carburetor problems need advice

[dave1shere]

Hello everyone. I would like to start by thanking all of the people who have helped me get this machine up and running. I was able to make it for bike week in Daytona but have been so busy at work have not been able to get on this discussion board for a while. It's starting to get cold here in Central Florida so now I am going to take some time and do a little more work on the bike.The one problem that I have noticed so far has been poorer gas mileage. I am getting about 21 miles to the gallon the only change from a stock engine is I have removed the air box and have put on K& N type of filters. this caused it to run to lean. A friend of mine at work who is supposed to be an expert on Japanese carburetors told me I do not have to change the main jet but to just change the clip on the needle so I move the clip down two slots. This seemed to help, the engine is running strong but I am deftly running to rich the spark plugs are black with soot. my question is should I just go back to the settings that were on the needle jets originally and go up a jet size. Thanks Dave

[Dennyz]

Dave,

The individual K&Ns do significantly lean the engine out. Moving the needles makes major changes. I suggest you only move it one position.

To better dial in your carbs, changing main jets provides finer increments than moving the needle clips. I found that one clip size is not enough and two is too much. So, if rejecting is needed anyway, might as well leave the needle in the stock location and buy larger jets (they all cost the same).

It is hard to recommend jet sizes without knowing what type of exhaust you have. It also matters what year of XS we are talking about. It has been my experience that '78 & '79 carbs respond differently to jetting changes than '80 & '81 carbs.

The link below might help you decide what to put in

Jetting info

[ratbyk]

Carb Tuning

Dave,
You mentioned K&N "type" filters. Some of us have installed K&N type filters and ended up with very rich mixtures. Some models of these after market copy kat look a likes have a large rubber shoulder where the filter attachés to the carb air horn. The shoulder of the rubber collar is so large that it obstructs air intake at wider throttle opens and acts like a choke greatly increasing fuel use. Have a close look at yours. In fact remove them all together and take the bike for a run, then check your plugs and see if it is running any leaner than before.
Ken/Sooke
78E Ratbyk
82 FT500 "lilRat"
92 ZX11 Temptress

[dave1shere]

My bike is a 1980. The air filters that I installed look like they are covered with a stainless steel mesh but have a filter material embedded between the stainless steel inner and our mash layers. I looked at the rubber shoulder it does not appear to be blocking the ports on the carburetor intake. The bike still has its stock exhaust system on it.

[Dennyz]

You say you have an '80, yet you where able to move the needle clips. The '80 and '81 XSs did not come with needle clips, only the '78 & '79. Is there a chance you have older carbs on your '80 (a good thing IMHO)? Where is the idle adjustment screw located on your carb set?

[thumperjsa]

Dave,

Yours would not be the first 80 with 79 carbs on it. Mine also has 79 carbs on it. One of the easy ways to tell is the 78-79 carbs have an air breather hose to let atmospheric air into the carbs. There is also a large adjustment screw on it. And oh yea, it uses 137.5 mains, not 115 and 120 like 80 carbs. Mine also had clips to adjust the jet, I just enriched mine because I was running lean.

I have an air box and 4-1 pipes. I am getting 140 mains to try out as well to see if that gets the plugs to a nice tan color instead of the very pale they are now.

[Ken Talbot]

Actually, both of my '81 Standards have adjustable needles, and that is the way the '81 fiche shows it too.

[Dennyz]

Interesting, the '80 and '81 Specials I have owned did not have adjustable needles. I can see the '80 having both sets of carbs, as I don't think I have ever seen two '80 a like.

Back to jetting:

We do need to know if the carbs on Dave's bike are early or late model design.

(Geeses guys, it is Christmas eve. Get a life LOL).

Have a Merry Christmas!