Old 23rd May 2020, 03:05 PM #1
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GS550 Head Wrecker

After a long time in storage working on bringing a 1983 Suzuki GS550 back to life. Carbs have been fully cleaned, a few times now.

Bike starts but as soon as it heats up climbs in revs and sits about 5k. Cylinder 4 seems to be cold and not firing. It has two carbs feeding 4 cylinders and the other cylinder from this carb seems to be getting fuel and firing. Similarly it has two coil packs and the other cylinder with the same coil pack is firing.

When out of the cylinder plug appears to be sparking. All other plugs have slight purplish coloration on electrode which I assume is causing by spark and not cylinder temp. Is it possible that a coil could still give enough to cause a spark but break down as it heats up in operation? I'm at my wit's end with it. Any suggestions?


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Old 23rd May 2020, 03:47 PM #2
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Re: GS550 Head Wrecker

Swap over the spark plugs from each coil. So if coil one is firing cyl 2+3, Remove plugs from 2+3 and swap over to 1+4. If misfire follows plug then plug is faulty. If still on cyl 4 then something wrong with the coil or HT lead, (or fault engine fault).

Last edited by vifferman; 23rd May 2020 at 03:48 PM.
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Old 23rd May 2020, 04:16 PM #3
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Re: GS550 Head Wrecker

Originally Posted by vifferman
Swap over the spark plugs from each coil. So if coil one is firing cyl 2+3, Remove plugs from 2+3 and swap over to 1+4. If misfire follows plug then plug is faulty. If still on cyl 4 then something wrong with the coil or HT lead, (or fault engine fault).
This
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Old 23rd May 2020, 04:39 PM #4
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Re: GS550 Head Wrecker

Originally Posted by vifferman
Swap over the spark plugs from each coil. So if coil one is firing cyl 2+3, Remove plugs from 2+3 and swap over to 1+4. If misfire follows plug then plug is faulty. If still on cyl 4 then something wrong with the coil or HT lead, (or fault engine fault).
Did this, swapped in 4 new plugs for good measure. Problem remains.


Visually checked 1 and 4 with plugs out and attached. Can see a clear consistent spark in both. They are firing off the same coil and making a visible spark. But the no.4 plug looks like it isn't colouring like the other 3?
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Old 23rd May 2020, 04:42 PM #5
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Re: GS550 Head Wrecker

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Old 23rd May 2020, 04:54 PM #6
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Re: GS550 Head Wrecker

Swap the leads over,if its no 4 then swap 1 and 4.
If it's still not firing you need a compression test.
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Old 23rd May 2020, 05:06 PM #7
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Re: GS550 Head Wrecker

The leads are different lengths and won't reach. Had hoped to do this.

Might try taking the coil off its mounting and making it fit.
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Old 23rd May 2020, 05:09 PM #8
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Re: GS550 Head Wrecker

Compression test.
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Old 23rd May 2020, 05:19 PM #9
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Re: GS550 Head Wrecker

Originally Posted by Gixxer
Compression test.
Swapped the leads after some fuckery and same result. Compression test it is.

Thanks for the advice.
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Old 24th May 2020, 11:58 AM #10
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Re: GS550 Head Wrecker

Originally Posted by pryrates
Swapped the leads after some fuckery and same result. Compression test it is.

Thanks for the advice.
Sticky inlet valve? Take rocker cover off, turn by hand and see if its opening alright. They were tappets right? Could need adjusting...
If exhuast pipe is cold and plug is dry then no fuel going in to chamber
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Old 24th May 2020, 12:22 PM #11
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Re: GS550 Head Wrecker

How are the inlet rubbers
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Old 24th May 2020, 03:40 PM #12
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Re: GS550 Head Wrecker

Originally Posted by joe williams
How are the inlet rubbers
Mwah, that's a bit personal....
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Old 24th May 2020, 09:28 PM #13
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Re: GS550 Head Wrecker

Originally Posted by pryrates
After a long time in storage working on bringing a 1983 Suzuki GS550 back to life. Carbs have been fully cleaned, a few times now.

Bike starts but as soon as it heats up climbs in revs and sits about 5k. Cylinder 4 seems to be cold and not firing. It has two carbs feeding 4 cylinders and the other cylinder from this carb seems to be getting fuel and firing. Similarly it has two coil packs and the other cylinder with the same coil pack is firing.

When out of the cylinder plug appears to be sparking. All other plugs have slight purplish coloration on electrode which I assume is causing by spark and not cylinder temp. Is it possible that a coil could still give enough to cause a spark but break down as it heats up in operation? I'm at my wit's end with it. Any suggestions?


what has it at 5000 rpm on 3 cyls? is tickover screw adjusted up or what ? sound a bit odd.if its not down to something simple like cable/tickover screw could it be float height flooding cyls .purple colour is odd too , maybe is burning oil , worth checking compression in case theres stuck rings.coils can break down as they heat up but it sounds like the coils not the problem here .
pics of carb setup please ?
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Old 25th May 2020, 11:05 AM #14
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Re: GS550 Head Wrecker

Running lean , high revs ?
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Old 26th May 2020, 01:28 PM #15
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Re: GS550 Head Wrecker

Originally Posted by joe williams View Post
Running lean , high revs ?
Doubt it's that lean that itll get it to 5000rpm on 3 cyls .

At op can you visually verify the slides are not open fully ?
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Old 26th May 2020, 09:37 PM #16
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Re: GS550 Head Wrecker

Idle screw has it sitting at roughly 1700rpm. At times it will idle low and as it heats up seems to want to climb. Or if you blip the throttle it'll hang.

The butterfly style valves open and close as they should.

I'm starting to think that the idle circuit for the cylinder 4 carb setup must be still blocked. Have it out and soaking in carb cleaner again.





Last edited by pryrates; 26th May 2020 at 09:39 PM.
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Old 26th May 2020, 09:44 PM #17
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Re: GS550 Head Wrecker

Does that cylinder pick up and fire when you raise the revs ?
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Old 26th May 2020, 09:46 PM #18
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Old 26th May 2020, 09:47 PM #19
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Re: GS550 Head Wrecker

Originally Posted by DavyH View Post
Does that cylinder pick up and fire when you raise the revs ?
Haven't checked as the revs were hanging so high. I've confirmed the main circuit on the carb for that cylinder is clear so will try on Saturday.
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Old 26th May 2020, 09:49 PM #20
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Re: GS550 Head Wrecker

Originally Posted by joe williams View Post
How are the inlet rubbers
Seem good, bolts to engine seem properly tight and can't find any cracks.
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Old 26th May 2020, 11:29 PM #21
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Re: GS550 Head Wrecker

Originally Posted by pryrates
Seem good, bolts to engine seem properly tight and can't find any cracks.
Will she fire if petrol is put in cylinder 4
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Old 27th May 2020, 01:07 PM #22
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Re: GS550 Head Wrecker

Originally Posted by pryrates View Post
Swapped the leads after some fuckery and same result. Compression test it is.

Thanks for the advice.
Did you do a compression test?
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Old 27th May 2020, 10:12 PM #23
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Re: GS550 Head Wrecker

Originally Posted by vifferman View Post
Did you do a compression test?
I do but I had to order the adapter to fit the 12mm plug. Judging off my thumb it is compressing.
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Old 27th May 2020, 10:13 PM #24
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Re: GS550 Head Wrecker

Originally Posted by joe williams View Post
Will she fire if petrol is put in cylinder 4
Might try chucking in a drop as I have the carbs off.
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