Fuel Starvation woes

[Rick S]

Will try to give you all the necessary info without too much:

My 68' Scout 800 304 runs great tooling around the mountain at low speeds where I live (9500') - a joy ride to my buddies house down the mountain today at ~7500' turned into an all day troubleshooting marathon. ONLY on hard pulls on grades and over ~30MPH I get a lean fueling surge. After eliminating everything else we determined that the fuel bowl was emptying on the hard pulls. We checked fuel level in the bowl running on flat roads and at idle. I keyed off and immediately pulled over and removed the top of the 2100 and the bowl was nice and full (didn't measure it). When pulling the hill and it started to die/buck, I again immediately keyed off and pulled over and the bowl was darn near empty (I'd say 1/4"-1/2"from the bottom?). This happened same hill on our test runs every time like clockwork. What's bugging me is that I know Scoutman uses a pump with even a bit less pressure on a 345 with no issues and he lives in the same area/same hills.

New Motocraft 2100 carb - everything perfect - float not sticking - no vacuum leaks
New good quality fuel lines with no obstructions
Well vented tanks (even removed caps to completely rule out vapor lock)
New fuel filter
New fuel pump(s) even temporarily ran pwr & grnd directly to batt to rule out any voltage or ground issues
Plenty of crystal clear fuel in both tanks
Fuel selector valve operating properly


Had a new E8016 Fuel Pump 2.5-4psi Electric Fuel Pump which I believe has 30GPH output. Then I purchased a Napa (Carter) P74021 3.5-5psi 70gph fuel pump and it still starved/emptied the bowl at the same point on the grade. Surely someone living near steep grades has experienced this before. The only thing we can think of is that the fuel pump simply does not have enough flow - I just took the back roads home (In the damn rain with no top ) -Steep grades the ENTIRE way at ~20mph with no stumbles/stalls. It only seems to happen above 30mph pulling grades. Any other suggestions..?

[Bill usn-1]

I didn't realize you were posting on both boards.
I just replied on the other one.

Here is a link to the tank write up.

[Jay Tabor]

does this happen when sucking fuel out of both tanks or just one?
wheres the fuel pump in relation to the tanks?

could be the fuel pick up location in a/the tanks, they get somewhat uncovered by fuel when on the angled roads. that shouldnt be the problem if the pickup tubes are stock in stock location.
maybe the fuel pick up socks have a lot of dirt stuck to them.
I never had any trouble with fuel starvation on my 800 with same engine, 3 tanks, steep grades.[ considering the fuel pickuptube size]

[Rick S]

Nice set up Bill

This happened on both tanks and both tanks were over half full. The stock pick up tubes are in place and in great shape and I never saw socks on the ends of them. Both tanks are from AZ and I cleaned them very well when I installed them. The fuel pump is mounted within 1 ft of the fuel selector under the driver seat area. The tank selector seems to be operating correctly - the next thing I plan on trying is running the pump directly from a can of fresh fuel on a grade just to eliminate anything from the pump back - frustrating.

[Jay Tabor]

theres a restriction somewhere, maybe at the selector valve?
easy enough to check is stick the end of the fuel line that attaches to the carb in a bucket, turn on fuel pump, and time its flow and quantity into that bucket for several minutes.
see if the pump flows the fuel as per its specs, try from each tank.
I thought I remember my fuel pick ups at the bottom center of each saddle tank on my scout 800. not coming out of the tops of tanks . .been so long since it went to its grave I dont know..
if its a long pick up tube hanging down into the tank[s] wonder if the gas is cavitating getting sucked up the tube?
only other thing to check is carb-

[Bill Bennett]

Can you add a fuel pressure gauge at the carb fuel inlet to show the fuel pressure while the engine is running under the same conditions where you are having the problem? If the pump is electric, it may need to be closer to the tank. Electric pumps are 'pushers', not 'pullers', so if you seem to have fuel starvation issues on grades, the pump location may need to be changed. How about going back to a 'stock' fuel pump?

[Will Marsh]

Originally Posted by Rick S

Will try to give you all the necessary info without too much:

My 68' Scout 800 304 runs great tooling around the mountain at low speeds where I live (9500') - a joy ride to my buddies house down the mountain today at ~7500' turned into an all day troubleshooting marathon. ONLY on hard pulls on grades and over ~30MPH I get a lean fueling surge. After eliminating everything else we determined that the fuel bowl was emptying on the hard pulls. We checked fuel level in the bowl running on flat roads and at idle. I keyed off and immediately pulled over and removed the top of the 2100 and the bowl was nice and full (didn't measure it). When pulling the hill and it started to die/buck, I again immediately keyed off and pulled over and the bowl was darn near empty (I'd say 1/4"-1/2"from the bottom?). This happened same hill on our test runs every time like clockwork. What's bugging me is that I know Scoutman uses a pump with even a bit less pressure on a 345 with no issues and he lives in the same area/same hills.

New Motocraft 2100 carb - everything perfect - float not sticking - no vacuum leaks
New good quality fuel lines with no obstructions
Well vented tanks (even removed caps to completely rule out vapor lock)
New fuel filter
New fuel pump(s) even temporarily ran pwr & grnd directly to batt to rule out any voltage or ground issues
Plenty of crystal clear fuel in both tanks
Fuel selector valve operating properly


Had a new E8016 Fuel Pump 2.5-4psi Electric Fuel Pump which I believe has 30GPH output. Then I purchased a Napa (Carter) P74021 3.5-5psi 70gph fuel pump and it still starved/emptied the bowl at the same point on the grade. Surely someone living near steep grades has experienced this before. The only thing we can think of is that the fuel pump simply does not have enough flow - I just took the back roads home (In the damn rain with no top ) -Steep grades the ENTIRE way at ~20mph with no stumbles/stalls. It only seems to happen above 30mph pulling grades. Any other suggestions..?

I had this problem with my 800 trail rig and the mechanical pump, turned out it was sucking air due to a missing hose clamp! It ran fine on the level or nose down, but starved for fuel going uphill. I'll bet you're sucking air somewhere on the suction side of the pump. That it does it on both tanks makes me suspect the valve or it's connections.

Check the hoses at the tanks for cracks and make sure the clamps are tight. And any other hoses on the suction side of the pump. Make sure the flared hard line connections are tight,and if that fails remove the valve and check the o-ring on the shaft.

HTH,

Will Marsh

[Rick S]

Just ran a little experiment and narrowed the issue down a bit more. I ran quality 5/16" rubber fuel line directly from the electric pump/filter inlet to a gas can of fresh fuel and went for a drive to a loooong uphill grade near my house and the lean fuel surge occurred again after less than 1/8 mile. This rules out the selector valve, tanks, fuel pick up, etc.

Next I will try running a new temporary 5/16" line from the outlet of the electric pump directly to the carb inlet. If it still acts up either the pump volume isn't high enough (which seems to go against what almost everyone has said - and getting two bad pumps right out of the box would be extremely unlikely) or there is some sort of adjustment that can be made on the carb. It will have to wait until next weekend though... Thanks for the replies

[Will Marsh]

This is a really dumb question, but since you said you have a MC2100 carb, I'll ask it anyway. Have you changed the filter screwed into the carb inlet?

Are there any old pieces of hose between the pump and the carb? They can deteriorate internally and not let fuel pass even though they look OK on the outside.

Beyond that I'm out of ideas.

Will Marsh

[Jay Tabor]

float level, too. raise the level of fuel if you can and try it again.

running at high altitude I guess the jets are adjusted for that alt> running way too large jets, could do it, but it wouldnt run very good at high altitudes . .