Carbed Fat Boy Issue
Thread Starter
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Stellar HDF Member
Joined: Mar 2009
Posts: 2,454
Likes: 58
From: Graysville, AL
Carbed Fat Boy Issue
Hello, I'm helping a buddy get his 2000 carbed Fat Boy up and running. The bike has the original carb, stage 1 air cleaner and Rinehart dual exhaust.
He said it has been running bad since he had a shop put on the exhaust and air cleaner.
I turned the mixture screw in till it seated then out 2 turns. I also idled the bike up a little. It cranked and idled perfect. I then turned the mixture screw back in until the engine got rough then back out till it ran smooth again. The throttle response off idle was great and did not hesitate. I took it out around the neighborhood and when under load it would sputter and hesitate. I pulled the bike back in and attempted to adjust the mixture screw. I turned the mixture screw all the way in until it seated and it had no effect on the engine.
I drained the tank and checked the in tank filter and it was clean.
I appreciate any help or suggestions.
He said it has been running bad since he had a shop put on the exhaust and air cleaner.
I turned the mixture screw in till it seated then out 2 turns. I also idled the bike up a little. It cranked and idled perfect. I then turned the mixture screw back in until the engine got rough then back out till it ran smooth again. The throttle response off idle was great and did not hesitate. I took it out around the neighborhood and when under load it would sputter and hesitate. I pulled the bike back in and attempted to adjust the mixture screw. I turned the mixture screw all the way in until it seated and it had no effect on the engine.
I drained the tank and checked the in tank filter and it was clean.
I appreciate any help or suggestions.
Intermediate
Joined: Sep 2015
Posts: 39
Likes: 2
From: Central Victoria
Sounds like it's running lean, if running ok at idle and at low throttle input.
The higher flow A/C and exhaust have increased the air flow so now you have to up the fuel as well. Just fiddling with the mixture screw won't be enough it seems. I'd say you need slightly bigger jets.
Start by replacing the pilot jet first and see if that fix's it. Go up only one size at a time, e.g. if the stock pilot jet is a #40 go to a #42. If stock is #42 go to a #45. Then go through the same process as you described with the mixture screw.
If you problem is only after 3/4 throttle then replace the main jet. Same thing here. Go up a little at a time.
If stock is a #185 go to a #190 etc. then adjust mixture screw again.
This should only be used as an initial adjustment. I would strongly recommend getting a Dyno tune on top of this as both over or under fueling can lead to excessive wear and engine failure. You mite get away with it being a little off for a while but really why risk it.
Good luck, let us know how it turns out.
The higher flow A/C and exhaust have increased the air flow so now you have to up the fuel as well. Just fiddling with the mixture screw won't be enough it seems. I'd say you need slightly bigger jets.
Start by replacing the pilot jet first and see if that fix's it. Go up only one size at a time, e.g. if the stock pilot jet is a #40 go to a #42. If stock is #42 go to a #45. Then go through the same process as you described with the mixture screw.
If you problem is only after 3/4 throttle then replace the main jet. Same thing here. Go up a little at a time.
If stock is a #185 go to a #190 etc. then adjust mixture screw again.
This should only be used as an initial adjustment. I would strongly recommend getting a Dyno tune on top of this as both over or under fueling can lead to excessive wear and engine failure. You mite get away with it being a little off for a while but really why risk it.
Good luck, let us know how it turns out.
Thread Starter
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Stellar HDF Member
Joined: Mar 2009
Posts: 2,454
Likes: 58
From: Graysville, AL
Just pulled the carb off and checked the jets.
45 Pilot Jet
190 Main Jet
So should I go one up on the Pilot? I also read on the forum that turning the mixture screw in with no effect is a sign that you need a smaller jet?
I plan on getting a few different sizes.
45 Pilot Jet
190 Main Jet
So should I go one up on the Pilot? I also read on the forum that turning the mixture screw in with no effect is a sign that you need a smaller jet?
