1983 Mercury 115 6 cylinder 2-stroke fuel pressure

[ahobbes]

Hi everyone,

I'm still troubleshooting a WOT bog issue (pretty common from what I understand). I've eliminated electrical problems so now I'm moving onto the fuel system, from there I'll goto the carbs.

What is a good place to hook up a fuel pressure gauge and what pressure should I expect? I was thinking of just renting one from AutoZone and they have some brass fitting on the end.

Thanks everyone!

- Al

[ssv1761982]

What do you mean by "WOT bog issue" ?
Bogging out of the hole? Carbs need a simple adjustment.
Bogging once it has been wound out for a while? Check fuel lines for deterioration.

[ahobbes]

With the throttle at full and the engine accelerating, the RPM drops suddenly around 4800. I thought it was the stator but then noticed it doesn't always happen so I thought it might be heat dependent and likely one or several of the ignition coils. I did a shutdown immediately after the condition arose and pulled each spark plug, none of them stood out as wet or sooty, they were all just a little oily. I just built myself a DVA adapter for my fluke so I can rule out the ignition system for good but it's a bit hard doing the tests on the water (I've been thinking about building a monitoring system which would read voltages and display them but also display RPM, fuel pressure and perhaps temperature data. It'd be nice to have a logger as well so I could plot voltage changes and correlate them with my RPM curve.) Anyway, I'm shifting my focus to fuel system and carbs. Also, I'm at 5000' and this boat likely came from somewhere near sea level. I plan to check out the jets in my carbs, not sure if they need to be messed with though.

[Mark75H]

Fuel pressure should be around 3 psi. Over 4 psi will cause continual flooding.

There is no temperature data, the motor has no thermostat.

Worn fuel pump components are common. I would rebuild the fuel pump before doing anything else. Also check for fuel tank vent and fuel line problems between the tank and motor.

[ahobbes]

Thanks Mark, I have a fuel pump rebuild kit on the way. Curious to see what that screen looks like. I also ordered an in-line fuel filter, not stock on the 83' 115 but looks to be common on other models. I wonder why. According to the factory manual (which is the best written manual I have ever seen btw), there is at least one other screen (possibly 1 on the fuel pickup in the tank) to check, that's the one in each carb (I have the back drag type carbs).

As to carb adjustments, I can adjust the idle screw, check the float level and/or swap out the main fuel jet. I am not aware of any other adjustments. Like I said, the standard jet is probably what's in there, I think it's a .074". The next size down is .072" for 2501' to 5000' above sea level. After that is .070" for 5001' to 7500' above sea level. SO I fall in the 5001' to 7500' range (5100'). Should I follow manufacturer recommendations (.070") or should I just go one step at a time (.072")?

Now, I don't think having the wrong jet in there is the source of my problems but it wouldn't hurt. I'm gonna try cleaning out the fuel system before I mess with the jets. I am getting a bit of surge in addition to the problem at WOT, according to the manual, this is a symptom of fuel pump problems or an obstructed fuel line

[Mark75H]

I wouldn't worry until after I got it running and could see whether the spark plugs seem to indicate an extremely rich condition at wide open.

[ahobbes]

It runs pretty well, just not at WOT. I checked the plugs and didn't find any wet ones after shutting down when the problem came up.