Cylinder #3 not firing 78 GS750

[Mike711]

Have you checked No.3 piston TDC is aligned with No.2, I've seen a slightly twisted crank (not on a GS though) cause very odd running probs.

[ledman_70]

After putting head back on I ran all the valve clearances again, but still nothing. I must have really pissed off the motorcycle Gods somehow!

[Dezza]

Have you tried swapping the ignition components from a bike known to run well, rather than the 1000 that runs badly? Most parts don't even have to be GS specific, e.g. a good set of 3 ohm coils, leads, caps, new plugs and so on. Last time I have a similar fault (1100 efe) it was the CDI. It sparked when checked in the normal way but presumably not under compression and ran on only 3 cylinders.

[ledman_70]

I have 2 1000's and a 750 and I have swapped coils and carbs all around.  Still the same issue.

[Arttu]

For ignition side troubleshooting it's good to keep in mind that cylinders 1 & 4 and 2 & 3 share the same coil.  So if another cylinder of the pair works then the problem can be only somewhere between the coil and spark plug.

Is that non-running cylinder completely dead or just "mostly dead"? Like if you put a new plug there will it get any black soot or does it stay completely bright? Or can you notice any heat on the exhaust runner?

[ledman_70]

This is hard to believe, but yesterday I poured a little Marvel down #4  and ran it for 15 minutes and once the smoke started to clear #4 started firing. I set the timing today so I still have to sync the carbs, but it's running decent now. Once I get the carbs set I'm going to drag the 750 back into the garage and try the same thing with it, since it has the same problem seemingly.  Can't believe all it took to clear #4 was Marvel! I would recommend that stuff for any bike that has been sitting. This is not the first time Marvel has fixed a bike for me, I guess I should have gone there first. Thanks for all your help guys!

[unfocused]

TL; DR: Did anyone mention the petcock? Vacuum tube goes to #3 carb if I remember comrrectly. However, I'm old and I don't.

[andyroach]

What is the update? I'm intrigued as to what the problem is on the 750.

Did the Marvel (Which in the UK is a powdered milk. I'm assuming it's something else in the USA)cure it? Is the 1000 still running strong? 

Come on give the latest

 

[Gixer1460]

13 hours ago, andyroach said:

Did the Marvel (Which in the UK is a powdered milk. I'm assuming it's something else in the USA)cure it? 

I suspect it's 'Marvel Mystery Oil' - widely used in the colonies to fix everything LOL's

[PanzerWomble]

5 minutes ago, Gixer1460 said:

I suspect it's 'Marvel Mystery Oil' - widely used in the colonies to fix everything LOL's

Along with "SEA FOAM"......as beloved of Youtubers "will it run after 20 minutes ?".....which is just ATF/Diesel .....Naptha ...and a bit of IPA .  Bit like all those videos showing WD40 doing everything from rusty locks to curing cancer ....

[Swiss Toni]

As PW says!

 

[1979SuziGS1000]

So did both the GS750 and GS1000 get fixed with an injection of Marvel Mystery Oil?  Did you squirt it into the carb without the airbox installed, or add it to the float bowl?  I assume you didn't just squirt some into the  cylinder with the plug removed.  Do tell me.  I have the same problem.