1984 GPz750 - misfiring during warming up

Hi - 

as mentioned in the last post in my "new member" introduction thread , I have an annoying misfire (engine not firing / running poorly / backfiring) after I set off for a ride. Beginning usually after half a km, and lasting for several minutes, especially when the engine runs at RPMs under 4000. After the engine has warmed up, it mostly runs beautifully. 

I see from the Factory service manual that the ignition timing advance is electronically controlled, from 10º BTDC at 1050 RPM to 40º BTDC at 3600 RPM. This coincides with the RPM range I have the misfiring, so I wonder if its the EI control unit (ICU) that is playing up. This is what my ICU looks like: 

 

I believe it is original, since the wires leading in to the block connector all match with the colour codes specified in the wiring diagram, and the male/female connectors are a nice matching pair. I've pulled the connector off the ICU to have a look at the individual pins, which look free from corrosion. There was some white residue in-between some of the pins, which I managed to clear out using a cotton pin

Are those ICUs know to misbehave like that? Or are there other components that may cause the described misfiring? I see from the wiring diagram that there is a 4-pin block between the ICU and the wires coming out of the ignition pick-up unit to the right of the crankcase, I haven't located that yet, to check for corrosion. Previously I've taken the kill switch assembly apart to clear out corrosion, and there is no hesitation with contact when I press the starter button to run the starter motor. 

Another minor issue is that sometimes the engine fires up immediately after I press the start button, other times if fails to start - just stumbles and sometimes almost catches, but dies again. This can continue until I risk draining the battery. This is probably due to a poor connection in the wiring loom that I still haven't located . . . 

You can check the pulse with an LED test light across the coil connection, you will need a strobe to check the timing

Sounds like a carburetor problem.  

Nessism wrote:

Sounds like a carburetor problem.  

Very true . . . . . but usually I don't hear backfiring in the silencers when the carbs are playing up, or see that the tachometer needle jumps up or down when the engine stumbles

Might be worth checking out the carb holders. 

These cracks caused me similar problems : backfiring through the carbs at start up and low rpm etc.

 

Nessism post=927705 userid=14026Sounds like a carburetor problem.  

+1

A seeping float valve can flood one cylinder while the bike is parked enough to foul the plug. A few miles of riding can clear the plug and if the seepage is small enough, the bike will run ok. (Yes, I have this t-shirt.)
I suspect a leaking enrichener could do the same.

After firing (backfiring but through the exhaust.) is perfectly likely as the unturned fuel from the misfiring cylinder is ignighted by exhaust gasses from the other cylinders.

put new plugs in it.
start it up..
let it idle badly then pull the new plugs before the idle smooths out.
are they coated in gasoline?
dry and white?

this may give you a clue as to what is causing the misfire.

Thanks for all comments Much appreciated, especially those that steer me in the direction of fueling problems, to counteract my belief that it's (only) the ignition system playing up. 

To Wookie58: 
Yes, I know that I can check for ignition pulses at the coils, but that isn't practical when riding, and so far there isn't misfiring/stumbling when the engine idles. However, it's a relevant test when the engine (in-between) fails to start - which may be due to ignition pulses only going to one coil/one pair of cylinders.

To Nessism: it may well be both a carburation and ignition problem. When I got the bike, the carbs were flooding, so I had them out and checked the float valves, and fitted new bowl seals. I still have to check the vacuum valve in the petcock, but I think it isn't leaking much since the bike have managed to do runs for several hours, after it quits misfiring/stumbling. 

To JR:
Good point, false air can lead to similar symptoms. When I had the carbs out, I noticed that a previous owner must have fitted new(ish) inlet rubbers, since they were flexible and without any obvious defects. 

To TexasKZ: 
Good point. My occasional checks of the plugs haven't shown obvious signs of flooding, but still cannot be ruled out entirely. However, during a three day round trip of more that 1000 km earlier this summer (when the engine happened to start fine and didn't misfire!), I ended up getting an average fuel consumption of 4,5 l/100 km (̉approx 63 UK MPG)- which doesn't indicate excessive fuel supply of the carbs.

To F64:
When I manage to start the engine, and it idles, it doesn't misfire or stumble, this happens briefly during the warming up phase. 

FURTHER TESTING:

Yesterday I fitted another pair of known good dual output plugs I used for a while in one of my BMW boxers, along with a similarly known good set of HT wires and spark plug caps. Primary and secondary resistance not far from what I measured for the GPz coils. Using them, the bike ran as before - i.e misfires and/or stumbles during the warming up phase, which leads me to think the GPz coils and HT wiring aren't faulty. 

Today I unplugged the ICU, and checked the resistance through the individual leads going to the female ICU connector block. I.e. from the 4-pin connector block halfway down to the ignition pick-up sensors, and from the green and black pulse supply leads to the coils. Plus the +12V and ground wires. No signs of anything but Zero Ohms everywhere, also when flicking, bending and pulling along the wires. So no apparent poor connections. 

 

Then I went for an in total five trips in my local area. Stumbling and occasional misfires were noted on every trip, but only during the second last was I able to keep an eye on the tachometer when the misfiring occurred (otherwise I had to keep an eye on my surrounding traffic, to not crash or disgrace myself). Going at 2 kRMP in 3rd, suddenly the engine stumbles, and at the same time the tach needle drops to zero. 2-3 seconds later, the engine picked up again, tach needle back to 2 kRPM. This happened again after a short while, and when the engine reverted to normal running, the tach needle jumped briefly up to 4-5 kRPM. After another while the same stumbling occurred, but with the tach needle staying at 2 kRPM. All changes from normal running to stumbling and back to normal again was instantaneous. 

My interpretation of this is that stumbling plus tach drop is due to lack of ignition signal to the left coil (that fires cylinder no 2 and 3). This since the tachometer gets its signal from a branch of the green wire that goes from the ICU and left coil. I suspect that identical stumbling without a drop in revs at the tachometer can be due to lack of firing impulses along the black wire that goes to the right coil, serving cylinders 1 and 4; without branching off to provide signals to the rev counter. 

I'm all ears for other than the above interpretation, especially how a fueling problem may cause erratic behaviour of the tachometer . . . .   

 

Could it be an intermittent short in the tachometer that also shorts out the power to that coil?