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View Full Version : Sportster backfiring at 1/4 throttle



woz
12-20-2013, 11:42 PM
Ok so on the way back from chopperfest my bike died just as I was rolling into a buddies in irvine, it seemed like a low fuel type of thing, no big deal, chilled inside for a bit said good bye and sloshed some gas over and blipped to the gas station (still running sorta funny) Fill the tank and go to hot the road, still running like crap, backfiring out the exhaust pretty bad. Push it back to my buddies and crash there for the night. Next morning check the plus, they were kinda wet, compression was ok pull carb bowl, looked clean. Put it back together, fired it up and it was running fine again. Hit the road, about 45mins in it starts popping out the exhaust at like 1/4 throttle while I was just cruising on the free way. Full throttle it ran fine and pulled like normal, so for quite a ways I would just be on and off full throttle ( just needed to get to SD) it seemed to get worse, eventually I had to pull off in Encinitas and stopped at a gas station, fuel was kinda low and I topped off, still was running sorta crappy. Got on the freeway and it only was popping a few times when I got closer to home.

The other day I pulled the tank (it's band new, so is the petcock) and let all the gas run out just to see if it was getting starved of fuel or something, it drained fine, I also pulled the peacock and took it apart, all seemed fine. Pulled the top of the carb off and checked that too, diaphragm was good.

Any other ideas? I haven't ridden it since I had the tank off because I've been working so much but I dele like the issue will still be there as I didn't find anything wrong...

It's a 2010 sporty with a cv carb and thunderheart ignition (runs off crank sensor) this exact setup did over 2000miles in September, only issue was the ignition module overheating because I had it under the seat ontop of the oil bag, it was relocated mid trip and have had no issue since. The issue I did have with it overheating was a complete shut down, so I don't think it's that.

roadgremlin666
12-21-2013, 12:07 AM
check for air leaks around the carb and exhaust.

Nosebleed
12-21-2013, 12:10 AM
You ever replace the cps? How many mile on the bike? Cps tend to fail around 20k miles. 35 dollar part, 6 minute install.

Spartan1
12-21-2013, 6:31 AM
Could be the battery. I would start by throwing it on a charger for a day andcsee if it clears up.

Easyricer
12-21-2013, 6:55 AM
At 1/4 throttle the carb is running off of the pilot jet, transition ports and just starting on the needle. If all of those are fine, then I'd be more inclined to think ignition problem. The cam position sensor on these, go to crap pretty fast, and like mentioned are cheap enough. A vacume leak will affect the idle and other rpm levels as well.
At that same throttle position does the bike vibrate more than others? Could easily be the ground to the ignition module. Trace out your wires, check all of your grounds and wiggle your ignition switch. I've also seen a bunch of Sporty ignition switches vibrate themselves to death.
A challenging issue for sure. Can't always follow the symptoms, have to reproduce the fault. If the sparkplugs look normal, a backfire through the pipes is usually electrical. Unfired fuel charges building up in the pipe and firing on heat or the next spark. Lean conditions are usually coughing through the carb.
EASY

BrokenSprocketGarage
12-21-2013, 11:13 AM
OP - did you clean the jets? You stated that you only checked the bowl.

woz
12-29-2013, 1:39 PM
checked main but did not have a screw driver small enough to get the pilot out, i know i would be running on the pilot at that point but it ran fine for like 30 minutes (in cold weather)

I am thinking its the crank position sensor for two reason, the night i started having issues it would not start or idle for crap, it would backfire like crazy, and was for sure getting fuel, so that seems like an ignition issue. i was also reading up on people that had issues with crank position sensors and they all said it would happen after the bike would warm up and it wouldnt run for crap at low rpm/thorttle but would run fine up top, which seems like my issue. every time i stopped to fuck with it, it would run ok for a little bit then fuck up again (as it heated back up)

i think ill pick up a new sensor today.

stoneam2006
12-29-2013, 2:10 PM
I have a 2000 sportster and its gonna sound crazy but I had same problem almost to a tee but my coil went bad. Even tho spark looked normal and it ran the coils readings were ok and everything checked out tried everything from carbs to plugs and everything between. Had spare coil on shelf so put in and It fixed it idk how to explain it but I happened twice in 6-7 years both times same problem the coil....40$ at dealer

