Does intake size affect jetting?

Angry Hipster /

Just curious ...

What would happen if someone stuck a 12mm 1hp intake on a 2hp bike?

It would obviously be limited in breathing therefore would mixture need to be changed?

It would probably need a hotter plug I guess. But how would jetting be affected?

Re: Does intake size affect jetting?

Chöschi21 C.K. /

Just think about it, you are restricting Flow to your Engine, what do you think would happen with the Jetting?

Re: Does intake size affect jetting?

We know that it requires X amount of air mixed with X amount of fuel to obtain an optimum explosion in the combustion chamber .

Changing that X to X ratio will produce either a lean or rich explosion . Change the ratio too much and complete explosive failure happens .

Re: Does intake size affect jetting?

well not sure what your doing ..

but if your using the same carb .. on a bike that’s running fine .. and you put on a manifold that’s a smaller diameter between that carb and the motor ..

The motor will suck in less air ,, there for the less air (suction ) will draw up less gas at any throttle setting .. the carb set up right will meter the fuel air mix based on air flow .... so no different that say your at half throttle with a 20mm in take and your opening is 10mm at half throttle.

It should not go lean and burn the motor up .

Not sure what your doing ??

but it’s as odd as this pic

6F266CD1-9698-4F2F-BDBA-354E3851CC0C.png

Re: Does intake size affect jetting?

explain more .... like what kinda bike is it ..

9099B84A-31BC-4704-8C97-C9B6E648C821.jpeg

Re: Does intake size affect jetting?

what kind of bike even has 1HP .. is it made in france ,

Re: Does intake size affect jetting?

Angry Hipster /

I was just trying to learn.

On the other hand what would happen with say a 14mm intake and a 12mm carb?

Re: Does intake size affect jetting?

Angry Hipster /

They sold 1HP bikes in the U.S.

Re: Does intake size affect jetting?

They sold 20 - 25 - 30 mph mopeds in USA depending on the jurisdiction

> Ted Kenny Wrote:

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

> what kind of bike even has 1HP .. is it made in france ,

Re: Does intake size affect jetting?

Michael Forrest /

The smaller the carb the higher the air velocity and the more suction so as you reduce size you have to make the jets smaller. The velocity is directly relational to the area so Carb Area=carb bore radius squared x 3.14

(radius = bore/2)

If you decrease the area by 10% then you decrease the jet sizes by 10%.

ps- Don't anyone argue with me unless you want to get destroyed in front of everyone. I know what I'm talking about after spending months developing my own jetting calculator.

Screen Shot 2019-10-15 at 11.12.29 AM.PNG

Re: Does intake size affect jetting?

> Michael Forrest Wrote:

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

> ps- Don't anyone argue with me

Why would we argue with you when you're supporting what we're saying ?

Re: Does intake size affect jetting?

Josiah Radebaugh /

> Ted Kenny Wrote:

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

> what kind of bike even has 1HP .. is it made in france ,

Dont Knock em'

Plenty of 1 hp bikes, just about every 20mph bike out there.

Re: Does intake size affect jetting?

> Michael Forrest Wrote:

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

> ps- Don't anyone argue with me unless you want to get destroyed in front

> of everyone. I know what I'm talking about after spending months

> developing my own jetting calculator.

https://www.reddit.com/r/iamverybadass/

Re: Does intake size affect jetting?

Dirty30 Dillon /

Michael Forrest has developed a calculator that can make your jet 10 % bigger in just 3 minutes. DM him your credit card info and SSN for details!!!

Re: Does intake size affect jetting?

Wesley Ambrosini /

> Dirty30 Dillon Wrote:

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

> Michael Forrest has developed a calculator that can make your jet 10 %

> bigger in just 3 minutes. DM him your credit card info and SSN for

> details!!!

Mechanics hate him!

Re: Does intake size affect jetting?

Never mind the science behind a peashooter intake: you will have difficulty cramming a 30 pound bag of shyte through a hole designed for 20 pounders.

