Mégacycle 560 nr performance

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Hi, I am working on a 1971 Commando 750 combat engine.
The motor was rebuilt (2,000 km ago) with a Mégacycle 560 nr camshaft, 0.060``+ pistons and all the other parts are std to this bike (except a pointless system).
There is good low and mid power but almost no top end (nothing over 4500 rpm).
I try a lot of carb tuning an timing (fire), a new pointless system was installed but with almost no change in performance.
I check the cam timing with a degree wheel and it is ok (may be 1/2 degree offset).
I have two questions:
1 = Is it normal to have that kind of power with this cam?
2 = I wonder if someone try to give 5 degree advance on this cam (playing with gears and number of links)?
Thanks!
 
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I have no experience with this cam, but your symptoms are familiar. My Combat engine had been detuned before it ever came to me and it ran very well with the standard cam and lowered compression. Eventually I rebuilt it with a good used Combat cam and with return to Combat compression. I found it was exactly as you described, went flat at about 4500 rather than starting to pull harder as it should. I was running with S pipes and the mufflers were apparently quite restrictive. I opened up the baffles so it could breath and everything performed as expected, pulls like a bear right through to red line, and beyond if I let it.
 

Carl H

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Hi, I am working on a 1971 Commando 750 combat engine.
The motor was rebuilt (2,000 km ago) with a Mégacycle 560 nr camshaft, 0.060``+ pistons and all the other parts are std to this bike (except a pointless system).
There is good low and mid power but almost no top end (nothing over 4500 rpm).
I try a lot of carb tuning an timing (fire), a new pointless system was installed but with almost no change in performance.
I check the cam timing with a degree wheel and it is ok (may be 1/2 degree offset).
I have two questions:
1 = Is it normal to have that kind of power with this cam?
2 = I wonder if someone try to give 5 degree advance on this cam (playing with gears and number of links)?
Thanks!
What were the lobe centers you ended up with?? All types of cams should give good power,
 
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I think Interbak hit it right. I built a 750 years ago with this cam and it ran like crap until I changed the exhaust system. Same thing on a stock 850 I worked on. Original mufflers, but when I swapped them out it was like a different bike.
 
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I have this cam in my Combat with free flowing exhaust and it has plenty of power, as said it should match other things in the motor.
 

HRD

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I have no experience with this cam, but your symptoms are familiar. My Combat engine had been detuned before it ever came to me and it ran very well with the standard cam and lowered compression. Eventually I rebuilt it with a good used Combat cam and with return to Combat compression. I found it was exactly as you described, went flat at about 4500 rather than starting to pull harder as it should. I was running with S pipes and the mufflers were apparently quite restrictive. I opened up the baffles so it could breath and everything performed as expected, pulls like a bear right through to red line, and beyond if I let it.
Were your S model mufflers original or some replacement type?
 

Ron L

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Hi, I am working on a 1971 Commando 750 combat engine.
The motor was rebuilt (2,000 km ago) with a Mégacycle 560 nr camshaft, 0.060``+ pistons and all the other parts are std to this bike (except a pointless system).
There is good low and mid power but almost no top end (nothing over 4500 rpm).
I try a lot of carb tuning an timing (fire), a new pointless system was installed but with almost no change in performance.
I check the cam timing with a degree wheel and it is ok (may be 1/2 degree offset).
I have two questions:
1 = Is it normal to have that kind of power with this cam?
2 = I wonder if someone try to give 5 degree advance on this cam (playing with gears and number of links)?
Thanks!
What cylinder head is on the engine? Since there were no "Combat" engines built in 1971, I assume this is an engine rebuilt to "Combat specs". If you are using the 1971 cylinder head, it would need to be milled to raise the compression and ports opened to use 932 carbs. The standard 1971 head had 28.5 mm ports and used 930 carbs. This may contribute to the less than satisfactory performance.
 
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Thanks for all your inputs, I will check the exhaust in the week end and give you an update.
 
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The mufflers are peas shooter std style (without balance pipe), I know they are aftermarket, probably the economy ones.
What I remember they are straight trough mufflers with only three rows of louvers inside.
You guys that had the same problems, did you try it without mufflers to find the trouble?
Thanks again!
 
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If it was stock in every respect and correct, and it was performing like that it would be breathing.
The biggest difference you can get is with breathing, all a hot cam is doing is trying to get a bit more air into it. If It gets more air the pressure will increase as with more compression. People get good flow from Commando heads without particularly huge ports.

I use a ported Thunderbolt head with twin 34mm PWK, cheap Chinese carbs around $78au a pair, on a stock Firebird, it has brilliant response and goes nuts on full throttle. To flow the amount of air it does it has to do so at higher gas speed so its very responsive and will also keep revving. The stock 30mm ports flow around 109cfm, the 34mm oval ports around 160cfm. What I think is happening is the high gas speed is forcing charge into the cylinder filling it with more air than it displaces and probably starting that at lower rpm. I did a similar head for a P3 outfit in Queensland which dominated the Queensland championship last year, running a 745cc motor on petrol against 1200cc Vincents among other things. They also lowered the lap record by 4.4 seconds. They were using 6,800rpm max. It has high comp and a hot cam and an A10 crank with longer stroke. The rider said it pulls like a train to redline.

