90hp mercury bogs down after half throttle

[Ginervitus813]

Like the tile says my motor seems to bog down after half throttle. I've changed my spark plugs, changed the needles in my carbs as well as cleaned them. Checked the compression (120, 120,120) I'm not sure what it could be I do have a bend in one of the blades of my prop which I'm going to get fixed soon. Any help will be greatly appreciated.

 

[philipjames5k5]

Re: 90hp mercury bogs down after half throttle

Its a 1988 90 hp black max, with oil injection disabled, and ran on 50:1. The boat stays Idling perfect. Runs half throttle perfect. but when pushed to get up on a plane, it bogs down and kinda sounds like it is missing. RPM gauge is currently not working.

 

[oldman570]

Re: 90hp mercury bogs down after half throttle

You need to do a spark test with a tester to make sure that all the plugs are firing even at idle. Use a timing light on each plug wire for doing the test if you do not have a tester that goes between the plug and plug wire with a gap of 1/4" so you can see the spark. Never run the motor with a plug wire not connected to the plug or to block ground, as damage to the switchbox can happen. I think you will find that one plug is not firing even at idle, and gets really noticeable when on the water and running. If the motor has ever been jump started and the jumpers were put on the wrong terminals even for a second, then this can damage the electrical ing. system. Once you test the spark for all plugs will tell you just what plug is giving you the trouble and you can switch the wire on the switchbox for that plug with one that is firing and see if the missing firing moves with the switch. This needs to be done with all the plugs removed and all grounded to the block. If the miss moves then the trouble is in the switchbox or back thru the trigger and stator. If the miss stays on the same plug then repairs from the switch box to the plug can be done.
If all the plugs are firing with the timing light, then fuel system needs to be look into. It could be that the carb has a plugged high speed jet or other troubles. JMO
Oldman570

 

[CharlieB]

Re: 90hp mercury bogs down after half throttle

Ditto what Oldman has said. Check spark first.

Fuel flow will limit power and a motor will just hang, sort of bog, at the limit of the fuel flow. It could be dirty jets, weak fuel pump, air sucking into the fuel hose.

Simple tests, disconnect the fuel hose from the carbs, point it into a suitable container and crank the motor. You just want to see how well the fuel pump squirts a nice fat stream of fuel. Now replace that hose with a clear vynal hose and start the motor. Watch for any air bubbles passing thru which would indicate air sucking in a poor clamp, cracked hose, etc.

REMEMBER to remove that clear hose once finished testing. It is NOT rated, nor recommended for fuel use, only short term testing.