CX500 GL500 650 Global

Since I've changed my motor and used it I've been keeping track of the fuel mileage, and it seems like its hovering around 38-40 mpg, (based on 3 tanks of gas), which from all I've read seems very low for these bikes. I took the carbs off and cleaned them back in June, and had them off again when I changed engines but I didn't open them up or clean them. I'm assuming its a carb issue, but don't know where to start. Ideas and suggestions?

The carbs are so important that just today I gave them their own section on the forum,

http://globalcxglvtwins.hostingdelivered.com/viewforum.php?f=19

If you are getting a decent spark and your compression is OK then it's the carbs.They are NOT as simple as people may think but it's not hard either and they need correctly cleaning.I don't go as far as LRCXed has gone preferring to soak mine in white Vinegar and then High pressure Airblow them out.

The pair that I've just fitted to the re-built engine I've just put in one of my CXs were one of my most crappy ones.I left them one side at a time(But still connected e.g not split) for a couple of days in a pan filled with white Vinegar.

These are the jets etc before and after being left in an egg cup full of vinegar for a few hours then a light brushing with a soft brass wire brush(The type used to clean suede shoes)








I finished tuning the bike today e.g Idle mixture correctly set using my Colourtune,

http://tinyurl.com/6dca5w

And then balanced them using my CarbTune2,

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

I have yet to do a mileage check but the bike/engine fires up on both cylinders on-the-button with 1/8th ish choke just to raise the idle speed on start up from cold.Engine will idle as low as 800 rpm but this is only as a test because they are designed to idle at 1100 rpm +/- 100 so as to allow the oil pump to correctly circulate the oil and gain enough pressure.

Also check state of Air filter.If the paper is ok these can be cleaned using clean petrol/gas and then dried out if they are oily etc or Carb/Brake cleaner.

Note:The holes in the Emulsion tubes were checked and made clear using a needle but I NEVER poke anything metal through jets.If they are blocked I snip a nylon bristle off a cheap hand brush I have and use that.

As I don't have an Air Compressor I usually take the carb bodies down to my local Test certificate centre after cleaing and borrow theirs.Mike actually did them for me there as he wasn't busy but they usually let me do them myself

Looks like I have a weekend of carb cleaning coming up. When I took them off the first time I did my best using the shop manual as a guide, but I've read a bunch more about cleaning on this site and the CC site, so I think its time I go back and re-do it. How does the ColorTune work? I looked at the ad and it says you can see the differences in idle, but I've never seen one of these before and don't understand what they do exactly.

kingston73 wrote:Looks like I have a weekend of carb cleaning coming up. When I took them off the first time I did my best using the shop manual as a guide, but I've read a bunch more about cleaning on this site and the CC site, so I think its time I go back and re-do it. How does the ColorTune work? I looked at the ad and it says you can see the differences in idle, but I've never seen one of these before and don't understand what they do exactly.

You buy them specific to the size of Spark plug in use.In this case a 12mm one.The kit contains a substitute hollow spark plug and a conductor cable that screws onto it(I had to put a bit of thread lock onto the small thrread it screws onto on the Hollow plug otherwise it would unscrew when you unscrewed the the plug) and a slip over hollow tube with flip mirror.

You Screw in the plug and conductor,place the outer supplied tube over this and the inner conductor cable which then has an end that goes into the spark plug cap.The hollow Outer tube also has a small flip mirror on it and you fire up the bike and you can angle the mirror ot just look down the tube into the Top of the cylinder(Weird right) but you then see the actual combustion going on in the Cylinder.You then just turn your Idle adjuster screw until you get,"Gunson Blue" as it's known.This is the perfect Blue colour of a correct Fuel to Air Ration.Too Deep a colour and it's too rich.Too white and it's too lean.

Note:The instructions that come with the Colourtune are worth the money alone as they are a fantastic troubleshooting chart as well.


This device is not really needed but takes all the guess work out of fine tuning the combustion ratio/Idle mixure on engines.
It was a,"Gadget" treat I bought and I'm glad I did.If you decided to get a Carbtune2 John Morgan also supplies the Colourtune but you may be able to get them cheaper on their own.


Note:The above explaination is way more difficult than the actual use which is Dead Easy

I have been getting very different MPG depending on the type of riding I'm doing. I ride to and from work daily with a lot of stop lights. When that's all I do, I average 36 to 38 MPG. Lately, I have been adding a lot of highway miles going to work my overtime at the hospitals, and average 48 to 52 MPG. I'm not sure if this is normal, but I did do a bit of modifying to the heads (porting) and the exhaust, Harley mufflers. I am also testing the difference between regular and high grade fuel. I will post if I find any changes.

The bottom line is that if all is in perfect working order, your riding style can make a big difference in the MPG. As Shep said, make sure the air filter is clean and all the mechanical issues have been set to factory tolerances. Valve clearances, cam chain adjusted, both exhaust pipes are even and not rusted out inside, no leaks, air or exhaust, and the tire pressures are up to specs. All these things make a huge contribution to the end result, of which is MPG.

This is great page for how the Colourtune works,

http://www.gadgetjq.com/ctune.htm

Correct float valve setting is also very important as this dictates the fuel available at higher revs when the needle valve comes into use, a friend of mine was getting 40 mpg his float level was way out about 11.5 mm if i remember correctly , Once we reset to the standard 3 tanks later now averaging just over 60 mpg

regards Roy

Float height is as important as good cleaning of the carbs and why I gave it a,"Sticky" in the Carb section.I too use a Colourtune.Takes all the guess work out of setting the mixture.Bad float height not only causes bad fuel consumption but can cause poor starting and single cylinder firing and of course fuel overflow.

The engine and carbs I've just rebuilt/built-up and put in my Main CX Gave me a 60 mpg(UK gallon) last Sunday on a 130 mile day out ride around.Normal Urban/Country road and a bit of Motroway/Interstate.Power-band a sheer joy.Pulled up the,"Hills and Dales" around where I live like it was on steroids

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.

I may have this wrong but the bigger the jet the more fuel sucked in but I thought that the small one controlled the low end and the big one the high end. But didn't these bikes come with 112 and 115 depending on where you are. My grandpa use to say that if you bought a car in a place like colorado that it wouldn't work the same in the lower altitudes so the car dealers put in what did the job to get it to work and then you fine tuned it the rest of the way.

Larry I wonder what a 118 or 117.5 would do?