Carb question!

[zedooo]

First of all, lemme say hi to all of you guys
This is my first post here. I own a Baotian scooter (ItalScooter), that's the brand name of the QMB's here in Europe (Croatia).

For my winter project I'm doing a bit of work on it (modyfing every single part) and getting the BBK on.

I was wondering if the stock Keihin carb with some modding will be enough for 47mm cylinder or can I get more out of this with a Mikuni VM20 round slide carb?

I modded my stock carb like this:
made the butterfly a lot thinner and smoother, along with the sharp edges, shortened screws and all (thanks to GTVert);
ported the intake side (no real gain here I suppose)
re-did the slide part (vacuum) like Kliff did. (but I didn't drill the other hole in it because I didn't get what Kliff was doing )

Here's my scoot ;D

[Rebel]

First, Welcome, it is great to have international members here.

For the most part I think your stock carb will do fine for you, you'll need to re-jet it. I also think that you need to port the exhaust side in order to realize the potential of the intake porting you did, without freeing up the exhaust it is kind of like taking a deep breath in, then trying to blow out fast through a drinking straw, too much resistance. It is my opinion that if you are porting one side only it should be the exhaust side.

As for the mod you did not do on the slide, I understand what you are saying, I'm sure Kliff knows what he was doing but it is above my head.

[zedooo]

Thanks for the quick answer, and sorry for the broken English

I know I'll have to rejet, already did that when I changed the air filter...this time we'll go even bigger

As far as the porting goes, I am very familiar with the process, and will be doing it soon, but I was talking about the carb Actually I didn't port it, just sanded it so it's much smoother now!

[Rebel]

Your English is actually very good. Dare I say, I'll bet you do better than a lot of us that have been raised speaking it.

Great looking scooter you have there too. Keep us posted on what you do to it and how it turns out for you.

[kliff]

Nov 29, 2009 17:03:48 GMT -5 zedooo said:

Thanks for the quick answer, and sorry for the broken English

I know I'll have to rejet, already did that when I changed the air filter...this time we'll go even bigger

As far as the porting goes, I am very familiar with the process, and will be doing it soon, but I was talking about the carb Actually I didn't port it, just sanded it so it's much smoother now!

Hell, your English already sounds better'n mine, and I was born here!

But back to the carb..... small details can help in small ways. Feeling just one is hard. You might see it on a dyno, but seat of the pants, not always, even though it may be there. Adding up all these small mods are where seat of the pants dynos are felt.

But back to the hole in the slide.... it serves severral purposes, that vary with the state of tune the engine has.

Pure stock, air cleaner, air box, exhaust...it will let the slide open faster, giving a slightly richer mixture quicker. It is most easily detected with an O2 sniffer shoved up the tail pipe, on dyno testing. It helps keep the engine from hitting a quick rpm surge, lean. More engine preservation than all out HP.

As Tuning progresses, and the aircleaner is openned u, and well as the exhaust, rejetted appropriately, it again serves the same purpose.

Add now, a ported head, and better cam. Intake manifold pressure waves just became a bit more erratic, especially at idle, and up through 5-6000 rpm cruising. This causes the needle and slide to fluctuate, disturbing the mixture between low end adjustments, and WOT jetting. Here is where the mod shines. with a single larger hole, or 2 smaller holes, the pressure wave between the vacume chamber and the carb venturi, can respond quicker, and keep the slide/needle steadier, less pulsing and surging. Mid range to immediate WOT hesitation is also done away with.

So the holes have more to do with making for an easier to deal with HP engine, than actualy adding any mysterious amounts of power.....

Hope that helps, but not all my mods are about pure power, some are about helping that engine smoothly produce that which it is capable.

Glad you signed on....always thought y'all Croatians was somehow a dislocated platoon of Conferate soldiers sent on a long range recon misssion, that got wooooed over by some of the voluptious farm girls I heard y'all have over there.....LOL

Welcome aboard, and hope we can be of some help from time to time.

[zedooo]

Oh yeah, you're the man!
Thank you very much, it's clearer now. So, off to drill that hole

We have the best girls here

So, Kliff...whaddya think? Modded stock or MikuniVM20? (for 47mm bore)

[kliff]

Ooooops, sorrry....modded stock.

[zedooo]

I thought round slide would do him better, but ok, I'm going stock

Thank you!