19 replies to this topic

[Brams96]

I dont know if anyone has asked this before, but appoligise if I'm repeating.

People talk about Port & Polishing the head to get better gas flow but what about the carb? I noticed their is quite a sharp angle where the air enters the carb body itself from the throat (hope that's described ok) and also inside the carb body (the part with the flap in it) there are casting edges sticking up. I have taken my dremel to the throat and smoothed these out a bit (not too drasticly) but I'm unsure if I should be messing with the carb body.

I was thinking surely this would improve air flow with less distrubance creating less turbulence?

Any opinions??

P.S.
I can get pics if anyone needs a visual for my description.

[james753]

Yes, it is a good idea i believe, have you got Vizards Bible? he talks about it in there. Apparently you can also 'slim down' the butterfly valve too. I'd recommend the book though, i'm not an authority on this by any means!
Pictures would be good for a look please

James

[eddbmxdude]

I picked up an exhaust for my Metro from a bloke and he showed me a carb that had work done on it similar to what you are describing. I was thinking of doing it to mine at the same time as fitting a stage one kit.
The screws holding the butterfly to the spindle had been smoothed right off and the sharp edges in the throat chamfered. Pictures would be great!

Edd

[Asphalt]

http://img146.images...28/hifvergl.jpg
before / after

Note the manifold protruding into the carb's throat! Most important is a smooth change-over from carb to manifold to head. Generaly called 'port matching'. If the diameter however goes from smal to large, that's perfectly ok. But if it goes from large to smal, i.e. you have a protruding edge in the stream of air - that's not good.

However; do NOT modifie the square 'bridge' in the carb where the jet's in! It has to stay sharp edged and 'chunky'. If not, the mixing of fuel with air get's disturbed -> bin.

Get the yellow Vizard book, he describes it to some extend and with many pictures.

Edited by Asphalt, 27 August 2009 - 10:10 AM.

[Ethel]

http://img146.images...28/hifvergl.jpg
before / after

Note the manifold protruding into the carb's throat! Most important is a smooth change-over from carb to manifold to head. Generally called 'port matching'. If the diameter however goes from smal to large, that's perfectly ok. But if it goes from large to small, i.e. you have a protruding edge in the stream of air - that's not good.

However; do NOT modify the square 'bridge' in the carb where the jet's in! It has to stay sharp edged and 'chunky'. If not, the mixing of fuel with air gets disturbed -> bin.

Get the yellow Vizard book, he describes it to some extend and with many pictures.

All true,


Be very careful around the jet and piston unless you're prepared to spend an age getting the fuelling right. Thinning the throttle only makes a difference when it's wide open & needs to be wide open. Polishing doesn't make any real difference and a rough finish can be beneficial in getting fuel off the surface and back in to the air stream.

[Big_Adam]

http://img146.images...28/hifvergl.jpg
before / after

I did similar but cut the spindle rather than smoothing it down. I highly recommend the carb mods as they do make a difference.

[m1tch]

I heard that if the surface is left a bit rough the air flowing over it has a tiny bit of turbulence that acts like a cushion surrounding the inlet to allow the larger flow of air going through the middle less friction - much like a golf ball

And also from the first post, the carb doesn't have a 90 degree angle going into it - its only if you fit an air filter and don't bother to fit the stub stack that is needed is where the airflow is rubbish lol

[bmcecosse]

It is well worth 'bell mouthing' the leading edge of the carb (but it must be done properly!) - to minimise the vena contracat effect - but be wary of doing much more than this until you read and undrstand what Vizard says in his books.

[icklemini]

fit a bigger carb...

Edited by icklemini, 27 August 2009 - 08:07 PM.

[liirge]

The rough edge promotes fuel atomization

[eden7842]

i simpily polished my hif44 and filed any casting lips away, it just made the carb smother thought the bore.

also ive always been told to get rid of the little butterfly valve thing as its not needed

[Retro_10s]

I'm having my HS4 'Vizarded' by a local firm...

I've worked the carb elbow myself,... I'll post pictures when the carb arrives back

[MiniMonty]

Steady as you go there...

The SU carb is a robust and beautiful thing.

I wouldn't try to adjust my Elephant because it happened to have four knees.

The SU carb is a landmark in automotive engineering.
Be careful before you imagine you can improve it.

If you JUST LOVE tinkering then tinker away but I suspect you'll get more from the inlet
and outlet in terms of "change" than you ever will from a stupidly well tried and tested carb
which is still at work (and still on sale).

If you want real fun try two.
Balance those babies and now you are really spitting it in their.
Got hot plugs ?
Got good proper suction out ?

Start with the exhaust - getting the nonsence OUT is the first step to performance.
Breathing your engine MUST start with how it breathes OUT.

Best wishes
Monty

[Brams96]

Wow, good responses and very helpful, I will definitely get my hands on a 'Bible' and start reading.

Minimonty - You may love your elephant but just because something has 'always-been-done-like-that' doesn't mean there isn't still room for improvement.
As for breathing out, how can you flow something out faster if you dont improve how fast it is going in? It only makes sense to me to flow 'everything' on the combustion side of things.

I have a 1 7/8" stainless RC40, with 3-into-1 1 1/2" manifold, minispares heated inlet and K&N filter to go on but I just wanted to see if anyone had 'improved'/modified the inlet side of the carb before. My thinking was why flow everything when your restricted at the start.

Edited by Brams96, 28 August 2009 - 06:29 AM.

[lawnmowercrap]

Wow, good responses and very helpful, I will definitely get my hands on a 'Bible' and start reading.

Minimonty - You may love your elephant but just because something has 'always-been-done-like-that' doesn't mean there isn't still room for improvement.
As for breathing out, how can you flow something out faster if you dont improve how fast it is going in? It only makes sense to me to flow 'everything' on the combustion side of things.

I have a 1 7/8" stainless RC40, with 3-into-1 1 1/2" manifold, minispares heated inlet and K&N filter to go on but I just wanted to see if anyone had 'improved'/modified the inlet side of the carb before. My thinking was why flow everything when your restricted at the start.

What carb do you have?
You want to try and use an inlet manifold used on the HIF44's on the metro's. These were the best inlet manifolds that rolled out of Austins factory and provide the best air flow and in most cases is better than most aftermarket units.

You must have a rough edge somewhere though else the swirl is greatly affected. You want it to get in there with less friction, but if you reduce the swirl effect then it will all be for nothing.

Lee