WOT Problem

M

mikeapi

New Member
Aug 12, 2009
1
Cincinnati Ohio
Boat Info
1986 Seville
Engines
135 HP Mercruiser
I purchased a 2006 Seville with a Mercruiser 140hp 3.0L engine. It runs good at idle and half throttle, but at full throttle it does a mix of running good and bogging down (mostly bad then good). Every couple of minutes it would have some power, then it would go back to several different degrees of being bogged down. It would back fire every once in a while. It sounded like it isn’t getting enough fuel.

I’ve done the following to it:
Replaced spark plugs, wires, points, condenser, distributor cap, all gas lines, fuel pump filter, carburetor filter

Had the carburetor rebuilt

Drained and replaced every drop of gas

Bypassed the gas tank with a portable gas tank to make sure there wasn’t junk still in the boats tank and to make sure there was no venting problem

I figured out if I put a product called SEA FOAM in the carburetor and let it sit for a while the engine runs great for about a half hour. So now my routine is pour Sea Foam in the carb before we leave home run the boat around the lake for a about a half hour. Stop and pour more Sea Foam in the carb let the kids swim for a while. Then the pull the kids around the lake on a tube, when it starts to run bad we go in.

The Sea Foam seems to temporarily clean something up. I’m wondering if my mechanic didn’t do a very good job on my carburetor rebuild, or if it’s clogging up somewhere after the carb. I thought about replacing the fuel pump, but it seems to me the problem is after the fuel pump. I didn’t check my timing when I tuned it up, but I wouldn’t think that would be the problem either.

Any advice would be appreciated.

Thank you.
 
Fuel pump does come to mind. There's a way to check the pressure from the pump, but I'm not the expert. Perhaps one will chime in.
 
Sounds like your choke may be acting up. See if you can find a knowledgable friend to help you monkey with the choke. That may be your problem. BTW, does it seem to be getting poor fuel mileage? If it is then I would say it probably is the choke.
 
The strange factor is Seafoam appears to at least temporarily resolve the issue. Seafoam is a top engine cleaner - does a great job at removing 'gunk' in the fuel system as well as on the intake valves. It typically will make the engine run rougher when first applied similar to what would happen if you sprayed carb cleaner in - you are changing the air/fuel mixture so the engine will stumble and run rough until the mixture evens out.

For the actual problem - you've done quite a bit which is definitely a good start to a tuneup. You say you haven't checked the timing but it could affect WOT operation. I would start with ensuring the choke has fully opened at normal operating temp as mentioned above and make sure the throttle plates are fully open at WOT. Make sure the linkages are not binding and you have a smooth movement of operation.

If the choke and throttle seems to be fine then I'd check the mounting surface of the carb to the intake. This can be a fairly simple check - while the engine is running you can lightly spray carb cleaner around the base to ensure you do not have a mounting/seating problem. If the engine RPM changes then you may have a torn gasket or the carb is not fully tightened down.

You may want to check the float levels and the accelerator pump as well. The float levels being incorrectly set will cause too much or too little fuel to be in the fuel bowls. If not adjusted correctly then the float valve seats could also let fuel in even if the float is up and supposed to have shut off the fuel flow into the carb. If the accelerator pump is faulty then it may have a torn gasket or the spring could be off which is allowing additional fuel to be introduced. What type of carb is on your engine? I'm assuming we're talking the 1986 in your boat info vs. the 2006 you posted originally about.
 
Assume you performed all the maint. items because of the problem, but then you said maybe the mechanic did not do a good job on the rebuild. Was it ok before the rebuild?
Start simple, how's the flame arrestor?
What do the old plugs look? Rich or lean?
You replaced evrything else but the coil, high Rpm or heavy loading can cause break up with a bad coil but you said it bogs down not break up but for a few backfires.
Tdappleman suggested you may have an intake leak, I would suggest that also,it's worth checking as he said with carb cleaner around the base, while your at it spray some around the intake manifold edges and visually check vacume lines. This problem usually causes a low unstable idle though.
Any Ideas on how many hours are on the motor? If it's alot try popping the distributor cap off and checking the distributor shaft bushing for play.
It's next to impossible to diagnose something like this from afar, but we've given you some things to think about and try. Hopefully some others will give a few more. Let us know how things turn out. Good luck
 
Didn't even look at the OP date. Oh well - 1 post and never came back..... wasted those few minutes.
 
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