Carburetor efficiency question..

[daverbmxer]

Well here's my first and probably only post.

Now, I don't have the most efficient vehicle ever made, but I bought it because it meets 2 conditions..:

1)It's 4x4
2)It gets decent mpg

The Suzuki Samurai

In the summer I've been getting about 30 mpg on the highway and almost as good in the city. It's has a 2 barrel carburetor, with a single primary and a single secondary.

***So my QUESTION is this:

Will an equal throttle position in different gears (4th or 5th) ALWAYS equal the same mpg?

I'm wondering this because I'm thinking the higher RPM can suck the fuel/air in faster and even though the valve is open the same amount, it gets sucked through it faster, resulting in more fuel burn.

I also have a vacuum gauge ready to install. With this, will I choose 4th or 5th depending on the condition (hills or headwind) based on which ever gear gives the highest vacuum?

Thanks.

[gone-ot]

...simple answer: No.

...because the engine RPM's are necessarily different in different gears...and, the faster the engine turns, the lower it's fuel economy will be.

..."Rule-of-thumb" -- best FE occurs in the highest gear at the slowest engine speed that doesn't 'lug' the engine.

...a vacuum gauge is an excellent indicator to have and use!

[SentraSE-R]

What Old Tele Man said.

[daverbmxer]

Well it's hard to tell if I'm lugging a 1.3L that puts 40 hp to the ground! When I cruise at 65 mph in 5th gear, I'm doing 3600 RPM. It's not lugging, it's just suffering from sheer lack of power.

So a real life example..

65 mph, 5th gear, 3600 RPM, 3/4 throttle
65 mph, 4th gear, 4100 RPM, 3/4 throttle

Even though I'm going the SAME speed and have the SAME throttle position, 5th gear should yield higher mpg?

I use 5th gear on the highway 90% of the time, but once in a while due to the barrenness of ND, winds can get crazy high and the truck doesn't have the power to hold speed at 60 mph even when it's to the floor. So I drop it to 4th.. Usually it's not that bad, and to go 60 mph I keep it about 3/4 though.

After all this is said though, once I get the vacuum gauge installed, I should just go by which ever gear produces the most vacuum to hold the speed I want, correct?

[gone-ot]

...what does the vacuum gauge say at each setting?

...bet there's a measurable difference with the higher gear showing a lower vacuum number, indicating heavier "breathing"--an indication of better BSFC.

[daverbmxer]

Originally Posted by Old Tele man

...what does the vacuum gauge say at each setting?

Dunno, it's not installed yet.

[bestclimb]

I have used a few samurais over the years, great little rigs but they do indeed suffer from a lack of horse power when runnin at highway speeds. With a webber carb nd 235/65s I was able to keep going on level ground at 70, on the first bbl in 5th any wind or uphill down shifting was pretty much required. My milage when in town was about 30ish till I moved to the bush then I was lucky to get 20mpgh. (4x4 engaged and slogging through dirt, snow, short trips and the like)

When cruising in 4th or 5th gear even with identical throttle positions you are sucking more air through at higher rpm which mean more fuel as well.

You can feel with the accelerator petal where the second bbl on the carb opens up ride wide open on the first barrel and let the truck do as best as she can, down shift when you have to and use the second barrel if that can't keep you moving fast enough.

Cal mini performance, petroworks, and rocky road outfitters have stuff that may help. a low rpm torque cam Will likely be high on my list for the next one. The help in fuel economy wont necessarily pay for a cam, but they do help in the off road drivablity department.

I am pretty sure that the 1.3 puts out about 65ish.

[bgd73]

is the carb progressive secondary?
if so, is it vacuum valve, thermal controlled?

if use of one barrel, yes you want the whistle of atomizing...and that also leads to a very decent surge into the second barrel. The samuri is not that old, I assume progressive.

30 is doing good. samuri with larger wheels still has low gearing, but it helps. and also, do not remove engine driven fan, that is a helper not a killer. the downdraft loves it. it is an "anti-thumper", carbs like smooth.

[bestclimb]

Stock carb is progressive iirc

[daverbmxer]

Originally Posted by bgd73

and also, do not remove engine driven fan, that is a helper not a killer. the downdraft loves it. it is an "anti-thumper", carbs like smooth.

I'm not too sure what you mean there. Can you rephrase it?