GK76 intermittently cutting out at idle.

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wullie3xv9
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Bike owned: GK76a GSXR400R & TZR250RS
Location: Fife, Scotland
GK76 intermittently cutting out at idle.

Post by wullie3xv9 » Sun Mar 21, 2021 1:03 pm

My GK76 has started playing up & I think I've narrowed it down to the 30 ish year old coils. :whistle:
After watching UToob videos on electrically testing the gennie system & battery, stator & reg / rec & finding nothing wrong with what's fitted to mine,
I'm wondering if I have a coil starting to pack in, but, it only manifests itself when the engine / coils are good & hot when out for a proper long run.
At home, the bike starts "straight off the button" & idles nicely at 1,000-1,200 whether the engine's cold, warm, or hot with the temp needle a third of the way into the white NORMAL zone. At idle it will idle all day long with no hint of cutting out when the throttle's blipped.

Out for a long run though, at about every fourth or fifth junction or roundabout, when having to stop, the motor refuses to idle & cuts out like I've turned the key to OFF or hit the Kill Switch & I have to sit at the side of the road for at least 15 minutes, (any earlier, the motor turns over but doesn't "fire") & then, low & behold, it starts & runs absolutely fine as if nothing happened for another few minutes / several miles at cruising speeds AND higher speeds, so reckon it can't be fuel starvation.
Acceleration in any gear above 6,000 is great, pulling progressively stronger the higher the rev's are & I don't hear / feel any "missing" either.

So, all you electrickery wizards, ( I'm not one ), is it possible for one of the windings inside one of the stock coils to have started breaking down when hot, causing the engine to die while stopped as above? :pray:

Ch'o the noo. Wullie. :grin:

wullie3xv9
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Re: GK76 intermittently cutting out at idle.

Post by wullie3xv9 » Sun Mar 21, 2021 1:09 pm

It's me again, folks. Just been looking on Blair's LiteTek site at the current GK76 viton seals kit, now coloured blue, & spotted these O-rings listed.

https://litetek.co/Guide_MikuniPilotMergeChamber.html

I don't remember these being included / replacing these in the green coloured seals kit I bought & fitted from Blair back in July 2015.
Could these knackered OE seals be helping to cause my engine to intermittently richen up & cut out at junctions & round-abouts ? ? :pray:
Blair / anyone, are you able to advise of symptoms if the above Pilot Merge Chamber seals are failing / have failed ? ? :pray:

Ch'o the noo. Wullie. :grin:

moforockband
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Re: GK76 intermittently cutting out at idle.

Post by moforockband » Mon Mar 29, 2021 2:18 pm

prime suspect will be ignition pulse coil or its connector wullie.

tailongwan
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Re: GK76 intermittently cutting out at idle.

Post by tailongwan » Thu Apr 08, 2021 2:02 pm

Hello all,

I have the same problem on the a GK76 '91 I just bought (see Suzuki - General). I have the same scenario as Wullie: "straight off the button" start, and nice idle when cold AND without starting lights. If I start lights, it just reducing but still revs at 2000-3000 with choke.
After a few km, then the idle is quite ok UNTIL i switch the lights on, or brake, on turn lights... then the idle revs reduce and the engine stop. If I switch off the lights before it stops, then idle comes back to 1,500 and it does not stop.
I checked the stator (coil and shortcut) and dynamic mesures (25-26v at idle and 65-70v at 4000rpm). I checked the reg/rec and 14.6-14.7v at the battery. The battery seems ok : 12.8v.
I thought it is the ignition coils: I checked resistance on the primary and it is 3 ohms for both. I did not check the secondary resistance.
What I found also is that the battery level reduce quite quickly when I turn the key without starting the engine and without light. From 12.7 to 12.4 within 20 seconds. I wonder if I have a leak somewhere...
Wullie: did you finally find a solution?
In my opinion it is not a carbs problem.

Any ideas ? How to check further the ignition coils? Or the wires to sparks?

Many thanks to all

MadMotoUK
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Re: GK76 intermittently cutting out at idle.

Post by MadMotoUK » Thu Apr 08, 2021 9:46 pm

tailongwan wrote:
Thu Apr 08, 2021 2:02 pm
Hello all,

I have the same problem on the a GK76 '91 I just bought (see Suzuki - General). I have the same scenario as Wullie: "straight off the button" start, and nice idle when cold AND without starting lights. If I start lights, it just reducing but still revs at 2000-3000 with choke.
After a few km, then the idle is quite ok UNTIL i switch the lights on, or brake, on turn lights... then the idle revs reduce and the engine stop. If I switch off the lights before it stops, then idle comes back to 1,500 and it does not stop.
I checked the stator (coil and shortcut) and dynamic mesures (25-26v at idle and 65-70v at 4000rpm). I checked the reg/rec and 14.6-14.7v at the battery. The battery seems ok : 12.8v.
This suggests an electrical short or harmonic inteference to me, probably caused by a bad earth or insulation degradation.

First port of call, check all the earths, make sure they are tight and onto bare metal. While you're at it, check all of the connectors for any signs of corrosion, water ingress can lead to shorting and the connectors don't tend to be waterproof.

