GS750 runs on 3 cylinders

[Guest]

Hi Guys,

I need some help with my GS750 '78.
I did a complete rebuild of the engine, after the rebuild the engine started immediately, however only on 3 cylinders.

Then I took the carbs apart and had the bodies ultrasonically cleaned.
I installed new O-rings everywhere and changed the main jets from 105 to 120 to go with the new open air filters.

I followed the "VM carb rebuild" that I found on this forum (this one: http://www.cycleorings.com/vm_carb_rebuild.pdf), from front to end.

Unfortunately, this didn't resolve the initial problem. It still runs on 3 cylinders. Cylinder #4's exhaust doesn't get hot.

This is what I tried already,

- checked compression, all cylinders get 9.5 bar which should be OK according to my manual.
- installed new spark plugs and checked the sparks visually (no change)
- swapped the spark plug leads for cylinder #1 and #4 (no change)
- checked #4 spark plug for fuel traces after the engine ran, the plug gets wet.
- checked the valve clearances again, the exhaust valve clearance was a little bit tight <0.05mm.
So I put in a smaller tappet to get the clearance to +-0.1mm (didn't have the size in between). However, no change.
- Checked the jets and blew out carburator #4 again. No change.
- Went for a quick ride and had it at full throttle for a few seconds to see if it could be related to the idle circuit. No change
- Checked and greased the (new) carb #4 manifold O-ring, was in good shape. No change.

I don't know if this could be related, but the petcock doesn't work properly either. When it's set to "on" the line is drawn empty when the engine is running.
So I have the petcock set to "prime" all the time now. I don't see the problem with this as long as the floats are functioning properly.

I'm not sure which type of carbs I have, I guess they're VM26s? Here's a picture:

IMG_20160430_173026054.jpg


I don't know what do next.
Your advice is much appreciated!

[Brendan W]

Swap the points condensors. Have seen a few only fire one of the coil outputs.

[jroels]

Swap the points condensors. Have seen a few only fire one of the coil outputs.

I swapped the spark plug leads of cylinder #1 and #4, which didn't change anything.
Cylinder #1 kept running and #4 still didn't fire.

Does is still make sense to swap the condensors then?
Thanks.

[sharpy]

I swapped the spark plug leads of cylinder #1 and #4, which didn't change anything.
Cylinder #1 kept running and #4 still didn't fire.

Does is still make sense to swap the condensors then?
Thanks.

You ask for advice then question it! Not a good start. lol

[jroels]

You ask for advice then question it! Not a good start. lol

Sorry, didn't mean to be an ass, just trying to understand the reasoning.
If a faulty point condensor affects one of 2 coil outputs, then wouldn't swapping the coil outputs also move the problem from cylinder 4 to 1?

[Dave8338]

Sorry, didn't mean to be an ass, just trying to understand the reasoning.
If a faulty point condensor affects one of 2 coil outputs, then wouldn't swapping the coil outputs also move the problem from cylinder 4 to 1?

Yes and no. Did you swap wires too or just move the plug caps back and forth ?

[jroels]

Yes and no. Did you swap wires too or just move the plug caps back and forth ?

I just moved the plug cap from 4 to 1 and vice versa, didn't disassemble the leads or anything.
So only the spark plug itself stayed in place.

[Dave8338]

Have you swapped the plugs with the cap change to verify it is not a "wet fouled" plug?

[Brendan W]

Sorry, didn't mean to be an ass, just trying to understand the reasoning.
If a faulty point condensor affects one of 2 coil outputs, then wouldn't swapping the coil outputs also move the problem from cylinder 4 to 1?

Yes and no. If the plug didn't move swapping leads is only a partial test. If the coil is having trouble from the condensor it will favour the easier side.
At the very least you eliminate something easy and non-invasive.

[allojohn]

Can't see from your photo, but where do the breather tubes from 2 & 4 go? They should run past your filters into open air. I've seen where some have connected #2 to #4 closing the circuit, this is wrong.

[geol]

If you swapped 1&4 plug leads and you have good compression, things point to the carburetors. I am not familiar with the oring rebuild stuff you mentioned but the vents must be clear or the hoses routed where they are not pinched else the floats won't work right. I don't believe it is a good idea to cross connect the vent lines. Next thing you want to do is check the fuel level in the bowls. If the carbs have been apart many times in the past, a float can be bent improperly and setting the float level won't give you the right amount of gas in the bowl. Last, change the pilot jet in the non-firing carb... even if you put new pilot jets in the carbs, they have been known to be poorly manufactured and may not flow as intended due to poor quality control. You should also check to ensure the fuel pipe leading to the #4 carb Ithink that is the non-firing cylinder) is not obstructed.

[Northern Mike]

I am running into the same issue, although haven't gotten as far into my testing as I have had the bike less then a week.
In my case, the guy I bought it from admitted to running SeaFoam in it just before I got there, and when I checked the air filter element, it was sticky and soaked. Also noted the spark plug wires in my case are not OEM.
I did notice with new plugs that the spark appears to be weak, so I'm thinking electrical in my case, but who knows.

Going through the suggestions in this thread and see if anything resolves my issue.

[jroels]

Can't see from your photo, but where do the breather tubes from 2 & 4 go? They should run past your filters into open air. I've seen where some have connected #2 to #4 closing the circuit, this is wrong.

John, that was it! Thanks!!
For some reason I had vent #2 connected to the petcock and #3 and #4 connected to each other.
I didn't cross my mind to doubt that since they were like this when I bought the bike.

I connected #3 to the petcock and opened up #2 and #4 and it ran on all 4 cylinders right away.
Feel pretty stupid now...

I'm gonna put the exhaust valve shim that I replaced back and then I can finally start the tuning procedure.

Thank you all for the suggestions!

@Northern Mike, maybe a good idea to check your vent lines too before going into all the other stuff

[Jedz123]

Glad to see you got it running!!! Thanks for restoring another GS and keeping it on the road!

[allojohn]

John, that was it! Thanks!!
For some reason I had vent #2 connected to the petcock and #3 and #4 connected to each other.
I didn't cross my mind to doubt that since they were like this when I bought the bike.

I connected #3 to the petcock and opened up #2 and #4 and it ran on all 4 cylinders right away.
Feel pretty stupid now...

I'm gonna put the exhaust valve shim that I replaced back and then I can finally start the tuning procedure.

Thank you all for the suggestions!

@Northern Mike, maybe a good idea to check your vent lines too before going into all the other stuff

Magic! .