Pre-bug carb 17.5 or 19?

[torinoman]

Hey all! Probably going to be carb time for me soon. Just did an upgear and need some some hp to utilize it!
Currebt build is a 2001 prebug, Black Trophy 70cc, Yasuni eco pipe, jetted stock carb, stock air box, Tops variator, 6.5g rollers, 13/44 gears, 1500 contra spring, and 1000 clutch springs. Stock clutch otherwise. Speed is about 50 and under 9k rpm.

I'm looking at Dellorto and Polini mainly, but the big question is what size do I need? 17.5 or 19? I know most on here like the stock air box but I'd like to hear input on those that have used foam pods. I know how to tune a carb.

[Zino]

A 17.5 would be plenty big enough for your build thta will work all the way up to a mid race build
A 19 would be better if you new you were going to upgrade beyond to mid race or beyond

They both will feed and cool your set up better vs stock

With a non mid race you may notice better throttle response with the smaller carb

[torinoman]

May 3, 2024 21:27:24 GMT -5 Zino said:

A 17.5 would be plenty big enough for your build thta will work all the way up to a mid race build
A 19 would be better if you new you were going to upgrade beyond to mid race or beyond

They both will feed and cool your set up better vs stock

With a non mid race you may notice better throttle response with the smaller carb

I was wondering if a 19 would take away bottom end. Let's face it, that's what this build is. Tourque. I want to keep it below 10k with the 10mm wrist pin! 

[Zino]

reference this the 3rd Zuma is a mid race set up with a 17.5 mm carb and Brett was impressed how it performed compared to his more powerful build
49ccscoot.proboards.com/thread/25866/endless-summer-scootin-strokes-video

I run 17.5 dellortos on most builds I have used a 19mm dellorto I liked them both top end performance and throttle response was so close for the level I build at .

My tie breaker is only have to buy one size of jets for my garage full of scoots and the 17.5 is easier to mount on stock airbox

[aeroxbud]

Pedparts did an interesting article on the difference between the 12, 17.5, and 19mm carbs. With a similar performance build to yours except for the pipe. But the PM tuning pipe is made for great torque with a big bore. Not a high rev monster. The 17.5 was the best choice.
Dyno here www.pedparts.co.uk/c/asset/aerox-carb.jpg

[bigmatthew86]

& you can keep your oil injection & auto choke!

[torinoman]

Cool, all good advice! I'm good with a Dellorto 17.5. Sounds like that's what I need. Do they take the same 6mm jets as the PHBN I have? That would be great since I have a bunch already. The other thing is I'd like to keep my cable remote choke.
Anyone have a favorite place they like to pick one up?

[torinoman]

This one maybe?

www.scootertuning.ca/en/carburettors-sport-midrace/5169-carburetor-dellorto-phbn-ls-175mm-1000000000447.html

[aeroxbud]

If you buy the PHBN, you keep your manual choke. 👍
That's cheaper than racing planet www.racingplanetusa.com/carburetor-dellorto-phbn-minarelli-p-210038-1.html
It will take the 6mm Dellorto jets.

[torinoman]

Good deal! That will probably be the one I get then. Looks like it comes with a 90 main jet. I'm running a 100 in my 12mm. Where should I start on a 17.5?
And I still need to decide if I want to keep wrestling that horrible stock air box every time I need to make an adjustment to the carb. 🤦‍♂️

[aeroxbud]

Keep the air box. It will run better.
On my BWS vertical engine, I just used to undo the air box, and remove it still attached to the carb. Taking it out as one unit removed the problem of struggling to fit the air box which is a pain on that engine.
You can still get to all the adjusters and float bowl.

[Zino]

A 100 should be on the rich side and cause it to be doggy
Work your way down from there if you have that jet

I have 80-100 range of jets that I use to dial in that carb with stock Airbox
I will bet you end up right around 90

I usually end up in the 88-92 range depending how racy the build and/or the riding style street light to street light can run leaner jetting compared to some one that rides wide open for miles at a time .

[torinoman]

Ok. I'll leave the air box on it. That know that would be better for fuel economy too. Probably need to take the snorkel out?

Good idea about taking it out in one unit. I'll give that a shot!

Thanks for the jetting range, Zino. My jet kit jumps in increments of 5, but I think I can get close enough.

[torinoman]

I've been on the bubble about deleting the oil injection since I put the BBK on. I like the convenience of it a lot. Will this carb change anything? Seems like it'll be more air, more fuel, and the same amount of oil?

[Zino]

The Yamaha oil pump has kept up with my builds all the way up to 11k rpms with the larger carbs