Misfire at idle

I have a problem with my 850 MKII that's got me at the limits of my troubleshooting skills, and with 37 years of owning this bike believe me that's a lot.

Short version: The left cylinder simply won't idle without misfire.

Long version:

Motor was completely rebuilt 2 yrs ago (about 5000 miles now). New cam, bearings, pistons, valves & guides. Head work done by Leo at Memphis Motor Werks, the rest was done by me with Norvil parts. Right from the first day I could not get the left side to dial in with a steady idle. The bike runs well otherwise and pulls hard in any gear.

I have changed virtually everything external to the motor. New coils and TriSpark ignition. New wires and plugs. Last weekend I installed brand new Amal 932 carbs, meticulously set floats etc. I flattened the mounting flanges on a surface plate and used new o-rings and RTV on the mating surfaces.

No change. Still misfires at idle on the left.

Compression is good. Leakdown test says the motor is healthy. Valve clearances are dead on. I put a ColorTune gizmo on it Saturday and the right side dialed in nicely while the left side showed spark but virtually no flame in the cylinder. Only when I screwed the idle mix way in did I see a yellow flame (rich) and it wanted to stall.

There isn't much left... my current logic is that the left side just isn't pulling in a charge at idle. Is it possible for the new cam to be off somehow? Like too much overlap on one side? I know that sounds desparate, but I'm really out of ideas.

What else can I do?

Blow out the left carb again. Validate pilot circuit is clear again, even if new. Swap plugs. Change the plugs. Swap coils leads?

What do the plugs look like. I assume sooty on the left?

I doubt the cam is the issue but what is it and where did it come from?

Fast and easy way to know if it IS a carb problem

first, as Peter suggests, put in new plugs or at least pull and clean both real good

then

simply switch the carbs right to left

yep, just take them off the manifolds leaving them completely assembled, and switch them

obvious if the right side then idles bad right?

then its that carb, and nothing electrical or motor related

Agree with swap carbs. I wouldnt use rtv anywhere near gasolene although
highly doubt it has anything to do with it.
If they are new carbs, sounds to me like you have a electrical prob. Under load,
misses that are minor can be hard to feel.

Take a look at in the dark when it's running and look for sparks.

Dave

I would be looking for a reason for the right cylinder to be over-running the left cylinder. Such as a vacuum leak into the RH port or a throttle cable that is not allowing the right slide to close far enough. Just a guess- long distance diagnostics can be tough. Jim

pvisseriii said:

Blow out the left carb again. Validate pilot circuit is clear again, even if new. Swap plugs. Change the plugs. Swap coils leads?

What do the plugs look like. I assume sooty on the left?

I doubt the cam is the issue but what is it and where did it come from?

Click to expand...

Pilot circuit is clean. I can't even estimate how many times I've cleaned them on the old carbs with no change in symptoms. Plugs look sooty, but I run with a rich needle (bottom groove) so that's to be expected. Plugs have been swapped, replaced multiple times. Wires are new.

This started after my rebuild and hasn't gone away. I first figured a valve wasn't seating but after 5000 miles that's not likely. Besides, a leakdown test says no issues.

Cam is a stock grind from Norvil.

Onder said:

Agree with swap carbs. I wouldnt use rtv anywhere near gasolene although
highly doubt it has anything to do with it.
If they are new carbs, sounds to me like you have a electrical prob. Under load,
misses that are minor can be hard to feel.

Click to expand...

Yes, new carbs. And they behave exactly the same as the old carbs. And I don't think you can swap Amals because the tickler gets in the way.

And it runs fine under load, it pulls to 6K easily. Only at idle is there a problem.

To add to the confusion, it doesn't want to start when hot unless I put the choke on.

First off you need to get the jetting to where the plug insulators run white. Trying to tune with sooty insulators is pretty tough to do. The spark under compression will travel down the side of a sooty insulator instead of jumping the gap. That will require a very rich mixture to run which will perpetuate the problem. Jim

Only when I screwed the idle mix way in did I see a yellow flame (rich) and it wanted to stal

Click to expand...

With pilots jets cleared open then above might just be a low float-fuel level as implies a too lean condition. Proper tune will require a tickle or choke for reliable cold starts, if not, then the over all tune is too rich. As stated must always be suspious of plugs run in misfiring conditions as enough of us have fixed the real issue w/o knowing it till new plugs installed. Once happy tune try the used ones again for more economy. Buy V8 packs.

Since the problem started immediately after the work was performed, the odds it isn't related to the work aren't good. Besides that, you've done a lot of work to eliminate fuel and ignition as culprits. Although swapping a known good carb to the left side could definitely eliminate fuel as a cause, it doesn't eliminate a possible vacuum leak (as was suggested already) which I would carefully check for while it idles.

In addition, however, I'd be concerned about the mechanicals. I'd put on a degree wheel and check the valve timing and lift. If you have a valve not opening far enough, you won't find it by doing a leakdown test, clearance check or even a compression test.

Probably time to check every part and note it. Eliminate each possibility.
Even if you doubt it is things such as valve lash, you dont know until you
know. Give it a complete tune up and isnpection of all . When you are done
it may not solve the problem but you will have few possibilities.
Maybe take a couple of days off and come back to it.
With a twin you can swap most of the parts side to side too.

Thanks very much for the suggestions. I've owned this bike since new and I know it intimately. I'm no stranger to fettling the old girl. Something is definitely different after the rebuild and I'm trying to narrow that down.

