Fuel in airbox

Whenever my 650 is pointed uphill, even at only moderate slope angles, overflow fuel from the float bowl flows back into the airbox and out the airbox drain. My overflow vents are not plugged, and I can't find any other problem. Has anyone else had this difficulty? Any suggestions for solving the problem, possibly through re-routing the vent hoses or something?

I am sorry I can't offer you any advice, but I would like to offer you a warning. The first time I met one of my buddies he was on his BRP trying a hill climb. He killed it on the hill and gas poured into his airbox. After getting the bike down to a flat spot he tried to get it started. We heard a backfire then he was yelling. Seems he lit a puddle of gas beneath his bike :) Of course we all cheered and laughed :D, but this could have been very serious. He ended up frying all of the vent hoses, but otherwise was unharmed. He soon learned that the first move after stalling on a hill is to reach down and shut the gas off.


The only thing I can think of is that for some reason your floats are to high. You might want to bend them down a little or check the pin to make sure it is not off set and moving freely :)

Dave @ www.winnersedgeracing.com

Could the carb be slightly leaning to one side? Could the float have fuel in it and be a defective one? I bet a call to one of the supporters here will result in a possible solution. Float hieght is number one on the check list.

My float height is correct (stock anyway). Maybe lowering it below stock specs :D? Also, in my research I have found that this is a common problem with the 650. Most others only mention uphills, but I also find that the machine loads up and floods on downhills as well. The only way to keep it from dying on downhills is by either blipping the throttle or just shutting off the engine. There must be a cure for this.

I have also had personal experience with fire in my airbox and airbox drain (last year) :). More careful starting technique when flooded has fortunately made it a one time event (reduced backfires).

Thanks for any advice!

Being that the bike is new, the dealer must have heard of this before. I'd call a few, and see what they have to say.

I lowered the float height on my XRR and it has mostly solved this problem for me. You want to make sure you have LESS fuel in the float bowl. I think this means the stock 16MM setting should be closer to 17MM. Correct me if I am wrong, but my friend at the dealership modded mine for me, I simply brought the carb to him. Took him less than a minute to do, he just bent the tangs by the float pin.

When restating your bike after flooding, ALWAYS keep the ignition OFF until you are ready to kick-start, just past TDC. If you don't you will get MASSIVE back-fires! Keeping the ignition off until you are totally ready prevents these problems, I have found. :)

I'll try your suggestions. I agree that lowering the fuel level is logical next step. Any opinions on how far max would be? I'll start with a 1 mm adjustment, but am curious what the limit might consist of.

The only downside to lower your float level TOO much would be starving the engine of fuel at sustained high speed, high rpm's. What happens is if the float level is too low, the engine will actually empty the float bowl completely, that would mean the engine cutting out, etc. I would say 1MM is a good start. It worked for me, but the bike still sputters when up a hill and you are sitting idling, trying to get going again. I think it may be a trait of the bike. Not a big deal, really. It doesn't puke out gas quite as bad a before, and it doesn't stall out at high rpm's either.

Be careful how you make the adjustment, it can be a little confusing. If you aren;t sure what to do, best get some advice from your local mechanic buddy first. It really helped when I was shown how to check float level firsthand.


Are there alternate carbs that can be used to rid yourself of this problem? Is there a block plate that can be made to keep the fuel from draining back into the air filter area when climbing a hill? Something to prevent slosh-back, and bowl drain. It seems like a safety issue with the manufactuer. I can't believe that Honda hasn't come up with a "Product Update" (a.k.a. Recall) on that item. If enough people have the same problem, and inform the dealers about it, a recall would go into effect. I'd get on the horn to Honda and tell them of this problem. I've worked in car dealer service departments before, and there should be a factory fix for it.

Corey forgot to inform you that he worked at Pedro's Thrifty Auto Repair, not an actual dealership. Pedro's was really a chop shop in California and that is where Corey's prison sentence began. Most recently, Corey has been employed with the state of California wearing an orange vest, riding around in a big white van with all of his soapy prison bitches. If you see a crew running weed eaters on the side of the highway stop by and say hi. :)

O.K. you got me. Car Dealers is code for Drug Dealers. I'm not employed by the state, but a resident courtesy of the state. Hey, I don't get to use power equipment! ("Shaking the bush Boss, I'm shaking the bush!") Ryan, how come you are jumping on the "Pick on the Fat Kid" bandwagon? :)

Hey thanks for your advice. Making the float adjustment is easy to do, so I'll lower the fuel level and see if that can help. I won't have any results until the weekend.

Also, I don't have the info yet for valve adjustments. Does anyone know what the clearance is supposed to be :)? It's about time to give them a look.

I swear I have never heard of this problem that you mention. I have ridden my XR up and down and all over Colorado for 2 summers now. Big, Huge, Steep Mountains! I've never had fuel flowing back like that at all. In fact, I've never had the overheating that I hear about also. I've never had any kind of mechanical problem to be quite honest.

So.. Sounds like someone, or something, jacked with your float height or something. That's the only thing I can think of causing something like that. Go to your Honda shop, and order a Honda Shop Manual for your bike. You'll love it. It's got every single thing you need to know about everything about the bike.


Isn't there an overflow tube for the carb bowl? If there is too much fuel in the carb, it should run out that overflow, and not back into the air cleaner area, right? Just a thought. This puzzles me. :)

Ryan, you'll get yours, regaurdless of your childhood problems! Can you say counseling? :D

Yes, there are vents for both sides of the carburetor, like other XR's. The only difference that I can see is that the 650's vents tee at the output, with one tube going down as usual and the other looping back over the top of the carb (to prevent siphon, I think). Your point is my major conundrum with this whole deal: even if I lower the fuel level, it still doesn't explain why the vents don't function properly :). Again, no, they are not plugged.

I can't work on it until Friday, but hopefully then some explanation will present itself.

This is my last thought here, and then I'll let it sit a while. I can visualize the float part by limiting the amount of fuel in the bowl, and therefore possibly limiting that amount of fuel being dumped back into the air filter area. What is routed to the airbox in the way of a fuel line? Is the fuel running from the carb itself in the path of the air box to the carb, or is it via a breather line? Someone has to know why!

A couple of years ago while riding with some pals in Tennessee the needle valve in my BRP malfuntioned. Anytime the bike was not running fuel would pour out the overflow tube. It was as if there was no float at all! The area we were riding in was some of the most extreme up and down you can imagine riding in. There was no fuel in the airbox and the bike ran ok - it was a bitch to start but ran ok.

My guess is your vent lines are restricted. My bike had no control of the float height and didn't put fuel in the air box. I fixed the carb by lifting it off the bike and putting a TM40 pumper in it's place.

One other thing to consider - my side cover is vented and the exhaust was wide open, no resrictions to breathing.

I could feel the bike pulling down on the seat while revving the motor off the bike and saw that the Honda race BRP's had that cover vented. Less vacuum at the inlet of the carb may help, (can't hurt).

One more reason why I dropped the cash for a EB quick silver. It doesn't stall on uphill climbs, it never dumps gas. It always starts first kick after dumping it etc etc... I usually skip all the jet, needle, starting problems, etccc posts cause they don't apply after you get the EB QS. This carb is the best, seriously the best product you can buy for this pig. It solves all the problems that you all keep writing about.

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