Starting Q: fuel vs spark

[geisterfahrer]

My '81 GS750L has been reluctant to cold start since I got it. I had always assumed it was a weak spark, and that I needed to do the coil relay mod.

Today, I went out to the garage to start the bike. It had not been started in over a week (I know, I know, it's been that kind of week). I wanted to test whether it was a spark problem or a fuel problem. I set the petcock to prime, and was going to give it a minute or so, and try starting it.

Just as I did that, some guy pulls up at the end of my driveway and wants to look at my Supra. After talking with him and showing him the car, about 10 minutes had elapsed. I went back to the bike, turned the key, pulled the choke, and hit the starter. The GS fired immediately, and started high revving! Usually, it takes several starting attempts, and then she runs raggedly for a minute or so even under full choke.

My question now is: do I have a fuel problem or a spark problem? What else should I be looking for to narrow down the starting issue?

[geisterfahrer]

Thanks, JC, that gives me some things to look for.

How, exactly, do I check coil voltage? Do I check across the primaries, or do I go from primary to frame ground?

The engine, once started and warmed, has no sputtering issues at all. It will pull hard and smooth to whatever RPM range I chicken out at . After the bike has been started once in the morning, it will fire easily all day long. It is only after it has been sitting overnight or longer that I have any starting issues.

I have completely rebuilt the carbs, including all o-rings, and they are balanced. The oil level remains consistent, so I don't think I have any gas leaking down to the crankcase.

I will do the battery check at 5k and report back. The bike does have an aftermarket R/R, installed by the PO, so I don't know if that is going to be an issue.

[TheCafeKid]

When is the last time the valves were adjusted? Hard cold starting is a sign of tight valve clearances...

[geisterfahrer]

Okay, here's what I've found:

CafeKid, the valves were adjusted within the last three thousand miles or so. They are the screw-type, not the shim-type, rockers. I've done lots of these types of valves, so I'm pretty confident they are very close, if not right on.

JC, I get about 13.4V across the battery at 5k RPM. One coil shows 12.3V, and the other only 11. On this bike, the coils have separate feeds, hardwired at the coil and pigtailing back to molex connectors, which is where I measured the voltage to ground.

Obviously, one coil is not getting the proper voltage, and that probably means it needs the coil relay mod. But why would it start easily after sitting on prime for 10 minutes?