The head was machined for new valves and guides and skimmed to be flat. Leo said it's the "last time" that can be done, since the valves are ground deep into the head. In fact with the combination of skimming and valve grinding I had to shorten the pushrods to get proper adjustment and reasonable rocker geometry. Is there anything that could have gone wrong with that process?

And is it possible to degree the cam in situ? I have a degree wheel and dial indicator etc. But I'm not in a hurry to take the motor down to the cases just yet. I can tolerate this misfire for the season but would like to identify the cause.

One thing I haven't been able to check is the vacuum at idle and I really want to do that. I need a damped gage, as my automotive vacuum tester bounces around wildly when used with one cylinder. Suggestions for that?

Thanks again.

maylar said:

Thanks very much for the suggestions. I've owned this bike since new and I know it intimately. I'm no stranger to fettling the old girl. Something is definitely different after the rebuild and I'm trying to narrow that down.

The head was machined for new valves and guides and skimmed to be flat. Leo said it's the "last time" that can be done, since the valves are ground deep into the head. In fact with the combination of skimming and valve grinding I had to shorten the pushrods to get proper adjustment and reasonable rocker geometry. Is there anything that could have gone wrong with that process?

And is it possible to degree the cam in situ? I have a degree wheel and dial indicator etc. But I'm not in a hurry to take the motor down to the cases just yet. I can tolerate this misfire for the season but would like to identify the cause.

One thing I haven't been able to check is the vacuum at idle and I really want to do that. I need a damped gage, as my automotive vacuum tester bounces around wildly when used with one cylinder. Suggestions for that?

Thanks again.

Click to expand...

About the only way to get an accurate vacuum reading on a Norton is with a mercury manometer. Jim

If problem was since new carbs applied, have you put on
the old one and tried it? Id just try each piece one at a time
since you have them handy.

...and that fuel tank could be replaced too. Since I have the
identical bike, maybe you coud cut your losses and sell it to me cheap.

Love the mysteries that ain't mine. In my own let downs after fiddling with one thing I routinely knocked something else askew, usually electrical related. If not mixture issue I'd suspect-inspect coil to plug conduction path. Night time might reveal something as it has with me a few time seeing leaks in HT leads or hints of flashes at terminals or through 'insulation'.

One way to check for a vacuum leak around the carbs is to put some propane around the carb and manifolds using an unlit torch. If it smooths out you have a leak. Not as messy as WD40. I have a problem with the right side missing and a hunting idle. I waved an unlight torch around the right manifold and it smoothed right out at a steady 1000 rpm, no miss. So I pulled everything apart earlier tonight, cleaned, rtv'd gaskets, carefully tightened, checked slide sync using 3/16 drills etc. We'll see how it goes tommorow, sparing the neighbours the noise tonight.

I recall doing the propane bit on a 57 Ford many years ago. I guess I got a little carried away because when it ignited I lost a lot of hair.
Not saying it is a bad idea. Jim

Hello Dave,
I feel for you mate, I have a Merc car that won't got right now, and am running out of options? Now back to the Commando, I had an occasion to fit new Amal bodies, and after much fiddling and carbs on and off, I was spraying carb cleaner through the pilot circuit and say bubbles coming from the welsh plug at the junction of the pilot circuit, of course you need the float bowl off to see this. So, does the left carb respond to idle jet screw adjustments, the same as the right?
I have a manometer I got off eBay for around $60 and I found it beneficial to really get the twin carbs synced. I would check that the throttle slides are mechanically the same height, a couple of 1/4" ball bearing are handy for this, adjust them up on the idle screws so the balls just slip through, carefully retrieve with a magnet. Then with your new manometer, check the readings, if they are a mile off, manifold pressure is different so you would suspect parts downstream of the carb, a worn cam lobe? I know you have a new cam. I found a valve leak with my leak down tester, I could feel and off exhaust valve, just holding my hand over the end of the exhaust for a minute.
The engine ran fine at normal rpm but was lumpy and uncertain at idle.

Cheers richard

stockie2 said:

Hello Dave,
I feel for you mate, I have a Merc car that won't got right now, and am running out of options? Now back to the Commando, I had an occasion to fit new Amal bodies, and after much fiddling and carbs on and off, I was spraying carb cleaner through the pilot circuit and say bubbles coming from the welsh plug at the junction of the pilot circuit, of course you need the float bowl off to see this. So, does the left carb respond to idle jet screw adjustments, the same as the right?

Click to expand...

No. The left side barely responds at all, the right side responds normally. I have a ColorTune device that lets you see into the combustion chamber and on the left side I see consistent sparks but virtually no flame. Just a puff once in a while as that cylinder fires.

I have a manometer I got off eBay for around $60 and I found it beneficial to really get the twin carbs synced. I would check that the throttle slides are mechanically the same height, a couple of 1/4" ball bearing are handy for this, adjust them up on the idle screws so the balls just slip through, carefully retrieve with a magnet. Then with your new manometer, check the readings

Click to expand...

A manometer is in my future. Carbs are adjusted meticulously for the same slide height. And again, these symptoms were exactly the same with my old carbs, so I've ruled them out as the culprit.

if they are a mile off, manifold pressure is different so you would suspect parts downstream of the carb, a worn cam lobe? I know you have a new cam. I found a valve leak with my leak down tester, I could feel and off exhaust valve, just holding my hand over the end of the exhaust for a minute.
The engine ran fine at normal rpm but was lumpy and uncertain at idle.

Cheers richard

Click to expand...

Cam is new, but possibly defective? Yes I'm at that level of uncertainty.

drc