misfire mystery

I have a 74 850 MKII, stock except for ignition. About 40K miles. I am the original owner, and very intimate with it.

A few years ago I did a major engine rebuild - pistons, bearings, camshaft, valves, guides, springs. Head work was done by Leo at Memphis Motorwerks, I did the rest. It's been about 4000 miles since.

Ever since that rebuild, I've had an issue with the left cylinder misfiring at idle. On cold start it will pop and skip, and it needs the choke for a long time. Once warmed up it's steady at 900 RPM, but I attribute that to the "idle compensation" feature of my Tri-Spark ignition. After a long highway run when the motor is really hot, idle will increase to an annoying 1300+ RPM and stay there until it cools back down.

The idle air adjustment on the misfiring side has very little effect. A ColorTune window shows intermittent combustion on that side. The bike pulls hard through the range otherwise, it's only idle that gives me problems.

My experience says it's running lean. After messing with float levels and idle passages to no avail, I replaced the Amals with new ones. NO CHANGE.

Both coils are new, as are the wires and plugs. Compression and leakdown tests are good. Valve clearances are to spec. I will note that Leo ground the head flat and with the valve job the pushrods had to be shortened in order to get clearance. He said the seats are at the end of service life.

I'm beginning to suspect some internal problem, like a cracked head, since this only started after the rebuild. But I have no idea how to prove that.

Getting desperate, I'm considering getting a 34 mm Mikuni kit just to finally rule out carburetors.

Any other ideas?

I had a carb puzzle for a while that turned out to be a loose main jet holder. I have had this twice on
two different bikes both happened with new amals. Once the jet and holder were nipped up problem
gone.
You may want to attend to the ignition just to be sure. Pain to go back to points but...
Failing that can you borrow another EI?

Id not go to another carb. I might put the old carbs back on.

Parts replacing isnt being a good mechanic but there are times that you get so frustrated that
this approach is worthwhile especially if you can borrow the parts.

You wrote:
" After a long highway run when the motor is really hot, idle will increase to an annoying 1300+ RPM and stay there until it cools back down.

The idle air adjustment on the misfiring side has very little effect. A ColorTune window shows intermittent combustion on that side. The bike pulls hard through the range otherwise, it's only idle that gives me problems.

My experience says it's running lean. After messing with float levels and idle passages to no avail, I replaced the Amals with new ones. NO CHANGE."

Sounds to me like an air leak distal (downstream) of the carbs. Whatever the source of the leak, it increases in size with increased engine temperature. First place to look is the manifold (possible crack) and manifold gasket. Spraying WD40 or starting fluid around the manifold may help find it.

Good luck.

Slick

As others have posted, Check for a air leak. Retorque the head bolts and then spray 2+2 gum cutter or something similar around the head and intake gaskets to see if RPM's flare from the added combustables.

You could also try resetting the ignition system timing so it all swaps sides,
and see if the problem swaps sides.
Lotta work for probably no result, but it would totally rule out the ignition.

Tried new spark plugs to throw at this problem. ?
Inc swapping them over, regularly.

It does sound like an air leak someplace.

At the cold start, when the left side is "skipping and popping", try easing off the choke and tickling that left carb. If the engine's left cylinder kicks in steady for a bit after doing this I'd double check the float level,,, both of them actually. My 850's engine is happiest with the carb's floats set level with the bowl top. That's what I would try with yours. I know there is a measurement that is somewhat lower that is supposed to correct, .080" I think, but level works great, without any flooding issues.

A friend had the same symptoms on his Triumph. Turned out to be a badly installed thread insert in the head for the spark plug. When the engine was cold, the insert was not grounding properly, and the cylinder misfired badly. When the engine warmed up, the insert made electrical contact and the engine ran fine.

Stephen Hill

Tickling after starting is a good idea, thanks. I'll try that.

I had a Boyer on it originally, and the problem was even worse. Idle speed changed constantly, and I carried a screwdriver in my pocket and was forever fiddling with screws. Switching to TriSpark was a major improvement. I even replaced the Lucas coils with TriSpark units ($$).

I've been into the manifolds many times. Replaced the gaskets and o-rings, checked for cracks, sprayed WD-40 etc with no effect. I have Stay-Up floats in the carbs and float level has been set meticulously.

I've considered changing the intake manifolds "just because", but the correct 32mm to 30mm manifolds for the RH10 head are rather rare. I don't have spares.

I once had a spark plug loosen slightly (on an auto engine) and suck air. Sounded like a turbojet. Check if plugs are cinched up. Sometimes the most maddening problem turns out to be some simple thing.

Did you try the WD 40 trick around the head gasket?

Slick

texasSlick said:

Did you try the WD 40 trick around the head gasket?

Slick

Click to expand...

Not around the head gasket, but around the intake manifolds yes. If the head gasket was leaky it'd show up in a compression or leak down test.

Went for a ride yesterday, and will add another data point - left side pops out the exhaust sometimes on backing off throttle, as when shifting. Not the continuous popping you get with an exhaust leak, just a single pop when quickly going zero throttle.

Sure sounds like a lean condition to me.

The carbs are a pain to access on a Commando. But all I can say that you do seem to have a lean
mix somehow. Time to pull the air cleaners and have at it.
When I was investigating my problem I changed everything in the ignition a piece at a time and sometimes
it would run just a bit better with a new piece installed but nothing cured it.
So since you seem to be pretty sure it isnt ignition all you have left is the carb. Gauling, as the carb
is brand new, but maybe once more check the idle jet circuit to be absolutely certain it is ok. Not just
throught the little jet itself but all the way up to the two tiny orifices forward in the throat. You should
be getting a good solid stream however small from there.

