Modern Vespa

It always starts immediately when cold or if it has been sitting for at least 15 minutes.

If I stop for 5-10 minutes, like when getting gas it usually won't start on the first try. It will just continue to crank without starting. When that happens the only thing to do is wait about 40 seconds. 30 seconds is usually not long enough, then it will start, or at least it always has.

It has never left me stranded but it is concerning. I feel like I don't want to ride it very far from home.

This has been going on for at least a year. It hasn't got any better or worse.

Last year I rebuilt the carburetor. I watched a few YouTube videos to make sure I was doing it right. I even bought an ultrasonic cleaner for this project. It worked great. It literally looked like new.

I replaced the gaskets, needle and seat and the float.

I made sure all the passages were open.

I checked that the auto choke was working properly.

It had no effect on the problem either way.

Any ideas?

When were the valves last adjusted?

Good question.

I've done it at least once and I'm sure I would have recorded it.

I'll look.

I adjusted the valves almost exactly 5,000 miles ago.

That shouldn't be an issue unless I didn't do a good job, right?

I suppose I should check it to be sure.

Edit: I know the recommended interval is 6,200 miles but as I said it has been doing this for over a year. I don't remember the mileage after adjustment when it started but I think it was less than 3K.

I am going to check it anyway.

Maybe the coil is failing?

I suppose that's possible. Is there a way to check it?

If you have a spare spark plug and 19mm wrench to get the cap off quickly and easily, you could check for spark when it won't start.

But, to actually test the coil properly, you have to build a test rig to run it, heat soak it, measure the temperatures and keep a record of volts and amps. My UK dealer built one just to prove a diagnosis of 'bad coil' - it was in action quite a few times over the years when heat sensitive coils were suspected.

Sometimes components can have just a 'heat sensitive window' of only a few degrees around a certain temperature - if cooler than that, all OK, if hotter than that, also OK.

Heat soak.

Engine heat is boiling the fuel in the carb float bowl. The fuel bubbles up through the main jet and floods the intake. Try flooded start technique next time it happens, throttle wide open while cranking.

My carbed S190 had this problem after I swapped in the Mikuni. Problematic hot starts and bike wouldn't run right for a 1/4 mile or so until it got cool fuel in the bowl.

Bypassing the carb heater could help with that in not-cold weather.

I bought my GT200 with 1800 miles on it. I checked/adjusted the valves at 10,000 miles, 3 of them were well within spec, 1 was right on the edge. That tells me that the valves tend to hold an adjustment well. I also eliminated the carb float bowl heater a long time ago. I live in AZ, where it gets HOT. I have never had a problem with it starting, no matter what the temperature.

However I do have a Kawasaki motorcycle that did not like to start when hot. It started fine when cold. It is a V twin, and has two ignition pickups on the stator. Turned out the gap between the pickups and stator were slightly too big, and it was getting a weak spark when hot. A slight adjustment of the pickup coils solved the problem. I don't know what the ignition setup is on the GT200, as I have never had any problems with mine.

That certainly could be it.

I'm guessing that the carb heat is to prevent icing. I don't think bypassing it would be an issue because I almost never ride when it's below about 45° F.

If that doesn't help I will check the valve clearance.