what size jets should I use

[jaydett]

My carburetors have the factory jet sizes right now, and I am wondering what size I shoud go up to? I have removed the air boxes and installed uni pods and have replaced my mufflers with these http://www.dimecitycycles.com/chrome-16-inch-red-tip-shorty-slashcut-muffler.html.

So Main jets are #130 and the Pilot jets are #42.5. My needles are the Y22's but I am replacing them with the 4JN19-4. Any suggestions on what sizes to go to would be appreciated.

Thanks

 

[CDNTX650]

Only testing can get you close. Why are you replacing needles.....basically. don't.

 

[5twins]

The usual rule of thumb is one or two up on the pilots, one to three up on the mains. Every one of these responds a little differently to mods and jetting changes so, as mentioned, testing is the only way to find out for sure what is best. I would start one up on the pilots (#45) and two up on the mains (#135). I would also get and try 137.5 mains next. One of the first rules of jetting is that when you think you're right, try the size above and below to be sure. If the 135s are OK but the 137.5s are better, you may need to try 140s. If the 135s are no good (too rich), you would need to try 132.5s. The earlier carb sets like yours came set up richer from the factory. In fact, you should try the bike with stock jets first. You may not need too much of an increase, if any.

The "4JN19-4" needle you mentioned is actually a 4JN19. The "-4" at the end designates the clip slot it is run in stock, in this case slot #4 from the top. If you end up several sizes up on the mains, you will most likely need to lower (lean) the needle setting to clip slot #3.

 

[XSLeo]

If his carbs had the Y22 needles then they should be changed to the right needles.
With the JN419 needle I assume your carbs are the 70-73 carbs.
With your set up I might try one size up on the pilots, two sizes up on the mains.
The carb guide has a section on "Tuning For Mods" It may help.
www.amckayltd.com/carbguide.pdf
We can give you advice on what may work, but you have to let the bike tell you what it needs. Make one change at a time, test, make a change, test.
The tuning section will help. After you test you use the results to determine what's wrong and what to change to improve things. Doing more than one change at a time, and you can't tell what change made a difference.
Leo

 

[jaydett]

Thanks guys.
They are the original carbs on my 73. At one point I was able to get the bike started with few sprays of fuel from a spray bottle. It idled high for a couple minutes then idled down, great!. I had no problem getting it started this way. Then a week after the last time I had it running I could not get it to start. It would sputter and the odd time spit a mist of fuel out the carbs on the uni pod side and the bike smelled of raw fuel. After that I did replace the floats because it was hemorrhaging gas everytime I turned the petcocks on. With the new floats in it was no longer puking gas but I still could not get it to start. That's what brings me to thinking that it may be a jetting issue now.

 

[5twins]

Well, they'll usually still start if the jetting is off, just may not run well throughout the whole RPM range. Hard starting is more likely related to incorrect float levels or the chokes not working. The chokes draw their fuel supply through a tiny jet off to the side in the bottom of the float bowl. If the bowls ever get gummed up or dirty, this choke feed jet is one of the first things to get plugged. Once plugged, the choke gets no gas and doesn't work. Checking that they're clean is easy .....

 

[jaydett]

I'll take a look. Thanks 5twins

 

[5twins]

On your '73 carb set, there is only one choke and it is on the left carb. The right carb gets fed rich starting mix through a crossover tube. You will only need to look into the left float bowl. There is no fuel feed for the choke in the right bowl. Well, there is the small chamber off to the side and there is a choke jet, but there is no feed tube hanging down from the right carb body. The feed to the choke circuit is basically blocked off and non-functioning in the right carb.