Good info Teazer.

I agree the vm30 is a bit large for the application despite open intake, exhaust, and a port cleanup. A 28 would probably perform much better with the increased velocity. Already having the richest needle jet I could find, going richer on the needle is possible to bring the clip setting back in the middle, but is that really worth it? I'm on the richest setting, but it's not like I have shims shoved under the clip either.

A dyno run would be wonderful, but that's just not available within a few hour's drive. In a perfect world however...

 

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.

 

Here's some followup after some more seat time. Never got a slight hesitation or miss, but I'm a perfectionist. The 30 pilot provided instant cold starts and ample power from the get go, but showed signs of richness with an idle rpm drop once nice and warmed up. A 25 pilot still at 2.0 turns out feels just right after a couple more rides.

The old timers may scream "Holy $hit it's not supposed to use such a massive main!", but I kept going up in my mains until a WOT roll off to 7/8ths didn't result in an increase in power. Currently I made it up to a, wait for it.... 260 large hex. A couple hours of in-town hooligan riding is showing excellent plugs. Color change halfway down the ground strap, toasty ring at the base of the insulator. Power is very linear and doesn't surge or fall flat at any rpm.

*I may play with going back to a 30 pilot but increasing the air jet, but does the air jet have much effect on the pilot circuit?

Jetting:
Main Jet: Large Hex 260
Emulsion tube/Needle jet: BLEED TYPE 176 series P-8, richest I could find
Slide: 2.5
Pilot jet: 25
Needle: 5F3, lowest (richest) clip setting
Air Jet: 0.5
Air Screw: 2.0 turns out

 

Repacked my mufflers and stuck the 6f5 back in. Run's perfectly and pulls at every throttle point. The packing was almost completely gone, so the exhaust must have been a little too open. Now she's fat and happy.

 

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

 

Thanks for sharing the info,
Marc

 

Experimented with the 6f4, but it was too rich down low. Even though its a nice taper, it might not be right for my setup. I'm gonna try a set of 6DH4's that I have. They look to be a little leaner in the middle and richer towards the top.

 

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....

 

Great information in here, thanks guys. Mc69, I'll mention that my efforts started with a primary type needle jet like yours. Idle, off-idle, and midrange performance were perfect, but I couldn't get away from eventually fouling plugs even with the air jet removed. I cut my losses and chose to focus on tuning with the more preferred bleed type needle jet.

 

Good stuff Teazer. I experimented with a larger pilot (30's and 35's) but it was super rich. I think yer right, the 6F4 is a good needle for this motor due to its consistency in the taper.

So I'm not sure if anyone else has run into this but my 32mm mikuni's are not threaded for an air jet. My 30mm's are but the 32's (even though they have the same port) don't have threads to accept an air jet. Anyways, I have jetted it to work around the issue. The bike will be going to the dyno/tuner so I'll post those numbers when I get them.

 

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.

 

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.

 

Thanks for the update, I've been doing some tuning as well on my CB350, but I'm still using 32mm mikuni's (I have both 30's and 32's, but 32's are working better for me)

My motor is a 362, bigger mild cam, exhaust etc, and carbs have k&n filters on them. Here's my specs right now, it's running really well, but I'm prob going to be changing my needle today to a 6F4 to richen up my 3/4 range or I might just have to step up the main to a 195 or 200.

- 190 main jet (hex style)
- 2.5 slide cutaway
- 6f5 Needle, Richest setting (needle lifted up, with clip on the lowest setting)
- 159-P-6 needle jet primary
- air screw--1/4 turns out
- Air Jet Removed
- 20 pilot jet
- Standard float height

From what I've learned, by removing the air jet it leans out the mid range so you can use a 159 primary.

 

I was also thinking that I needed to go big on the needle jet, and P-8 was the largest I could find so far. There's most definitely improvement to be found in the details.

 

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.

 

Glad to help. These numbers were the result of trial and error throttle feel with an emphasis on staying away from any lean conditions. Over time I may need to tweak it a bit based on plugs.

I also considered compensating elsewhere in attempt to get away from such a small air jet, but so far the bike's telling me it's happy so we'll see what the long term reveals.

 

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.