Single Mikuni Jetting '71 750 Commando

Hello Norton lovers. My '71 750 is a work in progress that I am methodically sorting. Next on the list is jetting. Single Mikuni. Starts and idles well. Runs great at partial throttle or even full throttle a low rpm's. But falls on its face at mid to higher rpm full throttle. It has 250 main. Just ordered a kit with 200 to 300 mains, and reckon i need to go up a new sizes, but checking in to see if I am in the ball park and headed in the right direction. Thanks in advance.

A single carb can be a disappointment in a twin above 1/2 throttle.

Big Red said:

Hello Norton lovers. My '71 750 is a work in progress that I am methodically sorting. Next on the list is jetting. Single Mikuni. Starts and idles well. Runs great at partial throttle or even full throttle a low rpm's. But falls on its face at mid to higher rpm full throttle. It has 250 main. Just ordered a kit with 200 to 300 mains, and reckon i need to go up a new sizes, but checking in to see if I am in the ball park and headed in the right direction. Thanks in advance.

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Forget about the RPM and just concentrate on throttle position
It's worth marking the twist grip in 4 positions from fully closed to fully open
Then you can see what circuit of the carburettor you are on
It could in your instance for example be the needle jet that's too small

I have a single carb (Amal 34 mk2). Pulls fine at higher revs. Can be done

As yours pulls fine on a wide open throttle at low revs, I'd be looking at what is different at high revs. As more fuel is pulling through, is the float level high enough? Are your tap and fuel lines fully clear. Try with both taps open. Check ignition is advancing.

Thanks for the suggestions. I plugged in a 260 and the difference is night and day. It pulls cleanly through the rev range at all throttle positions. Got lucky maybe. Will go up another sie and see if it pulls any harder, but the massive flat spot is gone. I wish it was that easy to fix the oil leaks.

Big Red said:

Thanks for the suggestions. I plugged in a 260 and the difference is night and day. It pulls cleanly through the rev range at all throttle positions. Got lucky maybe. Will go up another sie and see if it pulls any harder, but the massive flat spot is gone. I wish it was that easy to fix the oil leaks.

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Where are you leaking oil from?
What breather do you have?

It might be an idea to post the sizes of all the jets, air correctors and needles.
It may be you have a different needle. In addition, sometimes a flat spot like you had can be corrected
by raising the needle a notch or two in the piston. For example, you may be running a little rich now and want to drop the needle a notch (and then two) and see if the lean spot comes back.

Check your air jet first. 32 to 34mm carbs tend to work best on 4 strokes around 0.9 to 1.2. I have seen guys here mention 1.1 air jets working well. 36mm and 38mm carbs tend to like smaller air jets like 0.5 range. slide will most likey b a 2.5 or 3.0. 6DH3 or 6DH4 needle seems to be the choice for a norton. I would get a series of jets for pilot, needle jets, and main jet. I personally have a range of pilots from 20 to 50, needle jets from O5 to Q8, though the P0 seems common with these bikes. Mains tend to range from 220 to 260 with a single carb.

The single carb mikuni and amal run out of legs and struggle over 6000 rpm. This is due most likely to the small manifold plenum.