Modern Vespa

I have read numerous mentions of bogging....but it confuses me as 1) I am not familiar with the lingo, and 2) I have read conflicting opinions on it.

I have a Sprint V (in P body). I split the cases, done seals, gaskets, cruciform, clutch rebuilt and replace 6v points stator with 12 electronic setup. Suprisingly it started after this as it was my first Vespa engine dropping and splitting experience. Quite easy thanks to a few how-to websites and this forum Thanks guys!!

From about 1/2 to WOT I experience something between too much gas/not enough air mix and maybe miss firing on intermittend strokes (that's the best I can do ). New plug is nice choclate brown. It was somewhat better on my first 5km trip, but when returning home and doing a chop, I realised I never fastened the plug, only screwed in by hand. So, I thought I need some more air in the mix, so I fiddled with the air/fuel mixture screw (originally at 1 1/2 turns out). Seems somewhat better at fully turned in, but plug gets a bit lightish brown...scared of running into seizes. But with screw turned in it is a PITA to start. 1 1/2 turns out, start on first kick.

Something that is concerning me since cleaning the carb is that my slow running/idle jet doesn't have a hole in the top for air flow, only bottom for fuel...wrong slow running jet??

Dell'Orto 20/20
Main Jet: 160 / BE3 / 100
Idle Jet: 50

So......is it bogging?
How do I fix it....whatever its called

Thanks!!

It seems that you have the right idle jet. You don't have a hole in the top but you do have the extra brass hole on the left of the idle jet. It seems like you're running the right main jet combo too. You could try a 102 jet or see if your fuel cap air hole is clogged or gas line kinked. What type of spark plug are you running? It could be a number of things but I would start with fuel delivery and go from there.

Thanks Rowdyc

New fuel line, exactly 24 inches;
B6HS plug;
Holes in fuel cap open (blown with compressed air).

Maybe something worth mentioning: I cleaned my airfilter very well when everything was apart. Now, after about 20-30km it is quite dirty again from sputter back from venturi. Part of the problem, or normal?

Try changing the spark plug to B7HS. I had a B6 and it would backfire so I went back to the recommended B7.
Maybe with the blow back...too much fuel? Do you have extra fuel on top of air filter around the screws from the blow back. You can try bringing down the jets to a 98 but first I would try changing the spark plug and adjusting the mixture screw along with idle screw. Of course 1/4 turn at a time.

Yes, but not clean fuel, a bit soothed up. Why would the cooler spark plug make a difference (not second guessing you, will try that tomorrow, but looking for the rationale behind it )

Could it be a timing issue? I doubt it, as it did behave similarly with 6v points? Unless both had bad timing

Maybe your spark plug is OK but after experimenting, the B6 was too hot for me. The fuel and air screws can be different by engine so I would continue to experiment with them. The extra fuel on top of air filter maybe a sign of flooding and experimenting with a 98 main may help.

I'm sure others with more experience with this problem will chime in. I hope you can figure this out. Good luck!!!

Thanks Rowdyc

Tried a B7HS plug today, no difference...if anything maybe a tad worse. Plug was however too light brown for my liking

May this be a compression issue?? Hope not! It starts well, mostly first, otherwise second kick. Idle screw is somewhat concerning as it is only half a turn out.

So what is your compression at? Warm is best, throttle wide open, kick till it stops going higher, report back.

Still troubleshooting symptoms. Will make a DIY one this evening with tyre inflater and old spark plug....don't know how accurate that would be though. What is the recommended compression on a Sprint V? I see P's are about 110 ish?

I think I was in denial....I was scared it is going to be compression:

1) My DIY gadget shows compression at 78-80 psi
2) When doing it another way, with flywheel stopper in place and inflating the valve on the old sparkplug it loses pressure within 2-3 seconds.

SO now what am I to do....rings probably? I would probably need to go one size up, but haven't got a clue what my current size is. If I recall there was no markings on the rings??

Too soon to make rings the culprit?

Would a leak like this be bad enough for low compression? Just need a lap then? (Please say YES )

I think you can even borrow a proper tool from Autozone or some of those stores, I'd get a proper tool before you panic. Maybe a friend in the local scooter club? You want a minimum of at least 100 psi, 120-130 is much nicer.

^^^^^This. Borrow a proper compression tester. Make sure you open up the throttle all the way. Keep an eye on the gauge and kick it (usually three times) until you get your highest reading. Report back. Meanwhile it won't hurt to lap the head.

Lapped both head and barrel - made a 5 psi difference....but not there yet