Troubleshooting after Engine Rebuild

[DomsX20]

So I purchased this boat (X20 with 240) over the winter with a blown motor. Good winter project... I rebuilt the motor and am on my 3rd tank of gas.... The boat runs pretty good for the most part but has just been drinking the gas. I know these boats are not great on gas but today I was on the lake and burnt through probably 20 gallons in 1 1/2 hours running between 4000 and 4300 RPMs... Also, after my first tank of gas I tried to go full throttle and at WOT, the max speed is 40 MPH at 4800 RPM. The speed seems accurate with the actual speed. I have attempted WOT now multiple times with the same results.. During the engine rebuild I did replace both of the head temp sensors to be on the safe side...

Poor Fuel Efficiency + Low RPM + Low Top Speed = ?????

Thanks for the help!

 

[Dr Honda]

I think the 240 is about 28 gal/hr at full throttle. (just an fyi)

Since your RPM's are so low... I'd check....

1) the timing,

2) the fuel pressure in the high pressure side

3) compression. Just because you rebuilt it doesn't mean the rings set right.

 

[DomsX20]

Dr. Thanks for the input..

28 gal/hr at WOT would be ok but WOT is 5800-6200 RPM's. I have been cruzin about 4000-4500 RPM around 25-30 MPH. By the smell gas and fuel usage, I really do think the engine is running rich.

The compression is 118 to 120 on all 6 cylinders.

Timing, I am not sure how this could cause lower RPM's at WOT. The engine runs pretty good. No hesitation, no RPM jumping. Just low at the top end. If I understand the Merc Manual, timing advance stops at 3000 RPMs (if I recall)

Fuel Pressue... I will have to look at that but it doesnt add up. If it had low fuel pressure causing low RPMs then I would not be concerned about the high fuel consumption. I am not sure about effects of high fuel pressure unless it was flooding the engine... Just a thought....

 

[Dr Honda]

If the regulator is bad... and you have crazy high pressure... it would run rich.

As far as the timing... on the 240 engine, the triger unit is linked to the throttle. SO... at idle, it's down... and as you move the throttle... it goes up. If it's not moving, or it's just low... you wont' make full power.

Last thing... check the TPS and make sure it's set right. Along with that... make sure the throttle is actually opening all the way.

 

[DomsX20]

I dug in this weekend and troubleshot the boat. I tested all three fuel pumps, disassembled, inspected the vapor seperator and checked the fuel pressure (36 psi). All tested good. Checked the spark using a spark gap tester set at 7/16 inch and all 6 tested very good. I tested the TPS which was a little off (.496 volts at idle) and I adusted to specifications at .248 volts. TPS at WOT is 7.84 volts which is good (Manual says 7.46). I then tested the head temp sensor which brand new and found it working perfect. Finally I tested the Air Temp Sensor which at ambient temperature (75 F) was 5.8 M ohms!!! Tested in ice water which measured over 15 M ohms. When testing in very hot water I could only reach a value of 1.95 M Ohms.... According to the manual, an open temp sensor will increase fuel flow by 10%.... I am not sure if this is the root of the problem but it is definetly part of the problem.