idel to 5000rpm ignition problem | GTAMotorcycle.com

idel to 5000rpm ignition problem

A

Alex Dee

Member
My 1987 FZ750 is having a very interesting ignition problem, now first off let me explain the formation of my engine to my ignition coils. lets assume my enings piston are in order from left to right 1,2,3,4 and igniton coil on the left we will call igiton coil A, which feeds spark to cylinders 1 and 4. where obviously igniton coil B (on the right side) feeds spark to cylinder 2 and 3 anyways at idel i can disconnect spark plug wires 1 and 4 (fed by ignition coil A) and the bike will idel the same but obviously when i dissconnect spark plug wire 3 or 4 the bike shut off. so basically the bike between idel and 4000-5000 rpm is runnin on cylinders 2 and 3 but after that rpm the bike runs perfect! what would cause the coil to give no spark at thoses low rpm and spark at high rpm (keep in mind the bike has a generator (DC current) which i think may have something to do with it) its to my understanding the either a coil works or it does not. ALSO i must include that when i unplug cylinders 1 and 4 at idel that all bar metal i.e. the frame gives me a shouck when touched.

thank you for reading all that
 
Your firing order is wrong, inline-fours either go 1,3,4,2 or 1,2,4,3 but beyond that it doesn't make any difference.

If you have the cylinder 1 spark plug lead off, and you put a spare spark plug into it and lean it against a ground, are you getting a spark from it when cranking the engine ... or does it only spark when the revs are above 5000 rpm?

I suspect you have a bad coil or a bad plug wire. It's possible that the internal insulation is partially breaking down and at high revs, there's enough charging voltage etc to fire it, but at low revs, there isn't.

Try exchanging the coils (swap 1/4 with 2/3) and see what happens - hopefully the spark plug leads are long enough to reach.
 
im soory i did not meant the fireing order was 1,2,3,4 i was just trying to make it easy to visulize, and i mean it only sparks at high revs above 5000ish when u said internal insulation breacking down did u refer to the coil or the wire or both?
thank you for help
 
I just completed further tests, what i did was unplug coil pack A at idel and reved up and the bike ran ****** (as it should) then i pluged in the coil pack at idel (and obviously ideled the same) but as i reved up the bike started so sound normal! i think its a ground off the mani coil OR the spark plug wires as suggested by brian P
 
Sounds like a bad plug wire to me. Probably broken down internally.
 
Tripletrouble said:
Sounds like a bad plug wire to me. Probably broken down internally.
Click to expand...
the more investage the more i agree, i just ordered the spark plug tubes from yamaha but the problem is that the wires are intergrated into the coil so therefore i cannot replace the wires withe out replaceing the coil, but i was curious to if anyone had a simular situation and used some kind of wire union? so that i can cut the wire short and union a longer peice on. i work at partsource and there usless so if anyone got a suggestion to where i can obtain such a item,
 
Practically all motorcycle ignition coils are integrated with the spark plug leads. There is a fair chance that the bad connection is right at the spot where the spark plug lead connects to the coil, and it's not designed to be able to be serviced.

A common fix is to replace the ignition coil with an aftermarket one (Dyna is a common name) and those will have replaceable plug wires, thus making the situation more serviceable in the future. Make sure you match the primary resistance to the specifications for the original coil.

I wouldn't screw around with trying to fix the original coil and plug wires. You're just as likely to introduce spots for water to get in where it shouldn't, which will obviously create other problems later on.
 
I don't think the tests you did have narrowed it down to ignition. Do as Brain P said and check for spark. Could also be a fueling issue.
 
casacrow said:
I don't think the tests you did have narrowed it down to ignition. Do as Brain P said and check for spark. Could also be a fueling issue.
Click to expand...

^ +1 Idle jets plugged. Try adding a little propane to the inlet side of the suspect cylinders at lower RPM and see of that cures the rough running.
 
I thank you all for the reply's. This is a great site and im glad to be apart of it. It truns out it was the spark plug tubes were bad i replaced them a few days ago and the bike runs like a dream EXCEPT!!!!!! now when im ideling for about 20 seconds the bike will go from 900 rpm to 2100rpm I dont even know where to start with this new problem. i would turn my idel screw down, But how could the bike idel at 900 rpm for 20 seconds then continues to increase?
 
Sounds like what used to happen when bikes had centrifugal spark-advance.
As engine RPM increased, the spark-advance plate would spin faster and centrifugal force would cause little weights to throw out (overcoming the springs which held them at low RPM).
This advanced the spark at high RPM.
If everything was not perfect though, a bike could get a wandering idle that suddenly shoots up to 2000-2500 RPM.
The bike was said to be 'getting on the advance' inappropriately, and needed the spark-advance inspected and the ignition timing carefully set...
But 1987? Not sure - I guess I assume you're into digital solid-state spark advance and timing there.
But who knows; check it out. Might need timing.
Overly-advanced ignition timing is more rare these days - but not impossible, heheh...
Plus (as mentioned) the carbs are old and probably all the rubber parts are going rotten.
These include the rubber manifolds that connect your carbs to the motor, and also the connection from your carbs to the airbox.
Check these closely for cracks or deterioration, and ensure that all clamps are nice and snug.
Keep that 1987 running!
Hey if it was easy, anyone could do it...
Regards
 
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