"Dead" cylinders

Well, I've been trying to get my '81 XJ650 running. I've posted a lot on here for carb-tuning help. Last night I made a discovery....my two "inside" cylinders (2 and 3?) were not firing. Headers were cold. I think it may be intermittent because when I had the bike running the other day to adjust the pilot screws, it was running well. At least I thought it was. Sounded good, crisp throttle response, idled well. I didn't ride it, I just had it started in the garage.

Now it is starting very hard and when I finally got it started last night it struggled to keep running and sounded terrible. That's when I decided to check the headers and, voila, the 2nd and third headers were cold. With all the cleaning and monkeying I've done with the carbs, I am leaning towards a spark problem vs. fuel related issues.

I plan on pulling the plugs tonight or tomorrow to see how the "dead" ones compare to the ones in cylinders 1 and 4. I put in all new plugs when I first started trying to get this running earlier this year, but is it likely that I fouled two of them with my carb tinkering?

If I remember correctly, the coils operate cylinders 1&2 from one coil and 3&4 from the second coil and the coils fire both plugs at once, one spark being "wasted". IF this is correct, it would seem that there is spark TO the plug since 1 and 4 are firing.

Also, I need to check and measure the alternator brushes as I have not done that yet.

I do plan on studying the manual and some other reference material I have on troubleshooting, but if anyone has any trouble shooting advice or ideas, feel free to blast 'em out. Thanks.

 

The coils are setup so that one fires cylinders 1&4 and one fires 2&3. So with 2&3 dead I would check the coils. Check the connectors and make sure they have the correct resistance. Primay resistance is 2.5 ohms and secndary is 11Kohms. Measure primary at the small wires that feed the coil. To check secondary remove the plug caps and check between the plug wires of the same coil.

 

check for cracks in the coil, with all the rain weve had in the midwest , a little condensation can realy screw up a cracked coil and make it intermittent.

 

Thanks for the input. I'll be sure to report back once I get a chance to check. I'll measure the coils and ensure that they're well grounded, etc. and check for cracks or other visible issues.

 

easiest test to see if it is the coils. Swap the 2&3 coil with the 1&4. Now if the problem moves to cylinders 1 & 4, then you know the coil has a problem.

 

I agree with swapping the coils out, but, also check your connection to the ignitor. I had an intermittent spark on my father's maxim x, and I couldn't figure it out...a zip tie to maintain continuity between the harness and the coil wires solved the issue. worth a shot.

 

I agree with swapping the coils out, but, also check your connection to the ignitor. I had an intermittent spark on my father's maxim x, and I couldn't figure it out...a zip tie to maintain continuity between the harness and the coil wires solved the issue. worth a shot.

 

Boy, I think someone REALLY agrees with swapping the coils out.

 

Checked the coils and the readings don't match those stats above, but were strange multiples....2.5 ohms and 21k ohms maybe? Anyway, reconnected everything and took all the plugs out, cleaned them and put them back in. Long story short....I still had two dead cylinders, but now they were 3 and 4! I ended up swapping plugs around until I got three of four cylinders firing. Then, where ever I put the plug from the "dead" cylinder, the new location would go dead, and the other cylinder would fire. So......I'm going to go get four new plugs and see what happens.

 

New Spark plugs are in order. I ma guessing the 21kOhm reading is due to the fact that you measured with the plug boots on the wires. Remove the boots and measure again. The plug boots are like 5kOhms each.

 

Pretty easy to foul a plug with lots of starts/stops/enrichment on and no real running. Last winter I rode the 650 from the barn to the garage to put some lights on it (lots of fun in the snow!). When I went to ride back out only 1 cylinder running. 3 plugs fouled out.

All my 200 ft. trips from the barn to the garage had done them in.

 

This sounds encouraging....I'll find out today if this is one of my problems.

 

I've had a dead cylinder twice and both times it was a fouled plug. A new set of plugs fixed it.

 

I keep a good used set of plugs in the toolbox just for diagnostic purposes. Fouled plugs can cause mountains of problems. On these bikes, it takes less than two minutes to swap all four plugs out.

Something to consider the next time you're holding a set of used but still working plugs in your hand...

 

Just to make sure - 14.5 ft/lbs to tighten the plugs, right?

 

And they're CHEAP. 'Specially compared to some of the dirt bike plugs I run.

 

I actually would torque at 7ftlbs. I have a sevier stripped sparkplug thread phobia! I don't really use the torque wrench for that but it is not a bad idea.

 

I use the "Tip of my little finger on the Ratchet Handle" Sparkplug Torque Measurement.

When the Plug is seated and my little finger bends ... it's over.

I'm not waiting for a pointer or a click.
It's a "Feel-thing" for me.

I'd rather have to tighten it a dozen times ... than strip it once!

 

I use a torque wrench whenever it is available (and that is most of the time). A dab of anti-seize on the threads (or a drop of oil in a pinch), hand thread and tighten, then the torque wrench to finish it off. 11 ft/lb for me, aluminum strips too easily.