Stumped by a cap

[Czekus21]

Mabee someone here can elaborate on the cause. Here's the deal. I'm just about done with this XS650 and it runs great, except it fires intermittenlly on no 1 jug. Nothing wrong with no 2 jug. Heres the kicker. When I pull the plug cap off, there's no change in idle. Cool no spark. When putting the cap back on, No change in idle. Cool, no spark. In between pulling the cap off, and putting it back on, she fires great. It's got me stumped. I know the problem has got to be staring me in the face but I'm drawing blanks. What would cause this jug to fire ONLY with the cap partially removed????

[TopCatGr58]

Hey there Czekus21,

Well, I just looked at the parts fiche, and I see 2 ignition coils, and 2 sets of points, on the 79 XS650 anyways, you didn't mention what year/model you had! First you could try swapping coils and it's primary wires so that the contacts that are triggering #2 will trigger #1, but have the plug lead going to #2 plug! Then if #1 is still not firing, and you now know you're using a good coil, then it's probably the points!

But, if #1 is firing, and now #2 isn't, then it's probably a bad coil, or wire from the coil to plug, cap corrosion, etc.. With an intermittent spark upon movement of the suspect plug wire, I'd look hard at that wire, plug cap!

But also after trying to fire a while, that plug could have gotten fuel fouled. Try swapping it with the #2 warmed up and firing plug, and see if it keeps firing?
T.C.

[prometheus578]

Hand shoots up in classroom....

oooh... Ooooh... I know this one!
Ok, well sort of. What you're saying is... that the plug only fires when you have the cap pulled off, and it's a little away from the plug. You see a spark fly from the plug to the cap, and hear the cylinder fire. I must say that I can't really remember just what the cause is, but I can almost remember what is going on. Something like ... when the cap is away from the plug... the coil(?) has to make a hotter spark to bridge the gap.. which in turn makes a hotter spark in the hole to fire the fuel. I've seen this before.... that while holding the cap away from the plug, the bike actually runs better. Come on, this was H.S. shop class 30 years ago..
I can't 'member everything. Umm.. I think it has something to do with electricity, though.
(This has always bothered me. Please, someone straighten me out on this.) I believe this is also why gap was so important years ago, as determined the intensity of the spark.
Geez... I'l be up all night, now.

[Czekus21]

Thanks for the replies. I'll jack around with the coils, wires. caps and points this week and see if I can narrow it down. prometheus578 is right on the $$ about the problem. The thing is, this bike runs great if you hold the cap almost off of the plug. Mabee a wider gap setting on the points. The wires are new. The plugs were new but they were fouled pretty quickly with misadjusted float levels. I do have good plugs installed though.
This bike is the Ugly Duckling project that's just about done. This is the last hurdle to cross and she's out the door.

[CMA1]

You are adding "body capacitance" to the plug wire....

Check the ground on the points


(sounded good to me??? !!!!) :-)


- Larry

[Czekus21]

O.K. kids. This one is just plain weird, I checked the coils, plugs wires, point gap setting, grounding, even so far as to check and make sure that the motor is grounded to the frame. Everything checks out fine. Pulled out No.1 spark plug and fired her up on one cycl. Held the plug next to the motor to check spark and what a beutiful spark it had. Very blue, very good spark. No reason it should not work. Put the plug back in, Connected the plug wire and still will only run on cycl.2. Pulled the cap off agian just a little bit and she fired fine. Pressed the cap back on and nothing. Kinda like it's got an anurizm or a bad omen. I had this problem before on a truck but I can't remember what I did to fix it. I'm open for any suggestions to try. This is the last thing on the Duckling and she's done. Kinda like to have her done competely by this weekend.

[prometheus578]

Other mech at work said to pull the cap from the wire and check with an ohmmeter shouldn't be more than 5 ohms. resister caps... some time the resister goes bad?

[Czekus21]

I'll give it a shot tomorrow after work.

