BS40 CV carbs

[tz375]

The most popular mod to carbs seems to be retrofitting 32mm slide types from a J/K or VM34/36mm round slides. I want top set up a motor on OEM CV carbs.

Has anyone jetted a set of BS40 CV carbs for chambers and/or pods? If so where did they end up and what jet sizes did they try on the way? I have sets of 34 and 39mm flatslides and I know they will be much nicer, but that isn't the question here.

I have some alternative needles and needle jets that I want to try and I have a couple of sets of nice clean shiny CV carbs and boxes of slow and main jets just itching for a chance to show their stuff.

So if anyone has any experiences they'd like to share, I'm all ears - or eyes in this case.

[tz375]

Not to resurrect an old issue, but there are significant design differences between BS30/96 pilot jets on CV carbs and VM22/210 used in most slide carbs.

The actual metering orifice is at the threaded (fuel) end on a VM22/210 and at the outlet end on a BS. The VM slow jet orifice only meters fuel and the BS jet it modulates the air and fuel mixture which flows less fuel for a given size. A #40 for example in a VM will flow much more fuel than a BS and the flow rate will increase more as revs go up, that it would through a BS jet.

If you ever have to work out which type of jet you have in your hand, poke a length of thin wire inside the jet. If it stops close to teh threads it's a VM and if it stops close to the tip, it's a BS and should only be used in a BC CV type carb.

[oldjapanesebikes]

Has anyone jetted a set of BS40 CV carbs for chambers and/or pods? If so where did they end up and what jet sizes did they try on the way? I have sets of 34 and 39mm flatslides and I know they will be much nicer, but that isn't the question here.

I'm running BS40's with Gibson Allspeeds and a stock air box. With the benefit of hind sight (always 20/20 ) I'm not actually sure I really needed to do anything. I did go up a couple of sizes on the mains initially, and then went back to one size over (112 I think) with the stock needle position (centre). I was thinking of putting back in the stock main jet (110), but it seems to run well and pulls well at all throttle settings - but it really needs more miles on it as it has only had about 600 put on it since putting the engine back together. Not sure if that helps ?

[tz375]

Thanks Ian. That's an interesting and unexpected result.

[rngdng]

Before I put the 34s on Blueboy, I ran stock carbs, pods, and chambers. All I changed on the jetting was the main jet size. I went up two sizes, and made all three the same size. It ran well.



Lnae

[tz375]

Thanks Lane.

Just out of curiosity, how much better are the 34's on the street and would you recommend them?

[tz375]

While I was researching this and looking to see what others had done, I bought a pile of brass and aluminum AKA jets and needles to see what else might work.

Not surprisingly, teh answer was not much, but more than nothing. Based on what arrived and measuring parts as well as I can, I updated the page http://pinkpossum.com/GT750/carb/BS40carbs.htm" onclick="window.open(this.href);return false;

[oldjapanesebikes]

While I was researching this and looking to see what others had done, I bought a pile of brass and aluminum AKA jets and needles to see what else might work.

Not surprisingly, teh answer was not much, but more than nothing. Based on what arrived and measuring parts as well as I can, I updated the page http://pinkpossum.com/GT750/carb/BS40carbs.htm" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

Looks good ! For clarity - the anti-surging jets you list as being #80 and #90, are actually #0.8 and #0.9 on the image you've posted from the catalogue - correct ?

[tz375]

Correct Ian 0.8mm and 0.9mm. In an old parts list I was referencing they listed them as 0.80mm=#80. I'll change that page to make it clearer. Thanks.

[Suzsmokeyallan]

Thats a good write up Richard

[tz375]

Thanks Allan.