Mercruiser 5.0 losing RPM

79Invader

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Apr 23, 2019
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Hello. My 1990 mercruiser 5.0 throttles right up and gets on a plane just fine. Runs about 40 mph at 4600 RPM. After about 60 seconds of running wide open it starts to lose RPM’s. Once it’s down to about 4000 RPM there’s an audible “clatter/rattle” coming from the motor. Throttle down and idle for about a minute then gets right back back on a plane 40 MPH 4600 RPM then repeats. Seems to have the same issue if you’re not running wide open say 3800 RPM just takes a little longer to happen. Will idle all day no problems. What could be the issue? I’m thinking fuel delivery but does that explain the clattering? Any insight is appreciated
 

alldodge

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Your running out of gas in the carb and the rattle is detonation where the motor is starting to destroy it self.

Your fuel systems has a restriction or the fuel pump is weak.

Restriction can be
antisiphon valve on tank
collapsed fuel line
clogged filter (spin on or one in carb)

Pump pressure should be 3-7 psi idle to wot
 

Bondo

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I’m thinking fuel delivery but does that explain the clattering?

Ayuh,..... Welcome Aboard,....... As AD said, the clatterin', is yer motor eatin' itself alive, a lean condition detonation,.....

Dump yer fuel filter into a clear vessel, 'n look for anything but clean fresh gasoline,.......
 

79Invader

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Apr 23, 2019
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Thanks guys! I will change fuel filter and test fuel pump. Also inspect line to make sure not collapsed or damaged in any way. Will report back.
 

achris

More fish than mountain goat
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May 19, 2004
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Don't ignore the vent line. Clogged vent will do similar. If it keeps doing it, remove the tank fill cap, see if it goes away.

Chris.........
 

79Invader

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Don't ignore the vent line. Clogged vent will do similar. If it keeps doing it, remove the tank fill cap, see if it goes away.

Chris.........

Will do. Wish I would’ve thought to pop the gas cap while I was out there.
 

79Invader

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Alright took the fuel water separator filter off. Dumped the gas out and didn’t look too bad, nothing horrendous. No water either. Put a new filter on. Should I water test it and see how it does? And if it does it crack the fuel cap? How does a guy go about testing the fuel pump? Never tested a mechanical unit before. Thanks
 

QBhoy

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Mar 10, 2016
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Hi

sounds very similar to what would happen if you have crap in the tank and blocking the pick up pipe under high fuel demand. You then throttle down and it drops the crap out the suction for a while. Back on high demand and more throttle..it gets blocked again.
 

79Invader

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Just pulled the pickup straw in the tank, was clean as a whistle. Also checked vent tube, it’s fine. The rubber fuel line from the tank to the filter was all cracked and coming apart, so I will replace it and water test it tomorrow. If it still does it I have it narrowed down to fuel pump and possible carburetor issue.
 

79Invader

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Is there a float in the bowl of the carb? It’s a four barrel that’s all I know about it. Could it be stuck or broken preventing fuel from coming in full speed? I’ve never worked on a full size carburetor.
 

Scott06

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Is there a float in the bowl of the carb? It’s a four barrel that’s all I know about it. Could it be stuck or broken preventing fuel from coming in full speed? I’ve never worked on a full size carburetor.

Yes the float / needle and seat could be stuck closed or not open fully.measure fuel pressure at wot should be 5-6 psi, if ok rebuild carb, if low replace pump as it looks like you’ve covered any starvation issues
 

79Invader

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Finally got the boat out for a ride today. Same issue mostly. Will run at 30 mph 3500 rpm all day no problem. But once you ask for more and the 4 barrels open up, she picks right up but then starts clattering and drops back down to about 3500 RPM and will still clatter unless you drop the throttle down to where is should be for that RPM. Fuel pump is prime suspect at this point I’d say. What do you guys think? Thanks again.
 

Rick Stephens

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All it takes is crud in the carb or the filters.

I ain't giving an opinion on the pump - rather T the line and test it and know for sure. Pressure/vacuum gauge are cheap. Marine fuel pumps, not so much. If it isn't the pump, then the carb needs cleaning.

Rick
 

79Invader

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Is there a filter in the carb I should check? 4 barrel quadrajet. Anyone else think fuel pump? I'm at a cross roads to spend $50 or so on a vacuum gauge and fuel fittings to test the pump, spend $100 on a new pump, or rebuild the carb. Are these carbs relitvaly straight forward to rebuild? Like I've mentioned before, I have no experience with a
 

79Invader

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I will also add the flame arrestor was pretty dirty and there is some grime on the choke plate and 4 barrels.
 

alldodge

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Remove the fuel line and place it in a container, then crank the motor and see how well it pumps

Most Q-jets do have a filter at the carb, would check yours but don't have a serial number
 

rad1026

Chief Petty Officer
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May 1, 2006
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443
There is a small filter at the inlet to the carb, you can check that, but I doubt it is the problem. How long have you owned the boat and how long has it been since the carb was rebuilt. Lots of information out there on the quadrajet. I'm rebuilding mine right now. If it has been 10 years plus since the carb was rebuilt you might want to think about doing it. Ethanol takes its toll on some of the carb parts. Maybe your float needs to be adjusted. This is a good start if you are inclined.
 

79Invader

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I just purchased the boat at the beginning of this season, I have no idea how long since the card was rebuilt; but if I were to bet, it's been a few years judging from how dirty the flame arrestor and choke plate were. I am mechanically inclined, but being a younger guy, I've never messed with an automotive carb before. Just 2 stroke outboards and other various small engines. I will find a serial number on the carb and put it in here, since I'd bet I'd need it to buy a rebuild kit anyways.

When cranking the engine with the fuel line disconnected I should be looking for a strong stream correct? Also I should un plug the power wire going to the coil, not from the coil to the distributor right?

Thanks again.
 
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