If you have a 12mm carb then use a intake with 12mm bore or bigger, 14mm use a 14mm intake, etc. Otherwise you will have difficulty trying to tune your bike, bigger into smaller you will have vacuum signal loss over the main jet and you will have a boggy and frustrating mess on your hands.

Re: Does intake size affect jetting?

> Michael Forrest Wrote:

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

> If you decrease the area by 10% then you decrease the jet sizes by 10%.

WRONG

...and dont get too cocky, many of us have been wrenching, riding and racing for decades. Much longer than your Excel spreadsheet calculators and pin on bicycle motors. They are handy to point people in the right direction, but that would be the limit.

> ps- Don't anyone argue with me unless you want to get destroyed in front

> of everyone. I know what I'm talking about after spending months

> developing my own jetting calculator.

Re: Does intake size affect jetting?

Angry Hipster /

I'm so confused (edited)

Re: Does intake size affect jetting?

Whats so confusing?

Manufacturers used undersized carbs, intakes and exhause systems to bring down the speed and power of mopeds. Retro fitting components from a lower speed or hp rated engine will likely bring down the power or speed of a higher rated powerplant.

> Angry Hipster Wrote:

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

> I'm so confused

Re: Does intake size affect jetting?

Angry Hipster /

Ok so basically I just want to know what happens if you put a 14 carb on a 12 intake or a 12 carb on a 14 intake.

What would happen and why?

I happen to have a mismatched set of parts and was wondering if I can build a frankenbike without having to buy parts.

Plus it seems kind of difficult to find a 12mm square intake

Re: Does intake size affect jetting?

Dirty30 Dillon /

Either way is not optimal, but I have found through a lot of trial that tuning is much easier when your intake is slightly wider then the venturi. The opposite tends to be much more difficult to tune.

Re: Does intake size affect jetting?

Angry Hipster /

Thanks Dillon.. I'm throwing in the towel and am just going to find the proper intake.

I was interested in learning the theory but I don't know if I will. I guess it doesn't matter... the main thing to do is to just know that it's best to match carb to intake.

I was kind of thinking the smaller intake might restrict things a bit and also non matching ports is no good, and I kind of thought the larger intake might somehow help, but I think now that it would just cause trouble.

Re: Does intake size affect jetting?

♣Slew Foot♣ /

All this of course it's true but the back Scavenging hun....

Sachs need some restriction to work optimally...

Re: Does intake size affect jetting?

Jack Rutherford /

Just do it and find out. That'll be the closest thing you come to an answer.

> Angry Hipster Wrote:

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

> Just curious ...

>

> What would happen if someone stuck a 12mm 1hp intake on a 2hp bike?

>

> It would obviously be limited in breathing therefore would mixture need

> to be changed?

>

> It would probably need a hotter plug I guess. But how would jetting be

> affected?

Re: Does intake size affect jetting?

Jack Rutherford /

Guess I'm willing to get destroyed. I would think, however, that the carb is just one small factor in the amount of suction as it is at the end of the chain that begins with the piston/cylinder, reed block, reeds, intake, etc. If I go to a carb with a 2mm smaller Venturi, am I really increasing suction at all? Because of the items that comprise the "suction chain" (so to speak), that Venturi is only a very small percentage of it.

Let's say I go from a vm18 to an SHA 15.15. How does the 60mm air filter affect suction compared to the 28mm air filter on the vm18?

My experience differs from your results as well. I've gone from a vm12 to a SHA 15.15 which both use the same jets and use a significantly bigger main jet on the smaller carb (at least 10% bigger than on the SHA carb).

> Michael Forrest Wrote:

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

> The smaller the carb the higher the air velocity and the more suction so

> as you reduce size you have to make the jets smaller. The velocity is

> directly relational to the area so Carb Area=carb bore radius squared x

> 3.14

>

> (radius = bore/2)

>

> If you decrease the area by 10% then you decrease the jet sizes by 10%.

>

> ps- Don't anyone argue with me unless you want to get destroyed in front

> of everyone. I know what I'm talking about after spending months

> developing my own jetting calculator.

Re: Does intake size affect jetting?