Things like this are vexing.

Another possibility..the phenolic spacer is not known for being really air tight. I never use them because I have fought this problem before. I use the 1/8" fiber gasket (usually a Triumph part available from Rabers) between the head and the manifold along with a thin smear of Yamabond. It seals perfectly and works as a heat insulator. Then I toss the carb O ring and use a thin paper gasket instead, again with a thin coat of Yamabond. Now I can be assured that the only air coming into the motor is down the throat of the carburetor and no where else. Checking for leaks by spraying around the gasket surfaces works usually but not always.

By the way, if you use a pair of 5/16 allen bolts between the carbs instead of the studs, then it is possible to remove the carbs without disturbing the manifolds.

Long Shot , and likely ' in colaboration ' with other ' irregularities ' ( How UNUSUAL ) .

The infernal Ignition Wires ( and others ) if not a FIRM fit - the Spade Connectors ,
can give a missfire , more likely upper rpm / stutter though .

ANYWAY , ALL Spade Terminals should be a Firm Press Fit , not a free easy fit .

maylar said:

texasSlick said:

Did you try the WD 40 trick around the head gasket?

Slick

Click to expand...

Not around the head gasket, but around the intake manifolds yes. If the head gasket was leaky it'd show up in a compression or leak down test.

Went for a ride yesterday, and will add another data point - left side pops out the exhaust sometimes on backing off throttle, as when shifting. Not the continuous popping you get with an exhaust leak, just a single pop when quickly going zero throttle.

Sure sounds like a lean condition to me.

Click to expand...

FWIW, I'm chasing a VERY similar problem and new premieres have helped, but not eliminated the idle/misfire on the left pot. Same checks as you, ignition = check, timing = double check, float levels = triple check, fuel supply = solid, coil = no issues, wires/plugs = new, compression = 180 each cylinder

air leaks? none from exhaust or intake... but tonight I sprayed some carb cleaner around the head gasket area and BAM... leak detected....

I'm not sure how or why, but there's certainly an air leak at the head even though she's not smoking, no excessive oil consumption , and no oil leaks to be seen.

Will re torque tomorrow and see what happens.

acadian said:

maylar said:

texasSlick said:

Did you try the WD 40 trick around the head gasket?

Slick

Click to expand...

Not around the head gasket, but around the intake manifolds yes. If the head gasket was leaky it'd show up in a compression or leak down test.

Went for a ride yesterday, and will add another data point - left side pops out the exhaust sometimes on backing off throttle, as when shifting. Not the continuous popping you get with an exhaust leak, just a single pop when quickly going zero throttle.

Sure sounds like a lean condition to me.

Click to expand...

FWIW, I'm chasing a VERY similar problem and new premieres have helped, but not eliminated the idle/misfire on the left pot. Same checks as you, ignition = check, timing = double check, float levels = triple check, fuel supply = solid, coil = no issues, wires/plugs = new, compression = 180 each cylinder

air leaks? none from exhaust or intake... but tonight I sprayed some carb cleaner around the head gasket area and BAM... leak detected....

I'm not sure how or why, but there's certainly an air leak at the head even though she's not smoking, no excessive oil consumption , and no oil leaks to be seen.

Will re torque tomorrow and see what happens.

Click to expand...

Thanks for that. Since my problem started after an engine rebuild, an assembly related problem is a distinct possibility. Spraying around the head gasket is an easy thing to do (and might clean the motor a bit too ), so I'll give that a go. Do keep us informed please.

Your first post stated "compression and leak-down is good". Assuming that means equal on both sides, I'd say that any issue with mechanical condition because of assembly or otherwise would result in poor idle/running on both sides. Have you checked the routing of the throttle cables? It's a stretch I know, but intermittent hanging up of one can constantly change carb balance. I don't know if you have a 1 into 2 throttle arrangement, but if so, pay particular attention to the free operation of the junction connection.

maylar said:

Thanks for that. Since my problem started after an engine rebuild, an assembly related problem is a distinct possibility. Spraying around the head gasket is an easy thing to do (and might clean the motor a bit too ), so I'll give that a go. Do keep us informed please.

Click to expand...

I can see how easy it is to eliminate the possibility of an air leak, given my compression tests showed no issues, the bike doesn't smoke, and there's no apparent oil leak anywhere around the head/barrels... but sure enough, the carb spray around the head/barrel joint with the bike warmed up resulted in a stumble on the problematic pot...

Given the lack of oil related symptoms that would otherwise make a head gasket leak obvious, I'm assuming the it's a minor leak I'm dealing with that's most likely related to lack of sufficient torque... kind of makes sense, if the valve seals/guides are good, the pushrod tunnels sealed well, and the rings well seated... there's no reason for an oil leak. I'll try the backing off/re torque method with the head bolts this time around, but am curious if it's recommended to back off all the bolts first (in disassembly order), then go round and re torque?

note: using a solid copper head gasket which have proven to work well for me in the past, and I skimmed the head on a truing plate prior to the last install to ensure flatness...

Update: carefully retorqued the head, started her up and leak appears to have been sealed, copious amounts of carb cleaner all around base of head and manifolds.. no disruption to idle... adjusted idle air setting on problematic pot and she seems good

Awesome. I'm gonna check mine as well.

maylar said:

Awesome. I'm gonna check mine as well.

Click to expand...

Be sure to do it with engine hot. From your original post, it seems that is when the leak is pronounced.

Slick