Modern Vespa

Hey !

I just finished installing the Malossi kit. (cylinder+piston,cam,head,cdi)

What kind of improvements I should expect ?

Do I need to tune the carb ?

At a minimum, you should probably upsize the idle jet 1 or 2 sizes.

A little carb tuning will likely be needed to dial in the fuel/air mixture.

I got a wideband installed

I just finish the build, 218cc + cam + v4 head, I got the Malossi double red sponge air filter installed too.

Current situation is the wideband show super rich mixture on 1/4-3/4 throttle and at WOT very lean.

I got the WOT to be very rich by replacing the 118mm jet to 125mm I guess I need to be around 120 - I'll test.

But what can cause the middle range to be so rich ? the air filter ? I would think the whole range should be a little leaner then stock...

Would love some insight - thanks !

CVK carbs don't always respond as we like. If you have an air fuel meter, you will see that as you are riding at the same speed, with the throttle fixed in one position and the air/fuel ratio will randomly jump 4 points for no apparent reason. That's why a lot of people end up using the Mikuni TM33-12 carb.

With the stock carburetor, the jetting is usually fine as is. The Malossi single foam air filter is a better choice with the Malossi big bore kit.

Thanks for the info !

Do you think the double filter restricts the air to a point the mixture is super rich ? (3:1 (!!!))

When you say "4 points" you mean 14.6:1 to 15:1 ?

@ WOT I'll aim for 13:1~

No, the double filter has better flow than the stock filter and both of them work fine with the Malossi cylinder kit.
I've seen the air/fuel mixture ration jump from 12.X to 16.x for no discernable reason, then change back, or end up somewhere between the two.

So what can make it so rich on mid throttle ?

Just tune the niddle ?

From about 1/4 throttle on, it's pretty much all main jet.
It sounds like you've way overjetted it. Go back to the stock main jet and see how that works. It should be a lot lower than 118, I think. More like 92-105 depending on which 300 series cvk carb they used in your model.

It is a Runner VXR 200 2004.
Stock main jet 118.

When running 118 I got rich mid and lean wot.

I'll test again.

What model Keihin carb do you have? It's going to be something like 301 or 311, something like that.

Walbro WVF-7H

Pilot 34
Main 118

What model Keihin carb do you have? It's going to be something like 301, 305, 307 or 311 etc, something like that. Or the model number on the paper tag on the electric choke CM128 somthing.

Walbro WVF-7H

Pilot 34
Main 118

Got it. I've never tuned a walbro carb, but that explains the jet size difference. Different manufacturer.

well, I got it all sorted out, the needle got three notches - I installed the clip on the upper one (leaner) and the main jet went from 118 to 120 (125 is too much, and I don't have anything in between)

For now mid throttle is around 12-13:1 and WOT is 13-14:1

Pulls stronger and feels good - with the long final ratio I do not see and higher top speed - nothing significant - I think I'll try the stock shorter ratio.

Cheers !

Will I gain any performance from bigger carburetor ?

I would say so, the Malossi 34mm carb is meant to be good.

How are you getting on with the bike now? I have a 200VX, has a walbro carb but I am having difficulties with jetting. Bike idles fine on stand but as soon as I open the throttle, it bogs out and dies. I think maybe I have a too bigger jet in, tried 120 and 128. I am soon to put in an order for a 218 kit, Malossi multivar, 34mm carb, gear kit.

I run 218cc with the Walbro carb (measured 34mm...) I set the needle to the leanest position and the main jet is 120.

I got a wideband installed - very handy for tunning.

Have you de-restricted the airbox yet? That will make a difference.

How would I do that ?

At minimum delete the silencer tube and replace with a bellmouth. Can only do this if you have complete tuneability because you will be flowing a lot more air. Of all the mods I did to mine, big bore, cam, carb, pipe, ditching the Keihin CV for a tunable Mikuni and opening up the airbox made biggest difference. Some pics of mine.

I suggest I remove this tube ?

Yes. But need to open up the inlet as much as possible and use a bellmouth, airflow likes a radius.

Great - Thanks !

Anything I should do with this part ?
Any other modification I should do to the air box ?

My mid throttle is too rich - It will help ? I believe the new head and piston pull better vacuum and suck more fuel but air is more restricted.

Actually looking closely at your pics, looks like you might need to get more creative, looks like removing the tube will bypass the airfilter.

After some more testing - could be it is an ignition problem...
I get a cut at about 7000rpm - coil ? plug ? pickup ?

A little hole it all it took... maybe another to finetune.

The dip was still there, I took the carb apart and assembled it back.
The problem went away.

Malossi full 218cc kit, stock Walbro CV carb, had to lean out the needle and install a 120 main jet - works fine.

Dip is there...

I want to try a 34mm I got.

The stock needle is to aggressive - not linear...