Smaller bores anywhere in the system after the choke valve creates a restriction, not an increase vacuum, so it does not suck harder and give more air or fuel or air-fuel. It creates a vacuum behind the restriction, that is how a choke valve works to give more fuel in the mix. The slight increase in velocity is where the restriction is, which is how non CV carburetors work with it slightly necked down at the jetting, at the expense of having a restriction in flow.

You are not going for an increase in vacuum, you are going for an increase in dynamic pressure over the jets so that static pressure will push the fuel into the airstream, the dynamic pressure is created by the velocity of the air over the jets. Put in a restriction in the tract and the velocity drops.

The idea of increasing vacuum for performance improvements would mean if you had a restrictive air filter or run part choke you would go faster, which is very easy to prove does not work.

Re: Does intake size affect jetting?

Michael Forrest /

"If I go to a carb with a 2mm smaller Venturi, am I really increasing suction at all? "

In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.

Also right where you create a restriction there will be an increase in suction just because of the restriction. If you pinch a water hose then the water before the pinch will then have increased pressure. (reverse flow causes reverse pressure, otherwise known as vacuum)

Oh, and changing from a VM to an SHA is like apples and oranges. They are of different designs and so shouldn't be compared. Changing to a smaller VM would of been telling.

Decreasing the carb from 14mm to 12mm decreases the area by 27% so with 4/042 main jets for Mikuni, since their sizes are relative to their flow area, you could reduce the jets that much. With Dellorto and other carbs the jets are relative to their diameter and so you have to convert to area and then reduce and then convert back to diameter. Example: reducing from a 80 to a 68 will give you a 27% reduction in flow area.

Re: Does intake size affect jetting?

Jet resizing is NOT linear based on cross section area of the bore, but it does point you in the right direction as a base.

> Michael Forrest Wrote:

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

> "If I go to a carb with a 2mm smaller Venturi, am I really increasing

> suction at all? "

>

> In fluid dynamics, Bernoulli's principle states that an increase in the

> speed of a fluid occurs simultaneously with a decrease in pressure or a

> decrease in the fluid's potential energy.

>

> Also right where you create a restriction there will be an increase in

> suction just because of the restriction. If you pinch a water hose then

> the water before the pinch will then have increased pressure. (reverse

> flow causes reverse pressure, otherwise known as vacuum)

>

> Oh, and changing from a VM to an SHA is like apples and oranges. They

> are of different designs and so shouldn't be compared. Changing to a

> smaller VM would of been telling.

>

> Decreasing the carb from 14mm to 12mm decreases the area by 27% so with

> 4/042 main jets for Mikuni, since their sizes are relative to their flow

> area, you could reduce the jets that much. With Dellorto and other carbs

> the jets are relative to their diameter and so you have to convert to

> area and then reduce and then convert back to diameter. Example:

> reducing from a 80 to a 68 will give you a 27% reduction in flow area.

Re: Does intake size affect jetting?

Michael Forrest /

It gives a reliable base but then the type of exhaust pipe used makes a big difference. Somewhere around 25% difference on main jet area from header/ muffler to full blown expansion chamber. That is why my jetting calculator gives the option of just entering the type of pipe used or better yet to first play with different jets till the idle and main jet are just right. Entering that those jets are spot on the money allows the calculator to show if there are weird variations in jetting between those two extremes due to the needle. Then you can try different needles in the calculator till the graph shows the proper form.

Re: Does intake size affect jetting?

Michael - you need to get into some C++ or other coding and make an app from your spreadsheet calculators - just saying.

> Michael Forrest Wrote:

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

> It gives a reliable base but then the type of exhaust pipe used makes a

> big difference. Somewhere around 25% difference on main jet area from

> header/ muffler to full blown expansion chamber. That is why my jetting

> calculator gives the option of just entering the type of pipe used or

> better yet to first play with different jets till the idle and main jet

> are just right. Entering that those jets are spot on the money allows

> the calculator to show if there are weird variations in jetting between

> those two extremes due to the needle. Then you can try different needles

> in the calculator till the graph shows the proper form.

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