Roadster stalling

My 1970 Roadster stalls after it has run for a while, once the throttle is off. Otherwise it is running well; brand new plugs, idling well at start, running well at highway speeds; new amal twin carbs - which have just been adjusted by a mechanic - but only upon start up not after the engine is hot. Boyer ignition. Any thoughts?

Just a clarification. You're not saying that the mechanic adjusted the carbs when the engine was cold are you? That's how it reads. If that's the case, then he isn't much of a mechanic. Carb tuning should be done when the engine is up to operating temperature. If adjusted to run well when cold, then the mixture will be far too rich when the engine gets up to temp, hence the problem.

Are the plugs black? Turn the mixture screw out 1/4 turn and adjust the idle screw to suit. There is a procedure that involves turning the mixture screw to the highest idle point then adjust the idle with the idle screw and repeat till proper throttle responce and idle holds.

The other aspect is the pilot circuit. What does your mechanic say about the condition of the pilot circuit. Was it clear? What did he do the varify this? What does he say about the relationship between the throttle slide and body? Is it loose and sloppy or tight and smooth?

All Norton owners must become intimately familure with the Amals. These are simple carbs and plenty of information is available for the novice and/or seasoned mechanic to master these devices.

check battery voltage as electronic ignitions need a good battry to work propperly.

Fullauto said:

Just a clarification. You're not saying that the mechanic adjusted the carbs when the engine was cold are you? That's how it reads. If that's the case, then he isn't much of a mechanic. Carb tuning should be done when the engine is up to operating temperature. If adjusted to run well when cold, then the mixture will be far too rich when the engine gets up to temp, hence the problem.

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That's how one could read it, but I doubt that's what happened.

How did your mechanic adjust the carbs ? Are the slides synchronized ? Are the pilot screws turned out nearly equal amounts ? If not, I'd guess a blocked pilot jet. If your carbs are brand spanking new, check the fuel height in the bowls. The fuel level in new float bowls was too low to let my bike idle consistantly.

Greg

Thanks for the input guys. A number of good leads to follow up. I am away from the bike at work for the week, and will hopefully make some progress over next weekend, and report.

It may seem strange, but I set up my low speed tickover with the engine only just warm enough to enable it to start without the choke. As been mentioned, once the engine is fully warm after a good run it will run rich with these settings applied and the tickover will be found to be very fast - adjust back using the throttle stop screws to get a good even tickover as per the manual. I reckon as your mechanic, as he has said, done it with the engine fully up to temp, and the slow running screws are right on the edge and too lean to supply the fuel when the throttle is snapped shut causing it to stall. The float height, as mentioned could be the cause, but I thought it would have to be way low for this to happen, but would surely be highlighted when rapidly acellerating to high speed, high speed 80-90mph plus , with the throttle wide open as this puts a lot of sudden demand on the carburettors. The manual is very good, and has worked for everytime in the past 15 years and I now swear by AMALS.
If it was a problem with just one carb, it would manifest itself as lumpy / poor running which makes me think this will be easily sorted. The regular checks I do are, plug check to inspect colour, fingers down carb intakes to check both start to lift at same time and they both clear ( full throttle ) at the same time.
May be a mad idea but the insulators for the carbs are fitted, and not been omitted by mistake.
Hope you get it sorted and enjoy the performance as the AMALS poor fuel into the into the firey abyss

I spent some time with the mechanic this weekend looking at the bike. We have concluded that the stalling problem is not the carburettors, but a problem with the charging system. When the stalling occurred I had been riding for an hour or so with the pilot light on, on a rainy day. When we first started it was not properly firing in the right hand cylinder.

The mechanic put his voltometer over the battery and we found that it would not charge until revved quite strongly. Mechanic's view is that it should have been doing better.

The connections to the battery were loose. I took it home and recharged the battery, and scraped off the powdercoating on the frame to ensure that the earth wire was connecting properly; then rode on a sunny day at highway speeds for two hours with no lights on and there was no stalling or missing at all. I took note of the ammeter; at less than 2500 revs it was showing about -1 amp; at over 3000 or so the needle jumped to positive - it waves all over the place, but got to plus 12; which presumably means that the system is charging.

