Only goes 1/2 WOT

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Hi all,

I hope someone can help as two mechanics now have not been able to.

I have an 87 Cooper Prowler with a Mercruiser 260. I have a problem with the throttle and getting to WOT. When I reach 1/2 WOT, the motor sputters, catches itself and bogs to WOT. The RPM's reach only 3100-3200 at WOT. The entire 2nd half of throttle is a bogfest. I have in the last 4 years had the boat get past this problem about 5 times and RPM's reach 4200 and boat reaches 21 MPH. ALL the other times, I top out at the 3200 RPM and boat reaches 11 MPH.

According to the two mechanics it has been into see, the fuel pump is working propery, engine compression is good, plugs have been changed, carb rebuilt, distributer cap replaced, fuel filters changed and timing is good.

The boat starts wonderfully, the first 1/2 throttle runs wonderfully, BUT the 2nd half throttle is bit problem. There is a smell gas when idleing.

Anyone have any ideas? I suggested changing the fuel pump but the mechanic says the pump is working fine. He now ways that it could be water in the tank as he did find some.

One last tidbit. I hooked up a 5 gallon external tank to the fuel line that enters the fuel pump and ran it on that for a while, went WOT and same old thing. I then pumped the bulb to try to get more gas to it...no change. I would think that this would eliminate the water in the tank possibility.

 

[6meter]

Re: Only goes 1/2 WOT

Seperate tank eliminated a water problem. Any thoughts on a new coil? Some people say that there is alot of good used coils in garages that were replaced. But in the last 3 years i have replaced my coil twice for different symtoms. One being not able to get full rpm out of the motor. It wouldn't cough but would bog down and pop through the carb. The second one crapped out when it heated up. Another possibility could be the carb needs rebuilt, dirty and gummed up inside.

 

[Photo]

Re: Only goes 1/2 WOT

Hi 6meter,

Thank you for the suggestion. I am going to pass that suggestion on to the mechanic who's driveway my boat is filling up.

We did a new carb last time I clogged his driveway.

 

[Cuyose]

Re: Only goes 1/2 WOT

Is this a quadrajet? I would check for the secondary operation and make sure they are opening on WOT, it possible the spring tension is to great and they aren't opening, or they are opening way too early bogging the engine.

Only way to do this is have someone drive while underway with the flame arrestor off and watch as they throttle up to WOT. Be careful doing this though, have access to fire extinguisher and NEVER put your face over the carb, or engine for that matter while underway.

 

[FreeBeeTony]

Re: Only goes 1/2 WOT

Are you sure it's a bog?

I had a similar problem.......wouldn't go above 3200 RPM's.......

After many hours of troubleshooting and replacement parts I found I was loosing a cylinder at higher RPM's. Seemed like an intake valve was sticking opened, I readjusted it and it seemed fine but it hung up 1 more time and actually destroyed the engine.......piston hit the valve......

Good luck.

 

[Cuyose]

Re: Only goes 1/2 WOT

Good point, ive had this issue before too with bad plugs, you would be surprised how well a 6 cylinder V8 can sound until you actually try to get power out of it.

You can try a timing gun and check for spark on all cylinders at idle and at higher RPMs. Sure fire way is to check the plugs after the bog symptoms. Just idle down quickly and shut engine off then pull plugs. Don't shut off engine at wide open throttle though.

 

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Re: Only goes 1/2 WOT

I am not sure if it is a quadrajet carb or not...I am not very technical when it comes to engines but I am trying. I really think that if this latest trip to the mechanic does not work, I will need him to come out on the boat with me and try to isolate the problem. I hope it does not end up like FreeBee Tony's engine!

I can only describe it sounding like a bog. The interesting thing is on very rare occasions, I can get it up to WOT and runs great! Then after about 5 minutes at perfect, it slows and comes back down to the bog. When I say rare, I litterally mean half a dozen times in the last 4 years. I would think this could be a tell as to the problem...I could be wrong though to as again, I am not nearly as up on motors as you guys.

 

[wrosales]

Re: Only goes 1/2 WOT

Did you check for a leak between the tank and the carb?

 

[Photo]

Re: Only goes 1/2 WOT

For 4 years I have been driving with #2 and #4 sparkplug wires crossed! Mystery solved. Thanks guys for all your input. I have learned a lot from this.

Now I wonder if there was any damage caused by doing this for so long...

 

[libellav15]

Re: Only goes 1/2 WOT

change mechanic!

should really have spotted that - especially with the symptoms - a compression test and an ignition tune would have been the obvious place to start - both would have required the plug leads to be pulled and refitted - not good enough really ESPECIALLY considering the firing order is stamped on the block


oh well.......at least its solved

cheers

adam

 

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Re: Only goes 1/2 WOT

Hahaha...this was the 2nd try from the new mechanic. I am standing by him.