cam adjustment

I use the standard 850 cam advanced 12 degrees to compensate for a two into one exhaust pipe. The exhaust valve opens too early so the bike is very loud. With the torque increase, I was able to raise the overall gearing quite considerably, yet the bike still accelerates at about the same rate - the top speed is higher. I don't use a vernier sprocket. I simply took the original and softened it, then took it to a machine shop and had two more key-ways broached in at random. I didn't bother to re-harden it because I am racing the bike. With that arrangement I can get the desired timings within a couple of degrees. You will probably find that if you open the exhaust valve much before 85 degrees BDC, the noise will get louder and the bike will go slower, unless you are running a two into one exhaust. My inlet valve opens at about 65 deg, BTDC. It moves the start of the power band up a bit. I don't understand why you want to advance the cam 4 degrees - are you using a very restrictive exhaust system ?

You are way ahead of me here. Since you have asked briefly let me recap my situation.
After installing the JS 0 cam I set it to his specification. My main
worry was no valve clash and I measured with a sized wire and seemed to be ok.
It ran acceptably but as Ive not had a Commando in many years Im not able to
say it felt especially good or bad.
The rub came when I discovered it was difficult to start. Real difficult. Having had
lots of brit bikes and lots of miles I knew this was not standard. Once running it
was fine though.
So: decided to check everything from ground zero. You cannot trouble shoot if you
are guessing. Guessing will occur soon enough on its own account. I want to have
the valves as close to correct as I can. Since it seems that I have it late 4-5
degrees that needs to be rectified . Timing side stripped today and Ill dial in the
cam tomorrow I hope.
This isnt a racer (Id not be using a JS 0) just a streeter. Exhaust is AN peashooters
(no crossover) with mutes. It may just be that Ive developed modern sensibilities but the exhaust
seemed seriously loud when the revs rise. The mutes seem to make a small
improvement.

Correct cam degree is sliding scale of 'correctness' once no valve or piston clash. Retarded tends to move power peak rpm up while advancing moves it lower. For public use I'd suspect more fun advanced as you are aiming for. Would be informative if you could tell us what you sense response wise now and then advanced.

I've never had a situation where a bike was difficult to start because of the cam timing. It would have to be grossly wrong or tappets too tight so leaking compression. 4 degrees retarded cam timing would probably affect performance only. I usually run my 850 cam 12 degrees advanced, I've bought a combat cam which I will run 6 degrees advanced - to compensate for the exhaust pipe. It is not that critical. Fitting restrictive mufflers would have the same sort of effect as a 4 degree lag in the cam riming. You feel the edge go off the motor.

If the bike is difficult to start, I'd be looking at carburation and spark rather than cam riming. You are talking about 4 degrees error in cam timing, it is difficult to get repeatability when checking with less than 2 degrees variation.

acotrel said:

I've never had a situation where a bike was difficult to start because of the cam timing. It would have to be grossly wrong or tappets too tight so leaking compression. 4 degrees retarded cam timing would probably affect performance only. I usually run my 850 cam 12 degrees advanced, I've bought a combat cam which I will run 6 degrees advanced - to compensate for the exhaust pipe. It is not that critical. Fitting restrictive mufflers would have the same sort of effect as a 4 degree lag in the cam riming. You feel the edge go off the motor.

Click to expand...

FYI, Onder is looking to advance the cam, not retard.

Thanks, you guys answered my question and it doesn't relate to what I'm struggling with. Norton original cams have the timing dot alignment that is usually accurate. I did the alignment and also checked timing with a degree wheel and Starett travel dial. Was pretty sure I got is right, but you know how 60 plus age has that certain amount of doubt.

pete.v said:

acotrel said:

I've never had a situation where a bike was difficult to start because of the cam timing. It would have to be grossly wrong or tappets too tight so leaking compression. 4 degrees retarded cam timing would probably affect performance only. I usually run my 850 cam 12 degrees advanced, I've bought a combat cam which I will run 6 degrees advanced - to compensate for the exhaust pipe. It is not that critical. Fitting restrictive mufflers would have the same sort of effect as a 4 degree lag in the cam riming. You feel the edge go off the motor.

Click to expand...

FYI, Onder is looking to advance the cam, not retard.

Click to expand...

From my understanding, he is having trouble starting the bike. If it is a matter of getting the timing to spec., that is a pretty simple thing to fix