91 Fatboy Backfire Issue
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1st Gear
Joined: Nov 2020
Posts: 5
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From: United states
91 Fatboy Backfire Issue
I picked up a 91’ Fatboy recently. It had sat a while. So I refurbished the carb cleaned it, etc. replaced plugs did the oil twice, drained fuel and refilled with cleaner and ran it for a few miles... well barely a few miles.
The entire time it was backfiring like crazy. Anytime I give it gas at all I get backfire and I’m getting some kind of misfire when it is revved at all.
Any thoughts on how to fix it? Is it something simple I’m missing or am I in for a long overhaul here?
Thanks in advance for your help!
Cheers!
The entire time it was backfiring like crazy. Anytime I give it gas at all I get backfire and I’m getting some kind of misfire when it is revved at all.
Any thoughts on how to fix it? Is it something simple I’m missing or am I in for a long overhaul here?
Thanks in advance for your help!
Cheers!
Road Warrior
Joined: Dec 2007
Posts: 1,286
Likes: 64
From: SoCal
Does the bike idle okay?
If it does and the carburetor has been properly refurbished, then I think maybe you have an ignition issue like a coil going away or maybe the ignition module or ignition pickup crapping out.
If it does and the carburetor has been properly refurbished, then I think maybe you have an ignition issue like a coil going away or maybe the ignition module or ignition pickup crapping out.
Seasoned HDF Member
Joined: Apr 2008
Posts: 23,053
Likes: 12,327
From: Arkansas
Check the intake for leaks. Both where it connects to the carb and the heads. Start the bike, let it idle and spray some WD 40 around the manifold. If the idle increases, you have a leak.
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1st Gear
Joined: Nov 2020
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From: United states
The bike idles decently... but I noticed a major leak after I started it last. So I am thinking maybe it is the float pin that got stuck?? Would it backfire like that is the float is off?
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Road Warrior
Joined: Dec 2007
Posts: 1,286
Likes: 64
From: SoCal
If the fuel leak is from the overflow then the air/fuel mix may be going fuel rich.
If the fuel leak is from a low point on the fuel bowl then the air/fuel mix may be going fuel lean but that would have to be a severe leak.
Generally, if the backfiring is a fuel mix issue, going fuel lean can cause the engine to backfire out of the carburetor and going fuel rich can cause the engine to backfire out of the exhaust.
If the leak is out the overflow then yes, I'd suspect there is an issue with the float/needle/seat or float adjustment barring a bad gasket or crack on the fuel bowl.
So fix the leak, make sure the float and fuel inlet (needle and seat) are working properly and adjusted.
If you still have a backfiring issue after fixing the leak it is possible that you have an ignition problem.
If the bike has a steady idle, which is to say it isn't sometimes high and sometimes low, then I would not think there is a vacuum leak.
If the fuel leak is from a low point on the fuel bowl then the air/fuel mix may be going fuel lean but that would have to be a severe leak.
Generally, if the backfiring is a fuel mix issue, going fuel lean can cause the engine to backfire out of the carburetor and going fuel rich can cause the engine to backfire out of the exhaust.
If the leak is out the overflow then yes, I'd suspect there is an issue with the float/needle/seat or float adjustment barring a bad gasket or crack on the fuel bowl.
So fix the leak, make sure the float and fuel inlet (needle and seat) are working properly and adjusted.
If you still have a backfiring issue after fixing the leak it is possible that you have an ignition problem.
If the bike has a steady idle, which is to say it isn't sometimes high and sometimes low, then I would not think there is a vacuum leak.
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- Full rebuild kit in the carb, including new float valve.
- Polish the float valve seat with a q-tip and some polishing compound.
- Carefully set float angle
- Replace stock inlet nipple with a solid brass nipple. (Call this one a "may as well")
- Replace carb to manifold gasket and manifold to head gaskets. (Don't bother trying to inspect them. Just assume they are shot and replace)
If you're problem is fuel system related and if the bike was properly tuned before you got it, that should get you there.
While you've got the carb apart, make note of your jet sizes.
- Polish the float valve seat with a q-tip and some polishing compound.
- Carefully set float angle
- Replace stock inlet nipple with a solid brass nipple. (Call this one a "may as well")
- Replace carb to manifold gasket and manifold to head gaskets. (Don't bother trying to inspect them. Just assume they are shot and replace)
If you're problem is fuel system related and if the bike was properly tuned before you got it, that should get you there.
While you've got the carb apart, make note of your jet sizes.
Grand HDF Member
Joined: Apr 2007
Posts: 3,728
Likes: 3,073
From: Brighton, MI
- Full rebuild kit in the carb, including new float valve.
- Polish the float valve seat with a q-tip and some polishing compound.
- Carefully set float angle
- Replace stock inlet nipple with a solid brass nipple. (Call this one a "may as well")
- Replace carb to manifold gasket and manifold to head gaskets. (Don't bother trying to inspect them. Just assume they are shot and replace)
If you're problem is fuel system related and if the bike was properly tuned before you got it, that should get you there.
While you've got the carb apart, make note of your jet sizes.
- Polish the float valve seat with a q-tip and some polishing compound.
- Carefully set float angle
- Replace stock inlet nipple with a solid brass nipple. (Call this one a "may as well")
- Replace carb to manifold gasket and manifold to head gaskets. (Don't bother trying to inspect them. Just assume they are shot and replace)
If you're problem is fuel system related and if the bike was properly tuned before you got it, that should get you there.
While you've got the carb apart, make note of your jet sizes.
Double check your work on cleaning out the ports and passages of the carb since you will have it apart anyways. Idle port and transfer ports in the throat of the carb are pretty small and easily blocked by gunk.
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