valve timing / cam identification

I have been experiencing similar lack of acceleration in 4th gear with my 1972 Combat that was reported by @ILLF8ED in this thread:


(This is a very long thread but the relevant part begins at post #160.)

The timing side seems to be set properly with all the marks in the right places at TDC, but when I measure the actual valve timing, the results show that the valves are retarded approximately 11* from what is specified.
I measured inlet to be opening at 47* BTDC and closing at 79* ABDC.
For measurement, I set the timing side inlet tappet clearance to .017” and used .0025” feeler gauge to detect when the valve lift begins at .0145” (which is closest to the .013” cam lift specified in the Workshop Manual for this measurement when taking the 1.13:1 rocker geometry into account).
Workshop Manual:


So it appears that I may also have a 1972 Combat motor with a mis-marked timing pinion as did ILLF8ED.


The measured numbers are some what similar to the specs for standard (non-combat) cam (50* BTDC inlet opens; 74* ABDC inlet closes) so I remeasured valve lift using dial gauge on valve collar:

Timing side: inlet .439” (at approx. 70* BBDC); This is within 1% of Combat spec .441"

exhaust .365” (at approx. 82* ABDC); This is 6.6% below Combat spec .391"

Drive side: inlet .444” (at approx. 70* BBDC); This is within 1% of Combat spec

exhaust .368” (at approx. 82* ABDC); This is 5.8% below Combat spec.

These inlet measurement tend to confirm it is a Combat cam, but for some reason the exhaust measurements are considerably lower than Combat cam specs and actually closer to the standard cam spec of .375”, creating doubt.

Can anyone help me explain this?
Do I have a non-standard Combat cam with modified exhaust lobes?

I looked at @dynodave 's cam profiles on his atlanticgreen.com website but i don't have enough measurement precision to conclude anything from those.

I want to be as certain as I can about what I have before I go changing the valve timing set-up by advancing the intermediate gear.

Any help will be appreciated.

I suggest going through it again using the technique described here to confirm your numbers:

acadian said:

I suggest going through it again using the technique described here to confirm your numbers:

webcamshafts.com

webcamshafts.com

www.webcamshafts.com

Click to expand...

This seems to be a good resource for measuring cam lobe centers, but since my purpose is to identify the cam and I don't have a spec for lobe centers of the various cams available. I am left with the valve opening and closing specs from the manual that I believe I have measured.

Additional info:
I just measured the exhaust valve timing using the same method;
82* BBDC was measured when exhaust valve opens (spec is 88*BBDC), and 40* ATDC measured when valve closes (spec is 60* ATDC). Valve timing appears to be very close to standard cam and not close to Combat cam duration.
Now I'm thinking this might be a non-Combat cam. But the intake valve lift measurement is not correct for that, ??????????

Can’t you simply measure the max lift on inlet and ex? That will tell you whether or not it’s a 2S cam.

I posted this in 2015. Since then I went back to the drawing board setting the timing per manual with all the dots aligned and 10 pins between the intermediate and cam sprocket marks. The cam timing was now correct for the combat spec. It’s running as it should now with good acceleration in 4th gear. Don’t ask me what I did wrong several years ago.

Fast Eddie said:

Can’t you simply measure the max lift on inlet and ex? That will tell you whether or not it’s a 2S cam.

Click to expand...

Problem is cam timing not cam profile.

ILLF8ED said:

Problem is cam timing not cam profile.

Click to expand...

Yes and no, the OP is now unsure what cam he has (see post #4, which I was replying to) and IMHO measuring lift is the easiest way to ascertain this, especially if trying to establish if it’s a 2S or not.

Measuring opening and closing points can be massively confusing.

See pic below taken from dynodave‘s excellent website (such a shame he doesn’t post much these days, pissed off by all the bickering).

Opening and closing figures are quite close, most people would struggle to ascertain one or the other accurately using these figures.

Lift figures are hugely different though, and very easy to see.

Fast Eddie said:

Yes and no, the OP is now unsure what cam he has (see post #4, which I was replying to) and IMHO measuring lift is the easiest way to ascertain this, especially if trying to establish if it’s a 2S or not.

Measuring opening and closing points can be massively confusing.

Click to expand...

Absolutely agree with you regarding openning/closing points being massively confusing. That’s probably where I messed up 6-7 years ago. Got mine fixed now.

