74 commando idle rpm

Hey guys I am new to Norton motorcycles and this forum. I have a question about my 74 Norton commando 850, recently, I changed out the standard American style handle bars with a pair of Norman hyde m type no rise bars. After changing I took the bike for a ride and when stopped the idle is now at 1500 rpm whereas before the swap it was idling warm around 1000 rpm. I don't know if the two are related or even if I have anything to be concerned about, but what should the warm idle rpm be set at? Any help would be great thanks.

would it be that the change of handlebars has somehow impacted on the throttle cable so that it is not closing off properly?

I thought the same thing but it isn't hitting the reservoir, like I said I'm not sure if the two are related but idling at 1500 seems a bit too high.

Idling RPM should read 1000 or thereabouts. 1500 is too high and I imagine the engine would be jiggling around a bit at those RPM.

So to check what is going on, try screwing out the carburettor slide setting screws (idling screws) note where they are positioned before altering. If the RPM don't drop, you can bet the cable assembly is the culprit, or the twist grip is assembled incorrectly?

Unscrew the twist grip and see if that resolves the issue, if that does not work, check the routing of the cable to the junction box, you may have to remove the tank for access.

If you need to start from scratch, I assume you have the original Amal carbs? Remove the air filter, put some small drill bits under the carb slide cutaway, and adjust out the idle screws, then slowly adjust up so that the drill just twitches, from there screw in one turn. This is a base setting. Now set up the cables so you have equal free play, you can fine tune this with a couple of 1/4 inch balls, rest them against the cutaways, then carefully open the twist grip, the idea is to get the balls to drop past the cutaway, simultaneously, but don't let them go past the back of the slide, bad things will happen if you do!

The above will get the carbs mechanically syncronised, you may have to adjust the idle screws to get your desired RPM idle.

HTH Regards Richard

Decemberist said:

After changing I took the bike for a ride and when stopped the idle is now at 1500 rpm whereas before the swap it was idling warm around 1000 rpm. I don't know if the two are related

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I'm sorry but this type situation is a red flag for any mechanic. If your idle was steady at 1000 before the change was made and now it's 1500 and the change was the only thing different (as in, oh by the way, the bike sat for 8 months before I changed the bars), then the changes you made almost certainly caused the idle speed change. There isn't anything wrong with checking the carbs and making adjustments as necessary but I wouldn't touch a thing until I very carefully examined the throttle, cable, clearances for same, their run and connections to the carbs, etc.

VintAge nails it:

I wouldn't touch a thing until I very carefully examined the throttle, cable, clearances for same, their run and connections to the carbs, etc.

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I will check the proper routing and tension of the throttle cable. I don't think it has anything to do with the carbs, so I will leave that alone. Thanks for the advice I'm sure I'll have a few more questions at a later date.

I agree with the cable routing change being the culprit or at least the 1st thing to check. Putting a tighter loop in a cable will change the difference between the outer cable sleeve (that anchors to the bike) and the inner cable (that connects the carb to the throttle grip) distances. You may need to re-adjust the throttle cable adjustment on the cable.

Most definately look at the routing, and the way the slides snap shut onto the idle addjuster screws.
Possibly a hook up on the 2 into 1 junction.
If all is well, then it may be a classic case of worn carbs, and AUTOMATIC Idle adjusting.
AC.

I checked the throttle cable tension and routing and it looks good. Being that I switched to no rise bars I now have plenty of slack and virtually no tension. One thing I did notice today my rpm was steady at 1000 then after riding for about twenty minutes idle was not 1500. No idea! I don't think my carbs are worn out the bike only has 5800 miles on it and it was/is running nice aside from the rpm issue.

The rpms do tend to go up after they warm up for a while. I'd set it when hot and see if you can keep the air control on at startup a little bit. It takes mine a good 15 mile run to warm up before I can set the idle, I tend to turn off the air control right away after I start up though, just keep an eye on the idle until it's plenty warm.

Dave
69S

DogT said:

The rpms do tend to go up after they warm up for a while. I'd set it when hot and see if you can keep the air control on at startup a little bit. It takes mine a good 15 mile run to warm up before I can set the idle, I tend to turn off the air control right away after I start up though, just keep an eye on the idle until it's plenty warm.

Dave
69S

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Thanks Dave, I will definitely adjust the carbs down a couple hundred, I think that If I can get a steady 1200 rpm when warm I will be satisfied. My main concern was stress on the engine with the rmp's too high at idle.

I like to get mine down to about 1K. Everything else seems to high, but lower than that I don't like, it starts to make strange noises. At 1K the girls love it (as far as I know, at least that's what I like to think).

Dave
69S