I just put a Sportster muffler on my bike and rejetted the carb.  I am using a 144 main jet, the stock pilot, and have used the dynojet needle with about a 2/3rds spacer.  The bike runs great and I have no more backfires, but when I am "coasting" down a hill (meaning in gear but giving the throttle no gas) I hear sputters and little pops coming out of the pipe.  Now I am wondering if this is normal or something I have to work on fixing?  Its not obnoxious but I never noticed it with the stock pipe.

mine both sing pretty good on decl - no bangs or anything

By the way the dynojet needle has several groves right? - shouldn't need to sand the spacer down

Welcome to Thumper country.

All thumpers tend to rack off and pop to some extent on a decel.  Just the nature of a big single.  As Jim said, it is a serenade song to thumper lovers.  Mine makes all kinds of ruckus when I'm coastin'.  I love it

If it is running good and isn't backfiring or belching out black smoke you are probably just fine.

Just remember, "Suck, Squeeze, Bang, Blow".  That's as good as it gets

I live in Florida so probably I am at 0 feet above sea level.  I've read that with the dynojet needle you don't need to shave the spacer, but since mine was only shaved down only about 1/3 of the way, it fell in between two "grooves," making it only slightly deeper than what would be a half-spacer with the stock needle.  Since it was running great I didn't change it but kept the original needle just in case I need to go back.  I might be running a bit rich b.c. the muff has deposited a bit of black soot on my rear brake arm over the past three weeks, but the smoke itself comes out looking clear.

I am getting a shipment of jets in soon and thinking of going up to 150 for the main and maybe a 52.5 for the pilot (the one with the holes).   What do you all think?  Should I go back to the original needle and not use the dynojet?

It sounds like it is running fine and so maybe I shouldn't tinker with it.  I will enjoy the sweet music of the exhaust now rather than worry!    Here are the jets I ordered that I may experiment with:

MIKUNI PILOT JET BS30/96 #52.5
MIKUNI PILOT JET BS30/96 #55
MIKUNI PILOT JET N151.067 #52.5
MIKUNI MAIN JET N100.604 #150 AFTERMARKET
MIKUNI MAIN JET N100.604 #152.5 AFTERMARKET
MIKUNI MAIN JET N100.604 #155 AFTERMARKET

mau2wu wrote on 12/31/69 at 16:00:04:


With the Dynojet kit, the needle is completely different than stock...It isn't just the clip positions (which do what the spacer change does), but the taper of the needle is different too.  

You might notice that the main jet doesn't have the same numbering sequence as stock.  As well, the main jet and the needle work together.  Changing one does affect the other.

Since you invested in the Dynojet kit, I don't know if (or why) you want to bother with the other jets.  You should be able to just ride it now....

Here is a nice little web site for carb theory....

Carbs

I just checked the plug and it was sooty.  So I replaced the dynojet needle with my old needle and the shaved spacer.  Put in a new plug.  Then I did the carb tuning thing again, around 1 and 3/4 turns out or so.  Then I ran around the neighborhood a few times in mostly the mid throttle range in 2nd and 3rd.  Kind of loud but oh well.  Popped the seat and tank off again, looked at the plug.  This time it came back with a thin film of light grey.  Is that the color I am looking for?  Lesson learned from this--don't use the dynojet needle with a shaved spacer.  I think I have to fine tune it a little more.

Mine Kapops after shifting and upon deceleration sometimes. I think it give the machine character.
Don't worry about it.

Hey all, here is some good information on the sputtering/popping issue.  http://www.mikuni.com/fs-tuning_guide.html  

Basically it says it is normal to have moderate popping on closing the throttle hard.  This only happens when the exhaust is opened up with a free flowing muffler.  This would explain why I didn't notice the popping/sputtering until I put the Sportster muffler on the bike.

My bike is now running real good--some weeks ago I took it on its first road trip of over 600 miles.  I really enjoyed passing all those Harleys and found that the bike will do 75+ quite easily.  It actually shakes less than my other vehicle at that speed--a Geo Prism.

I found that checking the plug is a pain but necessary to really find out what is going on in the engine until it is tuned right.  I now am running a 150 main and half spacer with the stock needle.  In the end, the dynojet kit was a waste of money.