[Teazer]

Thanks. That's most useful. I was surprised that the Main jet ended up so large and air Jet was so small, but that's a great place to start.

 

[Teazer]

Isn't that needle the richest one in the 5 range already?

I suspect that the small Bleed nozzle is why it needs large mains and small air jets. Ideally, I'd expect/guess that it would need a larger needle jet but they are not generally available in 176 series. It would be interesting to see a set of dyno charts for the gas analysis at 1/4, 1/2, 3/4 and 100% throttle opening to see how the mixture looks.

 

[Teazer]

Thanks for sharing.

6F5 is a really lean needle from 3/4 to 100%. 6F4 is much richer all the way through the range. If that doesn't work for you, try larger needle jets. O-2 or maybe even O-6 to richen it all the way through.

Taking out the air jet basically changes from 2.0 air jet to say 3.5 and tilts the fuel slope ie it leans out the top end (revs) proportionately more than it does lower revs - for any given throttle opening. So at WOT, it will be much leaner at high revs without an air jet than with one. It's not so much throttle dependent, like most jets, but is load (gas velocity/engine speed dependent). It has more effect when gas velocity is high and pressure differentials are high (high engine revs).

If you did a full throttle run on the dyno with no air jet and with say a 1.5 air jet, the two A:F curves should move apart as revs rise - both at WOT. It's a great device to fine tune wide open throttle across the whole rev range and has much less effect at lower revs.

#20 pilot jet and 1/4 turn out on idle air screw indicates it's too lean at the bottom and would benefit from a larger pilot jet - say 30 or maybe even 35. If that makes it too rich just off idle, it needs a leaner cutaway (bigger number/gap).

But what this proves is that there are many ways to compromise and to cover the holes in the fuel curve.

 

[Teazer]

I should have added something about Bleed vs Primary type nozzles. At constant throttle opening a Primary type will be much richer at the top end IF the bottom end is right. Conversely if the top end is correct, a BLEED needle jet will provide more fuel at lower revs than a Primary type.

That makes a Primary type great for a piston ported motor. Reversion tends to make them over rich at lower revs, so a primary nozzle tends to lean that out somewhat.

If we use a Primary type on a four stroke with little reversion through the intake, it will tend to run really rich at the bottom end if the top end is close to correct. Using a smaller nozzle can clean that up a little but at low revs and large throttle opening it will still be rich if it's right towards the top.

Air jets, height of the primary tube and the use or Primary Vs Bleed types are more RPM dependent than throttle position dependent effects.

 

[Teazer]

Mc69, I have a pair of Power jet 36s with no provision for an air jet. They are on a TZ top end so that's not much of an issue. I think that TMX34s are like that too and they have the needle jet cast in place, so we'll be experimenting with needle shapes and sizes to try to work around that one.

I'll be very interested to see read/see what happens at the dyno. Hope your dyno isn't like Vegas. What happens on the dyno stays on the dyno room....

 

[Teazer]

Funny how we learn in fits and starts. This thread caused me to go back to notes on the changes we had to make to a set of VM26 carbs on a CB160 based race bike. It runs really well but after buying a whole range of air jets we left the originals in to try to better atomize the fuel.

We did change go through 4 or five sets of needle jets until we found a set that works well, but if I did it again I'd use different air jets. In fact I may just take it to the dyno and try a couple of things when ambient air s drops to something closer to S.T.P. There may be another HP or two hiding in there

 

[Teazer]

Air jet really only applies to the main circuit and applies more at the top end than low down.

The fact that you are on the richest needle clip and relatively huge main jet implies two things - insufficient signal to pull fuel through the jets (carb a little large for the application) and/or the needle and N/J combo is restricting flow. Now it's time to run it on the dyno and see what the A:F looks like.