1975 Mercury 85Hp

[fogie27]

I have an older '75 merc that I have been having problems with for years and just cant get it figured out. I was wondering if anyone has any other ideas.

History of the boat: This is a little 16' runabout playmate and this boat has always been coverd and ran dry in the winter. Impeller has alway been replaced regulary. The boat is in great shape and years ago we used to pull two skiers out of the water with no problem. It has a 15 pitch prop and used to jump right out of the water.

Couple years back we were having a problem where when we were trying to plain off it would start to cut out and someone would run back and squeeze the primer bulb and it would come right out of it.

I just want to mention that I jammed up the reverse gear a couple years back as well. I slammed it in reverse at about 2000 rpm. I got it to come out but had no revese.

So I had the carb rebuit, new feul pump, plugs, all new fuel lines, new points and the mechaninc said that the revese gear was probably gone so I swaped lower units with a 115 hp merc (which bolded up fine).

Now the carbs leak gas when over primed which it never did before and if I have more than the driver in the boat it will not plain off. If you have two people in the boat it just bogs down and falls on it's face and won't go much more than 2000 rpm.

I called the mechanic shop where I had all the work done (which costed me a small fortune) and they said that I probably just need to adjust the idle mixture screws because they can't adjust them under a load. So I did this with the boat in the water and in drive and I can't tell any differance from 2 1/2 turns out from taking the whole screw all the way out.

The disrtibutor is advancing when the throttle goes forward and I have also tried advancing the timeing hoping for a responce but had no different outcome.

The compression was the next thing I checked. They checked it at the shop but I wanted to make sure. most all of the cylinders were arround 100 psi so that seems good enough.

The only thing, other than the carb leaking; which is proabably a the float not being adjusted right, that I can think of for the lack of power is the differant lower unit that had put on.

The old 85 hp lower unit had slightly lower gearing ratio than the 115 hp that I had put on. I do not remember what the gear ratios were.

Any thoughts or ideas would be greatly appriciated. I'm just about stummed.

Thanks,
Dennis

 

[Chris1956]

Re: 1975 Mercury 85Hp

Dennis, First of all, you need a shop that is used to working on old motors. Your's sounds out of their league. Second of all, you need to make sure the timing is correct, and get your carbs adjusted correctly. There is a post on the link and sync from Clams Canino, which willl help you get on track, which I will add to this posting. Also check the sparkplug wires for arcing, while the motor is running (cowling off). Check the bottom spark plug for signs of water(bad) and fouling (not so bad). if you see fouling, your fuel pump may need to be rebuilt, but it is cheap and easy. if you see water, post back for the solution. oh yeah, you do not have any points on that motor, or more succinctly, "We don't need no stinkin points".

Now on to the link and sync.

Manual specs are different for each but this is close enough to get them all REAL close. This assumes nothing is broken or "wrong" with it. This also assumes that the timing pointer is properly adjusted (if adjustable) to accurately read TDC of the #1 piston.

This basic procedure will work for all the 2/3/4/6 cylinder inlines from 1960 to 1988. It's not for the newer 3 and 4 cylinder loopers.

Engine off

Disconnect throttle cable

Pull throttle arm to full throttle and verify carbs open fully. Adjust main throttle stop so that the carbs are JUST hitting thier own little throttle stops. The idea here is that you want the screw stop and NOT the carbs acting as the "final stop" for your throttle lever.

Return the throttle to idle, carbs should be closed. Set all idle screws to 2 1/4 turns out from lightly seated.

Remove all plugs except #1, hook a timing light to #1. Then follow the narritive.

When the throttle is in the idle position the carbs should be fully closed and the ignition retarded. It the amount of retardation that determines your idle speed and is set later with the idle stop screw.

Timing Event One - throttle pickup.
As the throttle is advanced, the spark timing advances toward TDC and then to before TDC. As the timing is just passing TDC the 1st (minor)throttle pickup should hit and start to open the carbs. Use the timing light. Adjust the tang or trigger screw to make this a reality. All of them are speced different, but if the 1st pickup hits at 3-4 degrees BTDC advance - you win. Use the timing light with a friend cranking it to complete this setting.

