GS450 with one cylinder out of timing other is OK

[Guest]

Hello guys! First post here after long time of lurking. I searched quite a bit for this one on the forums and haven't seen another problem like it. If you know a post, please point me to it! I've owned quite a few bikes and clean carburetors for people on Craigslist all the time. Safe to say I'm not new to bikes, let alone GS twins and 4 cylinders. I recently had something I have never ran across in the several years working on bikes. A GS450 (with ignitor box and electronic pickups) would start OK, and seem to run OK too. But you got on the bike and took it for a drive it feels heavily underpowered. I verified spark, fuel, air etc. All in good order it seemed. I checked spark outside the cylinder, and while the bike was running, pulled one spark plug at a time (left cylinder first) it would die. Then put that back on, and pulled right cylinder plug wire off and it would also die. After scratching my head and pulling the carbs out again to verify they were clean, I pulled out the timing light to have a look at the timing even though it was assumed it would be OK...they were electronic pickups afterall! The RIGHT cylinder would fire on the timing mark correctly. The LEFT cylinder was 10-15 degrees advanced...

So here's a short list of what's been done.

Cleaned carburetors (very well, even used the ultrasonic cleaner for this one the second time just in case!!)
Took out champion spark plugs and replaced with correct NGK plugs
Verified that cylinder 1 (left side) is not correctly timed. Cylinder 2 is OK.
Replaced ignitor box with used one, same problem exists. Both Ignitor boxes were cracked in the back, so we tried a 3rd box, which also didn't change anything.
Unplugged, cleaned and re-plugged all connectors in the circuit.
Trimmed spark plug wires and screwed caps into new clean copper
Verified that Ignitor box was grounded, no missing connections.
Verified that charging system is working OK and battery is OK.
Verified both cylinders have compression and both are within 2psi of eachother.

So, I'm stumped. Has ANYONE seen anything like this before??? The only thing I could see is if someone put a cam in wrong, or the valves are WAYYYYY out of spec. And being that I have not checked the valves/clearances, I just don't know if that could do it being that they aren't really doing anything to change the crank/cam timing.

Thanks!!

Bruno

[pete]

If one cylinder is more advanced than the other in terms of spark only, then the cam's can't be related... they don't affect when the spark is fired.

The only thing I can think of is that there is something wrong with the signal generator assembly under the timing cover.

If I recall correctly the 450GA has electronic advance and magnets under the signal generator plate. Perhaps one of the magnets is cracked causing the ignitor to fire early?

You've already proven the ignitor is not at fault and that would be the only other thing I can think of that would cause this.

[Brendan W]

Sounds like generator trouble. Is there any chance that the pickups have different polarity? Can't see how it might have happened but it could have advanced or retarded one signal.

[Grimly]

I'd be inclined to suspect that it's supposed to trigger on the rear of the pulse and somehow No1 is triggering on the front, instead.
Since you've verified by substitution the ignitor boxes are ok, the fault must lie in the triggers. The bikes you got the ignitors from, can they donate their triggers just for a trial?
There is always this option, if all else fails:
If the mounting for No1 pickup isn't adjustable, I think I'd make it so.

[Motorbikebruno]

Thanks for the replies guys!

I'm going to have a look into the advance mechanism then. Pete, thanks for the suggestion.
"If I recall correctly the 450GA has electronic advance and magnets under the signal generator plate. Perhaps one of the magnets is cracked causing the ignitor to fire early?"

Brendan, how would I check polarity? Or do you mean that the positive somehow became the negative and vice/versa on the pulse generators?

Grimly, I had given the bike back to the PO, and from what I remember he swapped out the pulse generators with another donor set but I will verify!

Thanks to you all for taking the time to help. This is just such an odd thing....ugh!

Bruno

[Motorbikebruno]

Just checked and he had swapped out the pulse generators already! So we are going to look into the advance mechanism to see if a magnet is out of place/loose causing this disturbance. Thanks again gents! I'll see if that is the problem!

[Brendan W]

Brendan, how would I check polarity? Or do you mean that the positive somehow became the negative and vice/versa on the pulse generators?

[/COLOR]

Normally there are three wires from the signal generator. The brown and green/white are connected to the top post on each pickup and the black/white common wire is connected to the lower post and looped across to the other pickup. If a new plug was fitted between the generator and ignitor the wires could have got crossed at this point as well.