I recommended that method because with it you can ascertain opening, closing and lift:



I can send you the spreadsheet that does all the calculations for you once the numbers are plugged in

Fast Eddie said:

Can’t you simply measure the max lift on inlet and ex? That will tell you whether or not it’s a 2S cam.

Click to expand...

I did measure the max lift of the inlet and exhaust valves. This is from my original post:

"The measured numbers are some what similar to the specs for standard (non-combat) cam (50* BTDC inlet opens; 74* ABDC inlet closes) so I remeasured valve lift using dial gauge on valve collar:

Timing side: inlet .439” (at approx. 70* BBDC); This is within 1% of Combat spec .441"

exhaust .365” (at approx. 82* ABDC); This is 6.6% below Combat spec .391"

Drive side: inlet .444” (at approx. 70* BBDC); This is within 1% of Combat spec

exhaust .368” (at approx. 82* ABDC); This is 5.8% below Combat spec.

These inlet measurement tend to confirm it is a Combat cam, but for some reason the exhaust measurements are considerably lower than Combat cam specs and actually closer to the standard cam spec of .375”, creating doubt."


I don't know what to conclude from these measurements.

ILLF8ED said:

I posted this in 2015. Since then I went back to the drawing board setting the timing per manual with all the dots aligned and 10 pins between the intermediate and cam sprocket marks. The cam timing was now correct for the combat spec. It’s running as it should now with good acceleration in 4th gear. Don’t ask me what I did wrong several years ago.

Click to expand...

Yes, I recently read your whole thread that started from 2015, but in your 2018 posts #160 and #163 I thought you determined that the timing mark was off by one tooth and so you advanced the intermediate gear one tooth to solve the problem. Did I misunderstand? I realize it was a long time ago and a lot was going on.

Fast Eddie said:

See pic below taken from dynodave‘s excellent website (such a shame he doesn’t post much these days, pissed off by all the bickering).

Opening and closing figures are quite close, most people would struggle to ascertain one or the other accurately using these figures.

Lift figures are hugely different though, and very easy to see.

View attachment 99967

Click to expand...

I agree that the lift should be a good way to differentiate the 2S cam. The problem I have is that my measurements show the lift of my cam is like 2S for intakes but not for the exhaust. Exhaust lift measures closer to non-Combat cam spec.

NickZ said:

I agree that the lift should be a good way to differentiate the 2S cam. The problem I have is that my measurements show the lift of my cam is like 2S for intakes but not for the exhaust. Exhaust lift measures closer to non-Combat cam spec.

Click to expand...

Maybe you just got a worn cam.

acadian said:

I recommended that method because with it you can ascertain opening, closing and lift:

View attachment 99968

I can send you the spreadsheet that does all the calculations for you once the numbers are plugged in

Click to expand...

I think my next step will be to use this method and get duration measurements at .050" lift and see if those fit with any of dynodave's profiles. After that, I might as well get the rest of it calculated from your speadsheet, but I still don't have a reference for lobe center spec and don't believe I can get that accurately from the dynodave profile graphs.

If you have vernier adjustment on your cam sprocket, whatever the cam is, you can make it work. If you have a restrictive exhaust system, you can advance the cam in 3 degree steps while testing the bike, until you get what you want. I usually use zero tappet clearance for checking valve timing. If I am timing a standard cam, I usually divide the errors between opening and closing points Moving a standard 850 cam forward can give more torque. With a 2S cam, the extra lift might cause valve bounce sooner in the rev range. Whatever the cam is, it's timing needs to be optimised to suit the exhaust system.
I actually have a 2S cam which I have not used because when compared with an 850 cam, it looks to be too radical.
The difference between a well-tuned 850 motor and a poorly-tuned one, might be more than the difference between a good cam and a bad one ? However, if all the good tuning hints add up positively, you should have the fastest motor on the planet.
How many variables are there ? There might be a pattern.

Fast Eddie said:

Maybe you just got a worn cam.

Click to expand...

Is it likely that both exhaust lobes could wear that much with the inlet lobes wearing virtually not at all?

You swing it over & check the OVERLAP , If itsnotdam near T D C somethings funny .

NickZ said:

Is it likely that both exhaust lobes could wear that much with the inlet lobes wearing virtually not at all?

Click to expand...

It depends.

I had a soft cam on my 74, that I replaced ~ 6rs ago that had much more wear on two lobes. One had a severe spike on the lobe edge.

I can't.recall which lobes. I should have taken pix.

Comnoz has noted he saw no pattern in more/less frequent wear on inlet vs. exhaust lobes.