Running on 3 Cylinders?

Hello fellow XJ'rs, my '82 750 Maxim has developed a problem, and I hope that someone here can point me in the right direction.
It has run very well (no issues) for the past 12 months; I've logged about 1750 trouble-free miles in that time.
After sitting for 2 weeks, I started it up to go for a spin, and here's what I noticed:
1. Rough idle, settles below the normal 1100 rpm (closer to 800)
2. Stumples upon acceleration, engine misses, then around 5,000 rpm begins to backfire
3. Loss of power, it cannot be ridden or the engine stumbles and dies.
I have maintained the bike according to general rules learned on this site; I've used Seafoam occasionally, and I only run Chevron mid or high grade (it's got that Techron).
Sooo...
I'm thinking I've either got a carb cleaning in my immediate future, or I've got a plug not firing.
My next step is to pull / inspect my plugs.
Any thoughts?
Thanks,
Bill in GA

 

1st guess plug not firing.
Pull and inspect plugs, it will be obvious if one is not firing.

 

Yes. Check the plugs. Check the caps are all seated well on the wire, i've had a loose one cause a similar problem.

 

I've had a spot of moisture in a plug cap cause a similar problem. Inspect them, clean them out with WD-40 and lube them up with dielectric grease (or a good silicone grease).

 

I had a plug cable that had come loose ifrom the plug end connector, you couldnt see ib because the cable hadn't come all the way out.
Fixing this made my engine run right.

 

An easy way to see if you have a dead cylinder is to start the bike and touch each exhaust header pipe,if you have one that is cold,that will tell you which one ain't firing.

 

Thanks for the responses- I'm hoping it is a plug not firing rather than a 'pull the carbs like you think you're Rick-O-Matic even though you're not' situation.
This weekend-plug inspection.
Gracias

 

yeah, I have a cold exhaust pipe on cylinder 3. I swapped the plugs between 1 and 3 and it got a bit warmer, but not near as hot as the others. I will check the coil wires, could it be a bad coil? Cylinders 1, 2, and 4 seem fine.

 

If it were a coil then 2 cylinders would be cold.

If youre suspecting ignition....
Check your plug wires where the end boot connects they should screw on securely.

Try swapping out the cold plug plug with one that works.

Try taping on the carb related to the cold cylinder just incase there is a stuck float. (careful not to break that fragile carb) taping on it.


I put my money on the stuck float valve!

 

oops, I din't realize I had already commented on this string.

 

10:1 its a blown petcock diaphragm. The petcock gets its vacuum signal off the #3 intake manifold. When the diapghram goes, it allows fuel to enter the port and floods the #3 cylinder. Best bet is to get a rebuild kit for around $20 and replace the diaphragm and other seal. I did this this afternoon, took about 1/2 hour and the bike runs fantastic. in the meantime since the ordering of the petcock kit till rebuilding, I just plugged the vacuum port off the manifold and off the petcock with rubber caps and ran the bike on reserve. Best to remember to turn the petcock off when not in use, though i never had a problem with gas leakage. cruzerjd

 

Mine has the vacuum hose for the petcock on #2 intake

 

1) I no longer have a vacuum petcock.
2) plug is wet
3) swapped plugs already and the problem stayed on #3
4) removed coil and found a crack down the body on the #3 side of the coil

any suggestions now?

 

Buy another coil?

Hve you tested the coild with a multi meter?

It might just be a faulty lead, they can be replaced but it involves grinding into the coil.

 

Did you fix your problem?

 

where can I get another coil? All I see are dynatek coils DC1-1, but I'm not sure they will fit or work well with the XJ550. Plus if I could pick up a stock coil, I would be happy.

 

Thanks for all the responses to my query-
The problem appears to be resolved. This past weekend, I pulled and cleaned the carbs, and replaced the spark plugs. After re-assembly, she started right up with minimal choke, and ran great!
Now, I need to Colortune to dial in the mixtures (it started with no choke) then it's on to bleeding the front brakes!
Thanks again,
Bill