Misfire Troubleshooting

My '74 850, 14k miles (last 6k done by me in first two seasons of ownership), has an intermittant misfire on LH side. Been through the Amal carbs (recently re-sleeved) and confirmed all jets/passages are nice and clean; jet holders tight in bodies. EI ignition so should not explain why only LH side issue. Swapped cables & plugs side to side...trouble stayed LH side. Miss happens at most rpm/throttle positions, so more evidence unlikely to be idle circuit related. Makes for a herky-jerky riding experience, esp at city street speeds. Checked for head & manifold leaks using copious streams of carb cleaner on a fully warmed engine, both at idle and at 2500 rpm. No detectable change in rpm while spraying. Checked head bolt torques, all good. Checked exhaust roses...needed a wee snugging up but this didnt solve issue. Checked compression on hot lump...both sides reporting around 150 psi with throttle wide open. On todays testing, I noticed for the first time smoke from LH while reving up...not on the de-cel side of throttle roll off.

What should I be looking to next? Headgasket blown? Valve issue?

Here's some viddy I captured today...LH side is a lot more chirpy. Actual misfire is not so obvious here, but there are few moments where a bit of smoke can be seen. In real life, both are more obvious....

Look at the plugs for a hint of what might be wrong. Then put new plugs in regardless of what you see. Sometimes a plug will have the insulation break up inside the electrode and will not spark at the tip reliably under compression. (I had that happen and it tortured me for 2 days)

If you see an oily plug, and you are now seeing oil out the exhaust, you may have lost a circlip and it's now scratched the bore deeply enough to be fouling the plug. If you suspect that, you can pull the header pipe off the offending side and you should see oil on the exhaust valve and port area. Pulling the header off isn't so intrusive, so you should give that a look if you suspect the circlip induced cylinder damage, rather than tear the head and barrels off first. Circlip damage will be well below the compression ring, so it could cause cylinder oiling early on and still have decent compression... for a while at least.

I know it's a gloomy possibility to have the circlip thing happen. I've had both of the above. Give a look, remain calm, drink a beer, inhale, exhale... You'll figure it out.

o0norton0o said:

Look at the plugs for a hint of what might be wrong. Then put new plugs in regardless of what you see. Sometimes a plug will have the insulation break up inside the electrode and will not spark at the tip reliably under compression. (I had that happen and it tortured me for 2 days)

If you see an oily plug, and you are now seeing oil out the exhaust, you may have lost a circlip and it's now scratched the bore deeply enough to be fouling the plug. If you suspect that, you can pull the header pipe off the offending side and you should see oil on the exhaust valve and port area. Pulling the header off isn't so intrusive, so you should give that a look if you suspect the circlip induced cylinder damage, rather than tear the head and barrels off first. Circlip damage will be well below the compression ring, so it could cause cylinder oiling early on and still have decent compression... for a while at least.

I know it's a gloomy possibility to have the circlip thing happen. I've had both of the above. Give a look, remain calm, drink a beer, inhale, exhale... You'll figure it out.

Click to expand...

THx. I did swap plugs side to side and also replaced with new. No change. LH side plug is black but not with carbon fuzz after short rides, RH side is chocolate.

Can you see the clyinder wall through the exhaust port to see if there is witness marks from a bad circlip? Maybe a cheapo bore scope might help here?

Well, neither black, nor chocolate plugs are good. With modern fuel, my plugs have barely any color to them, and I run non-ethanol fuel only. I would guess carb issues with plugs of that dark of a color, or a weak spark because a weak spark makes for poor combustion so the plugs would be dark in that case also.

A poorly grounded engine makes a poor spark. I have 2 extra grounds on my engine to avoid that possibility. I'd move the needles down a notch, put new plugs in and see how it runs. Then reset the idle air screws to compensate for the change in needle height.

I just watched your video,... I don't see any sign of oil out the exhaust. The bike doesn't sound bad at all, but I bet you've found the tell tale sign of what the problem is with the black and chocolate plugs. You're way rich. Maybe the needle clip didn't seat properly in the inner carb body and it wound itself up on the spring that holds it down. That happens occasionally upon assembly and causes rich conditions.

If you lower the needles two notches, you should get the cough. Then raise them one. You might be fouling a plug, especially if you have got a dodgy ring on that side.

LH dark, RH light. Carb needles correctly seated and on middle groove. Jets and passages clean and idle holes in throat spray nice from either air or fuel side. I did Bushmans guide drill out of blanked sides and made proper sure fuel sides are clear.

Note video doesn't capture smoke on throttle up well but it is easily seen in real life.

If it was running well, now it’s not, DO NOT start changing carburetor settings.
Keep troubleshooting.
Explain the misfire in greater detail please.

I assume you mean INTERMITTENT misfire?
And SLIGHT intermittent misfire?
Random? Versus happens at a certain load/RPM.

concours said:

If it was running well, now it’s not, DO NOT start changing carburetor settings.
Keep troubleshooting.
Explain the misfire in greater detail please.

I assume you mean INTERMITTENT misfire?
And SLIGHT intermittent misfire?
Random? Versus happens at a certain load/RPM.

Click to expand...

Well bike has had a chirp/different sound from LH for some months now. The misfire has been since all my test rides so far this year(4or 5). Misfire not a pop or bang, more like a huff instead of the standard pop pop pop. Happens in unison with power dips while riding along at steady speeds. Noted at most rpms.

