The dreaded 6 k RPM stumble pick up wires good?

[markb80]

Yes I am very suspicious of the 1-4 coil because the plugs on cyls 1 and 4 are sooty after I do a run where I make it start cutting out. I think the timing advancer moves pretty freely, I played with it alot while checking the wires. Also I have done some runs with the vacuum advance disabled. Problem still occurs, just a little higher in RPM.

My stator and field coil measure good, but my charging output is a little low at 2k RPM (13 ish). But I've had a few XS11's and they were all like that. Only way to improve that may be to replace all connectors in charging system, swapping regulators didn't help.

So am leaning toward 1-4 coil. I noticed this issue is vaguely heat related (maybe). First run not quite as pronounced as following runs.

Getting tired of riding around full throttle without my seat on, battery acid is hard on the a$$, plus you have to sit forward all the way so you don't sit on the ECU. : D I've heard of "butt dialing" but not "butt igniting"

[crazy steve]

Try trimming about 1/4" off the end of the plug wires on the 1-4 coil; sometimes that connection will degrade and giving some 'fresh' wire to screw the plug cap into will help.

[motoman]

One thing to remember is that there is a BIG difference in the amount of spark needed to fire under heavy load VS light load, so WOT will cause a miss that even steady throttle won't. Check that charging voltage cause 13.8 is NOT enough voltage to run the bike a the times it's requiring max out of the system, such as WOT at higher RPM's.

Not true. If batt. voltage is under 12volts, yes. If 12v and above, coil 'unloading' voltage is constant. It does not increase with added voltage. With 12v coils, storage capacity is constant for those milisec. before unloading to whatever they are rated at.

[TopCatGr58]

Hey Steve,

I think you read through his recent description of resistances a little too quickly, he said his caps are ONLY 1 k ohm...which leaves well over 20K values for his secondaries WITHOUT the caps. However, he didn't actually say that he tested the secondaries WITHOUT the caps, just that he had measured the caps resistances.

Like you suggested, he should trim off some wire from the end of the plug wires, then retest with the Ohmmeter, and if he is still getting 20+ K ohms, then I think his assumption of the coils being bad may be right.

Moto, his coils are the 2H7 series using the Ballast Resistor, so they are getting ~ 9 volts when running, but perhaps even less with a slightly weaker charging system.

Also, Mark would be nice to put your year/model in your signature for easier and quicker reference, had to view your profile to see it.

BTW fellas, he's an electrical engineer ! Whoda thunk?

T.C.

[crazy steve]

...However, he didn't actually say that he tested the secondaries WITHOUT the caps, just that he had measured the caps resistances...

He didn't actually say he measured the caps, just that he checked to see what they're supposed to be and used a 1K figure. NGK does make 1K ohm caps, as well as the 5K caps (and a 10K). Which one does he have? Those need to be measured separately....

[markb80]

Ok, I finally got back to it, and took coil caps off. I did that once already to cut 1/4 inch off the plug wires. But this time I noticed that the plug cap on cyl 4 was different than cyl 1. I must have broke on years back and replaced it with an NGK. Anyway plug cap 4 measuree 10kohms rather than 5 kHz, which explains why the 1-4 secondary resistance was about 5 kohm high.

I put a 5k (correct cap) on cyl 4 and the miss at 6k under hard acceleration was reduced some. It now only happens half the time and is less pronounced.

I found that I have a NOS set of 81 or 82 coils with wires attached. THey measure 2.5 ohms on primary, so I think I need to short the ballast resistor to get 1.5 ohms. I will try that next. I did notice this time that the hesitation did not happen on first run. Only on later runs.

So I am still leaning toward coils/wires.

Thanks

[crazy steve]

I found that I have a NOS set of 81 or 82 coils with wires attached. THey measure 2.5 ohms on primary, so I think I need to short the ballast resistor to get 1.5 ohms. I will try that next. I did notice this time that the hesitation did not happen on first run. Only on later runs.

If running the later coils, bypass the ballast resistor. Unplug it, and plug the two wires from the harness together...

[natemoen]

You need AT LEAST 3 ohms. Are you sure those are xs coils?

