Cam Timing

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Jan 10, 2005
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Found my timing stats from January this yr after I had the crank balanced etc. When I built the new 750 motor Dec 02 with std (was Combat) cam I knew the it out by a few degrees with gears and cam sprockets as per manual. I figured I'd leave it as it was but now I reviewed the results I'd like an expert opinion on whether I should try and get closer to the factory specs. I was hoping my fancy head job and new rings would give me back the low down torque I figured should be there with the std cam, but I still can't ride up 25% gradients in top gear (18.2/1000) like the stories tell us can be done! Nor does it pull like a train from 1500 in top. Could be down to the 32mm carbs and large inlets I have but I have to wonder if these tales are just that or maybe a stock 4.6:1, 30mm carbed Commando can do that. I had a 69 fastback and I don't recall it being that flexible and that was totally stock with maybe 19 or 20T gearing.

Inlet & Exh on 1 & 2 are within 1/2 - 1 deg of each other but are late opening on inlet by 4.5 deg and 14 deg late opening on exh. These are at .013" lift measured on the follower. It's the fact that the exh is so late opening (and closing) that concerns me. Readings as follows so should I concentrate on getting the inlet opening right or try to average out both so the +/- is equal? I assume getting the inlet right is more important since if you can't get the mixture in in the first place.....?

Inlet avg - Open 45 BTDC, (5 late), Close 73 ABDC (1 early)
Exh avg - Open 68 BBDC, (14 late), Close 51 ATDC (9 late)
Comp ratio = 9.3:1 (Combat head with std pistons, compo head gasket, 032" steel base spacer, no base gaskets).

I figured you can get within 2deg (engine) by manipulating the crank/idler/cam sprocket mesh. Does anyone have the gear/sprocket teeth count? Is it 26/52/18/18? So if I advance the cam sprocket 7T (+280) and retard the engine gear 20T (-277) I'd get 3deg net advance and at least get the inlet closer on opening but 4deg early closing. Does that make sense or have I got my maths totally wrong?

If you think I'm wasting my time let me know. I'd appreciate an honest opinion. As you can guess the expert advice (supplier included) is not to worry although of the four UK parts suppliers two said chuck the cam and fit ours, they're accurate. Oh well. Thanks for any input.
The late John Hudson was very insistent on correct cam timing and he was someone who was always worth listening to.

John used to recommend minimising the error by remeshing the pinions and I seem to remember that another of his solutions was a stepped woodruff key for the cam sprocket.

Don't Fair Spares now make adjustable cam sprockets again ?

Do you have Dunstall's tuning notes ? He used vernier sprockets with race cams and says that the best thing to aim for is the correct amount of lift on the inlet valve at TDC.

By the way, where do your performance expectations come from ? Don't forget that lots of those period road tests were done by skinny young men and you don't see many of them on Commandos these days :)
Thanks 79. What bugs me is the disparity on exhaust timing, being so far out from spec. Inlet is close to spec and opening accuracy achievable with remeshing but if I average out both cams that would advance the inlet too much I was thinking. Yes I am aware of the often quoted lift at TDC, but have never seen this stated for std cam, only the hotter grinds. Curves I've seen show about .125" std cam measured (not spec).
I'll have to get a copy of Dunstalls notes, see them on ebay often enough.
As far as performance expectations that was a bit tongue in cheek, no I don't expect to climb Porlock in top gear, not without an 85mph run up!
Lots of those tests were with 4.84:1 gearing and 8st riders (remember Charles Deane ?). For sure my Fastback could never do that and that was only 3 yrs old when I owned it. Cheers.
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