1981 carb Wont rev out. intermittently...

Hey Fellas,

Got a weird one for ya.

When Cold;
Its runs nicely.

When warm enough to feel the heat on your legs;
It will misbehave, hesitate greatly, and backfire on decel.
It wont rev out cleanly.
It appears all cylinders are not firing or not firing consistently.
Anything above 2K RPM is a struggle. It goes for a bit then dies then repeats,.
It will idle fine just anything off idle is a no go.

Things I have done to no success...
New Manifold Boots
Cleaned carbs
new ignition coils and leads(Found that one coil was from a GPZ900R ... had 5 ohms when behind the resistor...)
New spark plugs

I had time to investigate it this weekend. Once it was misbehaving I then pulled it up on the center stand stuck a fan near it and started going through the ignition system Electrics.

I Started with a timing light. Cyl 1,6 are not firing consistently nor 2,5.
After about 5 minutes of probing igniter connections, which where all in the norm. over about 5 seconds I could hear it coming back on all cylinders and like magic it was reeving cleanly. Took it around the the block a coupe of time and it was behaving nicely. Then I left it to cool.
I'll add that I was suspecting a fueling issue and drained about a bowls worth of Fuel from Cylinder 1,2/5,6 while it was running and plenty was coming out. This didn't affect the idle in anyway.


I then left the bike to sit for maybe 2 hours and wanted to confirm if this pattern was repeatable. The bike wouldn't run at all sounded like 2 cylinders only. Choke did nothing to improve the situation. It was left there.

4 Hours later at night when It was cold, it fired up nicely on all cylinders. couldn't go further as it was late.

I Though perhaps an oscilloscope would let me see the signals that the Igniter was putting out, or see if I can borrow the igniter from my DFI bike, to see if its the issue.

What do ya think?

Seems like you're on to something Razmo. Things that act up when heated up.
You've eliminated the coils.
Have you checked the resistor block cold and hot? Just in case there's an open circuit(s) when it gets heated up (and it will)
Hoping you're not piercing the wires when checking for voltages. I've seen too many faults show up down the road when moisture gets in to the insulation and oxides the copper wiring. Telltale sign is little bulges in the insulation with a tiny hole in the middle of the bulge from the pointy end of a probe. UGLY.
I would do some measuring across the pick up coil connector to ensure the voltage is getting across. It's only 8 volts at that point I believe and low amps. Just a trigger signal.
You mentioned that after poking around the pick up coils it started to clear up. That tells me something and you're in the right area. You seem to have a good idea on how to use your multi-meter. The 1981 Supplement has some good information to follow on your specific ignition. I'm thinking you're on the right track investigating the pick up coils.

First: a Q for kawboy who wrote: I would do some measuring across the pick up coil connector to ensure the voltage is getting across. It's only 8 volts at that point I believe and low amps. Just a trigger signal.
This somewhat puzzles me. Wondering if you can make a specific reference to "The Manual" regarding this. Can't find a spec. for the voltage you mention. As you mention: Just a trigger signal.
I've always believed that the Ignition Triggers (Pick-up Coils) to be "Passive" ...... no measurable voltage. Or to put it another way.....no supplied "harness" voltage but simply a small signal induced to the winding's within the triggers by the rotating Lobe, that the Igniter then senses.
razmo: The inconsistencies being experienced have me believing that you have a "connection" problem. Not discounting an Ignition Module failure but thinking of simpler remedies first. Can only presume at this point that all the related ignition components are functional.
Kawboy pointed out a valid concern about corrosion, due to insulation being pierced. This is something you can check for while you do the following from a strictly Electrical point of view:
I'd start with a very close inspection of the Ignition Module plug! Check to make certain all of the terminals are fully inserted and "locked" into the terminal block. It will be obvious if one has "unlatched" and been pushed back by it's counterpart. This alone would explain the inconsistency you're explaining. None-the -less I'd remove each lead and inspect the wire to terminal crimp. It's possible a wire(s) could be corroded!
Before reinserting the terminal(s) into the block.....clean them and make certain the small locking tab stands proud. This tab must be raised to ensure that particular terminal can't be pushed back out when re-connecting the male and female block.
Whether a single "Bullet Connector" or a multi terminal block....cleanliness for a low resistance connection is critical. Tarnish and corrosion is a :evil: So additionally, the same protocol applies to the 6 Ignition-Trigger connections and the ignition-coil connectors.
Terminal ends can appear clean but I've experienced poor continuity despite their appearance. Clean them to bare metal !
Male ends are easy ! Female ends, whether "Bullet" or "Blade", can be a bit more challenging. Simply use your creativity. For obvious reasons, don't use any type of metal cleaning solution !!!!! The short term gain will be soon lost when the corrosive solution that WILL wick up into the insulation, eventually "eats" the wire strands!
If this doesn't solve the problem then I'd focus on the individual ignition triggers and ignition coils. You may very well have a couple of "triggers" that are failing due to an "Open" in the winding's. Having two that are defective would be a coincidence but still possible. I've experienced two dropped cylinders due to one faulty trigger - similar to your explanation. I'd suggest removing the ignition triggers and do the following. Check each one for continuity per the specs. at room temp. If they check-out, put them in your freezer for half an hour. Check the continuity again. If they continue to "pass" then heat-up each one with a hair dryer (heat-gun if your careful - you want them hot-to-the-touch) and check them again. If they "pass" then you can logically consider them OK. Then do the same with the ignition-coils. If your issue is in fact an Electrical related problem then I think you may find the culprit, at this point.
hope this helps.

