Single carb setup runs on one cylinder

Carl H said:

The Commando uses the projected nose spark plugs Champion N7YC or NGK BP7ES

Champion N7YC - alternative spark plugs

www.sparkplug-crossreference.com

Even with an 1/8" skim off the head of a bike I built I didn't have spark plug clearance issues. You can see the tops of the pistons through the spark plug hole to see if you have a contact issue. Do you have stock pistons? with flat tops and have valve pockets in them. I would remove the carb and manifold and the exhaust pipes to let you see more of the areas where trouble could be. Are there any loose parts or missing screws / bolts in the exhaust or intake system. Do you have good and similar compression over 150 PSI in each cylinder??

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The compression is around 9,3 and 9,7. I'm not sure about the pistons used, all I know is that she got a replacement engine at some point. I will open in- and outlet, try to see something with the endoscope and will post pictures.
Aside from that I bought a digital TDC tool to see if one spark plug sits deeper than the other or one piston goes up further. Maybe It is just a thread issue.

Updates on monday!

Look at the spark plug seating area on both sides. Are they similar? Is the number of threads or the measurements of the thread area similar. You can see quite a bit of the piston top through the Spark plug hole. especially where the spark plug would contact it. You have to be careful about getting bootleg or fake parts these days. IF you really don't have the clearance for the N7Y, you could use the N4 Champion as they don't have the projected tip. and are a good heat range to try. The old Altas engines used them. A 1971 would have 8.9 to one OEM

Carl H said:

In a normal bike the spark plug is nowhere close to the pistons. The valves can too close or hit with high lift cams or if the cams are out of correct cam timing.

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Kind of obvious when you say it. My comment was based on the ability to take some material out of the plug hole when preparing the hole for timecert installation thereby possibly slightly lowering the plug in an extreme (and somewhat hamfisted) case.

So here is what I learned today:
The digital TDC tool was too long, so I couldn't really measure the exact distance between spark plug thread and piston, but it was very close to equal on both sides. I might modify the tool next week for exact numbers. The threads looked original to me without any modifications.

We looked into the cylinders and the pistons looked fine on both sides. Not much carbon, even visible metal at some spots. No intruders.

I then remembered that I ordered the new spark plugs online when the bike wasn't at my place, which means I just looked them up in the manual (-> BP7ES/N7YC - both prolonged electrode) and didn't look at the old ones (which you always should, I know that now). I digged them out of the junk box - she had B7ES when I bought her, that means that the pre-owner put in shorter electrodes already. So I tried that too. And what should I say - she fired first kick, idling easily even with a new carb in standard setting and revved smoothly without any problems. No smoke, no misfiring.

I'm equally reliefed and worried to be honest. I know that the pre-owner had an old friend mechanic who did all the work on the bike and I know already, that he apparently liked to improvise (and was terrible at welding). So since there is no unusual noise in the crankcase and she seems to run very well, I guess he put in larger pistons (or shortened/skimmed the cylinder) at some point, maybe because he didn't know better or couldn't find any proper ones? Would that be a high risk? As you see, the electrode isn't bent very much, but as far as I know, when it comes to cylinder architecture - every fraction of a milimeter counts. Whatever works or time bomb?

The weather today was very bad so I'm planning to drive her carefully around the block tomorrow to see if there is anything concerning happening in the engine. You will be updated.

So I went for a ride and checked the spark plugs. Everything seemed fine to me. There are other issues but the spark is no longer a problem and nothing touches the electrode. For now.

I just found this @andover. They seem to list the shorter spark plug there as recommendation for commandos. When I remember correctly I bought the longer champion ones from Andover by using the drop down list, where you can put in model and year. Whatever. I will eventually find out when we look deeper into the engine some day soon but for now it seems to work with shorter ones.

Thank you all for your help!

A picture of the head to barrel joint might give a quick clue to if the head was skimmed. Looking at combat motor and non-combat joints, the smaller joint gaps is readily seen.

Jacko said:

So I went for a ride and checked the spark plugs. Everything seemed fine to me. There are other issues but the spark is no longer a problem and nothing touches the electrode. For now.

I just found this @andover. They seem to list the shorter spark plug there as recommendation for commandos. When I remember correctly I bought the longer champion ones from Andover by using the drop down list, where you can put in model and year. Whatever. I will eventually find out when we look deeper into the engine some day soon but for now it seems to work with shorter ones.

Thank you all for your help!

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It looks like the info on the Commando and several other models is mixed up and wrong.