If that doesn't solve the issue, I would unwrap the loom and check all the wires carefully for the lighting and ignition circuits. Bear in mind that the wires are now over 30 years old and will have been exposed to hundreds of heat cycles so their insulation properties will be pretty rubbish compared to when new. Be especially careful to check areas of wiring that feel stiff, this could be an indication of an overload at some point during it's lifetime. I'm sure you've come across the usual suspect, a dodgy reg/rec or stator burning out the connectors? If you're unlucky, the damage from this sort of failure can spread to other parts of the loom and go unnoticed for years.


Just to put this into perspective, domestic house wiring has a recommended lifespan of 15-20 years before the insulation performance is adversely affected. House wiring isn't exposed to the elements like motorcycle wiring either! I personally don't think it's unfair to treat a 30+ year old bike to some fresh wiring.

tailongwan wrote:
Thu Apr 08, 2021 2:02 pm
What I found also is that the battery level reduce quite quickly when I turn the key without starting the engine and without light. From 12.7 to 12.4 within 20 seconds. I wonder if I have a leak somewhere...
This is normal, just the drain caused by things like the Oil Pressure lamp and Neutral lamp, possibly also a fuel priming pump if you have one fitted (my FZRs have them, not sure about the GSXRs). Nothing to worry about.

tailongwan
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Re: GK76 intermittently cutting out at idle.

Post by tailongwan » Fri Apr 09, 2021 7:29 am

Thank you MadMotoUK for your detailed answer! You are right, 30 yo wiring system might be affected. Do you think it can explain also the fact that when cold it seems better? Or maybe it is just because there is more choke ...
Anyway I will methodically verify every connectors. And I already know some strange things like: no oil pressure light (dunno if the bulb is broken or any trouble with the sensor), the thermal gauge is not going up enough, just a few mm above minimum (gauge or sensor ?), even when the engine is completely hot.
For your information, there is no fuel pump like on the FZR 400 (I have one also), which starts automatically when you put the key on, only with dpressure coming from the carbs (like the GSF400 bandit).
Thanks again, I will let you informed ASAP.

MadMotoUK
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Re: GK76 intermittently cutting out at idle.

Post by MadMotoUK » Fri Apr 09, 2021 9:58 am

Wiring faults won't always make themselves know when things are cold. A broken wire for example tends to become evident when the sheathing has warmed up and expanded, separating the copper cores.

SevenThreeSeven
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Re: GK76 intermittently cutting out at idle.

Post by SevenThreeSeven » Fri Apr 09, 2021 3:15 pm

All good info so far in this thread...

The truth about these older bikes is that because of their old-school Ignition Control technology they absolutely require a well-maintained electrical system (wiring, battery, and clean secure path-to-ground connection).

The reason behind the need for a tip-top functioning electrical system is that these older bikes do not have a modern ECU to compensate Coil Dwell Time when the electrical power in the bike's wiring drops (like during low-RPMs with high draw, like when all the lights are also on).

The required Coil Dwell Time to charge a coil to "saturation" is different based on system voltage. A coil that's not reaching saturation will deliver a weak spark. Modern ECU-controlled vehicles have computer controlled Dwell Time compensation, when the ECU senses low system voltage it will increase Dwell Time.

But our old-school bikes don't have anything like ECU-controlled Dwell Time compensation. The Suzuki engineers simply worked out the voltages, set a constant Coil Dwell Time and said, "That'll about do it" and they left it up to you to do the necessary upkeep on the bike's battery and wiring and path-to-ground.

tailongwan
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Re: GK76 intermittently cutting out at idle.

Post by tailongwan » Sat Apr 10, 2021 6:04 pm

Thank you for this information, very instructive, I like to understand why things are working / not working.
Some news from my saturday workshop:
- I did some check on different earths and most of them are properly linked to the frame, except 2 on the position lights (x2) where there is 11 ohms of resistance with the earth. Not sure it is a problem: what do you think ?
- All other light wires (headlights, rear, turnligts, etc), sensors, fans (x2) are properly grounded.
- Should I replace ignition coil with dynatek DC1-2 which have the same caracteristics ? (2-3ohms on primary)
- If I cannot find better way to improve electrical system, I think I will unplug one headlight low beam and keep the 2 high beam (same as my NC35), and keep the opposite position light. So I will have low beam on the left and position light on the right. Of course after improving earth on position lights ;)

tailongwan
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Re: GK76 intermittently cutting out at idle.

Post by tailongwan » Mon Apr 12, 2021 9:40 pm

I am wondering how position lights are working: I understand that when I switch ON the LIGHTS, it will activate LO or HI depending on the DIMMER switch. When LO is enable, it switches off position lamp since it pushes 12v both contacts. But in the intermediate position of LIGHTS switch, it should switch on position lamp. But how earth is provided on B/W wire? Is the DIMMER switch grounded when LIGHTS on intermediate position?
I use the Wiring diagram from the owner's manual, and I do not manage to get 12v on the front position lights, however rear position lights are OK.
Position lights also should activate on "P" position of the ignition switch. It works fine for the rear, but nothing for the front ...


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