Bog at mid range to WOT

[mc911]

Hey Folks
I have a 20 Ft Doral Cuddy with a 4.3L OMC Cobra 4V. The engine works great up to about 3200 to 3400 RPM. To go any higher results in a bog from the engine. It will recover from the bog on its own but my max rpm is about 4000.
To start out I bought the boat last year from the origional owner. It had sat for the past 5 yearsand required a ton of cleaning to bring it back. The engine looked to be in good shape but would not start. It was not getting any gas and after sitting so long I decided to put a kit in the carb. After seeing the condition of the carb I decided to replace it with a rebuilt (Rochester Quadrajet). The boat worked great with the exception of the bog but I decided to run it due to the short season up here in Canada.
I try to do my own work with my past experiences and advice from others. I was told it could be ignition, so I changed it all out with new cap, coil, plugs, wires, points and rotor this spring. I was also told it could be the carb.
I reset the accelarator pump to what my manual states. When working the throttle by hand I noticed I was only getting a spray frfom the accelarator pump up to about half throttle. From half to WOT I don't get any fuel from the accelarator pump. Is this right???
If anyone has run into this problem before or has any experiance with the Quadra jet I would really appreciate the help.
Thanks
Mark C

 

[Maclin]

Re: Bog at mid range to WOT

I am not sure on the pump shot, if there should be any after half throttle or not. I do know most carbs I have dealt with taper off by then on the pump shot.

This sounds like the secondaries opening too late or just falling open instead of a controlled open. The fact that you can only get 4,000 rpm leads me to believe they are too far open or the metering is not flowing enough fuel and the engine is leaning out. There are several components to the secondaries, there is a spring that keeps them closed, also a vacuum diaphragm and also some control rods that move with the plates to control fuel metering. I would have to try setting the spring tighter to where they do not even open at all and see if there is still a bog or if it acts more like it is just out of breath. My feeling is it would probably reach 3,800 - 4,000rpm's on just the primaries. If the engine behaves normally with no bog this way then you know it is in the secondaries actuation setup.

 

[Maclin]

Re: Bog at mid range to WOT

This could also be a fuel delivery issue at higher rpm, not being able to keep the bowls full enough to keep the mix from leaning out. If I were comfortable enough with the mechanism I would try the tighter spring setting to keep the secondaries from opening and observe. If it acts the same then I would look at the fuel delivery system.

With the secondaries blocked from opening the engine should respond smoothly up to whatever top rpm the primaries can support, and run there with no surging or other misbehavings.

 

[mc911]

Re: Bog at mid range to WOT

Hay Maclin
One of the books I have talks about the secondaries opening to soon. I had planed to check this today but got rained out (very wet summer this year). If the weather holds out for sunday i will try your recomendations and post the results.
Thanks for your quick post. This is my first forum post and was very happy to see such a quick reply.
Thanks again
Mark c

 

[xltier]

Re: Bog at mid range to WOT

Hay Maclin
One of the books I have talks about the secondaries opening to soon. I had planed to check this today but got rained out (very wet summer this year). If the weather holds out for sunday i will try your recomendations and post the results.
Thanks for your quick post. This is my first forum post and was very happy to see such a quick reply.
Thanks again
Mark c

also make sure the timing is advancing properly.module could be giving fits