What causes the unstable idle speed in Commandos?

Status
Not open for further replies.

Yellow_Cad

Joined
Apr 2, 2008
Messages
492
Country flag
I am fairly new to Nortons and have a '73 Interstate with a single 34mm Makuni. My idle is erratic over a range of about 600 or 700 rpm. I have cleaned the carb well so next suspected the linkage that takes a tight bend just after leaving the carb but then I read a comment that Boyer ignition would stop the erratic idle in Nortons. What could be causing erratic idle?
 
My first thought would be an air leak. With Amals this is often due to slide wear. Is your Mikuni manifold leak-free ? Try running an oily finger round all the joints while it's running and see if the tick-over alters.

A worn mechanical advance-retard could give an erratic tick-over.

Don't forget though that a Commando is a living, breathing beast and its heart rate can go up and down a bit. The carb or carbs are being given a good shaking by the rubber mounted engine as well, especially at tick-over. Erratic fuel levels could also give these symptoms. Is it worse on the stand ?

If it's crisp to ride, is it something that you don't want to live with ?
 
I have already adapted to it and work around it with no problem. Just thought I would look into the cause in case it is an easy fix or something I should know about the bike. Thanks for the response.
 
erratic idle

Over the 30 years of owning my Mk11 commando the idle has become more erratic. I can tune it , have it running sweet in the shed idling beautiful. Out on the road coming to traffic lights etc it idles anyware from 1800 down to 800 and a stall. Ive replaced rings and valves(oil scraper ring had collapsed) new amals installed, new throttle cables, new points and backing plates. Still erratic idle. but a joy to ride on the open road.
What I have not yet looked at is the timing chain & adjuster and also the bob weight mechanism for advancing the spark.
I have a trispark electronic ignition to install which will eliminate the mechanical advance which may be worn and sticking.
 
actually if I can achieve an idle of 600-700rpm without stalling then i would think i'd got her down perfect. Enjoy your machine !! :!: :!:
 
I don't like lugging air-cooled twins below 1,000 RPM.

With the single carb in proper nick, you should be able to EASILY achieve that.
 
Ditto--GrandPaul's 1000-RPM's
I too like it to idle around 800~1000 RPMS. It just seems to be smoother there and my oil pressure reading is higher which I also like. 600 may be ok for a Chevy small block--but not my Nortons.

Joda
 
Sorry, I wasn't saying that my idle is 600 to 700 but rather I was saying what dgwilson is saying: I set the idle at 1100 or 1200 and once in a while (not rare) at a stoplight the idle will be at 1800 or so. At the very next light, it can be right back down in the proper range again. Actually if it didn't move around I'd have it more like 1000. Since I am not always sure if I am at the lowest point, I keep it up a little, thereby 1100 or 1200 and it doesn't die.
 
If you have the heavy duty cable that comes with these kits it's no wonder your having problems. When you kink this cable as it enters the carb the slide can gall, chromed or not. This single carb problem was the motivation for me to learn to do my own cables from the lightest nylon lined products available. The good part is they seem to last for a very long time when properly done.
 
My 2 cents on this subject:
If the auto advance unit is worn in the weight slots and/or the mechanism is not serviced/lubricated it won't work properly and then sometimes the weight/s do not return properly at idle and you will have incorrect (too high) ignition timing causing the idle to get high.
Fitting a Boyer you remove the auto advance unit and will not have this problem but if you are servicing the auto advance unit it should not give you this problem. The problem here is to find good replacement parts when needed.

Using Amal carbs you will have more or less throttle air leakage but this can be fixed with a re-sleeve mod that if properly done, will give you the right clearance and also last for a long time. Buying new carbs temporarily solve the problem but it is coming back soon again.

There are a number of bikes here running with re-sleeved Amals and refurbished auto advance units and these bikes have a solid 1000 rpm idle.
I have the re-sleeved Amals fitted now and a serviced (but not totally refurbished) auto advance and as soon as the snow is gone and the temperature is at decent levels I will do more tuning and hope to have a relatively stable idle now.

Cheers,
Per
 
When my idle does inadvertantly go up to 1800, I have shaken the throttle cable where it suspends from the twist grip to where it runs along the frame and there is not the slightest change in rpms. I would think that if it was a throttle linkage issue even back in the carb, there would be some wavering of the rpm when I did this. I will look into the auto advance possibility.
 
I think I have a method to help get this sorted (please let me know if you agree with me?).

Try this when the engine sticks at a high RPM idle:
Select first gear, keep the bike a standstill with the brake, apply clutch pressure enough to load the engine forcing the idle RPM down.
Then you pull the clutch again to see if the RPM stays low or increase again.
If the RPM stays low I would say you most likely have leaking carb trottles (the extra airflow leaking by the trottles allows the engine speed to stay high until you load the engine).
If it goes up again the problem is to be found somewhere else.

