ANOTHER rejetting question, just want to be sure I have it right.

[knewsom]

OK, so I bought some HD Fatboy muffs, no baffles, and I'm putting in K&N pod style filters. I take it that I'm probably going to have to put in new main jets, and I want to make sure I get the right ones: SO, I looked in the tech tips section, and I THINK I've got the formula right, here's my math:

3 sizes for individual filters + 1 size for custom exhaust, -1 = 3. 3x2.5 = 7.5

137.5+7.5 = 145

Soooo, I should by size 145 main jets?? yes? no?

The impression that I get from reading is that my pilot jets don't need to be changed, because I can just adjust my idle mixture screws an extra turn out to compensate?

I found mainjets on mikesxs that I think will work, the part number is: Part #48-0123

...just checkin' with you gurus to make sure I'm doing the math right before I go for it.

[skids]

NO BAFFLES? Not to wish you any ill will, but you will likely have issues no matter how you jet it.

[knewsom]

well, now I'M baffled...

Originally posted by skids
NO BAFFLES? Not to wish you any ill will, but you will likely have issues no matter how you jet it.

wouldn't running muffs with no baffles be kinda like just running just straight headers? Haven't many folks done that without trouble?

what sorta issues?

[Harleychanic]

You need to provide the engine some amount of back pressure so as not to burn out the exhaust valves. Especially since you are running the pod style air filters. You are now creating a very low pressure free-flowing path for the intake and exhaust sides of the engine. You are essentially running a drag pipe setup. They are called drag pipes since the really only perform well on the "drag" strip. Plus the lack of back pressure makes it a PITA to tune for low and mid range throttle openings.

[knewsom]

ahhh, crap.

I guess I'll have to try and work up a baffle on those new muffs then.

[Harleychanic]

Are they the stock right from the factory mufflers or are they screamin' eagle mufflers or are they they one of the svo fatboy mufflers?

[knewsom]

I believe they're stock from the factory FB mufflers with baffles removed.

[Harleychanic]

If the baffle has been removed then you will be able to see directly throught the pipe, if not then the way to verify this is to look at the end and see if the weld has been ground out and the baffle removed. The factory mufflers have the baffles welded in premenantly on the newer models necessitating buying different mufflers for a more aggressive sound.

[knewsom]

Cheers, Harley - I'll take a close look at 'em when they get here on Monday or tuesday.

If the baffle WAS removed, do you have any suggestions for replacing it, or substituting a baffle on the end of the pipe for it?

Should I treat this as "custom 4 into 2 exhaust" for the purposes of jetting?

[knewsom]

Soo, I'm either going to weld in a new baffle or two on the new harley muffs ( if indeed I can see straight through it), or I'll knock out a baffle or two from my rusty stock exhaust.

Either way, I think rejetting may be in order..? Am I right with my guess of 145? Should I just GO for it and hope for the best??

Thanks,
Kris

[TopCatGr58]

Hey Kris,

The 145's should be pretty good. You also will have some latitude with the float heights, and also the vacuum needle, so you can go even richer without having to rejet again!
T.C.

[knewsom]

Hey TC, thanks much! I wanted to make sure I was on the right path before I bought new jets.


Cheers,
Kris

[Geezer]

Hogwash! Stock headers with no mufflers at all are long enough to prevent back breathing and valve warping. It's only a problem when you run the engine with no pipes at all.

If you can get away with the noise and you like the sound, go for it.

I used to run my Guzzi with blown out Dunstall mufflers. When they worked they were were a muffler in name only. After the baffles blew out, they were open megaphones. I had the loudest bike in Astoria, Oregon and I'm sure much of the town was happy when I left (or rather the Guzzi left.)

Geezer

[knewsom]

Originally posted by Geezer
Hogwash! Stock headers with no mufflers at all are long enough to prevent back breathing and valve warping. It's only a problem when you run the engine with no pipes at all.

If you can get away with the noise and you like the sound, go for it.

I used to run my Guzzi with blown out Dunstall mufflers. When they worked they were were a muffler in name only. After the baffles blew out, they were open megaphones. I had the loudest bike in Astoria, Oregon and I'm sure much of the town was happy when I left (or rather the Guzzi left.)

Geezer

I can, and I do, so I think I will.

remember, loud pipes save lives.

[Geezer]

So long as it doesn't wistle like a half burnt out VW bug exhaust, go for it. Just be sure to get the jetting right.

Actually you only need a little less than a foot of straight pipe to prevent backbreathing. Ever notice the lengh of the pipes on a drag racer???

Geezer