Modern Vespa

Hi All

Need some diagnostic assistance here, my Vespa bogs down on pull off. When pulling off on flat roads it bogs for a few seconds then pulls fine through all the gears. However when trying to pull off on uphills I can't get away if I don't rev up the engine and ride the clutch.

My set up is as follows, 200cc engine, main jet = 118 BE3, starter jet = 60, idle jet 160/45, 1750m above sea level, spark plug B8ES plug, Polini venturi, SIP road pipe.

These settings ran fine prior to me splitting the cases and changing the clutch and fly oil seals. The barrel was sent to a mechanic for a hone and the barrel and head were also lapped. Ring gap (0.55mm) and compression (100psi) were never great but the scooter ran fine before.

I suspected a dirty carb and stripped the carb and cleaned it with carb cleaner and a compressor. I have tried the following jets 116, 114 & 160/48 (don't have a 160/42). I also tried a different spark plug.

Any suggestions?

Is it getting colder in SAfrica right now? You may just need to let it warm up a bit longer. Check your idle screw. I lived at sea level and moved to a colder and 1000m above sea level. After I tinkered with these two setting and slightly upjetted (2 steps from my sea level base) bike runs fine. you'll find your sweet spot. Also, as far as your compression goes, that will only get worse the colder it gets.

*** fuel issue to on the uphill, the whole gravity fed fuel line can really suck if you don't have a full tank, i had an allstate that at less than a 1/2 tank if i went up anything beyond a 75 deg. incline would stall out, new fuel line and tap allowed for more gas to get to the carb.

It might be something as simple as backing out the mixture screw a bit.

Mix screw is currently 1 turn backed out. Yesterday I did runs with the following settings and still bogged
1/2 turn out
1/4 turn out
1.5 turns out
I will try 2 turns out and 2.5 turns out tomorrow.

My initial thought here is that the bogging is from being too rich and I first tried turning the screw in to lean the mix.

I have run out of things to try and will try backing out the screw further than 1.5 turns.

Yes we are heading into winter but our days are still very warm.

Engine was warm in the video. Bike does bog on the flats on pull off, you can't hear it much in the video. Tank was over half fill.

I would think the 55/160 would be the better idle jet, may be the reason you are so lean. the 45/160 is the most lean of the idle jets I know of.

I agree w/ JJ.. I'd try a 55/160
1.5 turns out (to begin with)

Vespigi ,how's it restart after its all warmed up,like its not getting gas?

Or anything weird on restarts like hardly will idle till it gets going.And unable to give it any throttle until it revs up and stays at a nice idle.

My tv is doing that and having to ride the clutch on take offs once in awhile.
It doesn't do any of that until it gets hot.

I'm thinking next size idle jet like said above

My mix screw was set at 1 turn out and I did runs with the screw turned out to 1.5 and 2 and the bogging did seem to reduce slightly. I swapped out the idle jet to a 160/50 (don't have a 160/55), set the mix screw to 1.5 out, and again the bogging situation improved. With the 160/50 there is no more bogging on the flats but pulling off uphill I do have to rev it up a little and ride the clutch but not as much as before. I will see if our local Vespa mechanic has a 160/55 and try that out.

As Mudnman asked in the post above, on one occurrence with the 160/55 I struggled starting the engine once hot. This is problem is shown in the thread below,
Engine Does Not Start When Hot

From the thread above, moving the fuel line away from the cowl definitely has helped but it seems the lean idle circuit is also contributing to the hot starting problem.

Before starting this thread I had it in my head that bogging occurred from being too rich and everything I tried never worked which was extremely frustrating.

Thanks for all the help.

from my clubs forum on carb tuning...

hotrodAl top tips:

"8. On tuned Vespas, if it takes more than 4 complete turns, then pop in a richer idle jet, and repeat. This is the smaller jet on the left. The PX200 runs a standard 160/55 idle jet. The T5 runs a richer 100/50. The richness of the idle jet is the ratio of the two numbers, the lower the number, the richer the idle jet. The 100/50 is 2.0, the 160/55 is 2.9. The 100/50 is therefore richer.

If it takes less than 2 turns on a tuned Vespa, consider popping in a leaner idle jet and repeat.


Main Jet Stack:
Air Corrector

This is the hole that a set amount of air comes in to mix with the fuel coming in. The larger the number, the more air is delivered. A 160 air corrector is therefore 'leaner' than a 140 because it has a larger air hole and lets more air in. Changing the air corrector will affect the mixture throughout the rev range from 1/3 throttle to full throttle!

Atomiser/Mixer Tube

This is where air from the air corrector and fuel from the main jet mix before going to the venturi. Moving from a BE3 to a BE4 mixer tube reduces the amount of mixture from the main jet stack to be introduced into the venturi to mix with straight air from the inlet. This reduction is caused by the BE4 simply having fewer holes for the mixture to pass through. So a smaller amount of mixture goes to the venturi where it is mixed with the normal amount of air coming through the carb throat, thus producing a leaner running condition. Therefore, the BE4 is 'leaner' than a BE3.

Main Jet

This allows a set amount of fuel into the atomiser/mixer tube to exit the carb and out the spray bar into the venturi then into the case inlet. On the Si Carb. the fuel exit on the main jet is a metric size. Eg a 116 main jet has an exit hole of 1.16mm. The larger the number, the 'richer' the main jet. A main jet of 118 is therefore richer than a 116. The main jet stack has its effect from about 1/3 throttle to full open. This is a much wider range than most other carbs that have three jet circuits and not two, like the Si."

good info:
http://www.lmlocgb.co.uk/phpBB3/download/file.php?id=298&sid=f5788e53e773863058da01443f7b61db

http://www.lmlocgb.co.uk/phpBB3/download/file.php?id=297&sid=f5788e53e773863058da01443f7b61db

and this from oopsclunkthud:


& from Garagerocker:
45/140 = 3.11 < Leaner
55/160 = 2.91 < Richer

Jet sizes
45/160 =3.56
36/120=3.33
42/140 =3.33
48/160 =3.33
50/160 =3.20
38/120 =3.16
45/140 =3.11
40/120 =3.00
55/160 =2.91
50/140 =2.80
52/140 =2.69
45/120 =2.67
50/120 =2.40
50/100 =2.00
55/100 =1.82

The older solid slow running jets without the hole was mounted in carbs that had an air intake hole in the carb body. These carbs could be upgraded with the new kind of jets by blocking the hole with a lead shot(see pic) When doing this operation it was advised to also adjust the idle air/fuel mixture adjusting screw by screwing it a ½ turn out.