1974 GT550 fuel mileage

[SuzukiSmith]

My question is when people talk about the main needle "E" clip being in the fourth slot is that from the bottom being the tapered end? Or from the top? How much would the clip height affect fuel economy?

[joolstacho]

Needle grooves are usually numbered from the top to the bottom (Bottom is the taper end).

[SuzukiSmith]

So the factory setting should be what? Its not in the manual what slot to start from.

[jabcb]

The service bulletins have the carb specs.
http://www.ozebook.com/compendium/techbuls/index.htm

The 74 GT550 jet needle is listed as 5DH21-4. That means needle 5DH21 with the clip in position 4.
Use a magnifying glass & look closely at the non-tapered section of your needles. If they are stock, you’ll see the Mikuni symbol & “5DH21”.

[SuzukiSmith]

Thanks for the link! Alot of good reading there. I'm guessing my rebuild kit needles O.D and taper may be smaller than factory. I kept the factory ones they were pretty nasty but they do have the Mikuni symbol and that number on them. I purchased a set off of eBay because I lost one of my original ones when I moved recently. I also put new floats in the carbs and set it with a float gauge as per the manual. But this thing is getting terrible mileage! 50 miles till reserve! I understand there are alot of variables but what is a average number of miles per tank?

[GTandcbr]

I have a 550 J and get at least 110 miles to reserve. I consider myself a spirited rider. 50 miles is atrocious and something is definately wrong.
First make sure there are no exterior leaks. Does the bike start and ride okay? Remove the plugs are they a nice tan colour or black and sooted up? Are your air screws between 1 and 2 turns out? Check your air filters are clean and clear.

[SuzukiSmith]

Starts great, idles great has a slight lull between idle and 1/4 throttle. My air screws are 2.5 turns out. The plugs are a brown color now since the air adjustment. My air filters are new uni filters. My thought was the main needle being an aftermarket piece being smaller in diameter that stock? Could that affect fuel economy that much? No external leaks either.

[GTandcbr]

Firstly i would buy Mikuni needles IMHO anything else just wont do. When tuning carbs everything needs to be accurate not near enough. I would try fitting a smaller pilot jets as having to turn the airscrews out 2 1/2 times means they are too big and i think that may be the cause of your heavy fuel consumption. The pilot jet has an effect through most stages of throttle opening.

[SuzukiSmith]

Thanks for the reply. So I have Mikuni needles coming. With your suggestion to run Mikuni parts I will also order new pilot jets. But since the ones in it are aftermarket I should get #25 Mikuni jets and start there before getting smaller ones.

[GTandcbr]

Yes. Aftermarket jets can be problematic and even sometimes come with the wrong threads. Once you have mikuni fitted then you can start to adjust the jetting if necessary.
Here is a link to the carb manual in case you dont have it
http://www.3cyl.com/mraxl/gt/manuals/su ... /cover.htm" onclick="window.open(this.href);return false;