Backfire/Poor performance as gas level drops


13 replies to this topic
  • Kamp

Posted August 25, 2012 - 12:52 PM

#1

I recently purchased a used 07 WR450F from a dealership. From what I can tell, the original owner did all the free mods and used a GTYR kit. It came with a 2Bros M7 system which was to loud for me, so I got a FMF Powerbomb Q4 system. I also added a JD Fuel adjustment screw. I rode it around town for awhile setting the idle and fuel screw. It ran great, plenty of power, way quieter than the 2Bros M7, but a nice growl when you get after it.

Time for a long ride test. I head for the country, with a lot of higher speed cruising, got the bike good and warmed up. Around the 30-35 miles, I start noticing backfiring on decel, and slowing down to turn, it almost wants to die. I rode the bike with it performing that way until 60 miles, when I switch to reserve, and head for my farm to re-fuel. I get to the farm, fill it up with gas and head back for town. Now the bike is running great again, NO backfiring on decel, and doesn't want to die when slowing down.

Thinking back I had the same issue with the 2Bros system, as the first tank of fuel got low, it started backfiring on decel and ran like crap. I thought bad fuel or gunk build-up, so I took the carburetor apart for cleaning and a "get-to-know" my carb session. When the bike ran fine after cleaning and installing the Q4 system, I thought I had solved the problem. I didn't link the backfire/poor performance to an emptying gas tank until it happened the 2nd time, and is working great again on a full tank.

Anybody ever had this issue or have any ideas as to what I should be looking for? Specs are in my signature.

  • GreenHornet450

Posted August 25, 2012 - 07:21 PM

#2

One of the guys I ride with has the same year bike and he has been getting crappy gas millage. He has had several people look at the bike and knowone can figure out the problem either. The bike runs well but fuel range sucks. If you find out what the problem is please post. Thanks.

  • allterra

Posted August 25, 2012 - 08:45 PM

#3

I have an 07 WR also and am getting about 41 mpg I can't say if that is good or not. That includes 40% asphalt. I have an occasional pop when dropping th throttle from high rpm to riding on engine backpressure. I also have all the free mods and ais disabled. I have not seen any change in performance between main and reserve. I have yet to take the carb out so I don't know what the previous owner had installed. The exhaust is stock minus the pea shooter.


  • miweber929

Posted August 26, 2012 - 04:40 AM

#4

I think you are onto something (and did some good detective work to conclude that) but I don't see the fuel level being a possibility unless you have the carb float above the fuel level of the tank.

It's a gravity fed system.....I'd say completely drain your tank, clean and check over your petcock (and any aftermarket fuel filters you may have on it) to be sure all is right there and you don't have water in the tank or something weird that way.

If it all is fine, I think it the level may be more coincidence, if not then fix the issues. Does the bike ever stall? Have you tried opening the fuel cap to see if there is a venting issue with the cap? Next time it acts up open the cap: if you hear a whoosh of air your tank vent isn't venting.

Edited by miweber929, August 26, 2012 - 04:42 AM.


  • MANIAC998

Posted August 26, 2012 - 06:32 PM

#5

I agree with miweber929. I would think that there is some poor fuel, water, ethanol, etc. in the tank, which tends to sink to the bottom. Then when the fuel tank gets low, you suck up that crap. That's my guess anyhow!
Maniac

  • Kamp

Posted August 26, 2012 - 06:48 PM

#6

One of the guys I ride with has the same year bike and he has been getting crappy gas millage. He has had several people look at the bike and knowone can figure out the problem either. The bike runs well but fuel range sucks. If you find out what the problem is please post. Thanks.



I have an 07 WR also and am getting about 41 mpg I can't say if that is good or not. That includes 40% asphalt. I have an occasional pop when dropping th throttle from high rpm to riding on engine backpressure. I also have all the free mods and ais disabled. I have not seen any change in performance between main and reserve. I have yet to take the carb out so I don't know what the previous owner had installed. The exhaust is stock minus the pea shooter.


