CX500 GL500 650 Global

That's what I'm trying to figure out Don. All of the guys that used the bigger CFM carbs are getting better MPG. I had a VW years ago that I pumped up the motor on. With a cam and all. With the stock carb, it got really crappy mpg.Like 10 to 15. I installed a dual 40 Delorto carb setup on it and I got over 50 mpg. That was over 4 times the amount of air the one carb could deliver. If a motor is creating a high vacuum, and it has to pull the air in through a small hole, (carb) it is telling the carb and jets that it is demanding so much fuel to air mixture , that the fuel jets deliver more fuel than it is supposed to because the vacuum never reaches a neutral even status. I know this is confusing, but if the carb opening (bigger) is not struggling to deliver the amount of air the motor needs, it is not telling the jets its running at a really high rpm and trying to give it as much fuel as can be delivered through the fuel jets. Its much easier to re jet a system when the air to motor demand is more on the level of what the carbs can deliver.
It's kinda like putting a jet stream nozzle on a garden hose. It is trying to push 75 pounds of water pressure through a 1/4 inch hole. When you take the nozzle off, more water flows through with less pressure behind it.

I usually have to fill up around 120 miles at reserve. I am now at 138 and have not done the reserve yet. It's a little nerve racking when I have cars behind me at this point, because I don't know when I'm going to run out of gas. But as each mile rolls by, I get a little better feeling that I'm moving in the right direction.
Update at 11 as they say!

It's a nice feeling knowing your getting more than you were till you sit there worrying when do I have to flip the reserve. Somethings up with my petcock since I only get 5 or so miles more with the reserve. The straw probably didn't float off again but now I have the 33 ounce fuel bottle in my saddle bags just in case. I think that the sag in the old forks might have been causing the gas not to flow as good due to the angle it was sitting at. Better fill up and run it dry to test it out with the new forks.

Good luck with the carbs.

Both my UK Cxs are circa 1980 and all my carbs I think must then be the larger type and as stated I can get 60+ mpg(50 US mpg).Take into account as well that the carbs that gave less mileage may not have been as well serviced/tuned as the present ones?

Interested to hear using a colourtune - I have one (donkeys years old) but it's for B sized holes not D.

Can you get them easily?

CXDave wrote:Interested to hear using a colourtune - I have one (donkeys years old) but it's for B sized holes not D.

Can you get them easily?

They are expensive for what you get IMHO but you are paying for years of experience and the instructions/troubleshooting chart is worth the price plus they work!so easy to use as well.You can get a joint deal from the John Morgen(The Legend that is) where I got my Carbtune2 from,

http://www.carbtune.co.uk/carbdtls.html

Testemonials
http://www.carbtune.co.uk/carbcust.html

Fastest delivery going all round the world and one of the best bits of kit I've ever bought.



You can get them off Ebay as well

http://shop.ebay.co.uk/?_from=R40&_trksid=m38.l1313&_nkw=colourtune

You need the 12mm type.

Blindstitch2002 wrote:It's a nice feeling knowing your getting more than you were till you sit there worrying when do I have to flip the reserve. Somethings up with my petcock since I only get 5 or so miles more with the reserve. The straw probably didn't float off again but now I have the 33 ounce fuel bottle in my saddle bags just in case. I think that the sag in the old forks might have been causing the gas not to flow as good due to the angle it was sitting at. Better fill up and run it dry to test it out with the new forks.

Good luck with the carbs.

Buy a new Petcock and filter

Thanks for that Shep, I have a good set of guages already so a colourtune would be a good investment.

Just remembered you can get them from Halfrauds.

http://www.halfords.com/webapp/wcs/stores/servlet/SearchCmd?text=colourtune&action=search&storeId=10001&catalogId=10151&langId=-1&x=0&y=0

and as good a price as anywhere for a change.Also you can then make sure you've got the right one

LRCXed wrote:I am currently doing a test on my original set of carbs for my 78. No accelerator pump. The CFM is bigger than the 1980 and up set that I had on it last week. With that set, I got around 40 to 42 with all in town driving and a lot of stop and go. Highway was about 49 to 50.

The 78 set is all ready past the point of when I usually needed to switch to reserve by 20 miles so far. I am going to do some jet changes when I get the final MPG on this first tank, but so far, it looks as if the bigger carbs are pulling less fuel through the jets than the smaller CFM carbs did. I even put in a 115 main jet. Up from 112.

I'll keep you posted as I figure out what more I can get out of them, but it takes me over a week to get to reserve.

LRCXed, I like what your doing here, very interesting, but I'd like to clarify since you made it a point to mention "No accelerator pump".

Are you comparing a non accel pump carb set (78) to the later set WITH accel pump? (80+)? If so, I see an issue there that IMHO, may provide for skewed data.

To simplify, I do believe that the larger carb will, what I call "stay out of the mains" longer and could indeed be more efficient. However, when you add to that, the fact that they are not throwing large droplet, non-emulsified fuel from an accel pump, it seems a bit of an unfair comparison.

sheppola wrote:
Buy a new Petcock and filter

Simply said. Having trouble convincing myself to spend the money since I still get over 100 miles on the first two gallons.