Jetting question

[mrmach5]

This 75 XS had just straight thru short slash pipes on it when I got it, way too loud for my taste. I replaced them with the reverse cone mufflers from Mikes XS, these are about the same length as the factory pipes.
In riding it today lots of back fires -pops during deceleration . I pulled a plug and they are pretty dark , not wet but much darker than the optimum tan (recent new NGK plugs). I'm assuming they jetted this thing up for the straight pipes,,, now I'm guessing I need to undo this. Two questions, what was the stock jetting on a 75 and can you change jets with the carbs on the bike????
Thanks for any help.

 

[figure8]

Yes you are likely running rich. Don't assume your carbs are 75, they were interchangeable on all year xs'. Post a pic and someone can identify them. You could change jets with carbs on bike but they aren't hard to get off. Remove the large acorn nut on the cam chain tensioner, makes them much easier to get off. Here's a chart of stock jetting for all years as well as a link to the complete carb guide.
http://www.amckayltd.com/carbguide.pdf

 

[5twins]

Mains are easy to swap through the drain plug with a stubby screwdriver, carbs still on the bike, provided they weren't installed with a big old screwdriver as tight as they could go. The float bowl will need to come off to get at the pilot jets. This too can be done with the carbs still on the bike using a stubby screwdriver again.

 

[650Skull]

A 75 is a clone of a 74. Do a visual to see if the carbs are the same. 76 and later carbs were linked........rough idea to start

 

[mrmach5]

OK , Pulled the main jets out and they are 125's , the chart above says that it should have 127.5 . Just to make sure I'm not messed up here, a 127.5 jet would be larger and make it run even more rich correct?
I read thru the Carb guide and just not sure what my next move is. It accelerates good , a little a hesitation just off idle but not bad , then lots of popping under deceleration, which the carb guide says
" Popping from the exhaust on deceleration usually indicates leaness in the pilot circuit. A larger pilot or richer setting on the mix screws (sometimes both) is the usual cure." Yet my plugs are dark brown
Like I said, not sure what to try next,,,, lowering the needles???

 

[weekendrider]

Reading plug color isn't all that useful for pilot circuit evaluation? Have you checked pj size? Know where your pilot circuit mix screws are adjusted?

 

[5twins]

Not sure what to try next? How about positively I.D.ing your carb set? Looking at jetting specs is pointless until you do that. Yes, a 125 main would be one size smaller than stock for a '74-'75 carb set but it would be one size richer for a '76-'77 set. You have to make sure your float levels are set right too. Any jetting, no matter what size it is, won't work right if the floats are set wrong.

You should also check your needle. Many of the aftermarket rebuild kits contain a generic Y-22 needle that is totally wrong for any of the '74 and newer carbs. It's about 5mm too short so is very rich.

 

[mrmach5]

I have a 74 TX650 as well, and the carbs on this 75 are identical. I'll check float height first, then check on the needles.

 

[mrmach5]

I decided before that before running down a rabbit trail with the carbs, I would go back and handle all the basics first (I've been fooled before). So adjusted the Cam Chain, Valves, Points, Timing, and a new set of plugs. Adjusted mixture screws for high idle,,,,,, and test drove. Like a new bike,,,,, ran perfect and the popping on decel has disappeared.
Thanks for all the input!

 

[grizld1]

Congrats. You eliminated other potential sources of grief, which is exactly what needs to be done before trying to tune a carburetor.