Intermittent spark when hot

Abraxxas1127 · Full Member Joined: Jul 01, 2009 Posts: 34

I just picked up a 79 650 Special, went thru the carbs, valves, points, timing etc and got the bike running perfectly but if it sits in the hot weather or in one spot idling while tuning or something it will start to intermittently miss. I had a timing light hooked up to it while it was doing this today in the garage and you can see the timing light miss a flash when the engine misses, on either cylinder, they both seem to do it but only while the bike is hot. If I go for a ride then it will pop/sputter or drop a cylinder for a couple revolutions and run poorly especially on acceleration but once going 50 or 60 mph with some air flow for a few minutes it seems to get better and go away after about 5 minutes. The bike has been at full operating temp right from the start as well.

Is this a sign of a bad coil? New plugs gapped correctly and all the timing/points settings are spot on and have been check 2-3 times.

Thanks!

pamcopete · Posted: July 5, 2009, 2:15 pm Post subject: Re: Intermittent spark when hot

Abraxxas1127,

It could be a bad coil, but, if so, then generally only one cylinder would be affected.

If you have a dirty kill switch, then both coils are not getting full battery voltage, resulting in low current and a poor spark. This condition would affect the coils worse as they heat up, because the resistance of the coils increases with temperature, further reducing the current.

So, I would reccomend that you clean the contacts in the kill switch which probably has not been done for 30 years, unless you have done it. Even if that is not the problem, it needs to be done and that will either fix the problem or eliminate the switch as a cause.

Abraxxas1127 · Full Member Joined: Jul 01, 2009 Posts: 34

Thanks for the suggestion pamcopete, I took the kill switch apart just now and used a stainless toothbrush on all the contacts and cleaned them and added a dab of dielectric grease to everything and put it back together, unfortunately it did not change my problem. Any other suggestions?

pamcopete · Posted: July 5, 2009, 4:19 pm Post subject: Re: Intermittent spark when hot

Abraxxas1127,

Wel, as I said, at least the kill switch is eliminated as a source of the problem, but it could still be caused by a voltage drop somewhere, like the ignition switch, but rather than just start tearing things apart, you could run a jumper with an inline 10 Amp fuse directly from the battery positive terminal to the red/white wire on one of the coils. The red/white wires are connected together, so you don't have to run thejumper to both.

Put the fuse under the seat so you can disconnect the fuse to turn of the coils. This test will eliminate all connections, fuses, switches etc. from the problem.

Other possibilities would include both coils being bad and low battery voltage, but, I would assume that you have already checked the battery voltage at 3,000 RPM?

Less likely possibilityies include the condensers because they are both in the same container.

And...just a sanity check...you did say that both cylinders are missing? Because if just one cylinder is missing, then it is most likely the coil associated with that cylinder. You could then swap coils to see if the problem follows the coil or stays with the cylinder.

EDIT: When doing the jumper test, do not turn the kill switch on.

Abraxxas1127 · Full Member Joined: Jul 01, 2009 Posts: 34

Battery voltage is 12.8 at idle and 13.5 at 3000+, I tried the jumper wire from the battery to the coils and it didnt seem to make a difference either. Yes, it seems that both cylinders do it because while it idles when its having the problem it will sometimes drop to idling on one cylinder for a second or two then back to two. Then every once in a while it'll drop to one and then the other will cut and it'll quit. Very strange issue... the bike itself is not losing power ever either, I have updated the old corroded fuse block with a modern 6 place ATO style and it's all soldered/heat shrinked not crimp connectors.

pamcopete · Posted: July 5, 2009, 7:20 pm Post subject: Re: Intermittent spark when hot

Abraxxas1127,

Well, it could just be that both coils are shot and not working well when hot. Of course, there is a sure cure available!

Abraxxas1127 · Full Member Joined: Jul 01, 2009 Posts: 34

Is there any way to test them? I suppose if it's only sometimes the only way to test would be to start throwing parts at it like any good service center lol.

Abraxxas1127 · Full Member Joined: Jul 01, 2009 Posts: 34

Update...

So I think I found my problem, I pulled the wires out of the coils and shined a flashlight up inside the coil posts and the left one was full of rust, the little pin is all rusty. I cleaned it off as best I could with a small pick and cut the wire end back about 1/2 inch and my problem seems about 50% better or more. So time for coils and wires I think!

