Main jet size

Not open for further replies.


I run 220 main jets on my 72 Roadster, standard 750 engine with 30 mm Mk 1 Concentrics. They are worn but they work OK as long as I coat the throttle slide with some oil every now and then. I want to ditch the original air filter and run with open bellmouths instead. What would be the most sensible jet size to start testing out with ? (I was thinking of buying 230, 240 and 260 for starters...) Do I need to go even bigger ?

Regards Thore
With worn slides you are likely sucking more air than normal. How much is a factor of how worn the slides are. It would be impossible for someone to guess what jetting you need with this situation. You should seriously consider having the carbs sleeved, buy new bodies and slides, or at least new slides.

Running without airfilters will soon turn your cylinders into condition to match your slides. I strongly suggest you keep your filter or replace it with one of the K&N filters.

Ron Leisner
I hear what you say Ron, and thanks for your concern ! I just think the bike looks better without the old filter box, and it should be possible to fit jets that compensates for the loss of the filter. All I want to know is what sizes other folks use on their unfiltered carburettors. The bike is a first kick starter, even without choke and at quite low temperatures too (5-10 deg C), so my guess is it is already running slightly too rich.

What K&N filters do you recoommend - I guess these will have to match the resistance in the old original filter if I were to use the same 220`s ?

Regards, Thore
The K&N I refer to is much smaller than the stock one. It clamps over both carbs and is said to flow more air than the stock paper filter. It's available from many Norton parts dealers such as Old Britts and others.

Jetting the Amal is more than just changing main jets. In fact, the main has very little effect until wide open throttle. At lower throttle openings, the metering of fuel is controlled by the pilot jet, the needle and needle jet, and the throttle cutaway. A worn slide will affect the performance of all these. That's why I don't think you will be successful by changing the main.

Incidently, my '73 750 with 932's and the high compression head has 220's in it (stock airfilter) and runs slightly rich. It starts first kick and idles at 1100.

K&N air filter


I'm using a K&N air filter that is the same size as the original paper filter. It fits into the stock air cleaner can. I also have the smaller type the clamps to the two Amals, but suspect the larger K&N will flow more air than the smaller. I'm using sleeved original Amal 932/19 and 932/20 concentrics and 230 main jets. The needle clip is in the middle position, I installed #25 pilot jets and new needle jets and jet needles. I may be running slightly lean as the pipes are discolored, but carburetion is very clean at all opennings and the idle is smooth. I've tried the top and bottom needle clip positions and both make a drastically poorer result.

I like the orginal air box as it looks like what was used in the late 60s on high performance America muscle cars with high rise intakes. It's the perforated surround.
Not open for further replies.