To drill or not to drill?

[Steppenwulf]

The rear wheel is off the bike and the sun is shining and all I can do is surf the 'net. So I came across a carb modification site with tips for improving performance and stuff and it seems well written and all. This the baby.

So far you guy's on here have served me right in regards to getting me wrenching my own metal. So now the question is, what kinda difference do you get by drilling a 1/8" hole in the Vacuum Port (step 3) as opposed to leaving it well alone?

Better ask before I get handy with the Bosch!

[girlyglide]

Not being an expert on CV carbs I think the difference is this:- by drilling the hole to 1/8" you will alter the pressure differential between the carb venturi and the diaphragm. It will then need a higher differential (wider throttle opening) to lift the diaphragm. The effect of this is that the main jet will come in at a higher rpm.

On the other hand I could be totally wrong and it might just make you dick bigger.

[Folkie]

T'other way about actually. Making the hole bigger means that a lower pressure in the venturi will transfer more quickly to the diaphragm. You open the throttle, the pressure in the venturi drops, and because of the bigger hole, the pressure above the diaphragm drops quicker than it would have. Result: slide rises quicker (remember, the air under the diaphragm is at atmospheric pressure, in effect pushing the slide up against the spring).

Both drilling the slide and fitting a lighter spring are intended to give a crisper throttle response. They do make the slide rise more quickly, but can easily make it rise too quickly, and overshoot, maybe even surging up and down trying to find the correct position. There will no longer be a constant velocity air flow, nor a constant fuel pressure to the main jet.

Most people are now saying you should leave the spring alone, and if you must drill the slide, ⅛" is too big. Some are advocating drilling the slide 7/64", but many more just say leave it alone.

[Steppenwulf]

Cheers Folkie, I had been looking at replacing with an SE spring as well as drilling. So thanks for persuading me to leave well alone, so that's today's excuse done for- suppose I got to take the missus out shopping now

[Phil]

Originally Posted by Folkie

Both drilling the slide and fitting a lighter spring are intended to give a crisper throttle response. They do make the slide rise more quickly, but can easily make it rise too quickly, and overshoot, maybe even surging up and down trying to find the correct position. There will no longer be a constant velocity air flow, nor a constant fuel pressure to the main jet.

Quite right. Drilling the slide MUST be done as part of a whole upgrade.
Low highway speed...The slide floats up & down as Folkie says....Buy Liz something nice John .

[girlyglide]

Looks like my theory is all shot to hell then. If anyone else is interested in the CV carb this makes good reading Cvcarbs

Oh! and it looks like it won't make your dick bigger either

I tried most of the usual upgrades to the CV carb
First was a dynajet kit, which is a drill bit to make that hole bigger, a weaker spring and replacement jets and venturi tube. That improved the throttle response.
The I picked up a thunder slide kit cheap. This is a light plastic slide to replace the heavier ali one so it responds quicker, plus a different spring, venturi and jets. This improved the throttle response still further but made the cold running rough.
Then I fitted a Mikuni which is much better all round but the slide rattles. Apparently it rattles worse if you've upgraded the cam, pipes and air filter like I have.

[Folkie]

Originally Posted by girlyglide

Looks like my theory is all shot to hell then. If anyone else is interested in the CV carb this makes good reading Cvcarbs

Hey thanks for that, that's a pretty good article. Although it doesn't really say what constant velocity is, or how this gives constant fuel pressure, in fact it implies that the slide rising increases fuel pressure, which is wrong, and it fails to distinguish between pressure drop through the venturi and manifold low pressure. But these are maybe picky points; overall it's a good article. Some technical details of the enricher and pilot circuits differ from the Keihin CV carbs

[dpb-RK]

Originally Posted by Spider

Then I fitted a Mikuni which is much better all round but the slide rattles. Apparently it rattles worse if you've upgraded the cam, pipes and air filter like I have.

ditto..........a bit noisy......but it works well

[girlyglide]

Best solution is to put a proper carburettor on in the first place