TC Valve timing

[Fotog]

Upon removing the cam cover and having a look, I find that the timing marks on the cam sprockets are not aligned as I would expect.  From some reading of old posts I've done here it seems like this isn't always as exact as I might have thought, but mine's mis-aligned enough for me to be concerned.

On my car the marks are on the rear-facing side of the sprockets, so can readily be seen.  I hope that there's no problem with the sprockets being assembled like that, as it seems that it isn't the norm.

When the flywheel is at TDC, both marks are about 1/8 in. below the level of the top of the head.  If I set things so that the mark on the inlet-cam sprocket is even with the top of the head, the exhaust sprocket mark is below the level of the head by almost a full tooth of the sprocket.  See the picture, in which I laid a straightedge on top of the head and set it to touch the exhaust sprocket.

When reassembling after correcting valve clearances, should I rotate that exhaust cam one tooth CCW?  What effect might that have on how it runs?

The thing has some sort of Iskenderian cams in it (P.O.).  As it is, it doesn't idle well until it's warmed-up, then runs fine; feels powerful and free-revving.  I don't have the impression that they're anything too radical.

I don't know much about cams.  I sent the info scribed on them to Isky and inquired, but they don't answer.  I'll have to call them when I imagine they'll be there.

Vince

[Bainford]

Do you have the cams positioned so that the sprocket marks are facing each other, or pointing away from each other? The marks should be pointing towards each other.

[Fotog]

Yes, they're both pointing toward each other (as close as possible, since they never actually point toward each other; that being my issue.)
Vince

[jbcollier]

The engine is an unknown so there are a lot of variables we don't know about.  First, I have never seen Lotus TC timing marks that line up perfectly.  Second, the engine is an unknown.  Shaved head, worn chain, different block, etc can all make a difference in where the timing marks line up.

What to do?

First look at the timing chain adjuster.  All the way in?  Chain and maybe sprockets are worn.  It won't make much difference for your issue but it is an important wear item to check.

After that, it ran well when warm, really well by your description.  So, if it ain't broke, don't fix it.  Put it back where it was and enjoy it.  It's normal for a high performance cam not to be happy at low speeds when cold.  Some race grinds don't idle at all.

No good enough?  Won't sleep nights?  Then screw the marks and get a degree wheel and dial gauge and check and see what the valve timing actually is.  Contact Isky for the specs and set it up as per their recommendations.  Offset woodruff keys are available to make adjustments.  I would assume someone makes vernier cam sprockets as well, if you are totally crazy.

On Maserati V8s and V6s, there are no timing marks.  The sprockets are infinitely adjustable and you set everything up with a degree wheel and dial gauge.  First you find TDC exactly and then you go from there.  Funny thing from back in the day.  The torque spec for the V6 sprockets was too low.  Normally not an issue as you don't spend much time at the redline.  However, people would go out and see what their top speed was.   Long straight, flat out, inching higher and higher when "WOOF" the nut would loosen and bend all the valves.  The spec was changed to hit the special spanner with a BFG until it doesn't move any more!  No problems after that.

[EuropaTC]

The engine is an unknown so there are a lot of variables we don't know about.  First, I have never seen Lotus TC timing marks that line up perfectly. 

Yep, I'm 100% with John on that one. I've got 2 TC engines and neither have cam sprockets that line up perfectly and yet both are within spec.  The Elan has a standard head converted to big valve & spring cams but has only had 2 skims to clean it up over it's life. So the head depth is well within spec. The Europa has had less than 0.005" skim, in fact the request was to take off the minimum to ensure it was flat. (after the water pump went and it overheated). That's got the original cams/sprockets it came with.

I've had old chains, fitted new ones and it made no difference to whether the marks lined up, not even before the Europa head was cleaned up. I wouldn't be surprised to find there are TC engines out there which do align perfectly but I've never had one that's perfect to the degree.

If you think about it, Lotus have a 0.040" difference between the small valve & big valve heads to give the higher CR. The BV head has a 0.010" allowance for skimming. so they are giving 0.050" between top & bottom specs which means a 0.050" potential reduction in the distance between the crank & cam sprockets. The width between the cams and the chain pitch are fixed.

In an ideal world you might manage to spread that 0.050" difference in chain length evenly on both sides but because the cam width dimension is fixed, you're on to a hiding to nothing if you want the sprockets to align perfectly without using either offset dowels or adjustable sprockets.

Miles Wilkins, in his famous book on the TC engine, devotes a whole chapter to cam/valve timing and makes a big thing about using offset dowels/timing discs. IIRC he wrote that deep head skims (presumably below Lotus specs) would mean you could be a couple of teeth out on timing and get inlet valves hitting the pistons.

I just accept the workshop manual with it's perfectly aligned sketch is a work of fiction.....   

Brian

[buzzer]

The best way is to check the cam timing by looking at the valve lift. First you need to identify the cams, which may not be that straightforward if the engine is unknown, you will need a dial wheel and a dial gauge to measure the lift on the valve and compare to the angle from TDC. You can get offset dowel pins for the cam sprockets to suit the required timing. I can’t remember the detailed method for determining the timing but the miles wilkins book covers it.

[Fotog]

Thanks, everyone.  The engineer in me wants to know what the situation is.  It seems like the important unknowns are:
1.  What is the cam setup supposed to be (from the supplier)?
2.  Has the head been altered (shaved)?

Maybe no. 2 isn't a requirement, but it would help in understanding how to achieve no. 1, or maybe why it might be difficult to do so.  I'll probably never know the answer to that one unless I take the head off.  But it's likely that it was re-surfaced, since the car has 54000 mi. on it and the engine has been re-built (but I know nothing about that, having bought it as a naive young man in the 80's). From some reading, I'm just starting to learn a little about cams and valve timing.

But I don't want to spend too long screwing around, and the front of the engine accessed from below the car is my least-favorite part of the whole thing.  So I'll probably just put it back together as JB suggests.  It's not hard to take the valve cover off if I want to investigate in the future.

As an aside, it's amazing to find that people with American cars sometimes want their engines to sound like they are race-cars, performance be damned.

Vince