I am not concerned about my gas mileage at this time. The backfiring and poor performance starts happening at about a half a tank as near as I can tell and continues at reserve. From what I read on I believe, an Aussie forum, 120 km or 75 miles for US per stock fuel tank is normal. With a fully open air system, aftermarket header/muffler, 178 main jet (178 is a big hole-drinks a lot of fuel) and the way this beast performs when it is running right - I don't expect to be able to go too long of a distance. I haven't put enough miles on to know what I am getting, and never had it under stock specs so I don't know what good mileage would be.

It's a gravity fed system.....I'd say completely drain your tank, clean and check over your petcock (and any aftermarket fuel filters you may have on it) to be sure all is right there and you don't have water in the tank or something weird that way.

If it all is fine, I think it the level may be more coincidence, if not then fix the issues. Does the bike ever stall? Have you tried opening the fuel cap to see if there is a venting issue with the cap? Next time it acts up open the cap: if you hear a whoosh of air your tank vent isn't venting.



I was wondering if it is a totally gravity fed system, or if the accelerator pump or a vacuum within the carb could be causing problems. The bike starts and runs good on a full tank. It did stall that day, but that was after I started having problems. I did open the fuel cap, but don't recall hearing anything and it still ran poorly after I did. The fuel cap is a Sunline with short hose and must have a spill check valve because I can only get air to vent in to the tank but it does not seem to vent freely, I can feel some resistance. So I might try attaching a long open vent hose up and over the handlebar. Checking out the petcock was going to be my next step. Wouldn't water in the tank cause issues for the whole tank? I have pretty much emptied it 2 times now.

Thanks all for your replies, I will post if I get it figured out.

  • Taxgod4u

Posted August 26, 2012 - 08:58 PM

#7

A leaf (or other foreign object like plastic shavings from the mfg drilling the holes in the tank) floating around can block the petcock when the fuel levels drop enough for it to slosh to the bottom and get wrapped on the screen.

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  • MANIAC998

Posted August 27, 2012 - 03:28 AM

#8

Interesting problem, that's for sure! I don't think that even if there is any amount of vacuum inside the accelerator pump that there would be any way for it to have the effect that your talking about. As for the gas cap not venting properly, lots of us have had some issues with that! In my case, the engine heat would end up "pressurizing" the fuel tank since the vent wouldn't let the air escape, and this would then force more fuel into the carb and would flood the engine out.

Your going about this in a nice methodical procedure, so we will get this figured out at some point!!!
Maniac

  • miweber929

Posted August 27, 2012 - 04:01 AM

#9

In reading the follow ups, couple things come to mind:

- yeah, you will mix the gas and water back together with a new tank but it will eventually seperate again as the tank level drops and time elapses. Remove the tank and petcock and completely drain the system. Also drain the float bowl as well. Get anything old out ASAP before spending too much time on moving to other things.

- try the tank vent line. I had a small Sunline one on my CR250 that the PO put on and it failed pretty quickly. Might be your issue there.

- when you say open system referring to the airbox, what do you mean? A 178 is a hella big main jet and even with a no filter airbox and full exhaust I think you might be way rich. Are you sure you jetted correctly and not just tried this, tried that kind of jetted? Not questioning, just trying to get a baseline.

- all your carb vent lines in place, clean and routed correctly? Carb fully tuned (squirt in the right spot and time, linkage o-ringed)? Wondering if the carb is struggling for air after a while.

- is there a chance your elevation changes, you're fatigued more or some other element in a more empty tank? I know my bikes always feel like they run different when I'm tired at the end of the day, especially my CBR1000RR after a track day. Sometimes just the way an arm moves to rotate the throttle can be perceived as a bike issue. Again, not doubting but just trying to pin point something outside the box since this is a weird issue.

Edited by miweber929, August 27, 2012 - 04:02 AM.


  • Kamp

Posted August 27, 2012 - 01:12 PM

#10

...... As for the gas cap not venting properly, lots of us have had some issues with that! In my case, the engine heat would end up "pressurizing" the fuel tank since the vent wouldn't let the air escape, and this would then force more fuel into the carb and would flood the engine out.
Maniac


Going to see if you are on to something here. Called a dealer and he also mentioned the gas getting too hot. Makes a lot of sense being that I went from running terrible and backfiring to running good and absolutely no backfiring - with a tank full of "cool" gas. I got some 1/8" self-adhesive foam foil tape and fitted it all over the bottom of the tank where it wraps around the frame above the engine. Hopefully, it will insulate the tank and reflect some of the heat away.