Abraxxas1127 · Full Member Joined: Jul 01, 2009 Posts: 34

Ok so I now have 2 brand new coils, wires, caps and another set of plugs and still have my intermittent "drop a cylinder for a second" at high rpms... but only sometimes. So to recap I have cleaned the carbs perfectly, set the float levels, adjusted the valves and timing chain, cleaned and gapped the points, adjusted the timing so it's right on for both cylinders, replaced the coils, wires, caps and plugs, cleaned my petc@cks and screens, flushed and cleaned my tank, new gas, fuel lines, checked for vacuum leaks on all lines and carb boots and still have this problem. Im running out of Ideas???

xsjohn · Posted: July 17, 2009, 12:53 pm Post subject: Re: Intermittent spark when hot

What do the plugs look like after say 10 miles......can you post a pic of them................xsjohn

Abraxxas1127 · Full Member Joined: Jul 01, 2009 Posts: 34

Went for a 20 mile squirt up and down the freeway on lunch...

Xumi · Posted: July 17, 2009, 3:40 pm Post subject: Re: Intermittent spark when hot

Abraxxas1127 wrote:

Ok so I now have 2 brand new coils, wires, caps and another set of plugs and still have my intermittent "drop a cylinder for a second" at high rpms... but only sometimes. So to recap I have cleaned the carbs perfectly, set the float levels, adjusted the valves and timing chain, cleaned and gapped the points, adjusted the timing so it's right on for both cylinders, replaced the coils, wires, caps and plugs, cleaned my petc@cks and screens, flushed and cleaned my tank, new gas, fuel lines, checked for vacuum leaks on all lines and carb boots and still have this problem. Im running out of Ideas???

Condenser.. bad points.. advance weights not advancing fully.

jayel · Posted: July 17, 2009, 3:47 pm Post subject: Re: Intermittent spark when hot

hot or lean, what do you have in for pilot jets? what spark plugs?

Abraxxas1127 · Full Member Joined: Jul 01, 2009 Posts: 34

jayel wrote:

hot or lean, what do you have in for pilot jets? what spark plugs?

Plug picture is after 20 miles of 75-85mph freeway riding...

Advance is working properly, it all checked out with my digital timing light, and with the marks on the stator housing and weights move freely. Points appear almost new, no pitting or deposits whatsoever and have been gapped properly. Condensor I have not replaced yet and dont know if you can and if so how to test it.

Stock pilots, 140 mains, needle clips on bottom groove. My problem is only at WOT at 5K or above it seems, it'll cruise smooth and not miss but at WOT running thru mainly 1st and 2nd gears it stutters and drops a cylinder for like 1/10 of a second or less, while pulling between say 5000-7500rpm. I dont think it's fuel related, I have access to a complete shelf of main jets and have tried everything from 135 to 160 mains and the problem stays the same, although the power delivery differs obviously.

Plugs are a fresh set of BP7ES

jayel · Posted: July 17, 2009, 4:27 pm Post subject: Re: Intermittent spark when hot

stock pilots would be 27.5's, I'd try some 30's, been playing with the jetting on my 74TX and the carbs don't seem as senitive to changing mains as they are to pilots ...... stock needle clip posistion is 3... most of your running is on the pilots and needle taper, the slides start lifting around 4500 rpm

Xumi · Posted: July 17, 2009, 4:32 pm Post subject: Re: Intermittent spark when hot

There is a procedure for testing the condenser in the service manual (can't look it up right now, I'm at work..).

Does it happen only under load?

If not, see if you can verify the ignition miss with the timing light at higher rpms?

ight also want to look at how much voltage you're getting at your coils.

The more I think about it, though, the more my mind goes back to condenser, because the coil ignition system is basically an LC circuit, building up a large charge then dropping it accross the sparkplug... the coil is the L (inductor) and the condenser is the C (Capacitor).

A capacitor will react differently to different frequencies based on it's capacitance value... so if the condenser has aged sufficiently, it will likely change capacitance, which will definitely change how it reacts.. it could even be shorting out intermittently due to heat/vibration.

It's not a common failure... but you've been through pretty much everything else in the ignition system.

