1980 GS550L still will not start without priming a cylinder!

[Guest]

Hi,
Posted a question a couple weeks back about the no-handle 1980 GS fuel petcock and how to get it to flow gas. Several people helped me out by explaining that you need vacuum even to get the prime position to work! Ok! So I used an external vacuum source to get fuel in to the carbs, but I still could not get the bike to fire. Plugs were still dry, although I got fuel out of the bowls if I took a drain screw out. Checked several and all had fuel. But no draw to the cylinders. I thought I smelled gas when cranking the bike. But again, plugs looked and smelled dry.
Only way to get the bike to run is to pull a plug and prime a cylinder (no way to access a carb mouth with that stock airbox in place!). The bike will fire roughly, but if I keep doing it, it finally runs long enough for all four cylinders to start to fire (draw fuel??), and then the bike runs fine. Even restarts immediately after a brief stop. But the next day, we are back to square one. I now have fuel in the fuel line from the tank, so again believe that I have fuel in each bowl. Starter seems to be turning over swiftly and properly, so I would think that would create enough airflow to draw fuel up into each carb, but that doesn't seem to be the case. Any thoughts on things I might check. Thanks!

[LAB3]

Last summer I used the original petcock in the prime position with an additional shut off valve that the previous owner put on it. The bike never did run quite right until I finally ponied up the $50 for a new one.

[Nessism]

What in the way of maintenance have you done? Rebuild the carbs per the carb rebuild tutorial? Did you check the Newbie Mistakes thread to see if anything applies?

[mtb1098]

Evening,
As for maintenance, I have rebuilt the carbs as per the instructions, with two exceptions: 1) bike came with carb rebuild kit jets and such, as PO has started a rebuild of the bike than gave it up (did confirm that they have the correct sizes for my bike and year, and 2) Did not remove and rebuild the intercarb connection pipes, so could have a vacuum issue there. Did replace the carb boot o-rings. Using stock airbox system with new oiled filter element. Using a MAC 4-into-1 system, as the stock system had rotted away. Have not rejetted for it yet. Replaced plugs and plug caps and confirmed spark. Checked battery voltage with the bike running, showed about 12.8 at idle and increased with RPMs. And tried to check the timing, but there appears to be no adjustment or timing marks on this bike. Have not attempted to set the valves yet. Plan to do that and some rejetting once I can get it running consistently. And I did replace the fuel petcock with an aftermarket version from K&B. Tank interior was clean, but I haven't dropped the petcock to check the filter.

[Steve]

As for maintenance, I have rebuilt the carbs as per the instructions, with two exceptions: 1) bike came with carb rebuild kit jets and such, as PO has started a rebuild of the bike than gave it up (did confirm that they have the correct sizes for my bike and year, and 2) Did not remove and rebuild the intercarb connection pipes, so could have a vacuum issue there.

#1. You need to do the carbs again. There is simply NO way to do them properly without unganging them. And which directions did you follow?

Have not attempted to set the valves yet. Plan to do that and some rejetting once I can get it running consistently.

Good luck getting it running.

Even if your carbs are PERFECT, if the valves are not adjusted properly, you will have a very hard time starting the bike.

On the other hand, with perfectly-set valve clearances, your carbs can be off by a fair amount, so get the valves set first.

.