80mpg?
 

I was browsing my copy of the Adventure Motorbiking Handbook when I read a bit that suggested that twin carb machines could achieve huge MGP figures by keeping the throttle just below the point where the second carb cuts in. Has anyone here done this, and if so at what revs and will this apply to my 3AJ too?

Mpg
 

"keeping the throttle just below the point where the second carb cuts in"?
:confused1: Must be a rare make. Anybody seen recently bikes working in that manner? Cars use two-stage carbs but I have not seen any bike (yet)?

Xt 600
 

I believe the XT600 does. No experience myself though.

Huh?
 

Methinks Patrick has never taken a good look at his XT600?
The XT/TT/Tenere 600 and 660 have the YDIS system. Yamaha dual intake system. Means, there's two carbs, each feeding one of the inlet ports. The slide on the first is operated by the cable, the second one by vacuum. Although, at about 1/3 turn of the throttle the valve downstream of the second slide is opened, creating an air flow, vacuum and causing the second carb to kick in.
There's no definite engine speed at which the second carb kicks in. You'll have to remove the tank to see when the throttle starts opening the valve in the second carb.
You'll be easier on the gas as:
- second carb is for the higher speed / power regions. Low speed makes for low consumption
- second carb is set richer than the first, lowering consumption

Auke

Yamaha
 

Sorry guys, I have an Africa Twin. Actually I started with a Yamaha LT2 back in -73 (100cc two-stroke). Amazing that I have not heard of the Dual Carburettor-system on the XT600. That´s the dark side of living in the countryside.
Yamaha has always been known for very interesting technical solutions.

Once more sorry, I believe you now.

PF:oops2:

Similar system was fitted to some uk fords using the webber carb ( Cortina for one ). There isn't much new, things go in cycles. Another way to get 80mpg is buy a new enfield, or double that and buy a diesel bike :)
see here http://www.dieselbike.net/commercialproduction.htm

No worries
 

Patrick. I just sometimes get fed up by unfounded opinions that provide the risk of confusing everybody.

Auke

I know its not a Yamaha but my 87 XR600 Honda has twin carbs arranged to come in sequentially.
Looking back over my records for the atlantic route (scribbled on a map!) I was getting 95mpg (10.6 litres for 227 miles), 72mpg (9.75L over 155 miles) and 73 mpg over 3 refueling stops.
The first one was so amazing I even took a pic of the pump readout. OK I wasn't trying to break any speed records but cruising between 50 - 60 mph on empty flat roads

So with BackofBeyond's experience to back up the theory this does seem to be something worth checking out. I'll use Auke's info to see if I can feel the second carb kicking in, I'm riding to Aberdeen tonight so I'll need something to keep me entertained on the motorway!

The secondary carb on my XT600E starts to open when the primary slide is only 5mm up in it's travel though this is adjustable. When the slide is at 5mm and the secondary carb butterfly is still closed I would estimate that the speed and revs in top would be down around the "ruin 5th gear" portion of the rev range. I don't think you'd be doing much more than 50-60km in top. Under normal use I get 16 - 17km/l and a range before reserve of 200km at around 100-120km/h.
Just my 0.02c

Andy.

Try 70 mpg..
 

In a 24,000 mile trans americas trip on two TT600RE's ( same carb etc) we averaged 20kpl (57 mpg) but the best was 24kpl. That was fully loaded with ally panniers etc. Cruising at approx 100kph.

Imperial - 5 liter approx - gallons or US gallons - 4 liters approx?

Google -

1 'imperial' gallon = 4.5461 litres

1 'US' gallon = 3.785 litres

Me? 1 gal = 4.5l, I don't track US gallons...

If anyone is thinking of modifying their bkes for more Mpg, I can offer this little bit of information.
back in the early sixties I had a speed twin, in standard trim it did 55 to 65 mpg fast touring. The motor was then hotted up After the motor spent some time in a different frame I got the motor back less most of the hot bits. The only thing was it went from 7:1 to 9:1 compression. instead of the twin 1 1/8 fuel injectors it went back together with a single 7/8ths amal carb. and the original cams.
after that it went well, better than a standard speed twin, but the big thing was it returned around 100 mpg. It was fitted with a one tooth bigger gearbox sprocket and avon touring fairing. I 'think' small bore carbs can have a big effect on mpg. The higher compresion ratio being compensated for by starving the engine of fuel/air so the internal pressures were probably the same but using less fuel/air mixture.

My 1VK has a little white line painted on throttle which will line up with another white line just before second carb operates. Previous owner did quite a lot of touring so presumably did this to get max MPG. Bike sounds different too when that second carb opens up. So the modification is two simple lines of white paint.