misfire driving me NUTS

I have an 850 MKII that has had major work done this Spring; head rebuilt by Memphis Motor werks, new cam and bearings.

I have not been able to get it to idle consistently since the rebuild. It's hard to start, wants the choke even when hot. Left side plug shows lean. Exhaust on that side is cold compared to the other side. Above idle it runs strong and pulls like a freight train.

I'm no newbee at this - I've owned it since new. It's got all the symptoms of a plugged pilot circuit. But I've poked and cleaned and drilled and sprayed the Amals 4 times now with no change. Yesterday I replaced the carbs with old ones I had and it STILL misfires on the left side.

The road I've traveled:

Float levels set and double-checked.
Verified that manifold and head surfaces are flat. RTV on the joints.
Replaced both coils with new Tri-Spark units
Replaced Boyer with a Tri-Spark unit
Replaced both plugs multiple times

I'm thinking that there's something amiss in the head? But in my experience that would cause a backfire, and there is none.

Suggestions?

Do you have two coils ? Are they the same ? 6V ? How are they wired ? Can you, did you exchange them one for the other ?

Are you using an air filter ? Balance pipe OK ?

New cam... can you degree both cylinders on both intake and exhaust ? Valve clearance OK ? Valves seal OK ? did you run a compression check ?

Jean

I guess you've done all the logical stuff of swapping over the ignition components systematically - plugs, leads, coils etc.

That done and carbs swapped for your old ones I would check the loom wiring to the coils and quality of the engine and coil grounds.
After that, as you say, there's head. I'd run a compression or leakdown test and check the timing.

What exhaust are you running? a collapsed baffle might give the symptoms you describe.

Tight valve clearances??

Cash

You've done pretty much what I'd do myself to look for a problem like that. A compression test would be interesting. I forget, can you swap carbs side to side just for testing in case you have two duff idle circuits? I can't remember if everything clears on a Norton to test like that.
I'm sure you also measured voltage to coils and tested the wires to the Tri Spark.

Cookie said:

You've done pretty much what I'd do myself to look for a problem like that. A compression test would be interesting. I forget, can you swap carbs side to side just for testing in case you have two duff idle circuits? I can't remember if everything clears on a Norton to test like that.
I'm sure you also measured voltage to coils and tested the wires to the Tri Spark.

Click to expand...

Yes carbs will swap side-to-side, you just will have a PITA tickling and adjusting will be near impossible

One only does it to see if it is a carb problem anyway. I just did it on my Norton and my problem followed the carb so I know which one it's in at least.

Thanks for the ideas. Sometimes you can't see the forest through the trees, and the community can hep one find the path.

Answers to some of the above questions:

Nope, didn't degree the cam. It's a stock grind from Norvil. Maybe I shudda..
Exhaust has no crossover, it's 750 style. Intake is stock and air filter is new.
Valve clearances set yesterday after a retorque of the head.
Haven't done a compression or leakdown test, but it sure feels sound at the kick lever (850 owners know what I mean).

My original carbs were sleeved about 10,000 miles ago. I had some issues with backfiring a couple years ago that prompted me to buy a new set off eBay and they ran great right out of the box. (That problem was bad valves). I have about 5000 miles on those. After fettling the left carb for this problem I put the original ones on again just to compare. No difference.

Both coils are new, as is the whole ignition wiring (converted to neg earth and switched the Boyer for a Tri-Spark). The only difference is that it doesn't stall at lights any more (way to go, tri-Spark!).

I'm gonna do a compression check tomorrow. I think I'll also replace the high tension wires, since those have not yet been changed and it's easy to do.

Maylar
Let me know how you get on. I have a similar problem, if you see my lead on 'idle missfire'
I have also not yet got rid of this problem.
Stu

Kind of late for this comment, but have you checked the timing?

It's possible that a stretched chain could affect one side and not the other.

Seems like your engine rebuilder would have renewed this, but maybe applied a faulty chain.

You've done everything else. You could check this with a timing light, moving from side to side.

But then you've probably fixed it, or sold the bike by now. Do you have any update?

2. Put on timing light on one cylinder and point the light at your eye while running. If there's an ignition firing problem with that cylinder you will probably be able to see it with the light. Repeat for other cylinder.

1. Replace spark plugs first. Can't tell you how many "I've got a problem, but I know it's not the plugs" problems get resolved that way....

Good luck

maylar said:

. . .I think I'll also replace the high tension wires, since those have not yet been changed and it's easy to do.

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You don't see too many Nortons now days with the old green OEM plug wires but we used to have lots of problems with them. Bad high tension plug wires can cause a world of head scratching problems especially at low RPM.

You seem to have eliminated both carbs and ignition.
Am not familiar with Tri-Spark but if anything like the Boyer it may not like low voltage at the ignition module.
Have you confirmed voltage at the box? I would expect to see nothing less than 11.5 V at idle to ensure proper operation.
You imply that this problem is new since the engine work. I have seen a similar problem due to a mistimed cam and even the best mechanics can screw this up. Over the years I have come to be be suspicious of cam timing, especially with aftermarket cams, so I verify cam timing on every British engine I do (I don't want to screw it up either!).
If you choose to use a degree wheel you must reset the valve clearance to a measuring clearance specified by the cam maker.
The most important specs are the opening timing of each valve (check all 4) a two or three degree error is tolerable.
If this is a bit much then at least pull the timing cover and visually check all timing marks for correct positioning.
Check your parts bill too. See if they billed you for new woodruff keys (both crank pinion and cam sprocket) as I have seen (rarely) these becoming worn offset.
Good luck.

A problem happens in 2009 and four guys solve it in 2011? Look up man.

norbsa48503 said:

A problem happens in 2009 and four guys solve it in 2011? Look up man.

Click to expand...

I thought for a moment my misspent youth was causing flashbacks or something. Big Stu isn't even with us any longer. RIP, and ride on, Stu!