Combat 2s Cam identification

Al wrote
"Glen That might be true unless most people are making the same mistake. A lot depends on the type of roads on which you use the motorcycle. Where I live the roads are long and boring, so I would use higher overall gearing"

Based on the two members here who have tried it, the Combat spec 750 does not like raised gearing. Both owners found that with a 21 tooth sprocket fitted, the Combat engine hit a wall in top gear and wouldn't pull past 80mph.

Glen

worntorn said:

Al wrote
"Glen That might be true unless most people are making the same mistake. A lot depends on the type of roads on which you use the motorcycle. Where I live the roads are long and boring, so I would use higher overall gearing"

Based on the two members here who have tried it, the Combat spec 750 does not like raised gearing. Both owners found that with a 21 tooth sprocket fitted, the Combat engine hit a wall in top gear and wouldn't pull past 80mph.

Glen

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Sounds like a tuner issue or too short a runway to me.

108mph and still pulling in 4th gear riding my Combat spec P11 when I backed out of the throttle with taller gearing in the primary and mostly the right stuff along with an SS cam in a 750 engine. Other modifications I made were also present in the carburetion setup and exhaust. Did all that around 1993.

Single row chain and old 3 spring AMC clutch was stock geared 19/42. I changed the engine sprocket to 21, so had a 21/42 sprocket combo in the primary. Gearbox sprocket was 19 and the rear sprocket was 42. The setup was about half tuned at that point. It's different now geared even taller. Have not finished the bike yet, but I doubt it would have any trouble doing better than 80mph top gear. It does 85 mph in 3rd without breaking a sweat.

Not bragging. It's not that fast and doesn't mean much. Plus, it's probably irrelevant. lol

My 850 motor pulls absurdly high gearing but the steps between the gears are smaller. With the heavy crank, you cannot afford to lose revs, and you cannot rev it high unless you rebalance it. I do not know what speed my Seeley 850 would do in top gear, and In think it would be useless at Phillip Island. On a small circuit, it is much faster than any Manx Norton and many four cylinder bikes. However that might be due to it's speed in corners - it enters the straights at a much higher speed. The slowest corner on Winton is taken at about 50 MPH when the motor is revving at about 6000 RPM in second gear - which is almost third in a normal gearbox.

Schwany said:

Sounds like a tuner issue or too short a runway to me.

108mph and still pulling in 4th gear riding my Combat spec P11 when I backed out of the throttle with taller gearing in the primary and mostly the right stuff along with an SS cam in a 750 engine. Other modifications I made were also present in the carburetion setup and exhaust. Did all that around 1993.

Single row chain and old 3 spring AMC clutch was stock geared 19/42. I changed the engine sprocket to 21, so had a 21/42 sprocket combo in the primary. Gearbox sprocket was 19 and the rear sprocket was 42. The setup was about half tuned at that point. It's different now geared even taller. Have not finished the bike yet, but I doubt it would have any trouble doing better than 80mph top gear. It does 85 mph in 3rd without breaking a sweat.

Not bragging. It's not that fast and doesn't mean much. Plus, it's probably irrelevant. lol

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Just looking at your sprockets. I also run single row primary chain and AMC clutch. My engine sprocket is 23, my clutch is 42, gearbox 22, rear 38. But with the closer gears the loss of revs on each gear change is less as I accelerate. So the speeds are probably higher. The engine gets a much easier task.

Schwany said:

Sounds like a tuner issue or too short a runway to me.

108mph and still pulling in 4th gear riding my Combat spec P11 when I backed out of the throttle with taller gearing in the primary and mostly the right stuff along with an SS cam in a 750 engine. Other modifications I made were also present in the carburetion setup and exhaust. Did all that around 1993.

Single row chain and old 3 spring AMC clutch was stock geared 19/42. I changed the engine sprocket to 21, so had a 21/42 sprocket combo in the primary. Gearbox sprocket was 19 and the rear sprocket was 42. The setup was about half tuned at that point. It's different now geared even taller. Have not finished the bike yet, but I doubt it would have any trouble doing better than 80mph top gear. It does 85 mph in 3rd without breaking a sweat.

Not bragging. It's not that fast and doesn't mean much. Plus, it's probably irrelevant. lol

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Was it fitted with a Combat head?
I believe the Combats in question went fast in 3rd, just not so much in 4th.

acotrel said:

Just looking at your sprockets. I also run single row primary chain and AMC clutch. My engine sprocket is 23, my clutch is 42, gearbox 22, rear 38. But with the closer gears the loss of revs on each gear change is less as I accelerate. So the speeds are probably higher. The engine gets a much easier task.

