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adas

Well-Known Member
Aloha, while we are still on the carb discussions topic.
What are your experiences with pressurized carbs?
I have used air scoops to "pressurize" the airbox
---- (I know it is not anywhere near turbo pressure),
but it seems to give a very noticeable performance boost.
You need to also pressurize the fuel float chamber
or the carb will not work properly.

Anyone experimented with this?

Frank
 
Bigger bikes with ramair enclose the carb in the airbox
so the entire carb has the same pressure as the air
intake. I enclosed mine this way, but I think you
could run a hose from the air intake "Ram" area to
the overflow tube for the float bowl.
frank
 
On my old mikuni vm26 carb there is two ventilation holes from the floatbowl so the airpressure inside the bowl is the same as the atmospheric pressure.
I guess it would work if you just connect these holes with the airbox with some hoses.

That is on my Mikuni carb but i guess most carbs has those ventilations...

I myself has been into that line of thinking, although I have to get the engine ready before I can try it...
 

I like the idea of pressurizing the bowl to the ram area pressure.
I have a scoop as well, and it makes a difference of about 2mph.
The main jet size is getting rediculous though, for the carb size.

There's a vent tube that comes up vertical from the top of the
carb, but there's also a small vertical tube in the bowl, attached
to the bottom of the bowl. This must be the overflow tube.. never
paid much attention to it. I guess both of these need to be connected
to the scoop area... and somehow still maintain overflow protection.

One other detail is that there's 2 pressure components in the
intake, static and dynamic. You would want (I think) to reference
static pressure, so the reference tube entrance would have to be
flush with the scoop wall, at 90 degrees.
 
Last edited:
The large jet size is probably because of pressure not the same in the float bowl as the carb mouth. I prefer to pressurize a large airbox rather than just the carb mouth opening as I feel it gives a residual available air supply when you crack the throttle, especially at high speed and cracking the throttle.

You have to watch yourself though as the scoop rise and fall in pressure will cause the engine to lean out fast and possibly overheat.
I had my mixture set nicely and EGT was at 1200F, and I went through a short tunnel where there was 30mph head wind. My EGT IMMEDIATELY jumped to 1400F and I had to hit the kill switch and apply WOT to avoid a meltdown.
Frank
 
adas said:
..I had my mixture set nicely and EGT was at 1200F, and I went through a short tunnel where there was 30mph head wind. My EGT IMMEDIATELY jumped to 1400F and I had to hit the kill switch and apply WOT to avoid a meltdown.
Frank
Click to expand...

Was that with the "pressurised" fuel bowl? If so, why do you think
it didn't compensate for the extra head wind?

Also, this might be a dumb question, but I thought EGT went down
with either rich, or lean mixtures, and was peak at 14.7:1 .
 
Last edited:
StepVino said:
Was that with the "pressurised" fuel bowl? If so, why do you think
it didn't compensate for the extra head wind?
Click to expand...

Aloha, It was quite a while since I did this and cannot remember, but I think it was pressurized because the bike would not even run properly without the fuel bowl pressurized. But it was one Hellava gust of wind and it threw the air/fuel mixture way off.
Frank
 
StepVino said:
Also, this might be a dumb question, but I thought EGT went down
with either rich, or lean mixtures, and was peak at 14.7:1 .
Click to expand...

EGT goes up on lean mixtures and down on rich.
That's why lean mixtures can cause engine meltdown, the combustion is too rapid and violent.
 
Hmmm

In preparation to building up my scoot to break all 50cc
speed records (only partly kidding), I have been
trying to build up as much reading knowledge as possible.

I thought this was an established fact.... When rich the EGT
drops because of the unburned fuel absorbing heat, when
lean it's the cooling effect or the extra air in the exhaust.

Maybe it's not so simple...
 
Hi, this afternoon I was considering the vent hose, for pressurizing
the bowl. It occured to me that that might hurt fuel flow into carb.
Normally fuel just flows down by gravity, but if you increase the
pressure in there, maybe it won't flow fast enough to supply what
is being used by the engine.
 
yup, pressurizing the float bowl will also go against the fuel flow from the gas tank, unless you have a progressive rate fuel pump that automatically raises fuel pressure when it detects any rise in air pressure inside the float bowl you can starve the carb of gas & run dangerously lean.
 

Aloha, I did not find this an issue. The "pressure" in the float bowl that we are talking about is so very very small that fuel delivery was not a problem. But it was enough pressure difference to cause it to not run right at all without equalizing the pressure. frank
 
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