re-rehabbing a 1978 GS750

[gwyon]

Back in 2015, I was able to restore a neglected 1978 GS750, including rebuilding the top end, carburetors, brake calipers, making new brake lines, a new wiring harness, installing new bearings, new shocks, etc. I couldn't have done any of it without the posts and tutorials written by some kind and generous people on this forum. The bike ran great until autumn of 2022. It's been sitting in a corner of my garage until a few weeks ago, when I wheeled it onto the floor to see what can be done. I've forgotten most of what I learned here, so I'll be asking for help from time to time.

Just before the trouble started, I'd cleaned and balanced the carburetors, adjusted the valves, changed the oil/filter, and put a new tire on the front wheel. As I recall, the bike started to sound different when it was running, a softer, sort of muffled sound. Then one day it would struggle to turn over, idle poorly, and stall when I blipped the throttle.

To start, I've removed the carburetors, dipped and cleaned them, used compressed air and fine wires to clear the openings, and used a sonic cleaner on some parts. New o rings and bowl gaskets. I've bench synced them, and now I'm checking the fuel service levels. My first question is: on the VM carbs for this bike, the vent on one of the carburetors is capped. I've lost track of which carb it is. I think it's #3?​

Thanks.

 

[Rich82GS750TZ]

Welcome back.
These are the hose fittings on the VM carbs.

The only reason the port on # 3 should be capped is if you’re using a non-vacuum petcock like a Pingel. Otherwise it should get a hose run to the smaller port on the petcock.

Reading the list of things you did prior to running poorly, I’d go back and check the valve clearances. Gaps should be at the loose end off the spec or even slightly beyond is OK according to most. BikeCliffs has a good tutorial for valve clearance check for VM carbs. https://gsarchive.bwringer.com/bikecliff/images/gs850valve_adjust.pdf
and a zip tie method for holding the valves open to swap out shims. https://gsarchive.bwringer.com/bikecliff/images/valve_tool_zip_tie.pdf

 

[gwyon]

Thank you, Rich82GS750TZ. Yes, I'm using a Pingel petcock. After the carbs are on, I'll check the valves again, and I want to check compression too. Thanks for your help.

 

[Rob S.]

Just before the trouble started, I'd cleaned and balanced the carburetors...

A clue, Clouseau! :soap:

 

[rphillips]

Hey Rob,,,, That's a pretty good thought... Who told you?

 

[Rob S.]

Hey Rob,,,, That's a pretty good thought... Who told you?

That's Chief Inspector Rob. And it is my business to know, monsieur.

 

[gwyon]

Carbs dunked, poked, blown out, washed, new o rings installed, bench-synced, and fuel service levels adjusted. While I had access, I adjusted the timing chain tensioner (I have a manual one, and it was fine). I checked the cam lobe clearances (also fine). I cleaned and re-oiled the foam air filter and cleaned the air box. I took the plugs out while adjusting the valves, and here's how they looked. Last time the bike ran was autumn of 2021, and the plugs were relatively new then. Left to right: #1, #2, #3, #4. I cleaned them, gapped them, and put them back in.

I'll need a new battery. My last one was a MotoBatt AGM, and that's probably what I'll buy again, unless something better is available now?

 

[rphillips]

4 and 1 too white, 3 and 2 too black. Get them all the color of a wet paper sack and you've did good