So, I conclude that somewhere the charging system is deficient, and that the Boyer ignition is not getting a consistant 9 volts when the lights are on and draining the system. I am very ignorant on such things, but have downloaded the Boyer trouble shooting notes. Next stage is to work out where the system is deficient; and how to deal with it. I guess a paliative solution will be to replace light bulbs with halogen which I understand use less power; but more important that I read up on the charging system and diagnosis; and how to use the voltometer that I bought some time ago, and make sure that the charging system is delivering sufficient power to the spark plugs consistantly. I guess the causes could be any or some of poor connections; failing battery (now two years old but the bike is used irregularly, so actually very little use on sealed battery); failing coils; failing alternator; failing Boyer system; would the rectifier or zenor diode have an effect on it? Rectifier is new solid state; zenor diode is original. Unfortunately I'm at work in the city and bike is currently at the beach; so it will be a week or so for me to sort.

First thing I'd suspect is the battery.

Next thing to suspect in old weak charger is the rotor magnets decay. A screw driver should snap to magnets and be a bit of jerk or slide to get it off. Rotor should easy carry is own weight on steel.

Of course you could find and fix both of those while at same time the carb bolts
seal got lose to suck in air on throttle close and fool ya some more.

Old Brit Iron motto, if it acts like fuel, check Lucas first.

Veddy Veddy interestinc but not stoopid. Its all good experience ol mate.

Come on over? I thought the weather was bad here. You guys have your hands full. God speed.

That's Godspeed.

Hi- one small note regarding your battery. You say that the battery has been "infrequently used"- that's actually one of the worst things you can do to a battery, particularly a sealed battery. Batteries do well when they are regularly discharging and charging. It's possible that if your sealed battery has been used infrequently, it's possible that the battery has been damaged and is no longer capable of putting out its full voltage. Additionally, the internal resistance of the battery may have increased and it's not allowing the full flow of power to pass through. I might suggest that you borrow a fresh battery and see what happens, or bite the bullet and buy a new one. Good luck!

Hi Chris T

For what it's worth, I've had charging / low battery problems (more than once...) with my Boyer-equipped 850 and the symptoms don't really match those you describe. First sign for me was a mid-range misfire which became rapidly worse, then lots of popping & banging and finally it expired completely. Attempts to restart result in mighty kickbacks (ouch!). Battery voltage by then was below 10v.

Cause was traced to a broken electrical connector so no charge reaching the battery, i.e. I ran out of electricity. Cure was a rewire.

As I understand (and please correct me if I'm wrong), the Boyer has an electronic advance which needs a solid 11v or more, otherwise the timing bounces around at mid to low rpm causing a misfire. Diagnosis is easy – charge or replace the battery and see if it fixes the symptoms. If so, you've got an electrical fault and/or a tired battery.

My view is that the Commando's charging system in standard form is marginal, particularly if you run with lights on all the time and this coupled with the Boyer's need for volts can give problems. If I haven't used my bike for a week or more, I put a trickle-charger on the battery for half an hour before kicking it up. It certainly makes it easier to start from cold!

Hope this helps

Mitch

I got some nasty backfire from a bad switch once. I took it apart and cleaned the switch took care of my problem. It wouldnt hurt to check the kill switch. Phil

askdoctorday said:

This exact thing happened to me this past weekend. Had me dumbfounded, and pushing my bike about half a mile home (fortunately I was not far away). Found a broken ring terminal on a ground wire. Replaced it- voila, no problem. SOmehow I don't think it will be the last time I push a Brit bike...

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Yep, this is the only thing that has left me pushing and it's happened to me three times on the main ground wire to the battery. First time, I replaced the ring terminal. It broke again. So I fitted a tougher ring and the wire broke. I now have three parallel ground wires connecting headlamp loom, cylinder head and frame which meet at a screw terminal under the seat. Fom this a single coiled wire connects to the battery (see photo). Coil should protect against vibration fracture and screw terminal allows easy replacement at the roadside using spare wire in my toolkit. Pushing stinks.....

Coiled lead is good idea. I also coiled the leads to ballist pack and from there to the coils as well as points trigger wires, having been show stopped a bunch of times d/t terminals or even copper inside failing to conduct. The funnest ones are when everything checks fine - until vibration starts and it fails right away or a short ways after testing all is well. I've also gotten into habit of once battery terminals nipped up double checked, I spray clear coat on them to seal and sort of locktite up.

Once again, you guys are way smarter than I am. Never thought of coiling the wire. Will definitely do that this weekend! Will also relocate the little plastic fuse case- the guy who wired my bike did a really nice job of placing it in a totally inaccessible sopt behind the battery- facing down, of course. Sheer genius. Thanks!