Mixer Tubes SI Carbs

[rosscla]

OK bit confused here. Discussing the mixer tubes on the SI type carb

SIP says -

The greater the stated number, the leaner the mixer tube. This means BE1 is very rich but BE6 is very lean.

Beedspeed says -

Tuning Tip: There is no particular rating for richer or weaker but as a guide: BE4 is the weakest, BE5 is richer than the 4, BE6 is slightly richer than the 5 and BE3 is the richest.

ModernVespa has this -

http://ec2-50-18-170-78.us-west-1.compu ... opic129142



They can't all be right...discuss

[ricalnic]

http://scooterotica.org/forum/viewtopic.php?f=8&t=27378

[rosscla]

Thanks for the response ricalnic.

I've seen (listened) to that before, Ginch links to it in the modern vespa thread. It doesn't specifically address the apparent confusion over the BE numbered mixer tubes, indeed in the thread you've quoted gaz_powell takes the SIP line and Norrie Bodge posts the pic which tends to support the Beedspeed interpretation.

The chart I found more interesting was one pdxjim added.

[Norrie Bodge]

watching this youtube video, explained better for me too understand things

http://www.youtube.com/watch?feature=pl ... pkFSA_rRFI

[rosscla]

Thanks Norrie,

In principle it explains how these work but doesn't answer the specific question of who's right or wrong about the BE tubes for SI carbs.


Sent from my iPhone using Tapatalk

[Ginch]

If you were to take David Vizard's word as gospel then you can read these tubes below. He says "Where there are no holes, it will cause richness". So that says to me that the BE4 & 6 are richer in the higher revs (remember he says to read the jet stack upside down starting with low revs at the bottom) than the others. Therefore BE2 must be leanest in the higher revs, and BE6 leanest in the lower revs. That seems to be verified in Rosscla's chart with the BE2 having the biggest holes in the top section at 14.1mm2. I don't really understand the 'behaviour' columns as the BE2 is listed as 7/very rich at WOT however...

So I guess it's not as simple as one tube being rich and another lean, they each seem to have their own richer/leaner sections within them.



This is all assuming that David Vizard is correct. But if you've ever had Mini's and are interested in tuning then you'd know him straight away.

[gaz_powell]

The atomiser on SI carb does seemed shrouded in mystery

I`ve only really worked on tuned p200`s so have had no reason to change from the stock BE3
I have altered the air corrector, main, pilot and filters to get where i`ve needed.

what set up are looking at ? - if using a p2 carb on a px (kitted) I would start with a p2 jet stack set up - 160/be3/main

px125 used a 140/be5/main

gaz

[rosscla]

Mine is a Malossi 166, fat tail Mikeck on an LML reed engine, with polIni carbon reeds casing ported to match the barrel.

I started with a spaco 20/20 which had a 42 closed slow running, 140/BE5/116 stack, this ran but latterly had developed all the signs of a bad airleak.

I laid hands on a genuine si24/24 e in good nick with no jets in at all, so we fitted that and put the jets out of the 20 in as a starting point but it was even worse. I started to search for jetting for this carb / kit combo and came up something from somewhere which I cant find now but was 55/160 slow running and 140/ BE6 / 122. I ordered the jets from BeedSpeed / wasp according to availability, but read the only the advice on BeedSpeed site re the leanest / richest jets (as above). Now a mate of mine who I'd normally defer to on anything Vespa related and who's built some fast Vespas said more or less what the SIP site says, that the BE6 was 'lean as fook'. So I start googling and get more confused as we go.

Anyway we stripped the whole lot out and found a busted gasket under the carb box, replaced that, fixed a 'too short' throttle cable and a dodgy stud and bolted the lot back together. Now it runs properly and the only thing that needs looked at now is I think I need a richer slow running as I had to put the mixture screw about four turns out to get a nice even tick over, it's crisp and clean and blows the arse of my brothers P2.

I like to try understand these things and there's advice on several dealers pages which is contradictory.

[rosscla]

If you were to take David Vizard's word as gospel then you can read these tubes below. He says "Where there are no holes, it will cause richness". So that says to me that the BE4 & 6 are richer in the higher revs (remember he says to read the jet stack upside down starting with low revs at the bottom) than the others. Therefore BE2 must be leanest in the higher revs, and BE6 leanest in the lower revs. That seems to be verified in Rosscla's chart with the BE2 having the biggest holes in the top section at 14.1mm2. I don't really understand the 'behaviour' columns as the BE2 is listed as 7/very rich at WOT however...

So I guess it's not as simple as one tube being rich and another lean, they each seem to have their own richer/leaner sections within them.



This is all assuming that David Vizard is correct. But if you've ever had Mini's and are interested in tuning then you'd know him straight away.

I think that the behaviour chart indicates the air to fuel ratio for that mixer tube, 1/13 being an 'ideal' figure at WOT. Obviously that would be altered as suggested by combination with different air corrector / main jet.

Smaller air corrector = richer stack, larger = leaner. Opposite for the main jet, for the same mixer tube?

[Jack221]

The mixer tube does the same job as the needle and atomiser in a PHBH or similar. Once you work out whats what, then tweeking the mixer tube by drilling a few more holes in the right area does the trick for tuned engines.