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2008 BF 750i Weak? Fuel Pump

smutty112010-08-11 00:39:35 +0000 #1
Hey all. Looking for help with my newly purchased used Brute. After i first start it up it runs great. After 2-3 minutes when I attempt a WOT run I can only get about 25mph out of it and a bunch of backfiring. Here's the interesting thing. If I shut it down and then immediately turn it back on and attempt a WOT run it runs awesome again for 2-3 minutes. I can only think of a couple potential problems and was hoping for some input. It's as if the bike is running off the fuel that is being pressurized at ignition turn on and then not by the ECU. I can hear the fuel pump running when I turn the ignition to run. I don't have an FI light on the dash. It is snorked but I don't know how this would cause these symptoms. It runs great by cycling the ignition when it's both cold, warm, and hot (while the fan's running). I'm leaning towards fuel pump because of how the condition goes away. If it were injectors or some other sensor (TPS) i don't know that cycling the ignition would fix those. Does the ECU "learn" anything in the first couple minutes of being on every time or does it just give a certain timing and fuel map based strictly on sensor input. I know there's two different timing maps for low and high throttle, but again I don't think the TPS is the cause. Any ideas???? Thanks....and I'll get a sig up soon just joined yesterday and goin out to load her for a ride today, limp her around the woods.


hondarecoveryman2010-08-11 00:51:32 +0000 #2
It is a crack in the pump housing , causing a pressure drop , I had one in my shop doing the same exact thing , but you cant buy just a housing , you have to buy the whole assy.
smutty112010-08-11 00:55:56 +0000 #3
Thanks HRM....The machine's got 94 hrs on it. Any ideay what I can do to keep this from happening again? Is the crack something I can see on a visual inspection once I get the assembly out of the tank? I'm not challenging your suspicion, but would like to confirm it before I go throwing parts at it.
smutty112010-08-11 00:58:10 +0000 #4
The kick in the junk of it all was that there was no way I could have identified this when I test drove it in the parking lot of the mower repair shop it was for sale in front of before I bought it.
hondarecoveryman2010-08-11 02:25:31 +0000 #5
Yeah there was no way you would have noticed it in a parking lot

It is not something you can see, I heard fuel spraying back into the tank on the one I worked on , rode it untill it started messing up , removed the fuel cap (dumb luck) and thats when i heard it . I tried univesal pumps,fuel regulators and every combination of the 2 before buying a new pump
Rack High2010-08-11 01:30:08 +0000 #6
Smutty, 3 months ago I rolled mine in a raging river current. Long story short, my bike acted just like yours where it would run good out to 1/4 throttle but anything past that or quick WOT and it would bog down, backfire etc.. Turns out sand had gotten into the fuel tank, picked up by the fuel filter clogging the fuel line and injectors. After a complete purge, she's running great now. Just another possibility.
smutty112010-08-11 02:22:15 +0000 #7
Thanks for the input Rack...Based on both your and HRM's suggestions, I'll be pulling the fuel pickup out today for a filter cleaning and pump inspection. I'd like to get a WOT fuel pressure reading before and after but I'm not sure if it's worth the effort. Wish I had my own dyno.
smutty112010-08-11 03:35:18 +0000 #8
....and clogged injectors. How'd you fix that? Did you clean them yourself or send them off?
smutty112010-08-11 04:08:35 +0000 #9
Just looking for backup on eliminating another possibility. In the service manual it states that with certain sensors failed or out of range that the ECU would lock into the "low speed" map. If this were the case my FI light would be on correct?

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