Besides all the possibilities, I am wondering if you have the vacuum valve set for extra burn in the exhaust connected. If so, are all the vacuum hoses not leaking? Is it functioning fine? It is connected to the intake. Cracking in exhaust is un-burnt gasses ignited in the exhaust .
Also, maybe those o-rings in the carburetor are affected by ethanol, and leak fuel enriching the mix? With higher rpm's (higher negative pressures), higher temperatures, it's too rich and misbehaves, letting the higher than normal 'extra' un-burnt fuel mix into exhaust, where it burns with the extra oxygen shot?
From my experience with other bikes, when they had irregular running and loss of power when hot, my first guess was the coils. Changing coils or upgrading to COP's always solved the problem. Then leads, spark plugs, damaged wires as mentioned. But that you eliminated (except wet wires).
When an engine runs with a couple or one dead cylinder, they get flooded with fuel and the engine runs in a somewhat similar fashion. Better cold than hot, and no power when higher rpm's. If all cylinders working but flooded with fuel, that be similar to what you descried.
Also good to check and clean all ground connections.

Scotch Wrote "First: a Q for kawboy who wrote: I would do some measuring across the pick up coil connector to ensure the voltage is getting across. It's only 8 volts at that point I believe and low amps. Just a trigger signal.
This somewhat puzzles me. Wondering if you can make a specific reference to "The Manual" regarding this. Can't find a spec. for the voltage you mention. As you mention: Just a trigger signal."

Seems I was bouncing between the 1981 and 1982 model supplements.,, 2 totally different ignition systems. My Bad. Back on track.

Proud to say Kawaboy that I don't break the insulation usually target the back of a connector block, When probing.

The Hall effect sensors check out by what the manual specifies at room temp.(yet to test when extra cold and hot)
When Running the bike has 8volts applies to the pickups. I was gonna try probe the pulses but doing so stops the engine running.
All the connector blocks are fully seated and I did sand some(igniter connectors) of then down to metal to ensure a good connection.
I don't think the Resistor block is shorting,the resistance does increase(Extra 0.7 ohms). I have measure it after a ride.

I know for a fact that the Timing Rotor is a bit iffy, 3/4 are on the F mark but 1/6,2/5 are before F by a bit. Really want to fix this but cannot source a new rotor. Kinda certain that this wouldn't cause it to drop cylinders just make it run rough, have less power and a bit retarded .

Wish it was something I could investigate Stan, but this bike has no vacuum hoses coming off the carbs.

I started it this arvo from dead cold it appears to have reset to its base behavior. Good Cold then misbehaving as its warming up.

This weekend or if I have time in the afternoons I'll go over the electrical connections thoroughly.