Commando decompressor

I think this modification may be of interest to you. It is a decompressor for Norton Commando, but it must be able to be mounted on other Norton twins.
I do not really understand the real usefulness of this system which seems very well done to me, but it is surely the work of an enthusiast,
because I've never seen anything like it on the market.

Hmmmm.....necessary? No.

But I'm curious, would it work almost as well if you just had a decompressor of this type on one cylinder?

SteveA said:

Hmmmm.....necessary? No.

But I'm curious, would it work almost as well if you just had a decompressor of this type on one cylinder?

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My thoughts exactly
Also the early model Yamaha xs650 only had a valve lifter on one side

Surely if there is alternate firing, then one valve will be open(ing) when the other is closed/closing.

So having it both sides superfluous, no?? (Or am I missing something..... for a change?)

EstuaryBoy said:

Surely if there is alternate firing, then one valve will be open(ing) when the other is closed/closing.

So having it both sides superfluous, no?? (Or am I missing something..... for a change?)

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I'd guess if you had a weak electric start for example
You could wiz the motor up then release the decomp lever
The momentum of the heavy crank would probably be enough to fire the motor
Can't see any advantage with kick-starting, with two valve lifters anyway?

hmm, interesting! I once had a BSA Victor 441 with a compression release and if you held it JUST right it was easier to kick start. They are also fairly common on big and/or high compression Harley engines...

bluto said:

hmm, interesting! I once had a BSA Victor 441 with a compression release and if you held it JUST right it was easier to kick start. They are also fairly common on big and/or high compression Harley engines...

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The 441 Victor....how are your ears? And the nerve ends on your fingers, and your kick starting knee?
I ask because I remember being launched a couple of times, among other 441 Victor "anomalies".

Lineslinger said:

The 441 Victor....how are your ears? And the nerve ends on your fingers, and your kick starting knee?
I ask because I remember being launched a couple of times, among other 441 Victor "anomalies".

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I got launched once by my Victor but I had been playing with the timing
And I did open the throttle whilst kicking it
Once sorted it was fine to start as was my b50

Persohn said:

I think this modification may be of interest to you. It is a decompressor for Norton Commando, but it must be able to be mounted on other Norton twins.
I do not really understand the real usefulness of this system which seems very well done to me, but it is surely the work of an enthusiast,
because I've never seen anything like it on the market.

https://www.leboncoin.fr/equipement_moto/2272328830.htm

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There is one potential downside.
The rocker cover studs were never designed for loading like that.
...another place to helicoil the head!

robs ss said:

There is one potential downside.
The rocker cover studs were never designed for loading like that.
...another place to helical the head!

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That's the reason the early xs650 motors had 4 studs as opposed to 3 on the opposite valve cover https://images.app.goo.gl/oLkHP6E6GeaQghWd8

I too had the 441 with the trigger under the handlebar

Torque to climb walls with, or is that rose tinted hindsight?

In the link below pictures of these decompressor covers installed.

Wouldn't it be easier to learn the knack of kick starting a Norton, if it's tuned right and kicked right no problems at all, most new owners of Norton's just don't know the knack of kicking one to life and really it's the same prince able with any big bore singles or high compression twins for kicking to life.

ashman said:

Wouldn't it be easier to learn the knack of kick starting a Norton, if it's tuned right and kicked right no problems at all, most new owners of Norton's just don't know the knack of kicking one to life and really it's the same prince able with any big bore singles or high compression twins for kicking to life.

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It's very different kicking over a big single compared to a big Brit twin Ash
As an example I could kickstart my b50 two and a half months after my knee replacement fine
But eight months in I'm still struggling to kick my commando over
And that's with an rgm Kickstart lever

When you start a big single, you use the compression release to get just past compression and then close it. Then when you kick you have no real restriction in the motor to get to the next compression stroke (two rotations). On a twin you get one rotation until the next compression stroke. Some electric ignitions do not fire on the first trigger - they are really bad on a single. Points or Tri-Spark work fine.

Smaller weaker riders carefully get just past compression and give a solid kick with follow-through to the bottom. I do it a little differently, big and still have a strong leg. I get it up to compression and kick being sure to follow through - the motor spins fast enough to make it to the second compression stroke if needed. This is because I'm applying much more force to start the kick. With points or Tri-Spark, that gives me two attempted firings and with other ignitions, you get one. That's with me on the timing side of the bike using my right leg.

My 50 something, 850 Norton owner, 6.0', strong, stepson, is always embarrassed when he tries several times to start a Norton or B50 and I tell him: "Move aside and let a real man do it" and the bike usually starts on one kick and never more than two.

I grown up with big singles from AJS's big single old thumper dirt bikes, they all have the same principles in kicking to life, yes knee injuries would make life harder, I am lucky only had 3 left knee injuries, I can't afford the high price to put an expensive ES so I just make things easier for kicking, big spark, RGM folding kicker and well-tuned motor and every knee injury I had took well over 4 months recovery for me, + it's a dying art of kick starting an old bike, every time I am parked up my Norton draws a crowd and the whisper in the back ground "wonder how many kicks before it starts" shock and behold it fires up first kick.
When I started riding in the early days it was cheap to buy an old big single British bike that everyone seemed to use as a dirt bike, the good old days are far gone, me and a few old riding mates that we been riding together for 50 years now are starting up a yearly ride for kick start bikes only, we are a dying breed.

Ashley

I could kick start an Enfield 500 I used to have. I've not yet managed to kick my 850 mk3 into life. Hot or cold, it's incredibly resistant to spinning. The kick-start will happily hold my weight! May be technique, but I've tried everything that I've read on the forum. Not sure how a decompressor would help.

Mart UK said:

I could kick start an Enfield 500 I used to have. I've not yet managed to kick my 850 mk3 into life. Hot or cold, it's incredibly resistant to spinning. The kick-start will happily hold my weight! May be technique, but I've tried everything that I've read on the forum. Not sure how a decompressor would help.

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Take your plugs out and try kicking it. Should be easy - if not, investigate what's causing it. Sometimes, the kickstart lever gets caught and will hold anyone's weight. If you know if works, then it is likely a mind over matter issue. Have you had anyone else who rides a pre-MKIII try? The 850 MKIII compression is not particularly high.

Mart UK said:

I've not yet managed to kick my 850 mk3 into life. Hot or cold, it's incredibly resistant to spinning.

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My Mk3 is much the same although it will start using the kickstart.

"Some electric ignitions do not fire on the first trigger -"

Please explain.

WHICH ignitions?