Lobe Center (Cam Timing) Help Needed

I am doing the cam timing on my rebuilt engine. I am using the standard camshaft.

I have set the timing going by the workshop manual (10 pins, etc)

I planned to use the lobe centre method for the cam timing.

I have set up the dial gauge on the head with pointer on the intake valve collar. Degree wheel mounted on the crankshaft. I was aiming for 102deg but I got 99deg.

The figures I had at 0.050" on the dial gauge with standard tappet clearances were:
23º BTDC opening and:
41º ABDC closing

For those in the know how much difference in performance would there be between 99deg and 102 or 103deg and is it worth the trouble of altering timing gears and sprockets to get the desired results.

Would I need to retard or advance the cam to achieve the results that I desire?

Does anyone know how many degrees the cam would need to be retarded or advanced to compensate for the difference between 99º and 102º on the lobe centre setup.

If I need to retard or advance the cam is the vernier sprocket made by RGM the best way to go? It sounds too hard for me to be filing woodruff keys to try and achieve the correct setting.

Thanks Don

Of course a vernier sprocket is the easiest way to make cam timing changes but offset woodruff keys have been used for that purpose for many years. They are available from cam makers for many common engines but I have no idea if they are available for a Norton cam/engine.

FWIW, it generally takes around 4 degrees of change in cam timing to feel any difference. So you are right at the point where it MIGHT be worth correcting. Obviously, like so many things regarding tuning, the best way to determine optimum cam timing for a particular engine is on a dyno. Two identical engines may work best with different cam timing using identical cams.

Valve lash also affects valve timing - generally, more clearance produces more low end; less clearance produces more top end. NOTE that either change SHIFTS the entire power band up or down. IOW, it adds power on one end while taking it away from the other end.

Cam timing does the same thing - advancing the cam shifts the power band down/retarding shifts it up.

Frankly, the published specs for a cam installation are a general guide and, for best results for your engine and how YOU operate the vehicle, the optimum settings of cam timing and valve lash may be quite different than the "spec" setting.

Thanks Mike

Fast Eddie said:

Don, what are you using the bike for? If it’s a road bike for normal road use I really do not think you’ll notice any difference at all!

Where did you get your 102 degree target figure from?

With the standard cam wheels you can change with 5 degree increments. So, if you are currently at 99 (3 away from target) you could get to 104 (2 away from target) just by using what you have (see JS web site for info on how to do this).

If you want to get closer than that, you can file down a woodruff key. Or use a Vernier wheel. I’ve heard bad things about those, so wouldn't risk it personally.

However, RGM do a nice looking cam wheel with 3 key ways allowing much more accurate timing: https://www.rgmnorton.co.uk/buy/special-camshaft-sprocket-less-weight-adjustable-timing_4641.htm

But I repeat, IMHO if this is a road bike, I do not believe 2 or 3 degrees either way will be noticeable.

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Thanks Eddie, Yes it is a road bike. An 850 Roadster (74) and all stock.

The 102 figure is what I used on my Suzuki GS550 when I converted it to 675cc with different head and carbs, etc. on advice from the Suzuki GS Forum and it works well in that application. Were you thinking of different figure?

When you say you can get 5 deg increment changes with the standard cam wheels so you mean by moving the intermediate gear by one tooth on the half time pinion?

Thanks Don

Read through this, post 14 is perhaps the summary you’re looking for:

https://www.accessnorton.com/NortonCommando/5-degrees-advanced-cam-timing.20069/

Based on your numbers you would be looking to advance the cam by 3 degrees.

Without knowing how many times you repeated the effort, I would suggest that the difference may actually be a measuring or instrumentation error.

It may be more accurate to take your measurements off the follower/pushrod rather than the valve collar, but if using the collar you may want to eliminate all valve lash to achieve more accurate results.

I would avoid the vernier sprocket (just me), but RGM does sell a cam sprocket with 3 different keyways broached into it which works well.

https://www.rgmnorton.co.uk/buy/special-camshaft-sprocket-less-weight-adjustable-timing_4641.htm

Generally speaking, around 5 degrees of intake advance will move power lower down, but at the expense of intake-piston clearance.

When I checked cam timing in my combat, I set up a degree wheel on the drive side crankshaft setting zero at TDC. Set the tappet clearance to .020”. Turn the crankshaft forward until the travel dial on the tappet just starts to move. Check and record the degrees. Repeat several times on both sides intake and exhaust. I wasn’t concerned about closing degrees as it’s fixed based on the camshaft grind.

thunderbolt said:

Thanks Eddie, Yes it is a road bike. An 850 Roadster (74) and all stock.

The 102 figure is what I used on my Suzuki GS550 when I converted it to 675cc with different head and carbs, etc. on advice from the Suzuki GS Forum and it works well in that application. Were you thinking of different figure?

When you say you can get 5 deg increment changes with the standard cam wheels so you mean by moving the intermediate gear by one tooth on the half time pinion?

Thanks Don

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Not sure why you would choose cam timing for a pushrod Norton twin based on an OHC 4 cylinder spec, but the specs for a stock Commando cam also shows a 102° lobe center, so it all works out. Like MM said, advancing the cam to 99°, as it appears to be in your engine, will slightly broaden the power band, while losing a little at the top end. Retarding it by 5° to 104° would in theory give you slightly more top end at the expense of a little less grunt lower down. I doubt if you would notice the difference either way in a stock bike. With the low compression of the 850 engine, I doubt if you need to worry about it causing valve-to-piston clearance issues, but if it concerns you, it's pretty easy to check by prying down on the valve, while watching the dial indicator, at several degree points around the area of closest approach. No need to tear the engine down and clay it.

Ken

102 is a very common target, same for Aprilla V twin is just one example.

I've seen factory and aftermarket cams for the Commando with specs for the intake lobe center varying from 99° to 106.5°, and 102° falls right in the middle of that range. Seems like a reasonable starting point.

Ken

How do you accurately find the lobe centres on a cam when you are setting-up the timing ? If you have a look at what happens with two-strokes, you might have a better idea of what is important. If you cut the exhaust port up higher, you shift the power band up. If you cut the transfers higher, in most cases you get more go. But the whole thing is a relationship between the port timings and the inlet and exhaust systems which resonate. If you fit a 2 into 1 exhaust system to a Commando, you should not expect the normal cam position to give the best performance. When I built my 850, I had two more keyways broached randomly into the cam sprocket. My inlet valves open at 65 deg BTDC ( not 53). The exhaust valves open at about 94 deg BBDC (not 80 something). The motor pulls like a train right through the whole usable rev range and right up through the top. It's only problem is that it is too loud.

Does anyone know what the stock timing is when measured using the lobe centre method?

Fast Eddie said:

Does anyone know what the stock timing is when measured using the lobe centre method?

Click to expand...

102°

Ken

lcrken said:

102°

Click to expand...

Thanks Ken.

In that case I’d definitely leave it alone at 99. I just don’t think you’d tell any difference, and if you did, it would be a slight improvement for the road.

But that’s only my lay persons thoughts....

Thanks guys for the replies. Considering your comments I think I will leave it as it is seeing that it is in range even though at the bottom of the range. l'll leave it at 99o. I might just check the maximum lift while I am at it.

Thanks again Don