Bad Fuel Pump??

[gofastsandman]

the most important adjustment in any carb job(rebuild) is the float height adj. i would almost bet the floats are set too low and the pumps cant constently supply enough fuel to run wide open. therefor the engine will run WOT until the fuel level is lowered, then it slows to an rpm that the pump can keep up with. all of this can be caused by the float too low and not opening the needle early enough and the pump is playing catch-up. what a coincidence the carbs. were rebuilt and this problem shows up. just my .02

Coincidence? Check the floats.
Bones is right on the bulb. I had an emergency buy on a Sat. that fell apart in my hand when replacing. Pulled 4-5" of vacuum @ idle.
Cheers,
GFS

[77SceptreOB]

Coincidence? Check the floats.
Bones is right on the bulb. I had an emergency buy on a Sat. that fell apart in my hand when replacing. Pulled 4-5" of vacuum @ idle.
Cheers,
GFS

Not sure I follow you guys on the above.

"if the engine consumes 10 gal/hour or 21oz/min and the bowls are full because the float levels are correct". And the fuel pump supplies more than 21oz/min all is well. If the fuel pump provides less than 21 oz /min then the bowl fuel level will drop until the a bowl or bowls dry up and the engine surges due to the engine starving for gas. Then I have to back off the throttle to 3500 rpm at which the Fuel pump can fill the bowls faster than the engine can dry then.

If the above makes sense, then it wouldn't matter what the float level was set at. The fuel supply simply can't keep up with the consumption.....right???

I replaced both fuel pumps BTW.

Am I missing something here?? Please explain.

Thanks,

Jim

[Bushwacker]

There are potentially 2 separate issues here, both related to fuel starvation, but for several different reasons.

1. If the floats are set too low, the motor will tend to run lean (not good if you run premix!), even if fuel pump is good. If they rebuilt the carbs, hopefully they would have also replaced all the internal fuel lines too. Otherwise some crap from old lines could break loose and plug up the inlet needle and seat or jets on the newly rebuilt carbs!

2. If fuel pump is weak and can't match engine demand, then yes, motor would start to lean out/surge as soon as fuel level dropped enough. Low float level would just let this happen sooner.

3. A fuel restriction is between pump and carb, like a partially plugged needle valve, might act like a low float or weak pump, but fuel primer bulb would be hard. A fuel restriction upstream of primer bulb would result in collapsed bulb.

Don't know if this helps, but a fuel starvation or blockage problem shouldn't be that hard to find. Pumping the bulb when engine acts up might give you a clue as to what's going on.

[77SceptreOB]

....If they rebuilt the carbs, hopefully they would have also replaced all the internal fuel lines too. Otherwise some crap from old lines could break loose and plug up the inlet needle and seat or jets on the newly rebuilt carbs!

No, the internal lines were NOT replaced.

...A fuel restriction is between pump and carb, like a partially plugged needle valve, might act like a low float or weak pump, but fuel primer bulb would be hard. A fuel restriction upstream of primer bulb would result in collapsed bulb.

That make sense.

... Pumping the bulb when engine acts up might give you a clue as to what's going on.

Thanks, I'll try that!

[76Red18]

If there was a fuel restriction, say a plugged filter or line, would the motor surge or would it just loose rpm at a constant level? For instance, "The motor used to hit 6000 now it only reaches 5500"

[Bushwacker]

I think it depends on amount of restriction. If filter is seriously clogged, yes, that'll reduce max rpm. If just slightly plugged, it could reduce fuel pressure just enough to reduce flow to the top carb, making it act like float was too low.

On my old 115, I had a slightly plugged carb one time; don't remember if it was inlet needle and seat or a main jet. It didn't show up until after about 10 seconds at WOT, when it began to surge, but it would still hit max rpm between surges.