Question About Tuning My 1971 Commando 750

Hey guys, I'm new here and haven't posted anything but have been a member since I acquired my 1971 Commando 750 months ago. I have a question about jetting and tuning my bike. It was sitting for around 10 years before I got it. When I got it home I adjusted the valves, changed the oils, put new plugs in, plug wires, rebuilt the original Amal 930 carbs with gaskets, cleaned the pilot jets, adjusted the float levels, new air filter, new balance tube connecting the carburetors. I checked the carbs and they are synced. I adjusted and cleaned the point gap too. And when I start the bike, it starts the very first kick and idles. But when I first took the bike for a ride, I would try and get it past quarter throttle and then bike would just start to blubber, lose power and pop and sputter. It sounds like crap. I've checked the plugs and they look like they are running VERY rich. So I checked and changed the jets and went down to a 200 on the main jets from a 210, and went to a 105 on the needle jets from a 106. I lowered the needles too from the middle to the top position. I took it out for a ride today and the bike is still running rich past half throttle now. The plugs are still very black, a slight improvement but still very black. Now the blubbering and popping is coming in past half throttle. So it's an improvement but still it runs bad when you get on it. It cruises through right along below half throttle but not when you go past that. The air screws on the carbs are set at 1 1/2 turns out from seated. What are your guys idea on what I should do next? Should I go down on the main jets again? Please let me know and thanks for your help!

Owned it for 750 months?
Back to the basics: do you have compression? Have you timed the engine? Have you tested the coils?
I think it's a big mistake to make multiple changes to the carbs all at once. There is a reason for the stock configuration of Amals on a Commando: IT WORKS. I'd clean the idle circuit again, look for air leaks and then be looking at electrical/ignition issues. You don't by any chance have the choke on, do you? Slides down, choke on. Slides up choke off.

I changed the jets which is the only adjustments I made that can be easily changed back to stock, or at least what were originally in the bike when I got it. As for the choke, I have no clue, I never touched it and now I feel like and idiot if that is the problem. Never owned a Norton before and am just learning as I go here. The first thing I need to do tomorrow is check the choke and see what position it's in currently. Thanks for the heads up on the possibility of the choke being on.

Next time you take it out, put a fresh set of plugs in.

The symptoms also sound like fouled plugs. Sometimes you can clear fouled plugs by giving it as much throttle as it will take without stumbling, them gently give it a little more then a little more until it will take full throttle.

You didn't mention cleaning the tank, petcocks and fuel lines. Sounds like you have a fuel delivery issue. Normal carb clogging will affect starting and idle, since those passages are smallest. As mentioned previously, you should also make sure ignition timing is spot on.

Are you operating the choke correctly? Cable pulled tight means the choke slides are up = choke off. This one gets a lot of new Norton owners. Some even say it is backwards?

Ok guys, so it ended up being the choke was on. Rookie mistake. I'm learning as I go with my bike. I have never owned nor ridden a British bike before my Norton. But now since I know what's on and what's off for the choke, the bike runs great. I'm in love with my Commando and can't wait to take it for a longer ride after I register it. I've only ever owned and collected vintage Japanese bikes but this bike may be now at the top of my list for my favorite bikes in my collection. The thing is a rocket too and the sound it make when you get on it is awesome! Thanks for the help guys!

Glad to hear it's OK now.

In case of mixture looking too rich, it may also come from sparkplugs being too cold. As far as I know, grade 7 on NGK scale (eg : NGK BP7ES) is OK for our bikes in average conditions.

L.

Cal-Neva Speed Shop said:

Ok guys, so it ended up being the choke was on. Rookie mistake. I'm learning as I go with my bike. I have never owned nor ridden a British bike before my Norton. But now since I know what's on and what's off for the choke, the bike runs great. I'm in love with my Commando and can't wait to take it for a longer ride after I register it. I've only ever owned and collected vintage Japanese bikes but this bike may be now at the top of my list for my favorite bikes in my collection. The thing is a rocket too and the sound it make when you get on it is awesome! Thanks for the help guys!

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it's an easy mistake to make , we all make mistakes ,I always throw those choke slides away and blank the carb tops, there are a few things that are different on a commando like the grease nipples on the swing arm that take oil NOT grease etc etc , learn how the isolastics work ,do your research on here before anything, cheers