Mikuni carb vs Combat engine 72 commando

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Jul 27, 2004
What a great forum. This is my first post.
I've owned a 72 Norton for 15 years but 10 of those years the
bike sat in a garage. I owned a '72 in '74 but this bike I bought
one day in 89' from a Vietnam Vet who couldn't start it anymore.
I know he was a performance freak.
While reading this forum I have learned more than I did in 20 years
of owning a Norton.
When this bike ran it was unreliable and finally in '92 it cracked the
cylinder walls. I took it to a shop that I thought knew about them.
I got a call during the repair that the pistons were oversize for the
new standard barrels. I ordered new standard sized pistons.
From reading I now realize that this engine is a Combat engine and
I probably put a standard barrel and piston on it.
I just now figured out that the Mikunis are 34 mm.
What should I do? I'm not changing the cylinders or pistons.
What will happen with a Combat head on standard sized cylinders?
What should I do with these Carbs? Replace them? With what?
Thanks in advance
Hello and welcome
I run a 72 Combat here in the UK with a 34mm SINGLE Mikuni carb. It's easy to start , idles perfectly, and enjoys great torque, and is only a smidge down on top end when compared to a twin carb setup. IMHO if you want to enjoy it as opposed to fiddling with it, I would get hold of a single carb manifold and set it up.

Have fun( and fit electronic ignition!)

Thanks for the reply.
I understand about the torque vs highend and the
need for a different manifold for the single setup.
Is the single 34mm Mikuni different than one of the two
I now have? Could I use one of my carbs or would it be
smarter to just buy all new?
What about having standard cylinders and pistions with a
Combat head? Is this now just a detuned engine.

And a bit of humor:
Norton's where always considered the
pinnacle of the motorcycle world in the 70's. We used to
laugh at anyone with a Harley.
( Good exercise bike while pushing it down a road. )
The fellow who sold me the bike had just refused the last
2 or 3 people's offer because they didn't know enough about Norton's.
I just had an offer from a fellow but he didn't know anything about
Nortons so I decided to keep it and restore it.

The combat engine didn't have special pistons or cylinder although the cylinder was painted black rather than silver. The differences from the standard engine were the combat head with .040" milled off for higher compression and 32mm intake ports. It also has the double S camshaft and 32mm Amal concentric carbs instead of 30mm. The engine cases were altered with stronger internal webbing and the "combat" engine breather, dropping the oil sump filter.

You can run a single 34mm Mikuni with little performance loss until you get into the highest rpms. The twin 32mm carbs are better at the high range.
David, Thanks for the reply.

I guessed than that the original owner had bored out the cylinders
and put oversized pistons in. I suspected that but got confused
when looking at the Norton parts manual.
It shows two different part numbers for the "standard" vs "Combat"
pistons. I guessed that they must have been bigger.
But you are correct the parts manual shows two different part
numbers for the cylinders but than says "silver" or "black"
no mention of size difference.

This info has helped me greatly as to where to go with the bike.
I do have Boyer Bransden ignition.
I took it out for a spin the other day after draining the gas and
changing all oils. Ran real good. Just the chrome is pitted.

Thanks again. Good riding to all.
Hello again
I believe that one of your Mikunis should be fine, you could try Allens here in the UK as they are Mikuni specialists. Then, sell the other one on E bay!

You can also reduce compression by using a thicker head gasket, which brings it back to standard spec, and, if you're still runiing the original pushrods, gives those an easier time too, as well as your knees.

Have fun

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