Emfour Posted August 1, 2018 Share Posted August 1, 2018 My R850R has a quirk. If the fuel levels falls below about half tank, it bogs and almost dies when I takeoff hard in from a start. It's fine having the throttle whacked open hard at all other times. The previous owner looks to have messed with the tank. Judging from his other workmanship, he screwed up something stupid. Any input? Link to comment
dirtrider Posted August 1, 2018 Share Posted August 1, 2018 My R850R has a quirk. If the fuel levels falls below about half tank, it bogs and almost dies when I takeoff hard in from a start. It's fine having the throttle whacked open hard at all other times. The previous owner looks to have messed with the tank. Judging from his other workmanship, he screwed up something stupid. Any input? Morning Emfour That is going to be difficult to diagnose over the internet. The first thing that comes to mind is that the fuel pump is not installed at the correct angle or someone bent the internal fuel pump mounting when trying to forcibly re-install it. So possibly now the pump pick-up isn't located near the bottom of the fuel tank. Your 850 is quite a bit different then the RT series as the fuel tank is shaped differently. Is your fuel gauge reading correctly?????? Basically is half-a-tank reading REALLY 1/2 full??? About all I can suggest is to remove the fuel tank, then MARK the fuel pump pass through for currently installed clocking, then hold the pump pass through/pump assembly at the same clocking & see if the pump pickup would be at the bottom of the tank. Link to comment
Michaelr11 Posted August 1, 2018 Share Posted August 1, 2018 Emfour When your fuel level drops to this half-tank level open the gas cap. Do you get a "whoosh", rush of air entering the tank? If you do, your tank venting is blocked. Link to comment
Emfour Posted August 1, 2018 Author Share Posted August 1, 2018 ........ The first thing that comes to mind is that the fuel pump is not installed at the correct angle or someone bent the internal fuel pump mounting when trying to forcibly re-install it. So possibly now the pump pick-up isn't located near the bottom of the fuel tank. Your 850 is quite a bit different then the RT series as the fuel tank is shaped differently. Is your fuel gauge reading correctly?????? Basically is half-a-tank reading REALLY 1/2 full??? About all I can suggest is to remove the fuel tank, then MARK the fuel pump pass through for currently installed clocking, then hold the pump pass through/pump assembly at the same clocking & see if the pump pickup would be at the bottom of the tank. No fuel gauge on this bike. No whoosh either. Had a GPZ 750 that came from the factory with a blocked vent. I put in about 3 gallons of gas last night, no bog. This answer sound like a good place to start. Thanks for the insight. Link to comment
dirtrider Posted August 1, 2018 Share Posted August 1, 2018 ........ The first thing that comes to mind is that the fuel pump is not installed at the correct angle or someone bent the internal fuel pump mounting when trying to forcibly re-install it. So possibly now the pump pick-up isn't located near the bottom of the fuel tank. Your 850 is quite a bit different then the RT series as the fuel tank is shaped differently. Is your fuel gauge reading correctly?????? Basically is half-a-tank reading REALLY 1/2 full??? About all I can suggest is to remove the fuel tank, then MARK the fuel pump pass through for currently installed clocking, then hold the pump pass through/pump assembly at the same clocking & see if the pump pickup would be at the bottom of the tank. No fuel gauge on this bike. No whoosh either. Had a GPZ 750 that came from the factory with a blocked vent. I put in about 3 gallons of gas last night, no bog. This answer sound like a good place to start. Thanks for the insight. Afternoon Emfour Do you have a low fuel light? With no reserve position petcock it needs some sort of low fuel warning. A blocked vent on high pressure fuel injected bike acts differently than on a gravity feed system. The BMW uses a high pressure pump inside the tank that is capable of pumping well over 40 psi OUT of the inside of the tank. As the pump is inside the tank the inlet side (pump intake) isn't as big of a deal like on an external pump set-up that is trying to suck fuel out of a negative pressure area. A blocked tank vent on a high pressure in-tank-pump system can actually suck the sides of the tank in. Problems with blocked tank vents on the high pressure BMW boxer bikes usually starts with more light throttle surging & worse steady throttle (fueling on the o2 sensor) leaner operational areas. Link to comment
Emfour Posted August 1, 2018 Author Share Posted August 1, 2018 Just have the low fuel light, which has never come on. Haven't noticed any tank deformation. I'm going to pull the tank and check the pump and vents. The previous owner lost all the trim screws as well as the tank mounting bolt itself. Link to comment
dave_a Posted August 2, 2018 Share Posted August 2, 2018 My bike had a bog under acceleration when fuel was low. Lower than 1/2 tank however. Previous owner had replaced filter and fuel pump with unknown sourced items. No filter sock or hose to extend the fuel pick up down. Also had large chips of the tank primer paint (99 R1100R) floating around. Beemer Boneyard replacement pump, filter, hoses, and a good clean out of the tank insides - never happened again. Been down below a gallon left. no bog. FWIW / Dave Link to comment
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