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Yours is a lot higher geared than my little 750. You'd never get out of 1st gear in town.

My bike is not configured with the gearing mentioned or the SS cam anymore.

worntorn said:

Was it fitted with a Combat head?
I believe the Combats in question went fast in 3rd, just not so much in 4th.

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I'd like to say I do not and never have had a tach on the bike. And at the time I did the little sprint, I didn't care if I blew it up. I don't feel that way anymore. Parts are too darned expensive to thrash it excessively.

Here comes my version of a phonograph record that is skipping:

Had a cigarette paper thin base gasket and a .020 shaved head ported by Fred Barlow with a composite head gasket. 31mm intake ports. 34mm Mikuni VMs, my hacked on for a straight up motor MAP Commando Mikuni intake manifolds, and a 2 into 1 exhaust.

I'm still using the same head on the latest rebuild with all the gotta have that parts. I have yet to find out how fast it actually is in 4th. I know I'd have to rev the heck out of it in 3rd to get up over 110mph in 4th.

worntorn said:

Was it fitted with a Combat head?
I believe the Combats in question went fast in 3rd, just not so much in 4th.

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A few years back my combat (original combat head, original 32mm carbs original cam, .040 pistons, 21t sprocket, etc) reached 118mph before I ran out of road. It would reach the ton easily but I had to wind it out a bit before I reached 118mph.

Schwany said:

I'd like to say I do not and never have had a tach on the bike. And at the time I did the little sprint, I didn't care if I blew it up. I don't feel that way anymore. Parts are too darned expensive to thrash it excessively.

Here comes my version of a phonograph record that is skipping:

Had a cigarette paper thin base gasket and a .020 shaved head ported by Fred Barlow with a composite head gasket. 31mm intake ports. 34mm Mikuni VMs, my hacked on for a straight up motor MAP Commando Mikuni intake manifolds, and a 2 into 1 exhaust.

I'm still using the same head on the latest rebuild with all the gotta have that parts. I have yet to find out how fast it actually is in 4th. I know I'd have to rev the heck out of it in 3rd to get up over 110mph in 4th.

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That doesn't sound like a Combat head. According to DynoDave, the stock Combat head suffers from very crudely shaped factory intake ports.

Glen

Come on you guys, it sounds like there are a lot of slow Commandos out there!, but it could explain why I had little trouble overtaking most Commandos on the race tracks with my 500 Domi racer. On the Norwich straight at Snetterton it could reach a genuine 117 mph @ 7300 rpm. (admittedly fitted with a fairing and weighing only 300 lb).

Have you tried poking a broom handle through the silencers to knock the baffles out?

acotrel said:

Just looking at your sprockets. I also run single row primary chain and AMC clutch. My engine sprocket is 23, my clutch is 42, gearbox 22, rear 38. But with the closer gears the loss of revs on each gear change is less as I accelerate. So the speeds are probably higher. The engine gets a much easier task.

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Geared for 165mph? I smell bullshit..........lots of it

hybridracer said:

Come on you guys, it sounds like there are a lot of slow Commandos out there!, but it could explain why I had little trouble overtaking most Commandos on the race tracks with my 500 Domi racer.

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Only if 2 is a lot. Both are overgeared, so that may be the cause.

Glen

Seeley920 said:

Geared for 165mph? I smell bullshit..........lots of it

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Have you tried gearing really high with a close ratio gearbox ? The gearing on my bike is stupid, but it works. With the 4 speed close cluster, first gear was hopeless, but everywhere else the bike was excellent. If you don't make the 850 motor pull - the bike will be slow, but if you lose revs the bike will be slow. My bike is slow in first gear, but once it is mobile, it is a different story. You could not use it as a road bike. You are either stationary or really going. The first few metres are very slow. And in towns, that is what you would be using the most. A Commando first gear would fix it, but you would not need to change from second down to first by mistake.

My bike probably is geared for 165 MPH, but normally I would get nowhere near it. At Phillip Island, it might reach 140 MPH on the front straight. at Winton, it might reach 100 MPH. The main thing is to keep the bike pulling hard in corners and around curves, so the back stays down a bit,while you are accelerating. That way, it is possible to accelerate from the beginning of the corner and right through it and up the next straight. My bike would never win a drag race, it is better if it accelerates over longer distances on shorter circuits.

