another jetting question

[robywan_kenobi]

Thanks for all the help TopCat and tab 1324! I finally took the plunge this weekend and pulled the carbs off my bike. Followed all your advice.
1. Used the "bread tie" bench sync to sync the carbs.
2. Screwed idle mix screws in to a gentle seat and then back out three turns.
3. Adjusted the floats to a height of 23-24 mm
Bike is running better than ever (since I took ownership anyway). Very nice, smooth idle. Very smooth and quiet at 70+ mph. Seem to have lost a little power from last week, but still way better than before the carb cleaner episode mentioned at the beginning of this thread. I still have some muffled backfiring from the pipes at decellaration. Do you have a recommendation for main jet sizes? I am assuming that would be the next step. I believe the bike is stock other than the exhaust, but need to check all the records I got from the PO and make sure I don't see a jet upgrade in there. Anyway, thanks again for all your help. I was really hesitant about pulling those carbs off, but found it to be a lot easier than I had anticipated. The biggest issue was the South Texas heat - 96 degrees and about 70% humidity made for one hot afternoon in my driveway.

[TopCatGr58]

Hey there Rob,

Congrats on the job! The Clymers and Yamaha web site both quote the main jets should be 115 for #1-4 and 120 for #2-3. The book also said that about the 81Special, but when I took mine apart, I found 110's straight across!?

So...the muffled backfiring/popping sound COULD BE a sign of a lean condition, a leaky exhaust pipe/header gasket. Mine after I put the 4-1 pipes on has done that decel pop both before I put the Indy filters and had stock jets, and even after with my 3 sizes up increase.....I'll be putting new exhaust gaskets on soon, but if it doesn't go away, I won't be worried about it!

So...depending on what you find in sizes, the jetting guide says to go up 2 sizes for modified exhaust alone. 2 if Indy filters, but if combining both, then subtract 1 size from total=3 sizes. So.. since stock air box, I would think 2 sizes up would be a good place to go! If you find staggered, and want to keep it staggered, you can save some $ and buy only 2 jets, cause you can swap the 120's from #2-3 into #1-4 and then buy 125's for #2-3. If you find same size across all four, then just pick a size 5 points higher than what you have!

Stay cool, HTH!? T.C.