I'm surprised that someone hasn't suggested swapping the coils, if you haven't tried that you may want to consider the maneuver.

You have 150 PSI measured from both cylinders so the wrist pin circlip idea seem pretty remote and would produce consistent symptoms, and if one of your cylinders did have a score you would expect an oily (more oily) plug in that cylinder and measurably less compression.

A bad ground, IMO, is a possibility that you can rule in or out quickly by simply employing some alligator clips nand jumping the engine to the coil bracket and the engine to the the battery.

If the pictures of the plugs were taken after the engine was idled for more than 30 seconds the data is not worth considering.

Did you replace the needle jets and the needles when you got the carbs back from sleeving? Did you sync them?

I like a bad coil or a poor ground loop. Don't go looking too deep or someone will come along and suggest that your drive side layshaft bearing caused similar symptoms...

Best.

How can a bad coil affect only one side in EI wasted spark setup? Problem did not change sides when running left coil to right plug and vise versa. Similarly a bad ground? Why smoke on LH only?

Jets and needles were new about 2k miles ago. Did not change them after resleeve. I did the resleeve in part to try to solve issue with rpm slow to return to idle, as thinking was air leak around worn slides. That does seem to have improved to some degree. Sync'd slides using bushmans method with dowels under them watching for both moving together. Also checked both reach wide open together.

Tornado said:

How can a bad coil affect only one side in EI wasted spark setup? Problem did not change sides when running left coil to right plug and vise versa. Similarly a bad ground? Why smoke on LH only?

Click to expand...

Both coils get a trigger at the same time and fire at the same time, doesn't mean that both coils are at the same state of health.

Best.

Found this thread were LH smoke was found to be blown gasket at rod tunnel:


Seems like a possible cause for my issue.

RoadScholar said:

Both coils get a trigger at the same time and fire at the same time, doesn't mean that both coils are at the same state of health.

Best.

Click to expand...

yes, and each coil is firing only one side.

Take a look at the O-ring seal on the air adjustment screw. My plugs showed the same signature. I found a cracked/bad O-ring giving me an inconsistent plug reference.

If the head gasket was leaking through to the pushrod tunnels I would have expected any smoking to be on deceleration not acceleration?

Are your silencers the muted type? They certainly look to have smaller ID outlets to me. The muted bikes ran the needles on the lower, weaker settings (higher clip position / lower needle position) I believe. So your needles, Ewing in the middle clip positions, may be too rich, causing the darker plugs.

I’m with Concours. You need to try and work out some correlation with other things. Did the misfire start after you had the carbs rebuilt? Ditto other work?

Whilst your plugs don’t look perfect, they do not look bad enough to be fouled. You probably therefore have two separate issues, a mild carb tuning issue and a misfire issue.

Also as already mentioned above, if you are running the stock 2 x single coils, you need to swap them over and see if the problem moves. Unlikely IMHO, but a possibility that needs checking out.

And which EI do you have fitted?

Obviously it’s difficult for us to know how accurate the sound is in your video, but that LHS pipe does sound strange. Are you sure the silencers are ok? Maybe remove and check for a misplaced baffle tube or similar. The other potential that comes to mind hearing the video is a possible exhaust valve issue, intermittently sticking partially open, perhaps due to carbon build up on the stem.

But you gotta rule out the simple stuff (again) first, carb settings, o ring condition, plugs, leads, coils, earths, silencers, etc.

Do a couiple easy things next: 1) check the ground from each coils with a DMM back to the battery, and check connections. This might be a good bet as issue is intermittent; 2) as suggested, swap coils. That elimanites most of the spark generation side of things that are r/l.

I did check carb mixture and idle screws today for o-ring probs, were fine. I put some dielectric grease on for good measure and replaced them left for right but issue remained. As stated previously I have also switched cables from left coil to right plug and right coil to left plug. Issue stayed on LH side. So not coil, not cable. Wassel EI. Starts nearly always first or second kick.
Silencers are muted. Headers are non balanced as the originals failed at balance weld early last year. Bike has run pretty well without the misfire most of last year, needles at mid point. Cant say exactly when issue began, only that all four last testrides havebeen herky jerky at city street speeds. Today I saw LH smoke on throttle up, not seenbefore but wasn't looking much for it until today.
Grounds will be checked but should be good. I fit fresh connections direct to a newly installed alloy DT headsteady at the head plate. Also poor grd should make both sides show issue.

I do have some opened silencers have not fitted yet, can try those.
In thread about HG leak I linked above, Comnoz states he's seen many times and smoke to one side was the only symptom.

What EI do you have fitted ?

I would also wonder about the inlet valve seal which may be worn or just popped off on the oily cylinder. As a side issue looking at the good plug I wonder if the timing is far enough advanced in the main area of use( mid range running). Might be that although the timing is set correctly at full revs you are getting a too slow advance. Not something you can normally adjust out. A feature of some systems.

PARTIALLY obstructed idle circuit could do this.
The plug color tells us ONLY that it’s not WAY TOO RICH, should run ok.
Do a 3rd gear roll-on from 30mph, the problems go away at 1/2 throttle or more?
The pickup wires had fatigued for my Boyer ignition gave random misfire.