Only other thing I could think of would be worn insulation causing a vibration related short.

xsjohn · Posted: July 17, 2009, 5:50 pm Post subject: Re: Intermittent spark when hot

I'm trying to get a grasp on white plugs with 140 mains and the needle it the 5th position (full rich) on a 79 carb....but it does take a few hundred miles for new plugs to show running color........should be dark plugs with those settings and running rough (rich) at steady cruise.......I would be looking at carbs.....needle jet clean.....

xsjohn

Abraxxas1127 · Full Member Joined: Jul 01, 2009 Posts: 34

xsjohn wrote:

I'm trying to get a grasp on white plugs with 140 mains and the needle it the 5th position (full rich) on a 79 carb....but it does take a few hundred miles for new plugs to show running color........should be dark plugs with those settings and running rough (rich) at steady cruise.......I would be looking at carbs.....needle jet clean.....

xsjohn

Been thru the carbs 2 weeks ago, poked needle jets good and blew thru them with 150psi compressed air, doesnt run rich/rough at a hwy cruise until I go up to 145's, then I have to drop the needles to the second from bottom, 150's in the middle, 155's third from top and so on. I'll take them apart this weekend and poke the needle jets again if my settings dont make sense with the plug readings. No air filters at the moment either, bike didnt have any and mikesxs are out of stock on the 52mm pods if that might help explain the plugs vs carb settings...

xsjohn · Posted: July 17, 2009, 6:14 pm Post subject: Re: Intermittent spark when hot

Because the needles are being set so high.......Make sure the pilot circuit is clean....the 3 holes in the throat blow cleaner out.....pilots do feed all the way to WOT....having to set the needles so high would suggest to me the pilot circuit is not working right.....maybe larger pilot jet........floats at 24MM I believe.........are the mufflers good........

Abraxxas1127 · Full Member Joined: Jul 01, 2009 Posts: 34

I believe I do need a larger pilot jet since I made the exhaust, not sure if you saw in my pictures or my other post but I cut the stock mufflers off right behind the mounting bracket under the footpegs. I am using the stock headpipes but the mufflers are now straight thru 2" ID automotive shorty glass packs so theres is very little restriction if any, hence the rejetting. I think I may have set the float level at 25mm, when I pull the carbs this weekend I'll change them to 24mm and see if it helps.

Thanks to everybody for their input and thoughts thus far, I'll be extremely happy once this is sorted out because aside from this one issue the bike is now running absolutely perfect, starts on the first kick, idles perfect, cruises smooth...

xsjohn · Posted: July 17, 2009, 8:31 pm Post subject: Re: Intermittent spark when hot

Exhaust is your problem..........along with geting the carbs to work with them.....xsjohn

jayel · Posted: July 17, 2009, 8:39 pm Post subject: Re: Intermittent spark when hot

xsjohn wrote:

Exhaust is your problem..........along with geting the carbs to work with them.....xsjohn

short open pipes and no airfilters makes me think it might be a wee bit lean,

Abraxxas1127 · Full Member Joined: Jul 01, 2009 Posts: 34

Except it did this same thing since I got the bike, before I touched the exhaust. At first I didnt think much of it, then I cleaned the carbs and it still did it, then rejetted and played with the needles trying to resolve the issue and have been to both extremes, way to lean and burbling fouled out plugs rich... and the problem still remained. Then I changed the exhaust, and the plugs again, and the coils and wires... and Im running out of ideas Smile

I do believe the midrange is a wee bit lean right now because of the fact that the needles are all the way up, I like to adjust the pilots until I can run good with the needles on the middle clip, that way I have a little adjustment each way for fine tuning with condition/temp changes. I just have trouble believing that the pilot jets being too small would create my issue simply due to the fact that when I had the 160 mains in it the bike was so rich it would barely rev to 7500 rpms... yet it STILL did this awkward little stumble. Thats what baffles me, even if the pilots are too small, with those huge mains in there surely there was enough gas dumping down the throats to cure any lean condition there could possibly be, and the exact same stumble was still there. The bike pulls hard and strong, then it just drops a cylinder for a 1/10 of a second or less sometimes two or three times in a row over the course of a second or two and then keeps pulling strong again. I shift to the next gear and it will pull strong all the way to redline without skipping a beat. It's not a consistant problem, that is what is baffling me and making me think ignition, in my experience if your jetting/carbs/mixture is off it will consistantly reproduce the problem every time until you correct it.

Actually I have captured it in a video clip now that I think of it, perhaps hearing it first hand will help.
Youtube
Fast forward to 1:05, I go wide open thru 1st and 2nd coming up to the camera from behind and it pulls smooth and doesnt skip a beat. Then at 1:20 I run through the gears again and you can hear it do it once in 1st, then a couple times in 2nd and maybe just slightly in 3rd then the rest of the gears it doesnt do it. And on the return run which is wide open back at the camera thru all 5 gears it doesnt skip a beat either.

xsjohn · Posted: July 17, 2009, 9:22 pm Post subject: Re: Intermittent spark when hot

Have you checked compression on both cyilinders........if one was different like 10 lower it could require a different mix than the highter one possibly causing it to intermitantly fail like that...although the plugs didn't infer that.......?

xsjohn

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