How do I diagnose a coil problem?

[Toppie]

I have an '86 GSXR400 (GK71F). It has the same engine as the GSX400X Impulse. I have previously posted a thread about some engine trouble I have with it. It seems to idle rough when it's warm (idles well before it gets too warm) and loses power above 8000 or 9000 revs. People have suggested the diaphragms in the carbs, sparkplugs and also valve clearances which I have inspected and seem to be fine.
I am aware the coils on these bikes are not amazing. Could worn out coils possibly cause this type of problem? How can I test the coils to see if they are faulty?
Cheers, James

[westie]

I dont mean to cover old ground but how did you go about checking the sparkprugs?
I have an rvf 400 and no one prugs keeps diying

[Toppie]

I just pulled them out when I checked the valve timing. The plugs were dry (as opposed to oily) and had a light black sootyness. The electrodes were clean though and had a light brown tinge to the porcelin. I'm not a mechanic, but I think thats how they should look from past experience.

[geoffm]

CDI problems are also a common fault - they pack up at high revs or when they get hot.
Check the battery is charging and the battery voltage is high enough. I used to race mine (total loss) and it was noticable when the battery voltage got below around 12.5V.
Coils are hard to check - do a resistance test first, then try some known good coils. I can't recall the corrent impedance, but swapping parts is really the only check as they fall over at high RPM or as they get hot.
You have tried a new set of plugs? They can look ok but be no good? leads and plug caps ok? Checked the resistance? The replacement coiils will usually have new leads as well so it is another check.
G

[sinfull]

I just pulled them out when I checked the valve timing. The plugs were dry (as opposed to oily) and had a light black sootyness. The electrodes were clean though and had a light brown tinge to the porcelin. I'm not a mechanic, but I think thats how they should look from past experience.

First thing i'd do is replace them plugs b4 blaming anything else !
Not a tech here but think you would find if a coil was breaking down it would be accross the range so my next thought would be rekit of the carbs 86 mmmm deffinately due with todays feul !
EDIT : there ya go someone who knows corrected me b4 i posted lol on ya shame he aint closer !

[Toppie]

CDI problems are also a common fault - they pack up at high revs or when they get hot.
Check the battery is charging and the battery voltage is high enough. I used to race mine (total loss) and it was noticable when the battery voltage got below around 12.5V.
Coils are hard to check - do a resistance test first, then try some known good coils. I can't recall the corrent impedance, but swapping parts is really the only check as they fall over at high RPM or as they get hot.
You have tried a new set of plugs? They can look ok but be no good? leads and plug caps ok? Checked the resistance? The replacement coiils will usually have new leads as well so it is another check.
G

Thanks for the help guys, it's much appreciated.

I checked the battery voltage. It was 12.6V before running and about 14.6V when running.
I also checked the resistance through the leads by measuring between the two spark plug caps on each coil. I got 13.13K ohms on the left and 14.33K ohms on the right. The resistance across the contacts on the left coil was 4.8 ohms and the right coil was 4.0 ohms. Do these measurements sound right?
I also noticed one of the coils is not standard (left coil). It is smaller than the other and someone has made up a bracket to attach it. Could having different coils cause problems?

Thanks again for the help!

[XRKID]

Thanks for the help guys, it's much appreciated.

I checked the battery voltage. It was 12.6V before running and about 14.6V when running.
I also checked the resistance through the leads by measuring between the two spark plug caps on each coil. I got 13.13K ohms on the left and 14.33K ohms on the right. The resistance across the contacts on the left coil was 4.8 ohms and the right coil was 4.0 ohms. Do these measurements sound right?
I also noticed one of the coils is not standard (left coil). It is smaller than the other and someone has made up a bracket to attach it. Could having different coils cause problems?

Thanks again for the help!

Yer mate that sounds about right, apprently i heard from another gsx400r owner that you use cbrr coils with these bikes, but shouldnt there be four coils? test all four. also with your ohms out on one of your coils that would produce a slight slow fire or speedy fire on one of the heads which could cause it not to run at full potential. Also check the condition of the connections on your leads by unskewing them from the coils if possible. other then that electrically it could be your cdi skewing up. or ya paulse generator.

[lostinflyz]

if your having trouble with coils and the like give kyle a ring at bayliss motor co. if your still in chch.

He sorted out my bike yesterday. He can get coils and the like the test out if things are buggered. electrics were never really my thing and he Seemed a pretty onto it feller.

[Max Preload]

...but shouldnt there be four coils?

Nah - they like most 4 cyl bikes fire on TDC in every cylinder in pairs so there are only 2 coils - like a 2-stroke. Saves having 4 coils and a cam angle sensor to determine #1 firing TDC.

[XRKID]

That's makes sense then, i have only owned single, twins, and v4s so didn't know that. Haven't had any problems with the XJ so that's why i haven't investigated straight 4s. Also that's prob one of the reasons the coils are stuffing up, being over utilized.