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?

:squid:

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 shit out of my girl when she was riding on back, lol.

I wouldn't say it's part of the personality but is a common thing amongst them.

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.

(:-})

Usually, if you get gas in that line, it's a sign that the petcock is on it's way out... But yeah, on occasion, you'd get it anyways...

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.

It did present itself after I did the PAIR removal. I blocked off the reed valves with block off plates after turning the reed valve over so it's held closed, I plugged the vacuum line left over on the front carburetor, plugged the air box hole left over from the PAIR removal and gone over the whole thing to be sure I didn't miss something but still could have, I guess. The only settings I've made are to the idle and balancing the carbs. I have thought about the TPS but I set that right after I got the bike. Is it possible that with the addition of the two brothers cans and PAIR removal the air mixture needs adjustment? The symptom shows up at 4000rpm when I keep the throttle at that position after about a minute or so. It could be that it's always done it but now that I am familiar with the bike I notice it. Could be that it showed up after modifications.:mad:

if shimming the needles, would it be to both or one?

After reading another post I am pondering the idea of me not having the sync exact. I thought I had it but maybe it's worth looking at again.

Is it a CA bike and did you also remove the evap stuff?

Just asking because if you did there should be 3 fittings on the carbs that should be plugged.

no, I got lucky. The bike came from Tennessee so the extra stuff was at a minimum. One vacuum line at the front carb, the PAIR system and a hole in the bottom of the air box.

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.

So close on the size......7mm is the winner but that set up is how I also adjust mine ;)

thank you! Since it's raining and I can't ride I'll be adjusting again to see if that takes care of the problem.

Thank you all again for the great advice!