Fuel pumps
09-12-2003, 02:58 PM
#1
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Fuel pumps
My recently rebuilt and slightly upgraded HP 500 carb motors seem to be short on fuel. I will be checking fuel pressure next time out.
Since the rebuild (which included Crane 168731 cams, bowl porting and milling the heads .010") the motors definitely want more gas. With the stock jetting the spark plugs were bone white. I first increased the primary jets 3 sizes and could feel an immediate improvement. I took a plug reading after a part throttle cruise and they were still pretty light colored but showed a hint of gray. I made my first full throttle run and the motors seemed flat on the top end. Checked the plugs and they were again very white. I then increased the secondary jets by three sizes. The motors felt strong in all ranges but still flat on top. The top speed did not change up or down. The plugs still looked white and there was little to no soot on the transom after a day of running.
I am wondering if the fuel pumps have gotten weak (they have 180 hours on them), allowing the motors to run alright until you get past three quarter throttle and into the upper rpm range.
If it turns out that the fuel pressure is falling way off at WOT, are there any larger mechanical pumps that will bolt on to the sea pump/fuel pump set up for the GEN VI motors? Has anyone else experienced this situation with an HP500 ?
I built a 540 for my last boat and initially used the fuel pump from a previous 502 mag. It did the same thing (ran strong until full throttle). I then put the big Holley 130 gph pump on along with -8 lines and the motor woke right up.
Rich
Since the rebuild (which included Crane 168731 cams, bowl porting and milling the heads .010") the motors definitely want more gas. With the stock jetting the spark plugs were bone white. I first increased the primary jets 3 sizes and could feel an immediate improvement. I took a plug reading after a part throttle cruise and they were still pretty light colored but showed a hint of gray. I made my first full throttle run and the motors seemed flat on the top end. Checked the plugs and they were again very white. I then increased the secondary jets by three sizes. The motors felt strong in all ranges but still flat on top. The top speed did not change up or down. The plugs still looked white and there was little to no soot on the transom after a day of running.
I am wondering if the fuel pumps have gotten weak (they have 180 hours on them), allowing the motors to run alright until you get past three quarter throttle and into the upper rpm range.
If it turns out that the fuel pressure is falling way off at WOT, are there any larger mechanical pumps that will bolt on to the sea pump/fuel pump set up for the GEN VI motors? Has anyone else experienced this situation with an HP500 ?
I built a 540 for my last boat and initially used the fuel pump from a previous 502 mag. It did the same thing (ran strong until full throttle). I then put the big Holley 130 gph pump on along with -8 lines and the motor woke right up.
Rich
09-12-2003, 06:07 PM
#2
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A common problem with these engines is the fuel pump arm wears out. It rides on a cam inside the sea water pump and if the oil isn't checked/replaced occasionally the arm and cam wears out. Remove the pump and check it out. I've seen them worn to a nub.
Dennis Moore
Dennis Moore
09-15-2003, 11:17 AM
#3
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Dennis,
I will check the condition of my pump arms and the cam lobe inside the housings.
When in good mechanical condition, are these pumps large enough for my application? I don't know what their rated free flow gph is, but with the small 1/4"npt lines it can't be that great.
I will check the condition of my pump arms and the cam lobe inside the housings.
When in good mechanical condition, are these pumps large enough for my application? I don't know what their rated free flow gph is, but with the small 1/4"npt lines it can't be that great.
09-15-2003, 11:40 AM
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They put out more than you might think because they run at crankshaft speed. When a mechanical fuel pump is mounted on the engine block it is run off of the camshaft (at 1/2 of crankshaft speed). The Mercruiser style, that mounts on the seawater pump, is belt driven off of the crankshaft and really cranks out some volume.
An engine mounted fuel pump really soaks up heat from the engine block, heating up the fuel. When the fuel pump is remotely mounted on the seawater pump it runs very cool (and keeps the fuel cool).
The size of the fuel lines is fine. Anybody that has ever rebuilt a carburetor and inspected the size of the inlet needle and seat soon realizes that huge fuel lines running up to these tiny inlet valves is being a little silly!
The carburetor can't supply more fuel than the inlet needle and seat can flow and they are pretty small!
If you think you are not getting enough fuel into your carburetor, and you have enough fuel pressure, you should increase the size of the needle and seat as your first modification. The drawback to larger needle and seats is that the engine will flood easier because the floats don't have as much leverage to keep them closed.
Dennis Moore
An engine mounted fuel pump really soaks up heat from the engine block, heating up the fuel. When the fuel pump is remotely mounted on the seawater pump it runs very cool (and keeps the fuel cool).
The size of the fuel lines is fine. Anybody that has ever rebuilt a carburetor and inspected the size of the inlet needle and seat soon realizes that huge fuel lines running up to these tiny inlet valves is being a little silly!
The carburetor can't supply more fuel than the inlet needle and seat can flow and they are pretty small!
If you think you are not getting enough fuel into your carburetor, and you have enough fuel pressure, you should increase the size of the needle and seat as your first modification. The drawback to larger needle and seats is that the engine will flood easier because the floats don't have as much leverage to keep them closed.
Dennis Moore
09-15-2003, 11:42 AM
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I run a 540 but with an Aeromotive HP1000 and a bypass regulator combination works great!
If you go the electrical route you will have to run a bypass line back to your tank.
I posted a thread on the project a while back.
If you go the electrical route you will have to run a bypass line back to your tank.
I posted a thread on the project a while back.
