Misfire at low revs

[turbopete]

My bike (xj1100) has developed a fault, it is not firing at low revs on No1 and No4. I guess the coil could be failing but before I set about changing it is there somewhere else I should look for the cause?

Thanks in advance
Pete

[TopCatGr58]

Round up the likely suspects!

Hey there Pete,

With both #1 and #4 acting up, the ignition coil does come to mind first. But it could be the pickup coil? The XJ got rid of the rotating timing plate, so there's no flexing of the pickup coil wires, so doubt that it's the wires, but the PU coil could be going bad?

You can do some diagnostics, by swapping both the power/signal wires for the coils, and then of course also swap which set of plugs they are plugged into. Then if the problem jumps to #2 and 3, then you know it's the coil. If the problem stays with 1 and 4, then it's the PU coil circuit, or even the TCI.

An economical replacement can be gotten thru MikesXS, he has an outlet in EURO, get the later high output coils for the 81-84 series XS650's with TCI ignitions, they are 3ohms, aside from a little adjusting for the mounting brackets, you may have to put it in facing opposite of the OEM, but otherwise, will be plug and play.
T.C.

[turbopete]

Cheers TC. I have a spare coil off a different model (XS) that I can swap out for testing. Just wanted to see if there was any other known troublespot. The bike was starting poorly this last week and I changed the plugs. It started a lot easier next time but not instant like it always has. When I checked today the plug cap connector had pulled up through the cap slightly causing a gap between the laed end and the plug top (I changed the plug leads months ago for some 5Kohm renault leads). I think this has toasted the coil.

[TopCatGr58]

Hey Pete,

The 5k ohm caps should NOT have done anything to the coils. The OEM caps are 5k ohms! Probably just coincidental failures occuring. The windings in the coils were probably going, and not putting out as much power, so it was harder to get thru the caps!

Your other coil from the XS, hopefully it's from the later years, 80-81, they also used the 3 ohm, the 78-79 used the 1.5 ohm with the ballast resistor in line!
Good Luck!
T.C.

[skids]

Are you sure 80 models had 3 ohm on the primary side? 81's used that different black box (what, 4RO?) and I thought they were the ones that used no ballast resistor because the coils were beefed-up(?)

Originally posted by TopCatGr58
Hey Pete,

The 5k ohm caps should NOT have done anything to the coils. The OEM caps are 5k ohms! Probably just coincidental failures occuring. The windings in the coils were probably going, and not putting out as much power, so it was harder to get thru the caps!

Your other coil from the XS, hopefully it's from the later years, 80-81, they also used the 3 ohm, the 78-79 used the 1.5 ohm with the ballast resistor in line!
Good Luck!
T.C.

[John]

My 80 SG does have a ballast resistor, and my 81 SH does not have the resistor.

[TopCatGr58]

My Faux Paux!!

Hey guys,

Thanks for that correction, was thinking carbs apparently! I do know this, I have the 4RO system, just forgot, damn Oldtimer's acting up again!
T.C.

[bikerhawk]

I now know waaaay too much about coils and the difference between the 81s and the others...

78-80:
part # CM12-08 (stamped on coil)
pri. resistance: 1.5 ohms +/- 10%
sec. resistance: 15K ohms +/- 20%
--MUST USE BALLAST RESISTOR--

81: (maybe 82s?? I can't verify that--anyone?)
part # CM12-09
pri. resistance: 2.5 ohms +/- 10%
sec. resistance: 11K ohms +/- 20%

(this info from the Yamaha manual itself--everyone genuflect--)

The plug-ins are different too...std. bullet connectors on the 78-80, molex on the 81.

If this differs from others...hey, I'm just reporting what I learned this weekend by comparing my H with the F donor bike. I suppose I could rig the F coils w/ballast resistor with some extra wire and some temporary connectors...but I think I'll just find the right thing.

[bikerhawk]

Re: Misfire at low revs

Originally posted by turbopete
My bike (xj1100) has developed a fault, it is not firing at low revs on No1 and No4. I guess the coil could be failing but before I set about changing it is there somewhere else I should look for the cause?

Thanks in advance
Pete

Dealing with your specific problem, Pete...mine would act the exact same way (stumble until around 2500 rpm, then clear up...I thought it was a carb problem for a while, until I discovered that it was on both #1 and #4). I'm 99.999% sure you're headed down the identical path that I am...that coil will fail completely, and soon.

Oh well--at least you don't have to clean the carbs again.

[turbopete]

Looks like it's the coils for certain. I hve closed up the plug gaps really tight and it's all but gone away, just a little rough at low throttle openings. Almost made it to the weekend when I'll strip the coils off and fit my spares.

Just so you all know I think what has killed this coil is my carelessness with the plug caps. The leads I have fitted to the coils have the straight-through ends that you can pull the plug lead clean out from. The lead had pulled up through the cap and it was working with a 8-10mm gap which the spark had to jumpto the plug top.