Running on one cylinder

I had a similar problem with my 1970 Commando replacing the coils with Tri spark ones fixed it for me.

Maybe this should have been in the singles section? !!!!!!!!!!

I would agree with Alan, although my Mk III is still on the original coils.

Is yours a high mileage specimen Mike? They could be breaking down?

Mike

Have you tried opening both fuel taps to discount fuel starvation? Also opening the tank cap to eliminate the tank breather. I had problems with the tank breather on a GT500 Suzuki in the french alps, when it rained the engine went onto one cylinder. Cured it by removing the breather pipe.Strangely, it wasn't blocked but come the rain it affected the engine!

regards

Peter Shand

Ihave had same problem on my coil firedtwin when the battery was on it's way out. More noticable when the lights were on.

Paul

Had same problem on my Mk 1A 850 a couple of months ago - turned out to be the1970s vintageChampion plug cap where connection to HT lead had broken down due to corrosion. Replaced plug cap and back on two cylinders , sweet as you like. If still scratching head , tryanewplug cap - and if that doesn't work , change HT lead which can also suffer core breakdown if old.

There is a high possibility of this problem being caused by a sticking valve.

I had a similar problem with my Dommie engine. The cause being Ethanol disolving fuel tubing and tank bits and then depositing the gunge on the valve stems.

Here is a simple check that costs nothing and will exclude the ignition system - swap the coils, plugs and leads from affected side to the good side and see if the problem swaps sides, then it has to be carburation. Sticking valve posiible, but I have rode with with knackered valve stem oils to Birmingham and back and used litre of oil and had no problems - I don't think the bike riding behind me was going to rust for a while as he was covered in oil.

Forgot to metion if the problem is still on same side - look at the carburation. Though my suspcion is on the ignition cct.

I have seen 2 Shropshire branch members struggle to get both new (but not Premirer) amals to work properly. I spent a fair bit of time with one of them investigating the duff carb and I couldn't see any reason why it would not work, but it wouldn't. Pretty frustrating to say the least. Both members have swapped to single Mikuni now. A third occasional branch visitor, only last week, has a similar problem. All carbs were supplied by Norvil. You would think that a carb is a carb and that's it. New carbs supplied from Amal come in a box with the jet and slide info printed on the lid of the box. That's one way of identifying the genuine article.

Where abouts are you in the country Mike? I have a spare 34mm Mikuni that you could borrow to prove your bike one way or the other.

Have you tried replacing the condenser onthat misfireignition circuit ? Note you have changed all other components in the circuit but no mention of condenser change.

I am still going for a dodgy valve guide as the root of the problem.

As a number 2 possibility I would be then check out the wires to the pick up points in the timing cover. This is a weakness of the Boyer system. The leads are very fragile and break internally.

Number three would be a duff carb or more likely the cables not lifting the slides evenly.

Previously mike_sullivan wrote:

Help! My MK III Commando is running on 1 cyclinder (not the junction box this time!).

After successfully checking for a sparkI've stripped, cleaned &remounted the carb (running twin AMAL Concentrics) for the offending cyclinder, After replacing the carb the bike ran on both cylindersfor about4 miles, thenmanifested the same symptons, i.e.one cylinderjust stopsfiring (no misfires or banging), the cylinder just falters,willthenoccasionallypick up, & then just dies away again. Happens with throttle wide open, and in between

Could this be a badly oiled plug? I am just about to pull the head as it looks like the inlet guide oil seal has failed on the cylinderthat is currently misbehaving (slots of smoky smoke). Plug when pulled was oily & smelt vaguely of petrol. I've tried a different plug - same symptoms, can't be the tank breather 'cos that would affect both cylinders, I'd expect more noises if the coil was failing (?).

Does anybody have any ideas, before I replace coils & leads as well as seals!

Regards

Mike

MkIII Commando

Hi Mike

Have you thought about replacing the valve stem seal with the head still on the bike?

I had a similar problem to you except my offside pot packed in after a 50 mile run with a sooty black plug. My problem turned out to be a duff coil so I replaced both with a single dual output one from RGM, Sorted. But I did have a smoke problem too and it turned out to be the seal had popped off the valve guide. I managed to get the old one off and the new one on without removing the head by making a tool up that went through the centre of a drilled through spark plug to hold the inlet valve up so that I could compress the spring, the rocker had to come out too but it will save you going through the head re torque process. I'm trying to remember where I saw the drawing of the tool, but other people have done this by stuffing rope into the cylinder and having the piston up at TDC or as near to as poss.

I'm away from home just now so I cant show you a picture of the tool I made up but maybe someone on here can remember where I saw the tool drawing in the forum.

Hope you get it sorted.

Mac.

Mac.

Before you go down the head off route, you need to prove that the RH carb is suspect by subsituting it with a known good one or a single setup. It would be a shame to go to all the hassle of removing the head just to have the fault remain after you've done all that work. The wet plug may just be the result of a non working carb. Or proove that the contamination of the RH plug IS oil.

Is the Boyer ignition system one of the ones with two coils on the stator plate?

As it seems to be heat related could it be the relevant coil, or its soldered joints are breaking down?

May be worth resoldering all of the joints on the back of the stator plate perhaps?

Maybe even remaking all of the electrical connections and checking to see that you don't have an intermittent wiring fault.

However if it is smoking as badly as you say it could be a valve issue.

Unless, possibly, it is only smoking from the side where it does not fire, and when it is hot, then it could still be electrical. Straws, grasping, at!

Never heard of a carburretor completely stop working when it gets hot though.

Does it smoke upon starting, is it blue or black smoke?

Good luck!

Jack

Bought my wife a ultra sonic jewellery cleaner, makes a great small parts cleaner for carbs and the like, when she's not looking.