Correcting the NT and Speed carbs

[Deleted member 12676]

Both of these carbs have design flaws that can be corrected for a more perfect jetting (not rich or lean) from closed to wide open throttle. I prefer to just replace them with the Mikuni VM18 but some people need to be as conservative as possible with their money. Using my jetting calculator I came up with these suggestions but unfortunately they are mostly for engines with reed valves fitted to them. I need to put a Speed carb on a piston port intake engine and see how the jetting compares. Any way here is the url for my sites newest page and I hope some people find it useful: http://www.dragonfly75.com/motorbike/cheapcarbs.html
By the way by using the jetting calculator I realized that the Mikuni I have is with an incorrectly sized needle jet, one too small which throws the jetting off. It came from Treatland with a N-0 needle jet instead of the needed O-0 jet. The N-0 overly reduces the free space between the needle and the jet when the throttle is mostly closed (well, up to about 30% open when the needle taper kicks in). I had previously tried to compensate for it by installing a really big idle jet which helped somewhat but still it was off. I'm disappointed that Treatland sold me the carb that way. Normally the VM18 is sold with an O-0 needle jet.

 

[FurryOnTheInside]

I don't really know what you mean by "increase the slide cutaway". Are you suggesting that I grind inside the slide so the needle circlip will sit lower?
I couldn't raise the circlip and modify the needle instead?

I have the NTTC which comes with a lot of the cheap kits, and that's entirely missing from your page.
It's functional with the clip on the second notch and jetted down. Yes it's still rich when dawdling at 1/4 throttle but I don't spend time riding with barely any throttle to be particularly bothered, tbh. It probably helps lubrication though.

 

[extremeodd]

The slide cutaway is that bevel on the front edge of it when you look in from the filter side. The angle of that can be adjusted to affect the low end fuel delivery of a carb. A shallow angle increases air velocity and increases fuel delivery where a high angle does the opposite. This is something that you must know when dealing with Dellorto carbs as most of them have several slides to choose from.

Technically all slide based carbs deal with cutaway sizes, its just many dont make a point to make them tuneable. But PHBGs are kind of unique due to how many tuning combos you can have. There are 26 needles with 4 clip positions, 3 slides, 5 (I think) pilot jets, and 60 possible main jets. That is 93,600 possible tuning combos. Personally I have the parts to do 4,320 (12 needles, 4 clip positions, 15 jets, 3 pilots and 2 slides. Technically the number is far far far higher as I have a micro drillbit set #60-80). To say that tuning one is tedious is being quite generous.

 

[FurryOnTheInside]

The slide cutaway is that bevel on the front edge of it when you look in from the filter side. The angle of that can be adjusted to affect the low end fuel delivery of a carb. A shallow angle increases air velocity and increases fuel delivery where a high angle does the opposite. This is something that you must know when dealing with Dellorto carbs as most of them have several slides to choose from.
View attachment 90285

Technically all slide based carbs deal with cutaway sizes, its just many dont make a point to make them tuneable. But PHBGs are kind of unique due to how many tuning combos you can have. There are 26 needles with 4 clip positions, 3 slides, 5 (I think) pilot jets, and 60 possible main jets. That is 93,600 possible tuning combos. Personally I have the parts to do 4,320 (12 needles, 4 clip positions, 15 jets, 3 pilots and 2 slides. Technically the number is far far far higher as I have a micro drillbit set #60-80). To say that tuning one is tedious is being quite generous.

A picture is worth a thousand words. Thank you for illustrating what it means to "increase the slide cutaway"
I don't even remember there being a slanted bit on my slide. Haven't seen that part of my bike in the last two years lol. ☺

 

[extremeodd]

It really is a strange term when you don't have anything to associate it with.
I'm fairly certain (especially after google image searching it) that the NT carbs don't have a cutaway to speak of, they tend to have flat bottom slides (well, no bevel) but that doesn't mean that one wouldn't create the desired effect of leaning out the 1/8-1/4 throttle area.

EDIT: I screwed up on the PHBG combo math, I also forgot there are 3 different sized atomizer tubes, so that brings up the potential 2 stroke combos to 280,800