annoying backfire through carburetors
Thread Starter
Senior Member
SuperSport
Joined: Jul 2012
Posts: 582
From: Santa Fe, New Mexico
annoying backfire through carburetors
I've mentioned in other posts that my hawk has an annoying little stumble/backfire through the carburetors at about 3500 to 4000rpm in 1st and 2nd gear. I've pulled the filter and air box to look for anything that might be causing the issue. I checked for kinked fuel lines, kinked vent hose, kinked vacuum hose. No joy! I've not touched the air mixture screws so I didn't mess with that. The petcock is on all the way and I didn't find any air leaks, (I have removed PAIR equipment and installed block off plates). The stumble is more obvious when engine is cold. Recently, I ran the fuel all the way down, filled up and the stumble seems to have vanished. (I have gotten into the habit of topping off due to the small tank size and fear of running out of gas). While checking everything I discovered a small amount of fuel in the vacuum hoses installed to make balancing easier.
Is it normal for there to be a little fuel in the vacuum lines used for balancing? Both of mine have it. I understand the line coming from the fuel tank diaphragm that I "T"d into might have fuel in it but they both do. Do I have a problem?
Is it normal for there to be a little fuel in the vacuum lines used for balancing? Both of mine have it. I understand the line coming from the fuel tank diaphragm that I "T"d into might have fuel in it but they both do. Do I have a problem?
Senior Member
SuperSport
SuperSport
Joined: Aug 2007
Posts: 524
From: Tarpon Springs, FL
Not sure if that conditions normal or not, but my carb balancing lines have that too... I realized this when the caps I used to close the lines rotted due to not being fuel-resistant like the lines themselves were. So the caps rotted, and I was getting vacuum leak there on the ends of both lines. Switched to bolts with clamps on the ends and that stopped that.
Good luck... backfires suck and they scare the **** out of my girl when she was riding on back, lol.
Good luck... backfires suck and they scare the **** out of my girl when she was riding on back, lol.
Senior Member
Superstock
Joined: May 2012
Posts: 397
From: UK
Carb farts are a part of the personality of the Superhawk
I believe the majority of carb farts are caused by bad tuning. My bike suffered with these quite bad when I first got it. I found they disappeared after adjusting my TPS which also got rid of the idle speed fluctuating. I then gave the carbs a strip down clean and re-adjusted the mixture (fuel) screws and synced (balanced) them up. Everything has been fine since and the only time when I ever have them now is after messing around (experimenting) with my carbs and it generally comes back if my needle height is slightly lean.
Iv'e never had gas in my vacuum lines. That's a new one on me, not heard of that before.
(:-})
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MotoGP
Joined: Aug 2007
Posts: 5,942
From: Lake View Terrace, CA
It is hard to diagnose over the 'net.
Is it a "cough" (lean) or a "fart" (rich)?
If I had to guess I would say the needles need to be raised slightly ( 1 shim) and you have a lean cough situation as that is the RPM range when you just start coming onto the needles.
Is it a "cough" (lean) or a "fart" (rich)?
If I had to guess I would say the needles need to be raised slightly ( 1 shim) and you have a lean cough situation as that is the RPM range when you just start coming onto the needles.
Thread Starter
Senior Member
SuperSport
Joined: Jul 2012
Posts: 582
From: Santa Fe, New Mexico
if shimming the needles, would it be to both or one?
Last edited by twist; Jan 7, 2013 at 02:10 PM.
Thread Starter
Senior Member
SuperSport
Joined: Jul 2012
Posts: 582
From: Santa Fe, New Mexico
The balance screw takes a little getting used to because if I use any pressure to turn the screw, it affects the rpm and the settings won't be correct. Difficult little bugger to set, IMO.
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MotoGP
Joined: Aug 2007
Posts: 5,942
From: Lake View Terrace, CA
Thread Starter
Senior Member
SuperSport
Joined: Jul 2012
Posts: 582
From: Santa Fe, New Mexico
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