Predator 670 dual 28mm Mikuni: Jet sizes?

[strantor]

Originally Posted by slonomo

Here's my 2 cents on jets:

1. Use only factory jets, unmodified
2. Drilling jets is not recommended, atomization will not be as good
3. Soldering and drilling jets is not recommended for the same reason as #2
4. Never use anything inside a jet like wire, bad idea

Carbs are very precise fuel metering devices that use vacuum pressure in a venturi to atomize fuel. Any alteration in the shape of the jet will cause a negative change in the fuel atomization. What you can't see is microscopic burrs and scratches that create turbulence and ultimately takes away from the correct flow of the fuel through the orifice.

Now I'm not saying it won't run doing what you're proposing, lord knows people do stuff like this all the time and engines run. But running is different than running correctly. I recommend using the correct jets for the application. If they aren't available, then you have to do what you have to do. But you won't find a "correct procedure" for resizing jets.....because it's not a "correct procedure".

Would it be better to drill/tap a port into my intake and install a needle valve? Basically a controlled vacuum leak to lean it out. I'm not afraid to do that, I made the intakes myself.

[slonomo]

I don't know, adding a non variable air jet will only lean it out enough at a given rpm level, say at idle. At higher rpm levels it won't be enough to keep it lean enough. It's not really the correct way to solve the problem.

If these carbs are exact copies of a VM Mikuni, they will have an idle fuel jet for your idle mixture, a low speed jet, and a diaphragm and tapered needle for the main jet for midrange and higher rpm at WOT. The carbs will automatically make the correct air fuel ratio based on the air that passes through the venturi, allowing more fuel as the amount of air passes over the orifices/jets. Adding an air leak will possibly fix the air fuel mixture at a given rpm, but will not be progressive enough to keep up with the fuel that the midrange and wot demand.

The correct solution is to install the correct jets. Beyond that I think you are simply experimenting and guessing at air fuel ratios. Typically jetting adjustments can be made by taking plug readings as you make changes. The modern and more sophisticated way is to use an O2 sensor and a wideband, very expensive, but very accurate and instant. There are other tailpipe sensors as well that require a laptop or scanner.

[slonomo]

With that said, if the correct jets are "not available", then soldering the jets and redrilling with high quality micro drill bits, is the only next step that I'd be willing to try. The trick with using solder is getting the heat just high enough to get the solder to stick but not too high as to melt the brass. Then the other trick is finding the right sized bits and not breaking the tiny ones as you drill, they break very very easily.

[strantor]

Yeah that's the problem. Apparently this engine requires very small idle jets. Smaller than what is available for these carbs. Smaller than the micro drill bits for drilling jets. Jet drill sets typically don't go down below a #80 (0.0135"/0.3429mm) at the smallest which would be about a #34 jet. The info I have right now suggests I'll need somewhere between a #7 jet and a #10 jet. A #7 would be a #105 drill bit (0.0027"/0.0686mm) and a #10 would be a #101 drill bit (0.0043"/0.1092mm). I can't find either bit for sale anywhere. I found the dimensions in a chart online but they might just be theoretical; the bits might not even exist. If they did, I would probably break them just trying to get them out of the box.

Only the idle is a problem (that I know of so far) so if the intentional vacuum leak only serves one RPM that's fine, that should be all I need it for. Just to get it to idle right.

[slonomo]

Sounds like you're venturing into the land of the unknown. I know Performance 670 uses 28mm and 24mm dual carb setups, wonder what jets they use in theirs?

[strantor]

Originally Posted by slonomo

Sounds like you're venturing into the land of the unknown. I know Performance 670 uses 28mm and 24mm dual carb setups, wonder what jets they use in theirs?

I sent an email days ago asking exactly that, got no reply. I expected I probably wouldn't unless I intentionally misrepresented my intentions to buy their kit, but I didn't feel right doing that. They were very responsive to my questions about their valve springs and steel pushrods when I stated my intent to buy them (which I did).