So these older bikes actually have great cams and components that work when they start breathing.
 
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Were your S model mufflers original or some replacement type?
They were aftermarket copies, possibly Campbell’s, don’t recall. They had an extra blocking plate at the end of the baffles. they worked great with the stock cam, fell flat with the Combat cam.
 
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My mistake it is a 1972 model, i will check for the number on the head in the week end.
 
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If your carburetion and timing is correct, and the exhaust is unrestricted...
you could probably benefit from better positive crank pressure relief. Could be what Mark is talking about, but I actually can't tell.

Essentially, your engine can't make enough power to overcome the positive pressure in the crank case at above 4500RPM where pressure should be 0. I have had that problem in the past and fixed it with PCV plumbing. Lots of products out there to choose from that can help.

A search on crank case breather or engine breather might get you some answers. I'm not that familiar with what would work on your motor exactly, and what I did is irrelevant for a Commando owner.

CNW Colorado Norton Works sells a couple of popular breather solutions as does JS Motorsports.
 
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illf8ed

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If your carburetion and timing is correct, and the exhaust is unrestricted...
you could probably benefit from better positive crank pressure relief. Could be what Mark is talking about, but I actually can't tell.

Essentially, your engine can't make enough power to overcome the positive pressure in the crank case at above 4500RPM where pressure should be 0. I have had that problem in the past and fixed it with PCV plumbing. Lots of products out there to choose from that can help.

A search on crank case breather or engine breather might get you some answers. I'm not that familiar with what would work on your motor exactly, and what I did is irrelevant for a Commando owner.

CNW Colorado Norton Works sells a couple of popular breather solutions as does JS Motorsports.
I hear you, but it doesn’t make sense as when new there wasn’t any need for aftermarket crankcase breathers. My new ‘72 combat in 1973 had no problem blowing past 100mph. What has changed with regard to crankcase pressure as the engines got older?
 

illf8ed

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They were aftermarket copies, possibly Campbell’s, don’t recall. They had an extra blocking plate at the end of the baffles. they worked great with the stock cam, fell flat with the Combat cam.
I’ve been struggling with this issue for 6 years. After putting a combat cam back in my ‘72 combat roadster it has not been able to pull in top gear. I did find that the timing mark on the combat cam wasn’t right. With 10 links between intermediate gear and camshaft gear the cam timing was 15 degrees off checked with degree wheel and travel dial. I opened to 11 links and timing is right on now. Most recently I repaired a helicoil that came loose on the left front head stud. I‘m slowly bringing the engine up to redline, but haven’t tried in top gear yet. In 1st to 3rd gears it pulls great.
 
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I’ve been struggling with this issue for 6 years. After putting a combat cam back in my ‘72 combat roadster it has not been able to pull in top gear. I did find that the timing mark on the combat cam wasn’t right. With 10 links between intermediate gear and camshaft gear the cam timing was 15 degrees off checked with degree wheel and travel dial. I opened to 11 links and timing is right on now. Most recently I repaired a helicoil that came loose on the left front head stud. I‘m slowly bringing the engine up to redline, but haven’t tried in top gear yet. In 1st to 3rd gears it pulls great.
If it was advanced the inlet would close early, so at high rpm stop the intake early as air was going in under the energy it generated moving, like everyone running for the door and lots being shut out with the elevator part full.
 
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What's interesting was testing a stock 1971 A65 head against one from 1970. Usually the 1970 and earlier 30mm heads flow around 109cfm. I got numbers from one of them and compared with numbers from the oddly ported 1971, if the early one is around 109cfm the late OIF one makes around 122cfm. Umberslade tested an engine they built for the new demo Firebird they did for 1971 and found by doing the head to the design drawing, rather than short cut it like the factory, they got around 8hp more on their dyno than ones coming from the factory. They found it much more responsive and lively when people rode it. So maybe they implemented that on the OIF twin carb models.
 
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Hi Mark,

I have read lots about your amazing A65 based bike. You seem very clued up about heads & porting etc. Have you ever done any development on Norton twin heads?

Martyn.
 

concours

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Hi Levais, " I wonder if someone try to give 5 degree advance on this cam (playing with gears and number of links)?"

Pull the timing cover, have a look? 30 minutes...
 
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Hi Mark,

I have read lots about your amazing A65 based bike. You seem very clued up about heads & porting etc. Have you ever done any development on Norton twin heads?

Martyn.
No I haven't tried porting a Norton, lots of people do it well, and the information is very interesting. Norton owners seem to appreciate how much it effects an engine. It seems a bit lost on many BSA owners as they sort of fail to see what it does. And are thinking cams and compression and stuff and do not realize what cfm does especially if the port is efficient not huge.
 
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