3AJ twin carbs

[dunc]

Hi Folks,
'Scuse me for being dumb but why are there 2 carbs on my 3AJ? I vaguely remember (and I'm showing my age here) something about them in the marketing brochure back in the year dot but I can't for the life of me remember what it was.
Anyway, reason for asking; I was up over 3000 meters for the first time today and the bike wasn't too happy. Coughing like, urm, me after my 3rd good morning Marlborough. I know I'm going to have to rejet, do I need to rejet both carbs?

Oh and btw, less air means less fuel and jetting down. Doesn't it?

Thanks,
Dunc.

[xtzrick]

Hi Dunc

It's a single, twin choke carb, "Choke" is the techy term for the carb bores.
Cars in the 80s used a similar set up, 1st bore is controlled by the throttle for quick reaction & low revs, the 2nd bore is controlled by throttle & atmospheric pressure, for fuel economy at higher revs.

Adjusting the mixture screw (turning in), under the left hand "carb" might be enough to settle your carburation at altitude, weather it's worth the time & effort depends on how long you're staying at that altitude.

Rick

[dunc]

Very good to know. So the left hand carb is the second bore?

[xtzrick]

Very good to know. So the left hand carb is the second bore?

Left is the 1st, it operates through out the rev range. The 2nd bore only comes into use after about 1/4 throttle.

If you do decide to change jetting, it'll be easier/cheaper to just do the left hand side. Dropping the needle (moving the clip up) will weaken the mixture aswell, just don't go to lean & remember to richen back up when you come down or

How long are you going to be at that altitude?

Rick

[dunc]

Hi Rick,
It's difficult to say how long I'll be at altitude, it depends on which route I take north. I guess it's likely to be some time though, from the map my intended route through Bolivia, Peru and Columbia is pretty much all over 2000m.

Dunc.

[xtzrick]

I'd try adjusting the mixture 1st, it'll be safer.

It's always going to be a compromise, rising then falling a few 100m.

When do we get to see the photos

Rick

[dunc]

I've got a blog with a load of pics (mostly of the bike and various beverages I have enjoyed) here: http://specialneedsdonkey.blogspot.com NTS: put it in my sig. I'll try and upload a few but the internet in this 'ere hostel isn't too reliable.

[Mezo]

Is that a small table or very large notes Dunc.



Like Rick was saying, the idea of the diaphragm is for bikes changing altitude & like he said you don't want to come back to sea level & hole a piston, its not just the piston but all the molten metal that drops down inside the gearbox (ive done it)

Mezo.

[Mats]

Maybe has something to do with this "thingy"?



I have one on my 3AJ but didn't know what it did until yesterday when I decided to google it.
I learned its an altitude compensator that is supposed to lean out the mixture when riding over 1000metres over sealevel.

Found this thread over at HUBB. Hope it helps a bit.

http://www.horizonsunlimited.com/hubb/y ... gles-49693

[Mezo]

Hey look its me giving the first reply.

That was a couple of years ago, long before this forum was even started.

Mezo.