fuel mixture on the S model Commando

My apologies if this has already been discussed (I tried searching the forum without luck).

I have a '69 750 S model Commando, with both pipes high on the left side. Obviously, the headers are asymmetrical with different lengths and curvature. I just removed the pipes to install some new headers and it looks (from the inside of the headers) that the two cylinders are not running at the same fuel mixture. The right cylinder appears to be running a bit leaner than the left side.

The bike has new flat slide carbs (common K&N air filter for both carbs as well). The bike seems to be running well otherwise (runs smoothy, accelerates fine).

My question is: has it been found to be true, for that model of Norton with such different exhausts for each cylinder, that it is sometimes necessary to run different settings (needle groove, for example) for each carb?

Thanks in advance for any feedback....

In my experience with a Norton it would be common to run different size jets in each carb if you want them to run at the same mixture -even with matched exhausts. There are usually variations in cam timing, porting and compression along with variations in the carb itself. Particularly Amals and Chinese carbs. Jim

Dunno about jets... the need to do that that would seem to indicate mechanical descrepancies (ignition, valves. compression) more than any difference in pipe length.

Adjusting the needles to suit individual cylinders isn't uncommon... but usually isn't necessary either. If you find spark plug "color" variations between cylinders, that supports what you are seeing in the pipes, you might try shimming the needle... rather than moving the clip a whole notch. Clip changes are p'robly too "coarse" to allow any kind of balance between cylinders. Shimming does things in finer increments.

Believe it or not... you might have better results trying an old exhaust "tuning" trick. Sounds crazy but temporarily wiring/attaching/taping a screw driver or bolt up against the middle of the outlet of the muffler on the pipe that's "odd" - then taking a ride and checking pipe/spark plug "color" again - is a pretty fair "shade tree" method of determining any differences in back pressure between one pipe and the other. (Some people have even discovered it helps mid-range so much they braze the "bar" on permanently. (Inside the muffler where it can't be seen.) Kinda of let's you "tune" back pressure without resorting to baffles or mutes.

Are you running genuine, original factory peashooters... BTW?

I've found it necessary to vary needle jets and needle positions to accommodate slightly different muffler back pressures when using other muffler makes, models or exhaust tract lengths--it's not ideal but effective between overhauls or carburetor replacement



Tim Kraakevik
kraakevik@voyager.net

Thanks for the replies so far.

Yes, Kip, I am running the stock factory peashooters (completely unobstructed)

coreyl said:

The bike has new flat slide carbs (common K&N air filter for both carbs as well). The bike seems to be running well otherwise (runs smoothy, accelerates fine).

Click to expand...

What type of flatside carb. These are more suseptable to variations and differences in intake and exhaust characteristics than the Amals. If they are the PWK's then these carbs can be inconsistant from carb to carb.

I recall a needle clip postion difference and slow jetting the from a 40 on one and a 35 on the other. If these are the PWK, as I suspect, jetting and the like are available at MikesXS.

http://www.mikesxs.net/products-82.html#products

They are the PWKs from JS Motorsport.

"clit" position? Not sure what that is, but I like the sound of it.

;-)

coreyl said:

They are the PWKs from JS Motorsport.

"clit" position? Not sure what that is, but I like the sound of it.

;-)

Click to expand...

opps! I edited that out.

OK. You need to go through these with a fine tooth comb in particular, the jet holders are funny (not in a "haha" way) and can be cockeyed dragging the needle on the main. Pulling the bowl is a trick in itself having to bring it apart on an angle. Once you do it a few times it will get easier.

Anyhow, expect slight jetting change from one to the other as stated before.
I believe the only thing proprietary on these is the needle where Jim can supply these if needed.