Sachs, carb restriction, and skavenging.

So as Im searching and reading about venturis, intakes sizes, and all that air velocity vs jetting mumbo jumbo. I keep running into "Sachs need a restriction to work well"

Can someone explain and give me an example?

Im running a D jug with matching piston, 15 intake, 15 Bing with one of those MLM velocity stacks, did I waste 35 bucks? What should I be using?

Re: Sachs, carb restriction, and skavenging.

I don't agree.

on one hand, your goal is to get as much air and fuel in as possible in the right ratio, so restriction shouldn't benefit you if it's tuned right, right?

unless some aspect of the carb provides insufficient venturi to draw up enough fuel without additional pressure drop across a filter, which ok sorta maybe, and maybe stock sachs have that to consider, I'd still think tuning could skirt that issue but youre not on a stock carb anyway so, whatever. I can see it more an issue in bigger carbs, I mean you need some negative to have fuel delivery

velocity stack helps there too, more velocity helps. Most airboxes have effectively a velocity stack that extends a little way into the dead air zone which adds to the airbox advantage. but your stack should be totally cool anyway

but there are advantages to an airbox:

it'll make it less sensitive to air changes if your neg pressure is more constantly provided by the airbox restriction vs just external air density; steadier air delivery.

and you can even theoretically net some benefit from resonance similar to the expansion chamber on the pipe

also an airbox can be one of the least restrictive ways to have an air filter. those velocit stack cover filtersare actually pretty restrictive, and also remove the velocity around the lip of the stack and limit it's function (ideally the stack sticks out into the filtered air a lil bit to allow good flow around the lip). Alternately an unfiltered stack just risks increased wear from dust & shit. you can have alotta filter area without much restriction with a proper airbox.

.

tldr: nah I dont think it needs any backpressure if it's tuned right, assumptions based on sachs needing backpressure for the stock carb are invalid with a different carb, and there are benefits to an airbox especially when getting bigger and when exposed to alotta atmosphere conditions, but generally it isnt critical for your setup.

Re: Sachs, carb restriction, and skavenging.

Sweet deal Will, I just couldnt understand what people meant.

My bike is running really well and when i read this it made me second guess where I was at.

I was guessing that the suction created by the piston would be enough with the correct Jets, slide and needle adjustment and any restriction would need a complete re-tuning.

Re: Sachs, carb restriction, and skavenging.

With all that being said, Will. Do you think going from a 14mm intake to a bored 15.5mm will need a re jet? Or would it be close enough to lean just enough to make a little more power without downjetting?

orrrrrr just plug chop it? I hate wasting plugs but its a necessary evil.

Re: Sachs, carb restriction, and skavenging.

yeah i'm pretty sure if anyone is saying that its because the midrange is hard to tune on the square bing- there is no needle so without an airbox it runs lean and you get a midrage boggy doggy.

i have never noticed a difference sweating +/- 2 mm or so on the intake. i've built intakes where i really sweat the diameter and they usually don't run any better or worse than ones that i just slap together out of whatever. you'll probably want to do a couple plug chops anyhow once you get your whole setup dialed in, everything from air in your tires to fancy neon grips on the handlebars.

Re: Sachs, carb restriction, and skavenging.

> Graham Motzing Wrote:

> -------------------------------------------------------

> yeah i'm pretty sure if anyone is saying that its because the midrange

> is hard to tune on the square bing- there is no needle so without an

> airbox it runs lean and you get a midrage boggy doggy.

>

> i have never noticed a difference sweating +/- 2 mm or so on the intake.

> i've built intakes where i really sweat the diameter and they usually

> don't run any better or worse than ones that i just slap together out of

> whatever. you'll probably want to do a couple plug chops anyhow once

> you get your whole setup dialed in, everything from air in your tires to

> fancy neon grips on the handlebars.

lol dont forget my clip in pedals

Re: Sachs, carb restriction, and skavenging.

I think the bigger intake can cause some deadzones that might make it stutter from idle, or affect how frosted the intake gets, and if you were at 14 with a 15 carb before it'll let the WOT preform better, but jetting i could se an argument either way, reducing velocity but also reducing restriction? Honestly though I don't expect it'd make a noticeable change, and if it di - well, you'd notice it! and can jet accordingly.

I was hoping jbot would chime in, he's thought alot about airboxes - you might be able to find some old threads of his on the subject

Re: Sachs, carb restriction, and skavenging.

♣Slew Foot♣ /

If you get it all working together right. At top end the back scavenging is noticeable with the expansion chamber it will boost by resonance with the carb. About 7 mph.

« Go to Topic — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account