Merc wont run under load

[Joe Luedee]

76 merc 850 runs great with flush hose connected and no load. have 120lb comp.all cyl., blu flash all cyl plugs, rebuilt fuel pump and carbs, timing static chks top dead cntr ok... dont have timing lt to check dynamics or carb sync. in the water, as soon i engage lower end and apply throttle eng dies... restart and it ildle fine, use warm up lever and it responds ok, put back in gear and it dies under load...

 

[FLOATY]

Re: Merc wont run under load

When you say "flush hose" do you mean water muffs for outside the lake cooling? If so, have you put the engine in gear while running with the muffs?
If you try that and it still dies the probelm is in the LU most likely.

 

[Joe Luedee]

Re: Merc wont run under load

Floaty I dont know the jargon but yeah when the muffs are on itll start, idle and will respond if i cobb it... it will do this in gear also... it runs tits in the driveway... put it in the water and it will start and rev with the warm up lever but put it in gear and it dies... im leaning toward a fuel problem as i cant believe the timing would be so far off it would cause this problem ???

 

[frozenokie]

Re: Merc wont run under load

I had rebuilta 75 85HP Mercury once before and I had a race bike/boat motor mechanic tell me that everything relies othe timing synch of those old Mercurys. Take the dist cap off and see if you can tell if the ignition point of time is causing the carbon to build up on the very edge of both contacts insted of right down the middle or the along the whole contact. He told me that "if it isn't just right then it isn't going give power and long life to the cylinders".

I admit after rebuilding one, it can be tedious but usually it's not far out. Here's another thought - it's possible that you need to back off the barrel shaped adjusting nut (thread it toward the front) to allow it just a little slack. It's possible that when pushing into gear under load, that the throttle it being pushed into the timing advance cam prematurely and jumping spark way too far in advance before the motor winds up. It would take a friend to operate the throttle and you stand back there and see when the advance cam is being moved. Without a timing light though and a shop manual, the odds are against you to begin.

Good luck...

 

[emckelvy]

Re: Merc wont run under load

The idle speed and mixture should be adjusted with the motor warmed fully, in the water, and in gear. You'll need 2 people in the boat to do this safely.

Your problem could just be a simple case of lean idle mixture; these motors like to run a bit rich at idle and will fall flat on their face if too lean.

So, you set the idle speed screw to a point where the motor will at least stay running in gear; then you 'tweak' on the first carb's idle mixture screw, running it in and out until you find the point at which the motor idles fastest. Then adjust the mixture screw about 1/4 turn CCW (rich) from there. Repeat adjustment with the 2nd carb. Readjust idle speed to just above the slowest speed the motor will run stably in gear.

Then hang on and mash the throttle! If the carbs are still too lean, it'll go Blaaaaaaaa................ and fall on its face. Richen each idle mixture screw 1/8 turn CCW and try again. If you can't find a place where the motor is "happy" and accelerates properly, it could be a problem other than carbs.

Or, if you find that moving the idle mixture screw around has absolutely no effect on idle speed, you're gonna need to service that carb as it likely has plugged-up passages.

BTW if you have no idea where the adjustments are for the idle mix screws, just turn them CW until lightly seated, then back out approx 2 turns. That should be more than rich enough for initial checks.

Anyway, it's a place to start. HTH & keep us posted on the results............ed

 

[Joe Luedee]

Re: Merc wont run under load

chief tnx for the tips... i think a found the problem in the spark plug wires... while running the motor and adjusting the throttle pick up and spark advance, i got high voltage shocked while holding onto the spark adv. lever. big time voltage bleed from the plug wires (which are snaked in behind the lever) . i replaced all four wires with beldon 7mm solid wire and new boots and clips. upon restarting the motor it fired immediately, something it has not done before, and when running there was almost no smoke in the exhaust, again a change from before. hopefully i found the problem. wont know until thurs. when i can get it to the lake.

 

[Joe Luedee]

Re: Merc wont run under load

tnx for the reply, i think i found the problem in the plug wires . i got a wicked shock while touching the spark adv. lever with eng running. all four plug wires run behind this lever and they were bleeding juice into it. ouch... that old sparky rule, always use one hand while working with electricity is true. i had hold of that lever with both hands and the shock went across my chest... guess i lucked out this time... wont get to the lake until thurs. will let you know how it turns out....