I plan on getting a few different sizes.
Last edited by lhgdale; May 20, 2016 at 04:52 PM.
Elite HDF Member
Joined: Jul 2010
Posts: 3,830
Likes: 302
From: Lorain Ohio
sounds like you have plenty of jet size to me, but what do I know...you can try adjusting the needle or getting the sportster needle.
http://www.nightrider.com/biketech/hd_cv_mods.htm
http://www.nightrider.com/biketech/hd_cv_mods.htm
Intermediate
Joined: Sep 2015
Posts: 39
Likes: 2
From: Central Victoria
Yeah 45 and 190 sounds on the money for a healthy stage one engine.
My own build has 95" with cams, more comp and head work and I run a 48 and 195 in mine and it is tuned great. So I wouldn't think you need to go up in jet size.........
Maybe check the float levels are adjusted correctly and needle and seat mod is a good thing too. If you have too much fuel going in when you open the throttle it will bog down briefly then go pretty well as the revs build.
Running lean is the opposite. It'll run ok at low throttle then as you crack it open or the revs get much higher it with feel lacking in power.
Still can't stress enough the value of a good dyno tune.
Sometimes depending on pipes the tune is a compromise anyway. Like with my 2+2 big radius it has a dip in the torque curve kind of early then gets up and goes. Just have to live with it.
My own build has 95" with cams, more comp and head work and I run a 48 and 195 in mine and it is tuned great. So I wouldn't think you need to go up in jet size.........
Maybe check the float levels are adjusted correctly and needle and seat mod is a good thing too. If you have too much fuel going in when you open the throttle it will bog down briefly then go pretty well as the revs build.
Running lean is the opposite. It'll run ok at low throttle then as you crack it open or the revs get much higher it with feel lacking in power.
Still can't stress enough the value of a good dyno tune.
Sometimes depending on pipes the tune is a compromise anyway. Like with my 2+2 big radius it has a dip in the torque curve kind of early then gets up and goes. Just have to live with it.
Tourer
Joined: Dec 2015
Posts: 338
Likes: 82
From: England
Does the carb have an accelerator pump? Is it working?
Depending on how long the bike's been sitting, it might be worth actually removing the slow and main jet, and also the emulsifying tube and making sure all the holes and orifices are clean.
Does it run better on acceleration if you pull the choke on a little or tape up a portion of the air cleaner to restrict some of the air flow? This might help verify a lean condition but as already suggested, your jet sizes seem pretty decent.
Bear in mind that it might not be a fuel issue but getting the carb clean is a good place to start if the bikes not been run for a while.
If it has a VOES, make sure the hoses to it are in good condition.
Check that the plugs are clean.
Good luck.
Depending on how long the bike's been sitting, it might be worth actually removing the slow and main jet, and also the emulsifying tube and making sure all the holes and orifices are clean.
Does it run better on acceleration if you pull the choke on a little or tape up a portion of the air cleaner to restrict some of the air flow? This might help verify a lean condition but as already suggested, your jet sizes seem pretty decent.
Bear in mind that it might not be a fuel issue but getting the carb clean is a good place to start if the bikes not been run for a while.
If it has a VOES, make sure the hoses to it are in good condition.
Check that the plugs are clean.
Good luck.
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Road Warrior
Joined: Sep 2006
Posts: 1,570
Likes: 45
Main jet could be plugged up too. Time to take the carb off and replace all seals and clean out the carb.
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Thread Starter
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Stellar HDF Member
Joined: Mar 2009
Posts: 2,454
Likes: 58
From: Graysville, AL
Quick update, I drained the tank, cleaned the carb, purchased a jet kit with new needle from the dealer, new plugs and fresh gas. I dropped the main jet to a 180, upped the pilot to a 48 and the bike runs great now. I have a little more fine tuning but it's close. No spitting, sputtering or backfiring on decel. Thanks for all the help.