/Per
 
If it's the cable setting up the problem you should see some scratches on the slide or body. The slide at rest on the idle screw tip takes a pounding when the bikes at idle. Checking some old Amal's with the slide at the bottom and a finger at each side of the slide will revel the rocking motion that is classic. Carb slides need about .003 clearance to keep them from sticking. If the cable has some pressure on it from the tight turn into the carb at the top the engineered slop can get used up. I have seen brand new chrome slides scratched after just 300 miles of use with no other symptoms than the ones you describe. Binding near idle. It's worth checking. Norton's shake their carbs more than other bikes like them.
The other suggestions about the AA unit are good. Many who have a single Mic would also have a E.I. of some sort they kind of go together. Same kind of thinking by the owner. The lack of real Lucas points keeps me away from the old set up. That and the expense of a good A.A. unit.
 
I guess the only thing keeping me from going to E. I. is the stories I hear about the reaction to a slightly low battery (difficult start, etc). Apparently, some E. I. types are better than others about this but right now I don't know enough about which one does which to make the leap.
 
The most susceptible seems to be the Boyer but I wouldn't have any reservations about running one, except on the Electric Start bikes.

If a misfire or hard starting develop then the battery condition and the wiring are the first thing to check but if they're OK then you're unlikely to have other problems.

In my experience, battery problems rarely begin without warning and first of all you'll have a tendency to pop and bang and kick back, giving you ample time to check things out.

I've run Boyers for more than twenty years and have never not been able to get home. The worst I've had to do is fiddle with connectors and wiring a bit.

If you try it, I think you'll find any of the systems a great improvement on a knackered mechanical set up.
 
As 79x100 said, go to electronic ign. Boyar or whatever you choose. I too have been running on Boyars since around 1986 and have never had a failure. But---I have a back-up black box with me whenever I'm out and away from home. Cheap insurance if ever the Boyar does take a crap. The difference it makes to the running, idleing and all around performance is well worth the price. Get rid of that antiquated mechanical advance unit and points. That's why the automotive industry got away from mechanical ignition systems and went electronic---cheaper to produce and less headaches for the owners.
You also may have an air leak around the carbs and manifolds---to make a fast check for this---BE CAREFUL NOW-- take a spray can of contact cleaner or brake cleaner and spray all around the carb area--if the engine starts to change rpms---you have a leakage problem to cure. You can also use starting fluid spray to check but be careful--it has a low flash point. Try not to spray around the intake or the air cleaner area so the spray doesn't get sucked in and cause a false reaction.

Joda
 
Per G said:

Try this when the engine sticks at a high RPM idle:
Select first gear, keep the bike a standstill with the brake, apply clutch pressure enough to load the engine forcing the idle RPM down.
Then you pull the clutch again to see if the RPM stays low or increase again.
If the RPM stays low I would say you most likely have leaking carb trottles (the extra airflow leaking by the trottles allows the engine speed to stay high until you load the engine).


When the idle went up (around 1800), I loaded the engine and the idle came down and stayed down. So if it is the carb (in my case, a single 34mm Makuni), is it leaking internally or would it confirm that it is the carb if I use starter fluid spay to see if the revs increase? The story behind my bike/carb is that the PO (a Norton guy) had not sealed the fiberglass tank and the carb had stuck open on him a few times. I sealed the tank and completely cleaned the carb slide and throat. Works fine except for the idle issue. There is definitely a tight bend in the cable after it leaves the carb. It is an Interstate so there is lots of tank over this whole area. I don't quite understand the carb sleeving option since I understand that the slide must be a precise fit. Thannks for any help.
Click to expand...
 
Sorry, I'm still getting used to using functions like quote, etc. Only the first part of what shows to be a quote is from Per G; the remainder is my question which is as follows:

When the idle went up (around 1800), I loaded the engine and the idle came down and stayed down. So if it is the carb (in my case, a single 34mm Makuni), is it leaking internally or would it confirm that it is the carb if I use starter fluid spay to see if the revs increase? The story behind my bike/carb is that the PO (a Norton guy) had not sealed the fiberglass tank and the carb had stuck open on him a few times. I sealed the tank and completely cleaned the carb slide and throat. Works fine except for the idle issue. There is definitely a tight bend in the cable after it leaves the carb. It is an Interstate so there is lots of tank over this whole area. I don't quite understand the carb sleeving option since I understand that the slide must be a precise fit. Thannks for any help.
 
Status
Not open for further replies.
Back
Top