Modern Vespa

The rebuild on my girlfriend's '79 P200 was completed yesterday, and we got the bike back in the engine and she was able to ride 30 miles last night without problems. New seals, piston, rings, cylinder bored out, etc. Stock P200, 24/24 SI carb, oil injected, Sito Plus is the only mod. 120 main jet. B7ES plug.

Today, she went to start the bike, and it idled for about 5 seconds then cut out, not to start again at all. Spark was good, but for shits and giggles we threw on my spare CDI and spark plug wire. Spark seemed a little better, but still wouldn't start. Tried with choke in/out, gas on/off, throttle open/closed, and all combinations of the three.

LOTS of compression, she was almost having trouble kicking it.

So we checked fuel. Plenty of gas in the tank, fuel line runs well.

Removed the carb, disassembled, cleaned (though it wasn't very dirty and everything seemed patent). Made sure to spray carb cleaner through all the small openings and verified it was flowing through. Jets cleaned. Float was intact and moved freely, needle in good shape. Filter at the top was clean. I could spray carb cleaner through the port where the banjo bolt attaches, and see it run down over the needle, so that was no problem.

Still wouldn't start, so we put on my old 20/20 carb. Same situation. Neither carb would even get wet in the venturi, though I could feel suction if I closed the top of the venturi with my fingers while it was being kicked. We kicked about 40 times for each carb to be sure there was plenty of chance for gas to flow through. Though after the 10th we were pretty sure no dice.

Then we put some fresh gas in the float bowl, which was still dry from being cleaned despite all the kicking. Kicked over within a few kicks - then died again. Opened the float bowl, it was almost empty.

My friends who also ride think air leak, or they're stumped too when I ask them. We took the airbox off and then back on just to make sure we weren't under or overtightening it.

One friend said the stator wiring might be bad, but my thought was that all the engine cared about was spark. If you have spark, the engine doesn't give a shit what happens with the rest of the electricals, right?

So, our problem seems to be between the fuel line and float bowl, she doesn't get gas flowing through. If we put some gas in the float bowl, the bike sucks it up and runs til it's gone. Same thing with 2 different carbs and 2 banjos. Everything upstream of the banjo flows well, downstream of the float bowl everything seems to work well.

Any ideas of what to check/clean/fix/replace? Thanks for reading this far even if you don't...

Fuel line or fuel tap prob.? One slug of crap is all it takes.

blockage on the air vent to the tank? Tried taking the filler cap off to see if that lets the fuel flow...?

If you had the carb re-built it may be the float. There was a recent post about some of the new floats needing the edge of the ridge shaved down. I assume that the float is moving freely after you've taken it out of the bowl. Sounds like since your getting fuel through the line that as soon as you put the carb back together that the float is possibly sticking closed.

if you've tried two carbs and neither get fuel in the bowl the problem must be upstream of the banjo....

Yes. I forgot you said you changed the carb. Take the fuel line off and check the flow. If they messed with the fuel tap the packing may have turned inside the petcock and not allowing the fuel to run through.

I know you said this: but I wonder whether you ascertained that the fuel was really pouring out with the line held exactly adjacent to the nipple where it is supposed to connect (not below it).

Since you tried two carbs, it is obviously unlikely that it is any part of either carb, but you should be sure your float moves freely and your needle drops when the float is down and opens the way for fuel (no need to search for zebras, but don't let one sneak up behind you and kick you in the head, either).

If we assume that there is no carb fault, no compression fault, and no electrical fault, we are left with major timing discrepancies, which are very unlikely in a motor that ran great just a day ago, or poor fuel flow into the carb. Since you found inadequate fuel in the float bowl, and had it try to start when you poured fuel in, this would tend to confirm fuel flow into the carb as your problem. Since this is a problem even at idle, a blocked air hole in the gas cap is unlikely. That leaves obstruction in the valve or fuel line, displacement of the fuel tap rubber (unlikely), or misrouting of the fuel line.

I'd play with the fuel line - check the routing, make sure it runs level (no up or down swoops), make sure it's no longer than 24" (22" if possible), blow through it if need be to confirm (wait for it...) patency.
The bottom of the fuel tank is no more than a cm above the float bowl on these beasts so there is minimal pressure to push the fuel past any obstruction, even very slight ones.

Good luck!

- Eric

Mis-routing of the fuel line would be unlikely since it ran for a good amount of time. A pinched line would let it run briefly in some circumstances. Obstruction in the fuel line would be a greater possibility including the fuel tap packing as this has happened to me. I over tightened the fuel tap cover plate which in turn spun the packing so the holes wouldn't line up.

Interesting dilema. Let us know.

I'm going to disagree with that. The amount of time it ran could have been just enough to bring the fuel level down to where the misrouted or too long fuel line is now causing problems.

Questions:

- Have you tried with tap in both On and Reserve postions?
- Have you tried topping up tank to full?

30 miles? That's great mileage or a real long fuel line.

Yeah, but if the fuel line was only recently installed, and shifted very slightly in position, or if it was just barely in the right position, and the lower fuel level after the ride reduced the pressure enough to no longer overcome the position problem, then that could explain it. These fuel line position problems can seem almost supernatural in their subtlety.

- Eric

Darned Murphy's law.

That was it exactly. Several people brought up the point that if both carbs weren't working, problem was upstream of the carb. I realized I hadn't tested the fuel line at the level of the banjo, just hanging down so it didn't pour all over the engine. So today, I checked - no gas coming out of the fuel line when it was still going to the carb with the tap on.

Reviewing the history - what has been done on the bike recently:

~1 mo ago, at least a couple tanks of gas - old green fuel line replaced since we were under the tank replacing one of the cowl locking tabs and the fuel line was super short and hard and pulled right off the tap. Ran through a couple tanks no problem, length wasn't an issue

This weekend:
- Engine rebuild, a garage/driveway job, no shop. Replaced seals, kick spring, kick gear, kickstart quadrant, cruciform. New piston/rings 2nd over, cylinder bored at an autorebuilders. Reassembled/reinstalled saturday.

- oil line replaced - from thick black fuel line to thin clear oil line tubing - and missing seat pin replaced while we had the tank out.

- no carb rebuild, though she did replace the idle jet (was slightly bent) and stepped up the main because we had to destroy the stock pipe swing-arm mount to get it off, due to it rust-welding to the swing-arm, and she replaced the stock pipe with a Sito Plus.

After all that, rode for ~30 miles around the city, then it wouldn't start the next day (Sunday) as described above.

So today at work, after having read everyone's suggestions, I was thinking it was probably the fuel line getting pinched between the frame and oil tank, so that when the engine was cold and the fuel level about half full there wasn't quite enough pressure to make it past that gap, even though gas did flow through the fuel line, possibly exacerbated by a too-long fuel line.

Anyway, got home, undid the banjo but left the fuel line in place. Turned on the tap, no flow. Pulled the tank, as soon as it was lifted a cm or so gas started flowing out the banjo. Rerouted the fuel line under the oil line, put the tank back in, checked fuel flow again. It was good! To be safe, cut a couple inches off the fuel line. Closed everything back up, it started after a few kicks, idled solidly for a few minutes before turning it off.

Thanks for the suggestions everyone, lots of them were good things that I wouldn't have thought of. Sorry I'm so long-winded, but I felt there was a lot of fine detail involved in the thought processes.

BTW: carb was clean inside, no varnish except a little on the slide, float was in good shape and moving freely, needle unworn, gaskets in good shape, so we just cleaned each piece and reassembled it before looking upstream.

Good job Eric! (I kneel before you)