Fuel pump suction hose, ID important?

[java230]

So I replaced the fuel pump suction hose when I had my powerhead out. The hose that runs from the fuel pump to the cylinder that gives the fuel pump its suction.

I used fuel line, and it was slightly bigger ID than the one that i pulled off. It fit the nipples fine so I figured it would be good to go. The old one had almost worn a hole though it.

Now I went out on the lake yesterday, it was hard to start (never had hard starts before), I had to bump the throttle up a bit and crank it a fair bit to get it started. The it ran fine at WOT for an hour. I stopped a couple times on the way back to the ramp, and it was hard start all times.

 

[Texasmark]

Re: Fuel pump suction hose, ID important?

You might be onto it. The suction volume is fixed. Since it is a suck/blow function, each revolution, the hose has to fill and discharge. Obviously (to me) the larger the volume of the interior of the hose, with a limited amount of suction volume, the less is available to operate the pump.

Additionally you didn't have this problem before and it repeated itself every time you tried to start.

So, assuming you didn't do anything in the rest of the operation, I will agree with you and keep this tidbit in mind when I change my hoses.

Mark

 

[java230]

Re: Fuel pump suction hose, ID important?

You might be onto it. The suction volume is fixed. Since it is a suck/blow function, each revolution, the hose has to fill and discharge. Obviously (to me) the larger the volume of the interior of the hose, with a limited amount of suction volume, the less is available to operate the pump.

Additionally you didn't have this problem before and it repeated itself every time you tried to start.

So, assuming you didn't do anything in the rest of the operation, I will agree with you and keep this tidbit in mind when I change my hoses.

Mark

Mark,
The only thing i can see is that i don't think the hose is actually emptied each revolution. There isn't anywhere for the exhaust to go as far as i can tell, ie. be discharged. Its simply a pulse. So the larger volume in the hose may be slightly slowing the pluses to the fuel pump.

The hose is not much larger, Ill see if i can get ID measurements of old and new tonight. but its about 1/16" at most

 

[java230]

Re: Fuel pump suction hose, ID important?

just to be clear, its this hose:

 

[ondarvr]

Re: Fuel pump suction hose, ID important?

The larger ID won't have an effect on starting, it would start the same even if you didn't have a fuel pump. Since it ran fine after starting the new hose probably has no relationship to the starting issue. Now it possibly could if the hose you put on was leaking air into the cylinder past the clamp, but that's about it.

By squeezing the primer bulb you have filled the carbs and fuel line, it will start and run for a very short time as normal until it runs out of fuel which after starting would be supplied by the fuel pump. If you continued to squeeze the bulb the fuel pump wouldn't be needed.

 

[Texasmark]

Re: Fuel pump suction hose, ID important?

5/16" hose has an area of 0.77"

3/8" hose has an area of 0.110", a 43% increase in area. So I guess it's a matter of how much volume of impulse it takes to move the diaphragm vs the difference in hose diameter.

But I do agree that your first start is via full carb bowls from the squeeze bulb and the fuel pump is not in the picture. Maybe you are looking in the wrong place for the problem. And squeezing the bulb with the bulb eliminates the need for the fuel pump as said, sooooo.

Mark

 

[java230]

Re: Fuel pump suction hose, ID important?

5/16" hose has an area of 0.77"

3/8" hose has an area of 0.110", a 43% increase in area. So I guess it's a matter of how much volume of impulse it takes to move the diaphragm vs the difference in hose diameter.

But I do agree that your first start is via full carb bowls from the squeeze bulb and the fuel pump is not in the picture. Maybe you are looking in the wrong place for the problem. And squeezing the bulb with the bulb eliminates the need for the fuel pump as said, sooooo.

Mark

I think this is right on..... It was the only thing i changed motor wise other than disconnecting the throttle cable, and cleaning the contacts in the coils. One was shiny the other hadn't been seated well and was corroded. Polished it with sand paper.

Well i did remove the powerhead, and left it sitting at a funny angle in the garage. Maybe something to loosened up in the carb. replaced the gaskets between the powerhead and leg. When cranking it almost seemed like one cylinder was much harder for the starter to move, if that makes sense, it would crank normal speed then slow for a revolution then normal speed again.

 

[ondarvr]

Re: Fuel pump suction hose, ID important?

Check your compression if the cylinders don't seem even. Also if extra fuel is dumping into one cylinder it can be more difficult for the starter to spin it. The other issue could be a weak starter or bad connection.