Tom - I have questions about porting my intake manifold for my twin. Basically I have a Y-manifold that is about 3" long. I'm not exactly looking to go Gung Ho, but I would like to be able to create a better fuel charge. As we all know these bikes aren't power houses, so every little bit counts. Last night I did a light porting job and nice polish. After doing so I discovered this was actually not the correct thing to do. So I did some research, basically freshened up and my P&P education. So, a perfectly polished finish like I did is actually good for forced induction, but as you previously said, a hindrance for N/A motors or anything that isn't direct fuel injection. I'm not going to touch the heads, but I would like to at least gasket match the runners into the heads. Also I noticed that the carb flange on the manifold is actually oval shaped and was thinking about opening up the lower and upper portions of the flange to make the mating surface to the crab more true and less restrictive. Then I'm thinking of going back and resanding everything with 320 grit paper and attempt to sand the runners in a spiral direction to try and achieve a bit of a swirl effect.
What are your thoughts Tom?
Scottie
Scottie,
First, before cutting any metal, we must define our goals and our approach.
Regarding carb size and manifold matching, assuming you want to keep the same carb, just make the manifold entry the same size as the carb throat I.D., and make a nice perfect transition there. Definitely do not make the manifold entry smaller than the carb. But larger than the carb is not real good either. Try to make it the same as the carb, so the flow "sees" it as going down one pipe with no transition bumps.
If the port entry in the heads is larger than the manifold exit holes, you can match them up perfect too. Just leave the port alone for now. The main idea is to remove any restrictions or flow obstacles from the manifold
Preferably use some 60 grit sandpaper to score up the inside of the manifold(ouch!) with the scores going cross-wise in the manifold in a tight spiral like you mentioned sort of like a screw-thread would look, but not as deep. The idea is that as any fuel drops out of the mixture(which it will), it drops into these scored scratches in the manifold, and the air moving above the tops of the scores will pull the fuel back up into suspension with the air, just like the carb jet works.
We also use a similar technique on the valve seats when we make the cuts for the flow angles. It's called "fuel shearing".
This technique also helps the air speed inside the manifold or port because it produces a "boundary layer" on the walls of the manifold or port, which reduces the surface drag of the walls by creating a thin little layer of turbulence all along the walls that acts sort of like little "roller bearings" for the main column of air to ride on, free of surface drag from the walls.
To go any further with real power mods, the single carb set-up needs to go, and you can move to tuned individual runner intakes. But this stuff should help with the single carb set-up that you have now.