading off the top of the piston. Readings should be taken at say 0.5 inches BTDC (Before Top Dead Center)and 0.5 inches ATDC (After Top Dead Center). True TDC will be in between these 2-readings. The reason measurements are taken before and after TDC is because it is very difficult to determine exactly where TDC is as the piston transitions through TDC. Reregister the degree wheel on the crank so that 0-degrees is at true TDC.

3. Remount the dial indicator to read intake cam bucket height. Rotate crank clockwise (normal direction of rotation). Note reading at say 0.100 inches lift and 0.100 inches before closing. Peak lift will be in between these 2-readings.

4. Repeat this process for the exhaust valve.

5. You now will know for sure the status of you cam timing. Compare it to your cam specs.

6. If necessary, obtain and install the necessary offset dowel(s) to correct any timing error.

7. Repeat 2 and 3 to confirm the new timing is now correct.

Most mechanics will not do it this way as it is time consuming. It is the most accurate method however. It also accounts for minor valve lash discrepencies and works for all (symetrical) cams.

Elan factory cams are the same for the intake and exhaust. I believe the lobes are also symetrical, meaning the opening and closing profiles are the same. If you feel ambitious, you can plot the entire lift profile for the inlet and exhaust cams with say a data point every 10-degrees for most of the profile and when nearing the point of max lift, increase the number of data points to every 5- or 2-degrees. This chart, makes it very easy to see where your cams are relative to the Workshop manual values for opening and closing.

Bill
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