350 mercruiser bogging around 3500rpm

[burke1]

Hi im new to the forum. Had my boat out yesterday trying a new size prop. Boat ran great gained 8 mph and lost 200 rpm. Boat ran for 30 min then i noticed the engine bogging out at 3500 rpm. It would bogg then go to 4000 then bogg back down to 3500 then back up? Then when i lowered the rpm just under 3500 it ran fine? I tried a wot take of and it was fine till it hit 3500rpm? I have a 22ft formula with a 350 mercruiser quad jet 4 bbl. Ive changed the fuel filter dident make a diff? The fuel tank was completly empty when i first took it out maybe sucked something up in the carb? PLease help thanx. Ok I think I finally figured it out. Pulled the filter out of the carb. It was clogged up with brown flakes? Cleaned it out and motor fired right up. Explains the bogging at 3500 plus not getting enough fuel. Motor ran good for 2 min and started doing thsame thing again. Pulled filter out again and was clogged. I gues the fuel filter/ water sep doesent catch many things. Opened up fuel sending unit on top of tank and looked with flash light. The gas level was 1inch over the fuel pickup. Had brown flakes floating around in the tank so I guess it was sucking the brown flakes. Do not want to pull 94 gallon tank out of boat. Drained tank again backed boat down my boat ramp so gas was all the way in the back of the tank. Installed new water sep. Installed inline fuel filter clear so I can see if any crap gets in there. Put 50 gallons of fresh gas in and ran it around my lake. Just like new. Dont forget the filter in your carb like I did. I will keep checking that inline fuel filter and carry spares on the boat along with my tool box lol. I will never get let my gas get that low again. I know theres **** floating around in there. Thanx for the input guys

 

[8USMC4]

Re: 350 mercruiser bogging around 3500rpm

Hey Burke1,
I am having the same problem you are having. I have a 24 ft Python with a 454 big block. It runs fine out of the whole then at WOT and high rpm's it starts to fall on its face. If I back off the throttle it seems to level off but as soon as I get back in it, it starts to fall off again. I have re built the carb, changed the fuel filter, spark plugs, oil, and used a few explitives along the way. Im thinking fuel pump at this point. Im told that the marine fuel pumps can still pump fuel but cant keep up with the high volume required at higher rpm(when they go bad). Any thoughts? Any help from anybody would be great. Thanks
ps, I also ran mine out of fuel (bad sending unit) a week ago. MAybe picked up trash or water? Filled it with good ethonol free gas and put phazer in to try to eliminate any water. I also need to check the fuel tank vent. You might also.

 

[cr2k]

Re: 350 mercruiser bogging around 3500rpm

Fuel delivery issue I would bet. Either Filter, pump or carb (if not blocked tank pick up).

Or possibly the advance weights in the distributor are stuck and not advancing the timing past a certain point.

 

[8USMC4]

Re: 350 mercruiser bogging around 3500rpm

Ok, Im not sure if I should reply here or start a new thread. Very new to this concept of information sharring. Thank you for your input and guidance. I thought I had fixed the above mentioned problem, but after a good day of boating on Saturday, the boat started acting up again. Very similar to Burke1's description. I have been all the way through the fuel system last week. Started at the tank. Pulled the pickup and it was clear. Pulled and replaced the fuel guage sending unit and visually inspected the tank. No trash observed with a flashlight. Aluminum tank. Fuel looked clean. Changed the tank vent with a new vent hose/ Removed and cleaned the vent itself and cleaned screans. Removed fuel pump. Pump was in very bad shape. Diaphram appeard ruptured. Replaced with new marine fuel pump. Removed the old fuel lines and replaced with all new lines. Replaced the pump overflow with new clear tubing to carb. Now, I did have the old quadrajet rebuilt a few weeks ago. It was flooding out. The tech that built it is very reputable and stated that it had a broken metering rod. He replaced and rebuilt and the boat ran better but not good.(bad carb?)The thing that bothers me is that it ran great all day. We ran a poker run and the boat was running most of about 4 hours. straight. Anywere from idle to WOT throughout the day and no hick ups. Then all of the sudden after sitting for about 3 hours, we took off and was fine around 3000rpm. Anything above that it wanted to bogg down. If I left the throttle alone it would surge up and down until I pulled back on the throttle. Temp guage just under 150 all day. Above water exhaust and good water flow. It will run all day at 3000 rpm. Pulled my son on his tube all day Sunday under 3000rpm. Anything above that and it wants to bogg down. Any ideas from anyone would be appreciated. Thanks

 

[burke1]

Re: 350 mercruiser bogging around 3500rpm

check your filter that goes into your carb thats where mine keeped clogging up. Sounds like its not getting gas to feed over 3000 rpms. Did you check your gas tank make sure **** ant floating around in there.If he rebuilt it and it ran good for 4 hours then back to the same 3000 rpm bogg sounds like u picked something up again maybe.

 

[Creekwood]

Re: 350 mercruiser bogging around 3500rpm

I had a similar problem a few years ago and tried all the same symptoms except that when it started to bog and I brought it back to idle, I would hear the engine "ticking" until it cooled a bit. In the end the problem was VERY simple. My mechanic had used the wrong engine oil. It was too thin and once it heated up the oil was too thin to work the hydraulic valve lifters and they were not fully opening, causing bogging. He changed the oil back to the proper viscosity and all was well again.

 

[burke1]

Re: 350 mercruiser bogging around 3500rpm

Wow thats crazy. I bet that scared the hell out of you lol