Help a newbie...mechanical issue on recently acquired '99 M750

[Howie]

Your problem could be a coil, ignition module or pick up.  Diagnosis would be easy if the problem was not intermittent.  Problem is, when everything is working all will test good, so you either need to duplicate the problem or diagnose on the road.  easiest way to do this is swap the two easier suspect components one at a time and see if the problem switches cylinders.  Pick ups no so easy, but if the other components check out good bring an ohmmeter.  Each pick up should read 95-105 ohms.

Probably a pick up.

[stonemaster]

I guessing its heat related, when I started it a few minutes ago, its purred like a tiger, nice and cool (comparatively) right now, a couple of days ago on my first ride on her its was cooler as well and no problems, the then second ride gave me the problwems, it was much warmer then, I'm gonna ride her in the morning for 20 miles and again in the heat of the day for 20 miles and see what happens

[stonemaster]

I may have figured it out, I couldnt get the cylinder to quit firing again but I now know how it got so hot, the other day it was warm up here (around 92) . I keep my bike in my shop which is one of those steel arch buildings, and if any of you have one of these building you know how hot they get in the summer. I'm guessing it was at least 100 to 102 in there and the bike immediately dropped a cylinder on startup, since I only had the bike a couple of days I wasnt sure.  After I got one the road I knew for sure but suddenly it kicked back in. The air cooled it down enough. Then at a light it dropped again.  Back up to speed, in the wind, and full power again. So after some research I read about the pick up sensor temporarily failing as the case heats up and expands.  I dont fully trust the source of this info but it sounds possible. Any inputs on this will be appreciated.  I dont know when it will be warm enough to replicate the issue as it gonna be in the upper 70's and lower 80's for a couple of weeks so I doubt it will get that hot in my shop.

[Howie]

A pick up or any winding such as an ignition coil can open at a specific temperature causing a temporary open circuit.  Even a wire can do that. 

[stonemaster]

yea I'm aware of potential coil issues as well, had it happen on and old kaw, will just have to chase it

[the_Journeyman]

Check your spark plug boots for cracking.  I remember one would run fine until it reached temp (like you could ride it a few miles then it dropped a cylinder) the, a crack in the plug boot expanded enough that the plug grounded out and the bike dropped a cylinder.

JM

[stonemaster]

they (THE PLUG CAPS) look good, I should clarify, when I said the case expands with heat I meant the "engine" case expands causing the pickup sensor to temporarily FAIL

[Howie]

Usually the pick up windings themselves open under heat.  Rarely, but not so rare on some older BMws there could be a break in the wire.  When the insulation is tight the wire has contact.  Insulation gets warm, wire opens.  Case expansion is not the cause, a failed part is.

[stonemaster]

thanx howie, for the info, like I said the source was suspect, same end result however, now the trick is to replicate the that heat and test it quick enough before it cools off

[stonemaster]

ok well that theory went down the tubse, today was cool and it dropped one, no rhyme or reason, reversed plug wires on the road and the bike wouldnt even start, changed back a I'm off, 95 % of the ride it purred like a tiger, bike's got some giddyup, so I can eliminate what based on today?

[Howie]

In this case of the pick ups hot refers to engine temperature.  Obviously ambient temperature affects engine temperature.  I had a pick up fail in my bike once.  After removing them I tested them by heating them in water.  The bad one would open at 190ish (don't really recall) ^o F.  This was after swapping modules on the road and and testing coil resistance on the road.  Do not reverse plug wires, swap components.  Oh, I failed to mention one suspect component,  A Journeyman said, spark plug cap.  There is a resister in it that can open when hot.

Coil specs:
      small terminal to small terminal 4.5 ohms + 10%
      one probe on ground, one on the spark plug wire 13.5K ohms + 20%
      spark plug cap 5K ohms + 15%

You could also bench test these components using a heat lamp and thermometer with maybe a little vibration.

[stonemaster]

thanx will do

[stonemaster]

ok problem solved I think, I decided to swap coils. Apparantly one of the wires from the CDI was loose, there are two on each coil one larger and one smaller. When I took one of the coils off the smaller wires kinda fell off. I went ahead with the swap and when reattaching the wires I noticed that it was really difficult to get the smaller one back on, when I got it on I double check and it hadnt gone into the metal clip, it lodged between the plastic case and the metal clip, I finally got it properly inserted when it hit me, someone, had not properly inserted it in the first place and thats why it fell out so easy upon removal of the first coil, so thats it

[plugmaster]

NGK DCPR8E #4339  I see also #4179 here: http://sparkplugs.com/results_cross.asp?pid=dcpr8e&x=0&y=0
Only difference I can find there is solid vs removable terminal nut. I know the part # is correct as it's on the dealership's maintenance record from 1 year/4000 miles ago. I used their application link and found our bike: http://sparkplugs.com/results_appOther.asp?otherMotiveID=224608&mfid=1 
Seem like the right ones. I'll also get a beam torque wrench tomorrow to makes sure they are fit properly. DAMN! So close...

Not to back track too much, but I'm a little OCD when it comes to links. Those links aren't working anymore, but I went to the liberty of finding replacement links since I'm nice like that. (Plus I was looking up that plug anyways)
(1) http://www.sparkplugs.com/SearchResults.aspx?kw=DCPR8E
(2) http://www.sparkplugs.com/product.aspx?zpid=9705