91 Merc 75hp cutting out

My 75 Merc idles well enough but at a load it surges, rpm dies off then comes back up on speed, when throttle is advanced it falls off almost to idle and then will (sometimes) regain previous power setting. I have changed the primer bulb, put in fresh gas, and changed the inline filter. Any ideas what I should check first?

Re: 91 Merc 75hp cutting out

Check that your fuel lines are air tight, fuel tank is properly vented and change out the fuel pump Diaphram kit.<br />If problem persists clean out the carbs and do a compression test, the variances should be within 5 to 10 psi between the cylinders.<br /> <br />ps: also check and make sure that you're getting good spark.

Re: 91 Merc 75hp cutting out

Thanks Vlad, I'll give it a shot and see what happens, by the way when I started it this AM to flush it out it had a hard time even maintaining idle.

Re: 91 Merc 75hp cutting out

Might be time for those carbs. As mentioned, check a couple of things first:<br /><br />Change your plugs and test for spark. Clean out coil post connections and coat with dialectric grease. Check coils as per manual specs.<br />Vent: If an onboard tank, blow in it. Air should pressurize/release freely.<br />Lines/primer bulb: Pressurize if needed to check for leaks; perhaps try bypassing the quick-connectors and run the fuel line straight to the pump. If you don't have a spare tank, get one. Use your new bulb and run the motor on the spare tank, thus bypassing the existing lines/pick-up tube in the gas tank. <br />Fuel Pump: Remove spark plugs. Ground each spark plug wire to the block (to protect coils). Run a fuel line from the output tube of the fuel pump to another tank or large glass bowl. Place motor on muffs. Turn motor over half a dozen cranks; fuel should pulse freely out of the line. Doesn't hurt to order the diphram kit and rebuild the fuel pump--it's easy. Make sure mounting gasket between the pump and the hole in crankcase is properly seated.<br />Carbs: Check that oil, &c. is not clogging the vent jet of each carb. <br /><br />Hopefully, you will find a kink in the system somewhere. Otherwise, remove front lower cowl, remove air box cover and plate, remove carbs, disassemble, soak in carb cleaner, reassy w/fresh gaskets and new needles and adjust floats as needed, and reassemble...not difficult, but not necessarily fun, either...<br /><br />Good luck!