[Ken Talbot]

Have you tried removing the cap, cutting off about 1/4" of plug wire, then putting the cap back on? This connection can get flakey so it makes the connection sometimes, looses it the next, all depending on just how it is being held or wiggled.

[TopCatGr58]

Okay, pull the offending plug, stick it in the plug cap, and lay it against the engine, and test fire the bike. See if you can see a spark jumping the plug gap? Even though it may be a new plug, you can get defective manufacturing! When you place the cap onto the plug, it sounds like it's shorting out along the outside of the plug, so that the spark isn't travelling thru the center electrode to cross the gap in to the grounding electrode that goes to the outer edge of the plug which connects with the engine case, to ground. The path the spark charge has to travel from the coil is just as I stated: coil wire, cap, center electrode of plug, across gap to outer electrode, to engine case to ground!

So....somehow when it's plugged onto the sparkplug, it's bypassing the center electrode and plug gap instead travelling along the outside of the plug directly to ground, but when you pull it off the plug, it's then having to contact/jump to the center electrode, which then makes it travel across the plug gap and fires!! Try another plug!?
T.C.

[randy]

I agree with T.C. Plugs that are fouled down deep on the insulator will spark in the insulator well, out of the flame front. Compression will make the problem worse. That may be why you see a good spark with the plug out of the engine.

Further, when you hold the cap away from the plug, you set up a high circuit resistance, raising the voltage of the spark. That may explain why it runs with the cap removed.

Have you cleaned the points? Or replaced them? You can dress them up with your wife's nail buffer (not the nail file, it's too rough) The buffers have a fine grit, so you don't remove too much material. And don't let her read this or you'll get us both in trouble! In a pinch you can use a new striker from a book of matches, it's the perfect grit. We old schoolers know that you can also use the matchbook cover to pre-set the point gap if you don't have a feeler guage handy, its just about .035 in. close enough to get the engine running. When I rode points ign bikes I always kept a matchbook in my tool case.

Change the plug, or clean it. spray carb cleaner and a wire brush will work if you can get the brush down deep. Make sure the plug is gapped right (watch out for metric vs. inches in your measurement) and re-check the cap resistance.

The mechanic who told you the cap should measure 5 ohms mis-spoke. He meant 5,000 ohms (or more).

[Czekus21]

This is all making sense to me now. I made new wires. and I've switched the wire and cap from the firing side to the non firing side with the same problem. I've tried a few old plugs but thats not saying a whole lot. I have not tried switching the plug from the firing side to the non firing side yet. I'll pick up a couple of new plugs today as well as a couple of resistor caps. We'll see what happens
Sure is nice to work on mechanical ignition stuff. I used to work on this kind of stuff all the time but you all know how that goes. If you don't use it, ya might loose it.

[Czekus21]

Alrighty now. I installed new B8es plugs gaped to .035, replaced the already new wires with new wires, also replaced the caps with new NGK 90 degree resistor caps. She's now fireing on both jugs. Went to take her out to the gas station, turned on the head lamp and she died.....nice......Looks like I'll be looking for a short. Also looks like I'll be cleaning some brushes. I put a volt meter on the battery and shows 100%, Turn the ingition on and goes to around 60%. This is understandable cause the motor was not running and this thing's got a teeny-weeni battery.
Turned on the new head lamp and goes to around 5%. I think that this is what was causing the bike to run on one jug. Power being taken and not fully replaced eventually detieriorates down to where there's not enough power to give a full spark under compression, Put a little bit of blow-by and a miss adjusted carb float, and you've got a fouled plug. I'll figure this all out tonight and hopefully have it fixed and ready to go by tomorrow.Thanks for the input guys

[randy]

Stephen,

Glad to hear you figured it out. Too bad about the elec gremlin, though. These old bikes can make you nuts with the elec problems.

[ratbyk]

Battery

Your problem could be the battery. If it tests OK (12V) at rest but drops like a stone under load I think the battery is toast. With no load and a 12V reading you are just reading a "surface" charge. As soon as you put the battery to work has nothing left to work with.
Ken/Sooke