Left cylinder not firing on 2 XS’s

[voelser]

Right, while I am sorting the jetting on my new Mikuni’s on my other XS I am also trying to solve a riddle on my chopper XS.

The engine in this was rephased, upgraded to 750cc by Howard Smedley before it was mine. It also has the Mikuni TM34’s, which I am trying to tune, but can’t until I solve the issue in this thread. It runs a Heiden tuning electronic ignition and A pair of XS performance coils (the big green ones).

The issue is that the left hand cylinder spark plug seems to fail every time it’s run once. The engine runs rich (which I am trying to solve by tuning), but the plug doesn’t just foul, it dies. Even after cleaning it won’t spark. This is the parrtern: set of fresh plugs, try to spark them out of the engine. Both give a nice fat spark. Put them in the engine and try to get it running. It is kickstart only, so it sometimes takes a while. When it finally runs, 9 times out of 10 the left cylinder runs cold. When I then test the plugs out of the engine again the left hand plug won’t spark or spark very little. This happens almost every time, with different plugs.

I’m a tad stumped. My first suspect was rich running engine. It does run rich at the moment, but not so mich it should cause this issue. Besides, the right cylinder plug looks the same and does keep running. Other suspct is the plug leads. When thing I did notice when testing out of the engine was that I got zapped, even though I held the plug cap at the back. Could be that the spark escapes? Yet to test new leads (ran out of time and will today). Suspect number three: coils. may swap them round and see. And last maybe the timing is out?

Open to other suggestions!

 

[Jim]

Take a plug (from the left) you suspect is bad and see if it will fire on the right coil. Could be the left coil is breaking down after running a bit. But definitely a try bad plug on the right side and let us know...

 

[voelser]

Did that. Tested both plugs to start with. When the left one went bad, moved it to the right. Little or no spark. Moved the good plug from the left to the right. Tested out of the engine: spark. Started the bike, plug got done.

 

[650Skull]

Just to cover all aspects change the coils to the other side before starting. if the problem moves with the coils then get the coils tested. Those green coils are to suspect for my liking

 

[voelser]

I agree, on my list of things to try...

 

[Jim]

Those green coils are to suspect for my liking

I agree, although I've never heard of a coil that would continuously burn out a plug after one run. I suppose anythings possible. That must be one hell of a fat blue spark...

 

[voelser]

Lol..it is a nice juicy spark out of the engine when testing. After a run it is tiny white, if at all....

 

[gggGary]

Take your "failed" plugs, heat good with a propane torch, wire brush the ash off, see if they go back to sparking. Could easily be oil fouling. A motor that's been washing a cylinder with gas can start using oil from several sources.
I got a basket case TX750 a couple of years ago, the "cardboard box of parts" had at least 15 fouled plugs in it. Same plug as the XS650 They all work fine after a bit of torch and wire brush.
An ancient "sold in the back of magazines" device was simply a spark gap placed in the plug wire. A fouled plug will allow voltage to drain off through the soot by creating a carbon path, it keeps the voltage from building up for the fat blue spark. The spark gap holds back the voltage until it's much higher, that quick surge will jump the sparkplug gap before the soot bleeds it off. Often staved off the death of an oil burner a few thousand more miles.

 

[voelser]

Ok, but that would make the spark plugs work again, but you are suggesting its because the plugs are getting fouled. So, getting the jetting to be better would help. Im not so bothered about the plugs, more about what is causing the issue. As for oil, no sign or smell of that. And would that not be both plugs?

Just saw your added text. I know that a sooted plug will hold the spark back. This is why I used brand new plugs. Didn’t even run enough to get sooted.

 

[Jim]

heat good with a propane torch, wire brush the ash off

Back in the R5 and RD350 days, that trick would at least double the life of my plugs.

 

[xjwmx]

Since a new plug fixes it, the problem is fouling, even though you think you've cleaned them. One time I had a stuck needle and was running super rich and the plug would last about 30 minutes. Did that with several new plugs, so I know! Sounds a lot like your problem. I think with normal cleaning there's still enough carbon left to bleed off the high voltage. I'm not saying you have a stuck needle, but you say you're running very rich which is similar enough.

 

[voelser]

Sorry, even a new plug doesn’t fix it. That also gets dinked as soon as it runs. And it does run rich, but its hard to tune it if it doesn’t run on both cylinders...lol

 

[xjwmx]

I meant a new plug fixes it temporarily, like minutes, right?

 

[voelser]

Until it runs, but even then the left cylinder doesn’t run and when I pull the plug it’s not fouled enough to not spark. Ive seen plugs way worse that still spark. Besides, even with the fouled plug the right cylinder runs and plugs look identical in terms of fouling. I’d expect the issue to be both plugs, or at least be left one time and right the next. But I’ll take it into consideration...cheers

 

[gggGary]

Like skull, and PamcoPete mentions it too, the green coils are SO hot they can break down wire insulation, find weird paths to ground etc.

 

[TwoManyXS1Bs]

A quickee.
Set the left plug gap to 0.015"-0.020".
See what happens...

 

[voelser]

So, here is my test sheet:

- New HT leads
- If that doesn't fix it, swap coils over and see if it happens on the other cylinder

 

[gggGary]

Fire it up in a dark room or at night, look for spark leaking out. Yes intermittent internal breakdown of a coil isn't rare. On the old K1300 stock coils that DIDN'T do that were rare. I truly hate to admit how long I was in denial of that needed repair on one I had.

 

[voelser]

Fire it up in a dark room or at night, look for spark leaking out. Yes intermittent internal breakdown of a coil isn't rare. On the old K1300 stock coils that DIDN'T do that were rare. I truly hate to admit how long I was in denial of that needed repair on one I had.

Lol...not an option as I work in a communal car park and I have no cotrol over the lighting. However as I mentioned above I did get zapped when I was testing the plug out of the engine while holding it at the back (where I should get zapped). So leakage is right up there for testing!

 

[voelser]

Right, I will be the first one to admit when I’ve been an idiot (well partially), so here goes..

Today (during work time) I had a moment of clarity which may solve this issue. I realised that a mechanic had swapped over the HT leads when the bike was there for tuning and I suddenly had the uncomfortable feeling the wrong spark plug caps were used. As soon as I was home I had a quick check and I was right: non resistor types! Of course there could still be other issues, but I reckon this is a strong candidate.

Although I’m not the one who put these on, I’m a lemon for not spotting it earlier. New caps on order, will report back. In the meantime I will hang my head in shame. Thanks for your input!