Lotus Elan

valve timing correction best action ?

PostPost by: Concrete-crusher » Wed Mar 22, 2017 9:33 am

I have finally got to a point where I'm getting repeatable results.

my intake MOP is 122 ATDC and exhaust MOP is 102 BTDC

so is it best to move both cams by 1 sprocket tooth and fit a 2 degree off set dowel to each

or just move the intake by 1 sprocket and fit a 2 degree offset dowel , whilst fitting a 8 degree offset dowel to the exhaust.

my head is spinning with numbers and I'm not sure what's best.

thanks steve
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PostPost by: rgh0 » Wed Mar 22, 2017 9:56 am

There is only one combination of offset dowels that works as they cover half a tooth spacing

Set the exhaust cam correctly with the offsel dowel it needs first ignoring the inlet cam.
Then do the inlet cam.

The inlet cam moves depending on timing for the exhaust cam so getting the exhaust cam right first is critical otherwise you just confuse yourself. :?:

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PostPost by: Concrete-crusher » Wed Mar 22, 2017 11:58 am

Thanks Rohan

I am managing to confuse myself , but see what you mean about one cam at a time.

So to bring my mop from 102 to 110 do i use an 8 degree offset or is it a 4 degree offset ?

I think I'm confusing cam and crank readings

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PostPost by: nmauduit » Wed Mar 22, 2017 12:18 pm

I believe all values are in terms of crankshaft angles.

What I would do is get as close as possible to the desired timing for both camshafts by jumping one tooth exhaust side (that should pull inlet one tooth), then do the fine tuning using dowels if necessary (in your case probably about 1? or 2? dowel - possibly with a similar dowel for inlet - to be confirmed with the chain in final location, one tooth being ~10.6?). [edit for the sake of clarity since all timing values are referred to crankshaft angles, 720? for a complete cycle : 1 camshaft tooth out of 34 is ~10.6? at camshaft level, so ~5.3? at crankshaft level // so jumping 2 teeth exhaust side may have been required in the case you described ]

instructions from one random supplier (no affiliation on my part) :

http://www.burtonpower.com/skin/fronten ... Dowels.pdf
Last edited by nmauduit on Mon Mar 27, 2017 10:18 am, edited 1 time in total.
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PostPost by: Concrete-crusher » Sun Mar 26, 2017 4:52 pm

Ok so I opted for Kent cams vernier sprockets.

There were no instructions included and burton power the supplier just said ask your engine builder ?

So 1st attempt checked and set tdc , the pulls mark looks spot on.

Then rotated to 110 btdc and set the exhaust cam to Mop locked sprocket

Then rotated to 110 atdc and set inlet in a similar manner.

End result both cams are facing the same direction so one is 180 degrees out .

Realising this I repeated the process and still got 180 degrees out.

So third time I rotated the crack 360 degrees between exhsust and inlet settings and that seemed to work.

So my question just to understand my error.

When the crank is at tdc , and you need to find 110 degrees btdc do you rotate 610 degrees ?

As the crank turns twice for the cams to turn once ?

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PostPost by: billwill » Mon Mar 27, 2017 9:23 am

With much hand rotation are you not in danger of the pistons hitting the valves?
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PostPost by: billwill » Mon Mar 27, 2017 9:49 am

If the valve opening diagram that you are trying to achieve is symmetrical about TDC I would have thought that you are only interested in getting the cam positions set at TDC to maximum valve opening. No mucking about with rotating to 110 degrees at all, that would be fixed by the cam profile.

So with the verniers, what I would try would be first set both verniers in their centre positions and assemble the cams & chain so that the timing marks align at TDC at their closest points (determined by chain stretch and head skimming only). I.e as if they were solid sprocket wheels.

Then remember that at TDC, the outer teeth positions of the verniers are fixed in space by the chain, so loosen the clamp bolts of the exhaust vernier and use some sort of double prong lever to move the inner of the vernier to-and-fro until the valve is at max opening. Then tighten its clamp bolts and go do the same for the inlet vernier. It's a bit tricky as you can't easily get at the lower clamp bolts as they are covered by the front of the timing case, so you do have to do various crank rotations to get at them and return it to TDC. I suspect that the maximum amount of movement of the verniers inner to outer is such that provided that you do not remove the chain there would be no danger of the pistons hitting any valves.

However this process is speculation on my part as my own verniers were set by my engine builder :shock:

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PostPost by: Concrete-crusher » Mon Mar 27, 2017 5:06 pm

I guess what I forgot is the engine is a 4 stroke ie first up and down is intake then second up and down is exhaust so the two cams are set on their respective cycles.

I hope I have not bent any valves , but with the engine at 110 degrees either before or after top dead centre the pistons are all down around midway. So rotating the cams should be safe.

Interestingly my sprockets now look exactly like yours ie intake lined up and no offset and exhaust slightly lower but with a small offset.

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PostPost by: billwill » Mon Mar 27, 2017 9:32 pm

As mentioned in other threads the timing marks on the sprockets are not precise, they merely allow you to get the chain on the correct notches.

Mine in the photo above line up quite well because (I think) the engine builder made fresh marks AFTER he had done the alignment. IF you look carefully at the exhaust sprocket you can see an old timing mark below the main one.
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