1990 Mercury 40 HP runs fine when choke is held in during load?

[mxsteve625]

Compression is good at 145 to 143 per cylinder. Carbs, Fuel Pump have been rebuilt and auxiliary tank is being used at this time. Motor runs good at idle but will not run under load unless the choke is held in and primer bulb is squeezed.Is this an accelerator pump issue? .

 

[Barnacle_Bill]

You don't have an accelerator pump. Recheck the fuel pump.

 

[mxsteve625]

Thanks I will recheck the fuel pump. That is exactly what I suggested to my son earlier.

 

[roscoe]

Also look for an obstruction in the fuel line, anywhere from the tank pick up, to the carb.

 

[mxsteve625]

Thanks I will recheck the fuel pump. That is exactly what I suggested to my son earlier.

 

[mxsteve625]

There was an obstruction in the line from the onboard tank which is why I am using an auxiliary tank at this time. I guess it makes perfect sense at this time to check the remaining lines. Thanks

 

[Bondo]

There was an obstruction in the line from the onboard tank which is why I am using an auxiliary tank at this time. I guess it makes perfect sense at this time to check the remaining lines. Thanks

Ayuh,.... Welcome Aboard,.... It could also be the carbs themselves,...

 

[mxsteve625]

OK. I took the fuel pump off and blew through the inlet port and air passed. Blew through the out port and air blew through. Took the pump apart and found I had installed the main gasket on the wrong side of the pump body. Re assembled with new dia and gaskets, blew through ports and air flowed one direction and not the other as it is supposed too. Thought great simple mistake. Took boat out and still will not rev up under load. Carbs were rebuilt a couple of weeks ago. Prop is a little chewed up. Can this be causing my problem?

 

[racerone]

Report the results of a compression test here.--Then test for proper spark ( jump a gap of 3/8" ) and report those results.-------It is not always the carburetors that are the problem.

 

[mxsteve625]

Compression was the very 1st thing I did psi as follows:

Cyl 1: 145
Cyl 2: 145
Cyl 3: 140
Cyl 4: 135

Had no Spark when I got it. Stator tested bad. Replaced. Then replaced switch box. Spark jumping 3/8" on spark tester.

 

[mxsteve625]

Just checked timing at idle 600 - 650 RPM: 4 degrees BTDC. specs say 3 to 10 degrees. Turned idle screws in on both carbs until starting missing then turned counterclockwise about a quarter turn until engine ran smooth again.

 

[racerone]

Often when motor idles nice the low speed jet are set too lean for good acceleration.----Open them 1/8 turn at a time and see if acceleration improves.