Idle concerns

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htown16

htown16

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My 72 Commando has been on the road for a number of years and it has an annoying symptom. When you start the bike it idles okay around 1000 rpm after about 30 seconds or so. However, when you stop at the first stop light after say a mile or two of riding the idle jumps up to about 1800. Everytime you stop the idle is in that range. But after riding for about 10 miles the idle at stops drops back to the normal 1000 rpm and will sit there for the rest of the ride. I've pulled my hair out trying to fix it. If you set the idle down when it is doing the 1800 bit, it will be too low and die once it thoroughly warms up. It did it with my original carbs and with a set of new Premiers, so I don't think it is the carbs. I've checked for intake leaks. The plugs look really good.
It has a Pazon surefire ignition. Could the advance curve be messing up the idle?
Any and all suggestions would be appreciated. I've basically learned to live with it. I guess it doesn't hurt it to have it idle a little fast for several stops but the obsessive part of me wants to fix it.
 
I guess you'd have to pack a timing light and check it at startup idle and when it's up to 1800 to see if the Pazon is influencing this.
My instincts lean that way as it hasn't changed with carbs. Does the jetting look OK based on plug color?
As you say, better than stalling at stops.......
 
If it did it with two sets of carbs that does point elsewhere except were the cables common?

I had a similar frustrating issue with an idle that varied up and down. In the end it was fixed by altering the cable runs. They like to be loose and flexible.

But if thats not it then it does point to an ignition issue.
 
I have the same symptoms. I’ll check my valve adjustment, look at the cables, and look for air leaks. My carburetors tend to “hunt” at stop lights, and I’m afraid to take my hand off the throttle.
 
1973x75 said:
I have the same symptoms. I’ll check my valve adjustment, look at the cables, and look for air leaks. My carburetors tend to “hunt” at stop lights, and I’m afraid to take my hand off the throttle.
Click to expand...
Sounds like new cables and junction block time to me.
 
I think Torontonian is on to something here. I'd like expand a bit.

The cables from the carbs need to be routed such that they have minimal bends (as minimal as possible). and must have some slack (about 2 mm) when the slides are sitting on their height adjustment screws. The cable that goes from the splitter/junction to the twist should also have some slack.

The lug inside the junction should be examined to make sure that the cables are a good fit and do not release the cable(s) to the carbs faster tthan the slide springs push the slides down.

Torontonian said:
Sounds like new cables and junction block time to me.
Click to expand...

If your throttle cable assembly is original I agree totally.

Note that the springs supplied with the new carbs are much stronger than previous versions. If your old springs are in good condition you may want to consider using them. My local Ace/True Value hardware store offers many choices.

Best
 
Good points, I would also check your throttle handle bar hardware for smooth action. I use red grease on the bar to tube area. Maybe yours is sticky until it's freed up by successive movement.
 
Bar end mirrors?

When I put bar end mirrors on, the right hand one had to be eased out 1/8th of an inch, or it created a friction resistance with the throttle grip, causing a slow return to idle and a feeling it was surging a bit between gearchanges. Just a thought.
 
Most of the times I've worked on that problem it's because there is too much slack between the carbs and splitter and one or both cables comes out the splitter and rests on the edge of the hole. Sometimes it's the splitter to throttle cable with too much slack doing the same thing. And, once in a while it's the junction being worn and sticking, especially when cold.

I greatly prefer a dual pull throttle and individual cables for that reason and because syncing is easier, but I understand it's sacrilege to do Nortons the Triumph way.

I haven't tried Don's gantry but it would certainly fix the problem I'm talking about.
 
It got new cables. I don't think that is the problem. The throttle snaps back with a clunk. I would think if it was cables it would be more random.
This follows a distinct pattern every single ride. A little warmed up idles okay, more warmed up idles fast, good and warmed up back to normal.
 
Yes, the bike came with no cables, so everything was new including the throttle.
 
Timing chain tension or worn?

I've had similar issue in the past. New premiers seemed to help. Then after using a carb vacuum balancer (Morgan CarbTune) to set the cable adjusters better than I could get by the drill-bits method, the idle variance seems pretty much gone. I have an after market, stainless splitter unit from Classic Bike Spares as the stock plastic one was pretty shot.
 
Well,
I have exactly the same problem. Perfect idle from start up but once warm hangs up around 1800rpm
very annoying. There must be a simple logical fix.
My suspicion is the carbs warping slightly as they warm up but you would think that in time they would wear into the correct spot.
I’ve not considered timing. My bike has a Boyer system on it that has not given any trouble. Any thoughts?
regards
Al
 
Alan L said:
Well,
I have exactly the same problem. Perfect idle from start up but once warm hangs up around 1800rpm
very annoying. There must be a simple logical fix.
My suspicion is the carbs warping slightly as they warm up but you would think that in time they would wear into the correct spot.
I’ve not considered timing. My bike has a Boyer system on it that has not given any trouble. Any thoughts?
regards
Al
Click to expand...
New carbies ? Sorry , spending your money once again .
 
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