Need a little help with Ignition Troubleshooting

[crawdad]

Hey folks. I've got an 81H that has some ignition problems. The trouble is that it's firing only on cylinders 3 & 4. Cylinders 1 & 2 do have a weak spark, but evidently not enough to fire the cylinders. I've pulled all the plugs one at a time, grounded them to the frame, and when I spin the engine over, I do get a faint spark. The thing is, 3 & 4 aren't much stronger, but those cylinders are firing fine. I would have thought that, since the left side coil fires cylinders 1 & 4, and since cylinder 4 is firing, that it wouldn't be the coils. The weak spark, however, seems to indicate that it is the coils. I'm saying "weak spark", because it looks weak to me. It's not a nice fat spark like you would find on an automobile. The thing that really has me stumped, is the fact that it is cylinders 1 & 2, and not, say, 1 & 4, or 2 & 3. I plan to pull the coils and check the resistance on them, but before I do, do you think that is where I should start, or could it be something else I'm overlooking? It's pretty obvious that I'm not much of a troubleshooter, so any help you could give would be appreciated.

[Bill__Woods]

Hi Crawdad,
I had the same problem on my 81 Special, It only happend twice at the beggining of the year. I belive the problem went away when I put a new shiney set of NGK spark plug caps on my factory coil wires. I cut the old wird back babout 3/8 inch.
forget if measurin the caps, if they are not already new - Replace them. A set of 4 is like $18.00 and they are avalible at any local bike shop

[sixtysix]

It could very well be a fuel problem and not coils or spark at all. Cylinders 1 and 2 are feed from the same petcock, try going to prime on that side and see if it changes. Sounds like a fuel problem and not spark.

[uk1.1s]

Not firing on one side

You are not alone...

I had this problem (on 3 & 4) last October after the bike had been stood for a while. It was a fuel problem, but I didn't do anything that I could say definitely cured the problem - just tinkered about a lot and decided not to trust the octopus / vac fuel tap.

I did spend some time looking at the sparks too as the spark on the side that wouldn't fire seemed noticeably weaker.

As Sixty-six says - try getting it running by turning the taps to prime. I eventually got some joy after heating up the plugs in the oven for 15 minutes (an old 'singles' trick...no that doesn't mean what you're thinking ).

If that doesn't work my next move was going to be to introduce a little bit of fuel into the dead cylinders (only a small amount mind you) - to 'encourage' some combustion. That's also worked for me before with dead bikes that seem to have everything they need - but still won't run.

As an aside - I've now re-routed my fuel system to get rid of the 'octopus'.

Good luck

[crawdad]

I tried it on Prime this afternoon. Didn't make any difference that I could tell. The #1 & 2 pipes are cold, and when I pulled the plugs on 1 & 2, they were wet with gas. I think it's getting gas ok. Any other ideas? I'll be putting new plug wire caps on this weekend.

[amatco]

Hi,
I had a similiar problem. I have a 81H. I replaced the plugs and that solved the problem. Sometimes the simple things work. If that doesn't work it would be nice if you had access to another ignition unit to plug in and see if that makes a difference.
Good luck. Allen

[schpiff]

I had the same prob..turned out to be a fuel height prob..mine would start on 3 and 4 then 1 would fire but 2 wouldnt til i gave it some throttle..after takin carbs off a zillion times and cleaning and setting..all the sudden the whol thing came to life. I set all my floats to 23 mm and did a quick sync with a piece of wire and found out i was pretty close.
hope this helps.......

[Bill__Woods]

I would not recommend setting your carbs with a piece of wire unless they are soo messed up from someones tinker that you have to do thet.
Throttle angle is only one of the factors when Synching your Carbs. The Main Thing is having all cyclinders pulling the same amount of vacuum. A weaker cylinder might require a little more airflow then a stonger one - to do the same amount of work. This lets you tune the airflow to the wear & tear of the engine. When you get them all pulling together, the old XS11 pulls like a mule on steroids!

[crawdad]

I'm embarrassed to admit the fault

Turns out that amatco was right on the money suggesting the plugs might be bad. I was out this afternoon playing around with it, and decided I would stick in two old plugs I had laying around. VAROOM!!!! Fired right up and ran great. I had just put new plugs in last fall and would never have thought of that being the problem so soon. Sorry for dismissing your suggestion Allen. You were right. I'll never overlook the obvious again. Thanks for all the help fellas. VAROOM!!! VAROOM!!!

[Bill__Woods]

Come to think of it I installed new plugs along with the NGk caps, when my problems went away.... :-)


Glad ya got your bike fixed!

[skids]

Yup, I agree with Gary. You might pull the plugs from the non-firing cylinders and see if they are wet.

Originally posted by sixtysix
It could very well be a fuel problem and not coils or spark at all. Cylinders 1 and 2 are feed from the same petcock, try going to prime on that side and see if it changes. Sounds like a fuel problem and not spark.

[Lord Grizzlon]

i have a similar problem, only its 1 and 4 w/ no spark. the plugs are good, at least they work w/ cylender 2 and get spark. I recently replaced the boots w/ ngk's bike cranked yesterday. ran fine (i guess). tried to start 2day and nada