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woz
12-31-2013, 2:28 PM
We'll the sensor was not it. Going to try and get another coil and hope that's the cause, it only has like 3000miles on it!

roadgremlin666
12-31-2013, 2:35 PM
it isn't your coil,been wrenching on bikes for 17 years go look for vacuum leaks at the manifold and carb check your exhaust at the heads to see if they are tight.

stoneam2006
12-31-2013, 2:41 PM
All the respect ...road ...but that's exactly what every shop said too...I am young and don't have even close to the experience but I had same problems 2 times and that was the last thing I tried and both times that fixed it....call it a fluke but 40$ is a relatively cheap fix

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woz
12-31-2013, 7:25 PM
it isn't your coil,been wrenching on bikes for 17 years go look for vacuum leaks at the manifold and carb check your exhaust at the heads to see if they are tight.


Would that be something that would come and go though?

I had an identical coil on my shovel, tried it an that wasn't it (didn't see your post before I tried it out) so that can be ruled out...

Someone was saying it could be a stator issue possibly. As it heats up it isn't working properly? Doesn't really sound right to me though.

I will check for intake and exhaust leaks tomorrow, I have new manifold to head seals.

One thing I'll point out is I am running a 04-06 cv manifold on a 2010 sporty. The efi heads are slightly further apart but guys have made it work, and mine worked fine for 3000 miles, it coughs out of the carb but that never seemed to hurt much.

Stlmikie
12-31-2013, 8:22 PM
Isn't a carb cough indicative of an intake leak or lean condition sometimes?

roadgremlin666
12-31-2013, 9:42 PM
yeah it can come and go like that,I had a vacuum leak on my shovel and it fucked with my mid range and the same on my evo sportie and just fixed the vacuum leaks on my van last week that has a carb was fuckin up in mid range and back firing through the exhaust. to the point that i blew up my muffler.Go google how to check for intake and vacuum leaks.

BrokenSprocketGarage
01-01-2014, 8:43 AM
One thing I'll point out is I am running a 04-06 cv manifold on a 2010 sporty. The efi heads are slightly further apart but guys have made it work, and mine worked fine for 3000 miles, it coughs out of the carb but that never seemed to hurt much.

This is your problem. You are using the wrong intake flanges and manifold. Causing an intake leak. I have seen many intake leaks that only occur once the heads have gotten hot.
When ever you have a problem like this you must diagnose before throwing parts at it. A bad CPS would have intermittent spark. A plugged pilot jet would idle poor and improve when choke is on. An intake leak has the symptoms you describe and will suck carb clean through the flanges when sprayed on after warm up ride. Causing fluctuation in idle (usually stalls) when sprayed.
Is there a MAP sensor ( Manifold Air Pressure) in the Thunderheart Ignition? Sensor or mounting seal could be suspect...

bobscogin
01-01-2014, 9:05 AM
At 1/4 throttle the carb is running off of the pilot jet, transition ports and just starting on the needle.
EASY

Yup, true. I'm putting my money on a problem in the pilot circuit. If it runs good at full throttle, it's probably not ignition related.

Bob

woz
01-02-2014, 12:40 PM
No maps sensor, the port iwas tapped and plugged on the manifold.

I'll probably have time tomorrow to mess around and check for the intake leak.

I know it's the wrong manifold, but guys use it and it works. And it worked for me for 3000miles. There is a company that makes a wider manifold, but it's $200, would like to avoid spending that if possible.

woz
01-02-2014, 12:41 PM
Yup, true. I'm putting my money on a problem in the pilot circuit. If it runs good at full throttle, it's probably not ignition related.

Bob


Wouldn't the issue be more consistent though?

meatfarts
11-12-2014, 4:05 PM
I know I'm an ass for digging up this old thread but I'm experiencing similar issues with my bike as described by woz.
I have exhaust popping / misfire from rear cylinder at idle until warm and then it seems OK, however it occasionally just dies as if it's out of gas or vapor locked, which it is not and I've checked for intake leaks while bike is at every range of temp and my carb is spot on, no coughs, bogs or anything through the rev range... maybe it's the thunderheart ignition which is running on setting 3 (carb conversion),coil?,Valves?... Trying to tune the TH ignition as we speak, just waiting on a serial to usb adapter. Wondering what the fix was in this particular case. Thanks for any info in advance.