Timing Event Two - max advance.
As you continue to advance the throttle the timing will advance while the carbs slowly open a bit more. The next adjustment is max advance, this is the point where the distibutor (or trigger) can advance no more. Set the max-advance stop to 21 degrees for "direct charge" inlines and 34 degrees for the older crossflows. Use the light, - err to the side of caution.

Right after max advance, further throttle motion should cause the second throttle pickup to hit and open the carbs to full. You already set that -now verify it.

Leave the throttle cable disconnected and go to the water. Leave the boat strapped to the trailer and back it in so it's deep enought to run. Have a friend crank it while you operate the choke and throttle. Warm it up at 1500 rpms's.

When "warm" adjust the idle stop screw for about 800 rpm and then ajust your idle needle(s) in (one at a time) till the motor stumbles. Quickly backing it out 1/2 turn from "stumble" will usually save it from needing a re-start. Do that for all the idle screws. Re-set idle stop for 800.

Now have your friend put it in gear (make SURE you are on the transom NOT in the water) and reset your idle stop for as low as you can get it. 500-600rpm is great if your old reeds will let it idle that low in gear. Now, re-adjust the idle screws, in to stumble, then 1/2 turn out. When you've gone back and forth with that a few times and have it "perfect" - adjust each one out 1/4 turn.

Reset the idle stop so that it's about 700-800 out of gear and 500-600 in gear. Sometimes bad reeds won't let you get that low, you've GOT to get it under 1000 in neutral (worst case) to avoid beating up your lower unit when you shift into gear, even 1000 is "bad" but like I said "worst case".

Now adjust (and hook up)the throttle cable so that with the shifter in neutral, there is light to moderate pressure holding the throttle arm against the idle stop.

Take the boat off the trailer and go out in the water. Try the hole shot. If it falls on it's face try adjusting the idle screws out 1/4 turn at a time (all of them together) till you can get a hole shot. You can kind of do this on the trailer at the ramp, but it's not really safe for you, the boat, or the tow vehicle.

You're done.

-W

 

[fogie27]

Re: 1975 Mercury 85Hp

Thank you for all the great information!

Do you not think there is that much differance in the gearing from the original 85hp lower unit to the 115hp lower unit that I put on?

Thank you again,
Dennis

 

[emckelvy]

Re: 1975 Mercury 85Hp

Dennis, your original lower unit had a 2.3:1 gear ratio. The 115 Merc (unless it was a '71 or older) would have 2:1 gears. This change would be the same as going to a prop with 2" more pitch, which would make the boat harder to plane off.

But still, on a boat that size your 850 should have no problems getting going. 15" prop is pretty low-pitched.

Now, if you got hold of an older 115 L/U, it has a 1.78:1 gear ratio which would be like having a prop with 4" more pitch. This might be a little more than the motor can handle.

An easy way to check this is (with the ign off and spark plug wires pulled), place in Fwd gear and turn the flywheel while watching the prop.

Count how many turns of the flywheel it takes to get the prop to turn once. This is your gear ratio and that'll tell you if there's an issue there. Correct direction of rotation is clockwise as viewed from the top of the motor. If you have a 1/2" drive socket set, a ratchet or breaker bar with 15/16" socket will ease the task. Or pull out the spark plugs and rotate by hand.

BTW, 2-1/2 turns out on the idle mix screws is usually far too much. 1-1/2 turns would be a good starting point.

With the motor warmed up and idling in gear, turn the idle mixture screws in (clockwise), one-at-a-time, until it leans out and wants to stall. This is Full-Lean. Then, go CCW; speed will pick up, then as you continue further rpm will drop and the motor will 'blubber'. This is Full-Rich.

Set the idle mixture slightly rich of the midpoint between Full Lean and Full Rich.

Do this for both carbs. If it bogs upon acceleration, adjust both needles another 1/8 turn rich until no bogging.

Anyway, give the above a shot, let us know what you find...........ed