Lotus Elan

Camshaft timing problems help required

PostPost by: mark030358 » Mon Dec 20, 2004 12:15 am

This is a long one Gents please bear with it,

I have a Big Valve engine with the correct (and standard cams). The
specs are:-

Inlet opens 22 deg BTDC
Inlet closes 62 deg ABDC

Exhaust opens 62 deg BBDC
Exhaust closes 22 deg ATDC

This gives a MOP of 110 degrees.

Ok so far, I think....

Anyway I have had the block decked and hence need to retime the cams
and have used the following method

Set engine to "true" TDC (use DTI)
Place DTI on number 4 exhaust tappet (always under tension due to
rotation of the engine)
Rotate engine to remove slack and stop at TDC again
Now slowly rotate engine 'till tappet moves (opens) to 50 thou, read
angle from timing disc.
Rotate engine 'till tappet starts to close and gets back to 50 thou
point on DTI, read angle from disc.

The results at 50 thou were:-

Angle when opening 64.5 deg BBDC
Angle when closing 16 deg BTDC

therefore MOP = (64.5+180+16)/2 -16 = 114.25 therefore advanced by
4.25
Now here is the problem,
When I repeated the exercise I used 60 thou of lift and got the
following results:-

The results at 60 thou were
Angle when opening 62.5 deg BBDC
Angle when closing 18 deg BTDC

therefore MOP = (62.5+180+18)/2 -18 = 112.25 therefore advanced by
2.25

When I repeated the exercise at 70 thou of lift I got the following
results:-

The results at 70 thou were
Angle when opening 60.5 deg BBDC
Angle when closing 20 deg BTDC

therefore MOP = (60.5+180+20)/2 -20 = 110.25 therefore advanced by
0.25

I have not checked the inlet cam yet but I cannot understand the
readings for the exhaust and I dont want to go any further. I have
also tried to do the same at max lift of the cam (ie either side of
the dwell) by the following method:-

Set engine to "true" TDC (use DTI)
Place DTI on number 4 exhaust tappet (always under tension due to
rotation of the engine)
Rotate engine to remove slack and stop at TDC again
Now slowly rotate engine 'till tappet moves to max lift, read angle
from timing disc.
Rotate engine 'till tappet just starts to close, read angle from disc.

The results at max lift were
Stop at max lift (on start of dwell) 132.5 deg BTDC
Start when closing (coming off dwell) 125.5 deg BTDC

therefore MOP = (132.5-125.5)/2 +125.5 = 129 therefore advanced by 19

Would appreciate some help
Thanks
MArk
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PostPost by: iancockshull » Mon Dec 20, 2004 11:08 pm

Mark

You did not say if the cams are new or used. If used I would suggest
you may be observing the effects of more flank wear on the leading face
of the cam than on the trailing edge. This might be more noticeable at
different amounts of lift. Also some cams have "Quietening ramps"
built into the profile that are noticeable when the valve starts
lifting, hence using the figures when the valve is more fully open (70
thou) may give more representative results.

Hope this helps
Ian





In message <cq55it+***@***.***>, mark030358
<***@***.***> writes
This is a long one Gents please bear with it,
???
I have a Big Valve engine with the correct (and standard cams). The
specs are:-

Inlet opens 22 deg BTDC
Inlet closes 62 deg ABDC

Exhaust opens 62 deg BBDC
Exhaust closes 22 deg ATDC

This gives a MOP of 110 degrees.

Ok so far, I think....

Anyway I have had the block decked and hence need to retime the cams
and have used the following method

Set engine to "true" TDC (use DTI)
Place DTI on number 4 exhaust tappet (always under tension due to
rotation of the engine)
Rotate engine to remove slack and stop at TDC again
Now slowly rotate engine 'till tappet moves (opens) to 50 thou, read
angle from timing disc.
Rotate engine 'till tappet starts to close and gets back to 50 thou
point on DTI, read angle from disc.

The results at 50 thou were:-

Angle when opening 64.5 deg BBDC
Angle when closing 16 deg BTDC

therefore MOP = (64.5+180+16)/2 -16 = 114.25?? therefore advanced by
4.25
Now here is the problem,
When I repeated the exercise I used 60 thou of lift and got the
following results:-

The results at 60 thou were
Angle when opening 62.5 deg BBDC
Angle when closing 18 deg BTDC

therefore MOP = (62.5+180+18)/2 -18 = 112.25?? therefore advanced by
2.25

When I repeated the exercise at 70 thou of lift I got the following
results:-

The results at 70 thou were
Angle when opening 60.5 deg BBDC
Angle when closing 20 deg BTDC

therefore MOP = (60.5+180+20)/2 -20 = 110.25?? therefore advanced by
0.25

I have not checked the inlet cam yet but I cannot understand the
readings for the exhaust and I dont want to go any further. I have
also tried to do the same at max lift of the cam (ie either side of
the dwell) by the following method:-

Set engine to "true" TDC (use DTI)
Place DTI on number 4 exhaust tappet (always under tension due to
rotation of the engine)
Rotate engine to remove slack and stop at TDC again
Now slowly rotate engine 'till tappet moves to max lift, read angle
from timing disc.
Rotate engine 'till tappet just starts to close, read angle from disc.

The results at max lift were
Stop at max lift (on start of dwell) 132.5 deg BTDC
Start when closing (coming off dwell) 125.5 deg BTDC

therefore MOP = (132.5-125.5)/2 +125.5 = 129 therefore advanced by 19

Would appreciate some help
Thanks
MArk






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