Misfire: Cryptic cause

Hi team,
May be useful (and may not!)
Chatting with a pal who has owned a few Guzzis and BMWs. We were discussing the many causes of misfires.
An interesting one that occurred on his boxer BMW.
Symptoms: Starts from cold and runs OK. Leave the bike for 10-15 minutes and it will start OK. Leave if for 2 hours or so and it is very difficult to start.
Cause: Heat dissipating from the engine rising to overheat the coils under the fuel tank.
May explain why there was a trend in the 1980s for moving coils to beneath the side panel. Have seen some Tonti bikes with from new.
All the best
guzzibrat2014-08-15 17:18:37

Or… … if it’s a 1980’s + Boxer like mine, then it’s most likely got electronic ignition. If so, the amplifier is under the nose of the tank and mounted on the front brake hydraulic line 1:2 splitter, which doubles as a heatsink for it. Not a lot of people may know this but, that amplifier has a safety shut-down feature if it gets too hot. This can be further aggravated if the heat conduction between amplifier and its heatsink is poor. Between the two is, or should be, a white coloured heatsink compound paste, same as is used for other power devices on heatsinks in electronics. With age this paste dries out and goes cruddy, which can cause premature overheating cut-outs of the amp due to the poor heatsinking. So if you’ve got a Boxer of some age with this type amplifier, you need to prise it off the heatsink (without breaking it!) clean off all the cruddy old paste, and put new on. This is most likely the actual cause for Boxers to refuse to run again having been parked on a hot day after a run. It caused a bit of head scratching at the time, as you can imagine. Ignition coils get ‘kin’ hot as a matter of course anyway, as they’re typically dissipating about 50 Watts total as purely wasted heat (4 Amps x 12 - 14V). So a few more degrees won’t make much more of an impression IMHO. HTH Edit: this is not to say tho that it can’t be a duff coil, because it can.
Mike H2014-08-15 23:56:10

PS: RT/RS fairings have a special small aperture behind the forks to allow some air to blow over the hydraulic splitter-come-heatsink, otherwise there’s a possibly increased risk of the over temp shut-down tripping even if the bike is moving. When I first got mine I thought what a stupid place to have a hole that would let rain get straight onto the amplifier, so I taped it over, before I realised what its real purpose is!

Cheers Mike,
I was thinking that the effect of retained heat upon the coil would be more interesting to Guzzi owners.

I am referring to the notion that hot coils stop working, then begin again after being allowed to cool down. Given the BMW mis-information, I’m trying to say that this is nonsense, it’s the Bosch amplifier over-temp shut-down feature that is causing the symptom. Is not a fault with a Coil. What we don’t want is to create yet another Internet myth, saying ign coils in general may stop working if they get “too hot” (and what is “too hot”) being generated by a specific thing that can happen only where this Bosch ignition amplifier is involved.

Unfortunate that this event occurred some time ago.

Sorry if I came on a bit strong there, but the specific example was BMW boxer, and I’ve explained what’s the most likely cause in that case. Nothing to do with ignition coils and not at all relevant to Moto Guzzis. Ths is the problem with 3rd or 4th hand hearsay, which gets further distorted with each retelling. This needs a more scientific approach, measure the temps of both undertank coils and under side panel coils in the same conditions, and compare. Also, measure the coil(s) current to see if there is any change there which may account for non-functioning sparks. Personally, given that a coil is merely two windings of enamelled wire, around a small bundle of thin steel strips by way of a core, nothing at all magical, I’m pretty sure that the wire cannot selectively decide to stop conducting electricity for whatever reason. “Oh dear I’m getting too hot, I’d better stop passing electrons for a while until I’ve had a chance to cool down.” Rollox. Either the wire conducts, or it’s broken. OK sure heat increases the resistance of a conductor, but as I’ve said an ignition coil, if used in the conventional way (applied DC, then a contact breaker or equivalent) is capable of getting flippin’ 'ot by itself, so I don’t think it’s that. But I am willing to be proved wrong
Mike H2014-08-17 23:06:44

The Spada and the LeMans models have the coils under the left side panel. I have a Pirhana electronic ignition box stuck to the inside of the side panel itself. That does get warm at times.

whilst we are on this subject does anyone have experience on replacing the 3 terminal starter relay with a modern 4 terminal 70a relay which is suituated under the right hand side panel on my Mk2 Le Mans ?

The old relay had 4 pins. Mike is yer man on relays, I’m sure he will chip in shortly with a compete explanation.I have actually found the old relays to be OK, it is usually the wiring to them that is the problem. This is mine, I took a wire from the spare permanent live way off the fuse box and connected it into the relay. Works a treat, especially with a decent battery.

Cheers Don, mine has a three terminal see through plastic cover Magnet Marrelli or something close to that, which has been water ingressed over the years it seems to still work but i would be much happier replacing it before become a problem.

I found the empty shell of mine during the rebuild last year. It is a completely daft place for a relay, right where the rear tyre can chuck all it’s krap onto it. Here’s that picture again! 85 - ignition switched supply, fuse 1, 2, 3? (brown wire?)86 - starter button. (can’t remember colour.)30 - battery plus by separate wire and 15/16A fuse. (aka, Starter Relay Mod.)87 - starter solenoid (red wire). Appendix (edit): I have mine behind the battery where it is much drier and cleaner. If it’s got a mounting tab for a bolt, can hang it from the top frame cross member using one of the rear mudguard fixing bolts. HTH
Mike H2014-08-17 23:19:40

Oh look I found a photo: Appendix: ~ on mine I have the supply side of the coil connected to ignition circuit supply, that way starter doesn’t work if kill switch is off. Hence white wire in the photo; think the other is white/brown from starter button ~ is actually supposed to be white/black I think, but Vehicle Wiring Products didn’t have that colour combo. PPS: and the red wire (motor solenoid) is ack-shirley red/yellow 'cause I’d used up all the red.

Mike H2014-08-17 23:30:05

I knew Mike would know. If yours only has 3 wires, I guess the ignition switch supply and power that goes out to feed the solenoid are joined up internally. Use your old switched power feed on terminal 85 and run a new power feed from the battery, via a fuse and to terminal 30. The other two will be as above.

Korrekt.      Give the man a cookie...

Thanks Guys