GS750 Cylinders 3 and 4 not firing

[Guest]

Hi
i have tried a search but can’t find anything so would appreciate any help or direction to an existing post if appropriate

I have just finished putting back together a 1978 GS750 which ran on all cylinders before it was stored.
the carbs a have been cleaned and bench synced and seem ok
on starting the bike is running ok on cylinders 1 and 2 but 3 and 4 are dead.
there is a good spark at both of these cylinders and after running on prime there seems to be fuel reaching them as the plugs are wet and smell of fuel.
the motor was not run for some time and I’m wondering if I’m missing something obvious or it’s time to get the head off and do a valve shim check
im an averaged skill mechanic so any help from the more experienced gurus would be greatly appreciated

[Big T]

Are the spark plug wires on the correct cylinders?

One coil fires 1-4, the other fires 2-3

You only need to remove the cover, not the head to adjust the valve clearances

[Tiff]

Definitely on the right cylinders and getting a good spark
going to compression test it tomorrow hoping that its not bad news

[Nessism]

Carb problem. Needless to say you need to check the valves but that's unlikely the problem. Checking compression is a waste of time.

[habsdoc]

Try swapping your 1 & 2 plugs for the ones not firing and see what happens. If 3 & 4 now fire up, then you know that the plugs were sparking but not enough to work properly.

[allojohn]

Carb problem. Needless to say you need to check the valves but that's unlikely the problem. Checking compression is a waste of time.

+1. .

[Tiff]

All sorted now thanks
blockage in fuel pipe between carbs 2 and 3 starving 3 and 4 of fuel