In reading the follow ups, couple things come to mind:

- yeah, you will mix the gas and water back together with a new tank but it will eventually seperate again as the tank level drops and time elapses. Remove the tank and petcock and completely drain the system. Also drain the float bowl as well. Get anything old out ASAP before spending too much time on moving to other things.

- try the tank vent line. I had a small Sunline one on my CR250 that the PO put on and it failed pretty quickly. Might be your issue there.

- when you say open system referring to the airbox, what do you mean? A 178 is a hella big main jet and even with a no filter airbox and full exhaust I think you might be way rich. Are you sure you jetted correctly and not just tried this, tried that kind of jetted? Not questioning, just trying to get a baseline.


I completely emptied the tank, cleaned the petcock with carb cleaner. Didn't notice anything abnormal. Going to refill with all new gas and bought 2 feet of clear tubing to vent the gas cap.

Sorry, about my poor terminology. Snorkel was removed by PO. The jetting that is in the bike now (in my signature) is the way it came to me. I did try a FMF Power-up jet kit which consists of a Dynojet needle and 4 main jets that I ordered with the Q4. That kit was causing all kinds of sputtering and surging at mid to full throttle. I am a novice at jetting and didn't know if there was incompatibility with the Dynojet mains and the stock pilot and had no other jets to test, so I just went back to the way I got the bike. I need to order an assortment of mains and pilots to play around with, but like I said, when the bike is running good, I like the way it performs.

Edited by Kamp, August 27, 2012 - 01:14 PM.


  • MaxPower

Posted August 27, 2012 - 05:34 PM

#11

I'm really sure it's your little one way no spill thingie in your vent line.

  • Kamp

Posted September 17, 2012 - 09:52 PM

#12

Thought I would post an update after going on another 70 mile ride yesterday. Changes I have made since my last ride: Cleaned and re-oiled air filter - decreased main jet from 78S to Keihn 70 - decreased pilot jet from 50 to 48 - vented gas tank cap with clear/open tube, no spill check valve - temperature was at least 20 degrees cooler than my last ride - foam backed aluminum tape covering underside of fuel tank.

This time there was no sputtering or cutting out, although it still did want to die when slowing down to turn a corner after it was good and warmed up. But I don't think I had my idle and fuel screw set right at the time, played around with that quite a bit during the trip. Also after going hard the first 9 miles, I noticed that it was backfiring on deceleration and did so to a certain extent, until the last 9 miles. So when the gas was at the reserve level, and if heating of the gas was a problem, it still should have been backfiring, but I did not notice much backfiring on decel then. I filled my tank again, at the end of my ride, and on the way home there was no backfiring at all. The one thing that I am still suspicious of, is the Sunline tank cap. The id of the vent hole seems very small. How big is the vent on the OEM tank cap? Still need to do some playing around with the idle and fuel screw as they appear to be pretty sensitive after warming the bike up. Overall, I would say the bike performed better. If I get my idle and fuel screw figured out, I can probably live with the amount of backfiring that was happening. Thanks to all for sharing your thoughts.

  • MANIAC998

Posted September 18, 2012 - 01:47 PM

#13

I don't know what size the gas cap vent hole is, but I always drill mine out as large as possible, while still maintaining structural integrity. It gives me peace of mind knowing that plenty of air is making it into the tank to replace the fuel leaving.
Maniac

  • cjard

Posted September 22, 2012 - 12:31 AM

#14

One of the guys I ride with has the same year bike and he has been getting crappy gas millage. He has had several people look at the bike and knowone can figure out the problem either. The bike runs well but fuel range sucks. If you find out what the problem is please post. Thanks.


if i lean my bro's wr450 more than about 30 degrees away from vertical, fuel runs out of the carb overflow - havent had time to investigate yet but suspect float height. if the bike is running well, perhaps he should re-route the overflow pipes out of the side and on top of his boot - he'll find out pretty quick if stuff's running out of them - stands to reason that if fuel mileage is bad it's going somewhere - doesnt have to be the engine.. :)




 
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