Dyno Tuners With A Carby Fetish??

[ATOYOTA]

Ok, so I've had a bit of a play around with the main jets and needle heights and now I've hit a wall.

 

I changed to the biggest jets you gave me (the overdrilled ones) and then it ran more or less stoich on 1/4 throttle, but completely bogged again when I went to full or even half throttle. I raised the needles up the last notch and then spaced them a bit further with a washer, and now it goes through the rev range fine on 1/2 throttle but again goes lean at full. If I open the throttle up from 2000rpm and let the revs climb up it starts off sounding a bit lean, then at 3000rpm it gets a bit leaner again, then by the time I hit 4000rpm it's like hitting the brakes. I've tried lifting the needles with one more washer but then it stutters and misfires due to a rich mix. The idle circuit seems fine.

 

Could the main jets still be too small, or is it the wrong set of needles? And would it be an idea to drill them out bigger or buy a tray of jets/needles?

[altezzaclub]

I've tried lifting the needles with one more washer but then it stutters and misfires due to a rich mix.

 

What is it like further up the rev range with the needles lifted? Does it stay rich right through??

 

You can see that if you lift the needle you change the mixture right over its length, the whole rev range, although the smallest change is probably just off idle where the needle is straightest.

 

To change the mixture in part of the rev range, say, just in the top end, you need to change the taper on the needle, so either sandpaper yourself a new taper or get a new needle set.

 

There is also the rate of throttle opening that is a variable in the SU, and that is controlled by the thickness of the oil in the dashpot. If you open the throttle quickly the oil holds the piston down and makes the mixture rich for the couple of seconds it takes the piston to lift. If you open the throttle slowly the piston moves up and it stays around 14.7 . I'm not sure if you can do that on the CV.

[ATOYOTA]

It struggled to even get to the higher RPM, but when it got to 4000 it still leaned out.

 

I reckon it's the needles. It seems like what I need is a steeper taper, right near the tip, so at open throttle there's more fuel flowing though without affecting the other throttle openings. So yeah, bit of wet & dry would probably do the trick. Thing is, I'm not game enough to try sanding them, let alone trying to get 4 of them identical. Could irreversably cock it up.

 

How can I tell which jet needles I have, and which ones I'll need?

 

CV needle slides are vacuum operated. The more air that passes through, the higher it lifts the needle. There's no oil involved. That's probably why they're so responsive.

[altezzaclub]

So yeah, bit of wet & dry would probably do the trick. Thing is, I'm not game enough to try sanding them, let alone trying to get 4 of them identical. Could irreversably cock it up.

 

I marked them against a graduated sheet of paper every 3mm, and at those levels I measured them with a vernier. Then I started with a strip of 400# about 3mm wide in my fingers and rotated the needle back and forth. Soon figured out that 20rotations took off a thou or two and did each the same. Still not absolutely identical, but the difference doesnt show on the fuel mixture display. Doing four would be painful, so see what the options are.

 

The SU oil gives the instant enrichment as you accelerate, so maybe that is part of the problem too. Opening the throttle gives instant leaness that doesn't matter so much with a mo'bike weighing 1/3 what a car does.

 

Once the needle has lifted as high as its going to for that speed, the mixture leans back to whatever the needle allows. I suppose you get those cruise conditions sorted first and see how the acceleration works after that.