[TopCatGr58]

Hey Nate,

I've ohmed my working OEM 81 coils and found they only had 2.5 ohms also, and worked just fine, but of course I'm also running the 81 TCI. Davinci hasn't ever said whether the 81 TCI has a different tolerance vs. the 78-80 TCI's for the circuit that controls the coils. The aftermarket coils like Dyna and Accel are listed/rated at 3 ohms, so we know they are safe to use on the 78-80 systems with bypassing the resistor. I'm thinking and trying to remember whether I've read folks getting the 81 coils and putting them on the 78-80 series bikes, bypassing the resistor, but I've not read of anyone frying their 78-80 TCI's while using the OEM 81 coils!?

But his miss is most probably because he's running 12 volt coils at 9 volts, not enough power to provide a full strength spark...and his 10k ohm plug helped to bring the miss out more prominently!

I'm thinking the 2.5 ohms of the OEM 81 coils are sufficient for the 78-80 TCIs so as not to draw too much from them to burn them up.

T.C.

[markb80]

Well I'm back again, I changed coils to the new set (81 and up 2.5 ohm style). I shorted across the ballast resistor by connecting the wires together. Same cutting out problem at 6k (or so). I have never seen anything like this. It is definitely related to holding the throttle all the way open through the RPM range. If I open the throttle a little more gradually than whacking it open, it does not happen. Earlier I tried disconnecting the vacuum advance, seemed to have some effect, but problem still exists. I'm going to try swapping in a completely different set of carbs, but saw no issues with mine. Thanks for ideas, will let you guys know if I figure it out.

[TopCatGr58]

Hey Mark,

The blocking off of the vac. adv. is to see IF keeping the timing plate from moving...and therefore the pickup coil wires from moving/bending can help. IT sounds like it did a little, which points towards problems with the pickup coil wires...cracks/breaks in them that cause drop out of the timing signal at different locations in the movement of the timing plate...so most of the folks here would probably say that you need to look closer at he pickup coil wires, doing a more thorough testing, pulling of the wires checking for breaks which allow the insulation to hourglass.

T.C.

[markb80]

So yesterday I swapped in the 2.5 ohm coils and shorted ballast resistor to get the desired 2.5 ohm total resistance seen across the primary side of the coils (XS11 ignition spec with 1 ohm ballast resistor in series with original 1.5 ohm coils for a total of 2.5 ohms across primary). No effect.

I was able to make the problem worse tonite. I have a set of 82 XJ1100 carbs. These carbs use the same emulsion tube/nozzle but smaller main jets due to different needle design. I went ahead and swapped the whole slide assemblies in just to see what would happen (knowing it would make the mixture a little rich).

Now it stumbles at 4k RPM, and I can make it stumble at will by simply holding the throttle all the way open.

This tells me I probably don't have a good enough spark for WOT operation. The stator and field coil measured in spec (resistance), and I swapped 2 different regulators in. Could be inadequate ground somewhere , or my wiring harness has a high resistance somewhere. Pickups could still be an issue, but seems like something else is wrong, cause not yet known. I am going to put my dynojet needles back into the xj's diapragms, as they were in excellent shape. I could then swap field coils and stators, hoping for more output. I have a rewound field coil from customrewind.com

One clue, my turn signals don't blink at idle, I have to bump up the RPM a little, but its been like that for 10 years, and did not have this stumble

Thanks for help





Thanks for help.

[BA80]

It's really starting to sound like something other than carbs or ignition. Have you checked valve clearance? Too tight valves can cause a similar problem.

[markb80]

Yes I have only 1-2k miles since setting valves. But I have heard of the cam chain tension causing strange running problems. I should retension the cam chain., thanks

[Big Beadle]

TC had helped milk me through this same issue...I tried every thing!!! It turned out being by pod filters were to close to the carbs and I had to take the stock rubber manifolds ans extend them out a bit. It was ok on an easy accel or above 5-6K, Try removing the pods and giving it quick spin around the block to see if it clears up. I was totally puzzled after I found out what the problem was!! I guess it was making a vacume and sucking the slides shut instead of allowing them to open. If you can pull just the pod filter off and leave the rubber connecter there you will see that it's blocking an air passage in the mouth of the carb....Good Luck!!!