Modern Vespa

Ok completely stumped now.

Engine fully rebuilt but just cannot get to start.

6v standard vbb 150.

New stator, points coil, plug, lead. Points gap checked and rechecked, timing set and rechecked. Spark looks good.

Carb dismantled and cleaned, all jets clear and definitely fuel delivery through intake.

Compression tested and all good.

Can get to fire with shot of cold start through air cleaner but cuts out straight aware (presumably once cold start burnt).

Have I missed something? Any other suggestions?

How sure are you about fuel delivery?

Does gas squirt out of the fuel line when you loosen the banjo bolt?

If it doesn't, check that the tap works and the fuel line isn't pinched under the tank.

If gas is flowing into the carb, then idle circuit is clogged. Clean it again, with compressed air.

Definitely fuel from tank when banjo connection removed and in float bowl.

Carb cleaned with carb cleaner and compressed air but will check again on idle circuit.

How was the crankcase pressure test?

Man this is a real head scratcher. One thing that has not been mentioned is what is going on with the choke. I think most VBB's were a fuel enrichment circuit style. If the choke is not on or failing to enrich, this would make sense on why the engine only starts on starter fluid. On the VBA's it had a flapper in the air cleaner that would cover the inlet hole and no fuel enrichment circuit. With air cleaner off, you could simulate a VBA choke flapper with you hand once you get it going on starter fluid. Cloquirk, you said fuel comes out when the banjo connection is removed. I think what SoCal is referrencing is having the fuel hooked up like it is ready to run, fuel tap on, then cracking the screw on the banjo and ensuring fuel weeps, then tightening banjo screw again, then starting the engine. Honestly I'm not sure if it makes a difference between the 2 methods, but I do it the SoCal way.

If the idle circuit is non functional, could he get RPMs high enough on starter fluid to be running mostly on mains as a test? I'm getting my carb tech confused in my head and can't remember if this would work for these Vespa carbs. +1 on pressure test for any newly rebuilt engine. You can search the forum for more info.

Is the float chamber filling with gas?

Check that the needle isn't sticking.

Clean that idle circuit again ... If it starts with starting fluid, you have compression, spark, and timing. The carb isn't delivering fuel.

Lots of good suggestions . Thanks.

Haven't done a pressure test so will look into that , but all seals and gaskets were new and everything torqued correctly.

Just to clarify, when I say it fires on starter fluid it does just that but doesn't run so it must be something to do with fuel delivery I'm assuming.

Am going to strip and clean carb again but can see fuel coming from main sprayer in Venturi so assuming delivery is ok to crank case.

Is the plug wet? You could be fouling the plug.

Try a new plug and starting without the choke.

Has anyone said piston upside down? Worth a check.

Double check the points gap (and get them as close as possible)

Does leaning the scoot towards engine side for a count of 5 get it started???.....this is a good test for choke issues diagnostics

Leaning the scoot on an angle dribbles enough petrol into the carb/ barrel area to get a start

On one of my scoots its the only way to start it I lean until the kick start touches the ground..then stand it up straight and it will fire up first kick


push start nothing
ten kicks nothing
adjust points nothing

one lean over..bang shes away

set that gap first

Have personal experience with this one--in my case engine started easily enough, but wouldn't run without throttle wide open.

Had same prob on my 6v vespa. Changed the condenser and all was good Instantly.

So an update.

Carb removed , stripped and cleaned again.- all good.
Fuel delivery from tank good.
Stator plate and condenser are new and spark looks healthy.
Still no start. Tried leaning as suggested - still nothing.

Piston up right way

Don't have kit for pressure test so still to sort.

Plug is not wet strangely which suggests fuel delivery to cylinder may be issue.

I think SoCal may have had it right all along. When the carb is off the bike, I check if the float is working by blowing into the banjo bolt barb with carb level and you should hear air escaping. Then I turn upside down and blow into the barb and you shouldn't be able to move any air. Use your breath and not your compressor.

Yep tested float valve just like this - all good.

I'd try take the spark plug out, using the Kickstarter, pump it with your hand and smell for fuel... a waft of fuel smelling air should come out... if not, you still have a fuel delivery problem... doesn't matter if the fuel is coming out of the hose or banjo, if it's not at the piston that's the issue...

What's the history of the carb?
Was it on the engine and running before the rebuild?

If you can't borrow a known good carb, might be time to shell out for a new one.

Yupp, I spent a week trying to get the original carb on my P125X going, couldn't get it to run properly however many times I cleaned the damn thing. Bought a new carb and it ran perfectly instantly.

Can also actually be too rich.

Some starting fluid can ignite over a larger range of AF ratios than regular gasoline.

Also make sure the reeds are working.

You should be able to stick a finger near the carb intake and feel only suction. If you feel pulsation on your finger (other than small amount when engine is not sucking) you can see if the air is flowing both ways.

Again the starting fluid ignites more easily than gasoline over a wider range of AF ratios.

Also check the timing with a timing gun to make sure flywheel is situated properly. Again starting fluid burns differently than gasoline and if the timing is way off it can likely start with starting fluid due to changes in flame front propagation.

If you put regular old gasoline on a rag on top of the carb air filter and crank it over, what happens? This is a real test because the gas from the rag will burn the same way as gas from the carburetor.