350 Magnum acceleration problem

VeroWing

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May 16, 2009
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I have a 1989 Grady 241 with a low hour (300ish) 1997-98 350/260 Magnum carbed engine. Boat sat for a few years on trailer prior to my buying it, but was starting occasionally according to prior owner. I'm been going through everything for over a year, including fuel lines, exhaust manifolds (raw), plugs, wires, Ignition p-up, separator, lower unit serviced, and on and on. Drained fuel tank and replaced with fresh fuel, and pulled Weber carb, cleaned and installed new cab kit. Set fuel/air screws at 1 1/4 turns out, and set floats at 5/8" closed, and 1 1/8" open. My problem is, when I take boat out on water to determine correct prop, rpms get to 3900-4000, and then engine bogs down to almost completely stalling, and actually stalling several times. I put drive in neutral and it starts right back up, and idles fine, and will start off again up to the same rpms and then abruptly fall off again and stall, or almost stall. I tried several different props, and it doesn't matter. It seems to me like I may be emptying the carb, like maybe the floats are not letting enough fuel in, OR, there is a rev limiter somewhere that is holding rpms to a 3900-4000 limit. it has the Thunderbolt V ignition, and I replaced cap, rotor p-up, etc. Engine has good even compression, starts right up and runs smooth and stays around 180ish degrees. Does anyone have any suggestions on what my problem may be? Do you thing setting the floats 5/8" closed, and 1 1/8" open (as per rebuild kit instructions) may be the problem? Any suggestions are appreciated.
 
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alldodge

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I would suggest starting with your fuel pump, check pressure is between 4 to 7 psi. Leave the gauge inline and run the boat while watching the gauge, my guess is if your not low when you start at idle, pressure will drop as speed increases. If pressure drops only as speed increases then you have a restriction.
 

Bondo

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Ayuh,.... To go along with AD, hook up a remote tank of fresh clean gas to the fuel filter barb,....

That'll eliminate the boat's side of the fuel system,.....
 

VeroWing

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Good ideas. Thanks guys, I'll give it a go tomorrow
 

VeroWing

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Well, I did a few changes on boat trying to chase down high rpm engine bogging problem. Pulled carb once more and reset floats again, rechecked timing, visually looked into fuel tank for contaminants, and installed new fuel pump. Took boat out again this morning, along with spare portable fuel tank and hose. Engine did the same bogging at around the 3000rpm mark as it did before. Went back to shore and changed fuel line going into the fuel/water separator to one from spare fuel tank and went back out. Engine ran great all the way up to the 4500-4600rpm mark without any bogging or hesitation. Looks like my problem is between fuel/water separator and fuel tank. (thanks Bondo) All fuel lines are new from fill line to the line from tank to engine, and I personally changed them, and I'm certain they are good, without any kinks, bends, etc, so that leaves me with the check valve on tank nipple, or a clogged pickup tube.
Is there any kind of screen or mesh at bottom of pickup tube in fuel tank? Do those check valves go bad? When putting suction on fuel line to engine, I can feel a little back pressure, but thought that was pressure needed to open check valve ball. I checked tank vent a few weeks ago and I could easily blow air into it, and it seemed clear, but I'll double check it. Any other suggestions will be appreciated.
 

alldodge

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There is an anti-siphon vale attached to the tank and yes they go bad. You can also have a clogged pick up screen in the tank. If your getting any suction with the valve removed from the tank you probably have a clog on the pick up tube
 

NHGuy

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The way your description reads it seems that you ran fuel from the separate tank through the water separator and things worked correctly.
If that is the case you should focus on the fuel pickup and anti siphon valve.
Take the anti siphon valve right off the tank and push the ball in against the spring a few times to be sure it moves easily.
While it's off see if the pickup can be removed from your tank. If so pull it out of there and clean it out. Might be able to get it clear with gun brushes.
Remove your new hose at the fuel water separator and blow through it in case any thing got in there.
And obviously the tank has to be clean too.
 

NHGuy

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OTOH if you fed the carb AFTER the separator, focus there.
 

VeroWing

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Thanks AllDodge and NHGuy. yes, remote tank was hooked to inlet of fuel/water separator and ran fine. I'll pull anti siphon valve and see if pickup tube is removable on tank, but I don't believe it is. I'll let you know how I make out. Thanks again!
 

Bondo

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Ayuh,..... Usually,... when ya pull the elbow at the tank's outlet, you'll have the anti-siphon valve on one side, 'n the dip-tube on the other side of the elbow,....
 

VeroWing

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May 16, 2009
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Good news! I double check vent line as well as new fuel line going from tank to fuel/water separator, and both were good. Next, pulled anti-siphon valve from elbow attached to tank, and temporarily replaced with a straight through barb and short piece of fuel line. Blew through it into tank, and it was clear. I replaced anti-siphon valve although old one seemed to be ok, but for $11, figured what the heck. took boat out this morning and it's running great throughout the rpm range. Kept out on the water for a couple hours running all rpms trying to make bogging happen again, and she running great! Looks like that anti-siphon valve was the culprit. Thanks again for all your help. I'm on to the next problem that looks like questionable oil pressure intermittently showing on new oil pressure gauge.
 
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