mercury 305LX carb popping/backfiring

eclipse210s said:

Hi everyone,

Here's my situation. I boat my first boat last week knowing it had some motor problems. The boat had sat for the last two years. I'm familiar with gm motors and figured I'd be able to fix it. Well, so far no luck.

The problem is backfiring through the carb when increasing the throttle. The backfiring only seems to happen under certain conditions.

The motor starts right up both in the driveway and in the water. The motor will throttle up to 4600 rpms with no problems as long as I don't jab it too quickly in the driveway. In the water, increasing the throttle slowly, the backfiring happens regardless of how quick/ slow the throttle is pushed. It happens less if the throttle is pushed fast though. The boat will get to 3000 rpms and stop backfiring. It will cruise fine and tops out about 40mph. Also it idles fine in gear (headway speed up to about 10mph is good too).

The vacuum is steady but low, about 11in. It does have a slight vibration less than a 1/8in swing.
The coil, cap, rotor, plugs, wires have all been replaced.
I rebuilt the carb (rochester 4 barrel) and all seems fine. Used new gaskets, new needle valve and seat, new accelerator pump, new carb fuel filter. The inside looked good before as well.
I've checked the timing 8 bdtc as it should be.
All the fluids have been changed too.

I've ordered a new rebuilt carb. It should be here friday. I'm hoping the vacuum will go up after I replace it.

Any ideas?

Thanks

Click to expand...

Bond-o said:

Ayuh,... Welcome Aboard,.... Odds are, even with the rebuilt carb, it's goin' Lean causin' the backfirin'...

Make sure the tank is givin' up it's fuel, 'n the fuel pump is pumpin' 5 psi to the carb...

Did ya change the fuel filters,..??

Are ya usin' Fresh gas,..??

Try a remote tank, 'n fresh gas...

Click to expand...