Vespa Polini 210, bgm PRO 60mm ,Dellorto SI26 – Tuning Test engine – part 2

Today we took a closer look at the Air intake filter / bellmouth from Polini as well as for the Dellorto SI 26 carb. And we will answer the question what the glory hole on top of the carb is good for?

In the end it has a good effect on the working of the carb. Simplified it works like a bigger carb, the carb sucks harder and gives more fresh air for the engine. The result is well known: more power, good!

If your engine gets more air you need to richen the mixture with the right amount of more fuel. The delivery of the Polini air intake includes a138 main jet for the SI carb (this is at the P200 version).

At the idle jet and atomizer there is a relief groove at the venturi. This guides the fresh air thru. This is similar to the mods done to the SI air filter base. To get a bore in there gives a better and easier jetting and better throttle response. This does the groove in the Polini kit as well.

Because you can’t put the air filter on top anymore, Polini found a nice and well working solution for this too.

An adaptor kit is tightened down at the carb box. This makes it easily possible to fit a modern day foam filter.

The adaptor should be fitted in a way that you still can get to all connectors, fuel pipe connection as well as cables for choke and throttle cable.

The fact that Polini included a 138 main gives a rough indication on how much more air will get thru with this neat mod. Experience on engines with bigger SI main jets than 130 showed us that some mods to the carb are needed to get the show reliable. Otherwise the fuel starvation will happen.

You need to sort this out properly and start where the action starts. At first the fuel tap should deliver enough fuel. We recommend for tuned engines the bgm FASTER FLOW taps. These are capable to deliver more than enough fuel. If your fuel tap gets 280-300 ml/min thru everything should be fine. But check the video at the fuel tap out to get the full story.

Except for a working air vent in the fuel tap cover you can’t do much more for the fuel tank. It is sorted with the bgm FASTER FLOW.

Now we come to the blue printing of the carb that is needed-at least- from 130 main jets on.

The small float bowl of the SI carb needs to be topped up with fuel all the time. Even under full throttle for long distances. The main bottle neck here is the float bowl valve. The best way is to swap this for the Vespa Cosa one. A straight and easy fit that cures this problem.

To the right: the standard SI item. At the left corner: The Cosa one.

Except for the larger float needle the bore diameter for the valve is bigger to. And for sure the of the valve.

The fuel channel for the float valve is opened up to 3mm.

Now all the needed mods are done to have a proper blue printed SI carb that gets enough fuel even on powerful engines.

One hidden design fault is still there though.

The small hole in the middle of the picture has a 1.5mm diameter. This is the channel for the main jet. All the fuel for the main jet has to go thru this.

This is too small as well. It is good for a 150 main jet but a design fault. We drill this out to 2mm. Better safe than sorry!

The bore at the left is for the choke system only and fine as it is.

This post is also available in: Deutsch

3 replies
  1. rosso says:

    hello i have question. you use not modifid (24mm)polini venturi system?
    si 26 on 24mm veturi?

    Reply

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