If you race, always take note of where the other guys get on the gas as they leave corners. If you come out of a corner having gassed the bike much earlier, at the beginning of the straight your bike will be faster, and the other guys need much more power to get to the end of the straight ahead of you. I think some guys might concentrate on getting around the corners and only racing down the straights. When that happens, the most powerful bike wins. Most road bikes oversteer slightly when you gas them on a lean, except for two-strokes.

worntorn said:

That doesn't sound like a Combat head. According to DynoDave, the stock Combat head suffers from very crudely shaped factory intake ports.

Glen

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OK my bad and not apples to apples relevant to large degree. My head is the original P11 head with a mild port job by Fred Barlow. Barlow knew what he was doing as far as I can tell. Barlow was recommended by Phil Radford. Phil actually took the head to Europe with him in a suitcase on a vacation/parts purchase run, or at least that is what he told me. I think he was probably joking about the suitcase.

Speaking of crudely shaped intake ports. I have a stock Commando 750 head with only a few miles on it and the ports are not impressive. I ran the engine with that Commando head on it while the P11 head was in Europe getting ported. Had to tap the 4 5/16" holes in the barrels close to the spark plugs for 3/8" bolts to use the Commando head. The Commando head did not get along with the 2S cam like the Barlow ported head does. It's the little things that make a difference sometimes.

Ive heard that name before in conjunction with Norton porting and I recall that he is one of the gurus. Effective porting seems to be something that only a small number of people are capable of.
Jim Comstock is at the top of my list as he has a lot of experience and makes full use of a boatload of flowbench results.
Herb Becker did a good job with my 650ss but he no longer takes on work.
Is Fred Barlow still with us?

I suspect that at least as often as not, heads have been ruined by porting too large for the engine displacement.
I had a pair of heads like that on a Vincent. It ran fine but wasn't as strong as my stock Vincent anywhere except full rpm, where the bikes were about equal.
Overall the stock bike was faster and nicer to ride.

Glen

Fred Barlow is no longer living. I believe but am not 100% certain that Fred Barlow preceded most of the Norton gurus currently breathing and still taking on head port work. I told Andy Molnar the head I'm using is ported by Fred Barlow and he said Barlow was good. I doubt Phil Radford would have used him if he wasn't a good resource. Phil probably didn't want me coming back and whining about the head being ruined.

All that BS out of the way, I'm sure the new CNC FullAuto copies are better, but I'll baby that Fred Barlow Spares (FBS) ported head anytime I have it off or am putting it back on and keep it untill the end of my Norton ownership, or I forget where the garage is, whichever comes first.

worntorn said:

Only if 2 is a lot. Both are overgeared, so that may be the cause.

Glen

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Agreed, I was probably being a bit hasty so I dug out the motorcycle performance software I wrote some 30 odd years ago to aid me with analysis of the Norton rotary F1 sports road bike. Naturally I wrote it to accept raw data for any bike so had also created data files for the Dominator and Commando. I used the dyno data in your post #27 and assumed a 21T g/b sprocket and a 4.10 x 19" TT100 tyre. Cd.A was the best I could determine back then but bear in mind that a lot depends on the riders position on the bike at those speeds.
The top speed came to 111 mph with gear changes from 1st and 2nd at max revs, and from 3rd at 6600 rpm for max acceleration. Max speed with optimum gearing was 115 mph.
Naturally the software also gives output data (time/distance/speed) but the extra graphs don't work in windows.
The higher speed that my Domi 88 reaches is entirely due to the much lower Cd.A therefore the two cannot be directly compared.

The software, being ancient, was written in turbo pascal (for dos, not windows and I think a 640x480 vga screen!) but works fine in a dos window in windows xp. I haven't tried it in 7, 10 or 11 yet but I expect it to work (I will check). It also needs a standard keyboard. Gear ratios, etc, C.of G. position.weights, tyre sizes etc are all user editable. I can give copies away to anyone who is interested once I have determined what versions of windows it runs on.

I have been following Illf8ED's ( David) posts on this since 2015 or so. I know he has checked out everything possible in trying to track the problem down, all to no avail. I believe the problem arose when he replaced a 1s cam in his Combat with a 2s, or stock Combat cam.
It sounds like he is going to try reverting to the 19 tooth sprocket.
It will be interesting to see what the results are.
Here is a thread from 2015


Glen