Carb syncing with a manometer.

[DaveO]

Wow! Every day is a school day. The bookmarking facility is better than most browsers. After decades of browser wars which browser allows keywords? Fantastic.

Dave

(Someone is going to tell me. Or say, "all of them")

 

[Kevin Werner]

That 1 1/2 turns mix screw setting is just a suggested starting point that should be close. From there you'll want to fine tune the setting for best running (smoothest and fastest speed). I work the screw out (richer) about 1/4 turn at a time until the idle speed starts to fall off and the cylinder starts stumbling, then I turn it back in just enough to get it running good again. I find setting the screw at the rich end of this good running area helps with eliminating popping on decel.

You want the mix screws to dial in pretty close to that suggested setting. If you need to go more than a turn or so from that to get it running right, that would indicate a different size pilot jet is needed.

The mix screw lets in more air? I got best revs turning the mix screw in but lots of popping on decel. Opening them up to 1 turn out got rid of popping. I know something is not quite right. I dropped the float bowls and removed the pilots. They are stamped 25 but there are no bleed holes in the jet. I will order new 25s and 27.5s from Niche Cycle.

 

[Jim]

The mix screw lets in more air?

No. Rule of thumb: a mix screw on the air filter side of the slide is an "air mix" screw.
A mix screw on the intake side of the slide is a "fuel mix" screw.

I got best revs turning the mix screw in but lots of popping on decel. Opening them up to 1 turn out got rid of popping. I know something is not quite right.

There's nothing wrong with that. As you turn the "fuel mix" screw in, you're leaning the mixture.... that increases the revs. But lean mixtures cause popping on decel....
So, you turn in until you get the highest idle, then screw it back out to richen the mix enough to prevent or at least minimize popping.

 

[Kevin Werner]

Thanks

No. Rule of thumb: a mix screw on the air filter side of the slide is an "air mix" screw.
A mix screw on the intake side of the slide is a "fuel mix" screw.



There's nothing wrong with that. As you turn the "fuel mix" screw in, you're leaning the mixture.... that increases the revs. But lean mixtures cause popping on decel....
So, you turn in until you get the highest idle, then screw it back out to richen the mix enough to prevent or at least minimize popping.

Thanks Jim.

 

[Jim]

Thanks
Thanks Jim.

If you think about it, 5twins and I explained the same procedure. We just approached it from different directions.
Either way works just fine.

 

[Kevin Werner]

If you think about it, 5twins and I explained the same procedure. We just approached it from different directions.
Either way works just fine.

Yes, in the tech section, I try to go there first, the carbsection has a pilot jet explanation suggesting that upstream of the slide is a fuel milxture screw and downstream is an airmixture screw. Regardless, I suspect I have incorrect pilots. Most of the info suggest up 1 on pilots and up 2 on mains as a starting point for pods and pipes. I ordered 76/77 correct 25 for baseline and 1 over 27.5s. I can only go from there. When I am finished I will have plenty of jets to offer up to others.

 

[5twins]

No, your pilot jets were correct. Yamaha had special "no hole" pilots made up for the '76-'79 650 carbs, not sure why. Maybe they got them cheaper because Mikuni didn't have to put bleed holes in them? Basically, these are VM22/210 pilots without bleed holes down the sides. The way the bowl is set up, the pilot flows only fuel, so the air bleed holes aren't needed. Air is added above the jet .....

But don't worry, you can use normal VM22/210 pilots with the bleed holes, they work just fine. Good thing too because you can't buy the "no hole" ones anywhere, lol. So, no need to get new 25's, But you probably wouldn't need them anyway. Stock pilots can work, until you lean the needle a step. Then you usually create a flat spot just off idle and you need a larger pilot, sometimes even 2 sizes larger. You have to keep in mind that the 3 circuits in your carb overlap a little. Changing one can effect the one next to it. This is what happens when you lean your needles a step to fix the upper midrange stumbling brought on by larger mains. That also leans the idle to midrange transition area .....

 

[Kevin Werner]

Well then, assuming the 27.5s are an improvement I will have 2 styles of 25s for the cost of a stamp.