Modern Vespa

Okay, I'm exasperated. Tried everything, and have even run out of ideas to try.

Here's wassup: I finished rebuilding a VBB a couple of months ago and it ran wonderfully. Used a 55/60 idle, then went down to a 48/60, and the engine seemed a little happier. Two days ago, out or curiosity, I tried a 42/60. Didn't work. Engine bogged on acceleration.

So I went back to the 48/60. And now: Nothing works. At first, the engine kept dying after a minute, and after a lot of experimenting, I realized there was a big "U" in the fuel line next to the carb, and that, because the fuel was low in the tank, gravity was only feeding a dribble of fuel into the carb. I straightened the line, topped off the tank, and all seemed well. The engine idled happily.

So I decided to take it for a ride this morning. As soon as I applied any throttle, the engine bogged and died. Adjustments seemed to do nothing. So I tried the bigger idle jet. With that, the engine would only run with the choke open. As soon as I close the choke, the engine sputters and dies, unless I 'lm holding the throttle pretty far open. But it won't idle at all.

While trying to solve yesterday's problem, I tried everything—cleaned the jets and carb passages, replaced the float and needle with known good ones, checked the tank valve, checked the fuel hose. So all those things have been looked at.

The thing ran fine until I tried a 42/60 idle for 15 minutes. Now it's unridable, and I'm stumped.

Tho, now that I think of it, I didn't clean the fuel/mix passage. And this maybe does seem to be an idle fuel issue. Okay, back to the shop and I'll try that too.

Also, prayer. Stopping short of animal sacrifice, of course. Tho, I gotta say, the hamster is beginning to look nervous.

First thing I'd do is isolate the fault.

You're assuming it's the carb, but that could be correlation, not causation.

You have two ways to check this:
1) Go through and verify that all parts of the system are working correctly:
- Spark
- compression
- timing

That takes about five minutes and saves you chasing the wrong problem.

2) Get a fresh plug and some starter fluid. If it's a fuel issue, that should get a few seconds of run time out of the motor.

Also, I'm assuming this is Known Good gasoline.

You went to a crazy-lean idle jet. The fact the bike would only run with the choke out confirms that (the SI carb has an enrichener choke circuit) also supports that.

Have you tried going back to the 55-160 yet?

What's the rest of your jet stack? Is it a stock 150 top end? What kind of ignition? Points or electronic?

Finally, did you pressure test the motor so you know it's tight?

Let us know what you find and that will point toward the next steps.

He said it ran great until he started messing with the idle jet. Start there.

Sounds like something's plugging up the idle circuit on your carb.

It's super easy for a piece of crud to drop into the carb when you're switching jets. It only takes a microscopically tiny particle to clog the idle circuit.

When you clean the carb, you've got to really clean it. Take the it off. Take all the jets out. Blow cleaner and compressed air through every orifice. Then do it again from the opposite direction. Then do it one more time.

I don't necessarily disagree, but I also like to make sure.

At least test it with starter fluid. If it's just a clogged idle, it should also start kicking it at WOT, too.

Hi everybody. We have a verdict: Clogged idle circuit!!

Been quite a journey tracking it down. Tried everything yesterday—cleaning the jets, replacing the float and needle, checking the tank valve and hose. When I took a U-bend out of the hose just before where it attaches to the banjo, the engine started to idle again and I thought that, obviously, I was a mechanical genius.

Then I gave it some gas and it died.

As you can see in my original post, the light went on for me in real time. As I was writing that note, it occurred to me that all the problems I was having seemed to relate to idle circuit. It would run with the choke on; it would IDLE with the smaller jet but not the large one; it would seem to rev well about where the main would come in. Finally dawned on me that maybe something was up with just that circuit—and that when I had disassembled and cleaned the carb and jets, I had (this is difficult to type . . . ) FORGOTTEN to remove the #^**%#^ mix screw and clean THAT circuit!!! D'oh!!

Cleaned everything well, put the carb back on, and everything began behaving as it should. Engine ran happily for 15 minutes before I shut it off, and it responded predictably to throttle and mix adjustments.

I realize now I had missed another clue: The mix-adjusting screw was getting squirrelly results. Didn't seem to be working right at all. So that was another signal, if I had been thinking it all thru better.

But all good now. Will actually try the 42/160 idle again. Was boggy before, but, in retrospect, it may have been a fuel problem.

Think I'll go a few blocks down to the school parking lot, tho. My neighbors may have had enough throttle-blipping for now.

Thanks everyone. Quite a lesson in what even a small speck can do. Cheers.

Congrats, man!

Make sure that you're not bogging lean with that 42/160, though. That's a crazy lean idle jet.

Thanks for the heads-up. The mixture screw was pretty far in with the 55/160, so I went down to 48/160, and the engine seemed a little better.

The 42/160 was def lean-bogging when I tried it for a few minutes, but it happened, as it turned out, just as my idle circuit was going squirrelly. I just want to make sure the bog was from the idle jet and not from the clog.

But I'll be even more careful now. Any issues at all, and I'll 86 the jet. Appreciate the warning.