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possible Head issue.

Submitted by gerald_kane on Wed, 08/06/2016
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Commando

This relates to a Mk1 850 with a single carb.

On a ride last weekend the previously good running suddenly changed to a misfire which got worse, so I decided to head home, the performance dropped off and I was down to one cylinder, I just made it home. On examination I found the left hand spark plug wet and oily.

I ordered a compression tester, fast forward a week.

I changed to new plugs and after confirming a good healthy spark, the bike started as usual on the second kick. It warmed up and idled well for ten minutes.

I have checked compression and I am getting 155 PSI on the right and almost 160 PSI on the left which seems pretty good to me, although I don't have a baseline from the bike. Can I assume good compression means the piston rings are ok? Before I dive in and remove the head, is there anything else I can check first to confirm what this might be? Would a failed valve seal be enough to stop a spark? I assumed it would just turn the bike into a heavy smoker.

Submitted by john_holmes on Wed, 08/06/2016

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Compression figures look good, a difference of more than 10% would have required investigation eg a leak down test to find the leak eg valves or rings. Sounds more like an inlet valve guide seal which then caused the plug to foul.

Submitted by gordon_johnston on Wed, 08/06/2016

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Are you sure it's not just a duff spark plug? If the spark plug had failed (not unknown) it would become wet and oily as the engine continued to run on the other cylinder. I would try new spark plugs before taking the head off. I have watched someone remove a Triumph twin head trying to find a problem when it was in fact a contact breaker problem. I tried to tell him.

Submitted by paul_knapp on Thu, 09/06/2016

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I'll second that what Gordon says, always start with the simple, easy things first. Knackered plug was my first thought, particularly since you say the engine now starts and runs fine after replacing the plugs.

Submitted by mark_woodward on Thu, 09/06/2016

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Hi Neil - my Commando had a 'mystery misfire' at high revs until I swapped from Champion to NGK - the mystery of spark plugs!! I've found the 5k resistance of 'suppressed' plugs range from 5k to 1m ohm!

Mark Woodward.

Submitted by gerald_kane on Thu, 09/06/2016

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Thanks.

I normally run ngk iridium plugs, because they are supressed and I have a Tri Spark ignition which either wants supressed plugs or leads, and I don't have the leads. Although it started ok, I never ran more than 10 minutes and not under load, so I will take it out for a run this weekend and see what happens.

Submitted by gerald_kane on Sun, 19/06/2016

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Update:

While testing the bike last weekend, I noticed a new issue with the bike cutting out when I applied any other electrical circuit, like a brake light, I traced this to a pretty much dead battery, so I ordered a replacement, with that fitted and, new plugs and HT leads, I tested the bike on the road and it started to pop and misfire within half a mile under load ( I get no issues with it idling on the main stand), so I returned home and swapped HT leads over, so the left coil is firing the right plug (I guess this does not matter in a wasted spark ignition, but it was worth a try) and that made no difference. When I got home this time, I left the bike running on the side stand while I got out of my kit, when I returned to the bike, it was smoking out of the left exhaust, I pulled the plug and small wisps of smoke came out of the plug hole for a while, if this was un-burnt fuel vapour it would of evaporated quickly I think. Bearing in mind my battery problem could an electrical/ignition issue cause anything like this do you think, or is it more likely to be a leaking head?

Submitted by david_evans on Sun, 19/06/2016

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Try wiring your trispark direct from the battery (via a switch of course) Trispark are very finicky when it comes to the power supply and other things in the electrics can effect the power supply to it. I had misfire problems with trispark that were not happening with either the previous boyer or points. My Boyer powerbox delivers volts in a stepped manner, I suspect there should be a capacitor across the output to smooth the supply. The momentary steps, shown on a digital voltmeter, coincided with the misfire. Using the battery as a smoothing device cured the problem.

Submitted by paul_knapp on Mon, 20/06/2016

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Hi Gerald, sounds increasingly like an electrical problem or lack of volts. Have you checked the voltage across the battery terminals with the engine running through the riding rev range with lights off then on?Your voltage metershould be 13.5V- 14.2V (from memory) engine off the idle.

Submitted by gerald_kane on Tue, 12/07/2016

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Update 2

Some good news, I have managed to fix the misfire, and it was a problem with the Tri Spark, All the connections looked good, but whileI was using the electrical test probe I pushed it down the connector boot of the Black/yellow wire on the Tri Spark and the connector started to come away from the wire, so it looks like although the connector was attached it was only by a couple of strands. I have now crimped on a new connector and soldered it, plus I have tried to minimise the sharp turn of the wires, which I think was the initial cause of the wire starting to come apart. I also checked the valves and they were outside tolerance, so I brought them back to the recommended setting.It's not all great though, I still have a smoky left exhaust if I leave it running on the side stand, the smoke clears if I lean it over to the right hand side though, so I guess may be a valve seal does need doing at some point.

Two other problems have cropped up, the clutch is dragging, I think I slipped it a bit too much when I was limping home that time a few weeks ago, and the speedo gearbox is no longer working. I am getting the clutch tool so I can take it apart and clean the plates, and I have a new speedo gearbox to fit.

So thanks for all the advice, it definitely pointed me in the right direction.

 


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