1978 Mercury tower of power 115 bogs under load

[ak.man1]

Thank you fellas for all your advice. Looks like new pistons are not readily available anyhow for this old girl. I have decided to rebuild the carbs, throw a new impeller, and check the timing. Then run it and hope the pistons tops do not continue to deteriorate. If they do then I guess I will have lots of good parts to sell to help others out and look for another used motor. Thanks again.

 

[ak.man1]

Ok fellas sorry for the delay on report. So to recap my 1978 inline 115hp mercury fouled #2 and #4 plug while running wide open with aluminum frags.

I have done the following in an attempt to repair:
Changed gas
Changed plugs
Rebuilt fuel pump
Rebuilt carbs
Dropped pitch on prop to 15
Ran seafoam to decarb
New coil wire to distributor
Checked TDC and BTDC
Checked compression = only +/- 6psi

Ran on hose and fired right up and idled fine but any throttle would cause it to run rough. Took it to open water and same thing. Opened it up all the way up and I could only get 3500 rpms and still ran rough. Most of the shops around here refuse to even look at it and the other couple are too busy and said wait until winter.

What else does a basic handyman look for? My guess is it is electrical. Stator/trigger/resistor? I only have an amp/ohm meter for testing. Any testing tricks or Any other ideas gents???

 

[charakel]

From my experience with a 75HP Chrysler motor that had a not so good piston was that even if your top of the piston is good, the sides may be shot. The motor would run and idle fine, but when in the water no power what so ever and it would motor would die off. Pulled the head off and the top of #4 piston was ok and looked like it was starting to detreated, however if I looked closely at the sides they were going and I could see the rings.

 

[ak.man1]

I still have enough power to get up on plane and my compression is good on all cyclinders. I had my transfer covers off and piston and rings look good. I appreciate your response but not sure that is the problem.

 

[Chris1956]

Check/set the timing very carefully. You will want 4-6* BTDC for idle pickup timing and 21*BTDC for max spark advance.

I am surprised pistons are NLA. They were easily found aftermarket, just a few years ago.

 

[ak.man1]

I planned on setting the last timing adjustments on the water with a light when we were out but I didn't because it is noticeably running rough with any throttle given which is way before my primary/secondary adjustments even come into play. And since I can't get the rpms up to WOT I don't think I can set the timing correctly for these? maybe I am misunderstanding the book or you as I am very new to this motor and getting frustrated the summer is in full swing and I have yet to take my family on a successful trip. Thankfully all of you are willing to share some advice as my local mechanics pretty much tell me to pound sand cause it's too old for them to waste their time on.

 

[Chris1956]

You can set the timing successfully on land, with the motor not running. Remove all spark plugs and prop. Set motor to TDC using a steel ruler or caliper, thru #1 spark plug hole. Note position of flywheel. It should show close to TDC, Note measurement to top of piston from cylinder head. Back the piston down until ruler or caliper shows it has traveled .464 inches. Timing pointer should point to the .464 mark on timing decal. If not set it to point to it.

Install spark plugs 2-6 into their wires and wire their bases together and to ground. Install #1 plug and put timing light on #1 spark plug wire.

Using a clip lead, jumper the starter solenoid so the motor cranks. Chek timing and advance throttle (in gear) until timing is 4-6*BTDC. At this point carbs should still be closed, but ready to open. There are two bolts on the midsection of the distributor that loosen (5/16" wrench) and allow the idle timing collar to rotate to adjust the opening of the carbs. Set this snugly against the carb lever. That is idle pickup timing.

Advance throttle to full. Check timing. Back throttle down until you get 21* BTDC, if necessary. Tighten the max spark advance set screw (upper right of dist. housing) to stop further advance of the distributor. Alternatively, you may need to loosen the set screw to allow more advance.

Set throttle stop set screw to allow carbs to open fully, but no more.

Install plugs, install flusher and start motor normally. Set idle stop screw (and adjust throttle cable, if necessary) to allow motor to idle at 900-1000 RPMs.

If she idle well in the water, and will accelerate, you are ready to rumble....

 

[ak.man1]

Thank you so much Chris! I will keep you posted once I get a chance to try this out. Fantastic that people on this site take the time to help each other out. Seems like these days that is disappearing.

 

[ak.man1]

Happy 4th fellas. So I began timing as instructed by Chris and realized I do not have the plate that has the throttle stop screw, maximum spark advance crew, or idle adjustment screw. if I am looking at this correctly there is no way I can keep the distributer base set in time to advance the throttle to set my others points. Am I correct? No idea what happened to it or why it ran fine when I tested it to buy but here I am none the less. If this plate is necessary what is this plate called and does any one have one available to sell? Thank you.

 

[Faztbullet]

Ya gotta have it!!! Where ya located???

 

[ak.man1]

I am in WA State. I figured I needed the plate. No idea why someone would remove and not replace. I have found one online locally and will get it put on and finish the timing sequence. Thank you for the reply!

 

[ak.man1]

Well I broke the cardinal rule of do not do multiple repairs until each one is complete and tested. I got my idle screw plate today and installed and followed Chris instructions to the letter. I thought I found the problem as wot was showing 28 degrees so I adjusted to 21. After that everything seemed to be just right by the timing light so I fired it up and it idled super high then cut out and ran crappy and died. Pulled the plate put things back to where they were and still horrible. Light smoke came out #2 carb and when I pulled the plugs the block was way warmer then it should be and light smoke came out #1, #4, #6 plug hole. Then I remembered a few weeks ago I noticed that my rotor had a rough blemish on one side and the cap was full of dust. I thought this might be the problem with it running so bad and since I can't afford a new rotor I touched the end up with light sand paper and scuffed the tabs in the cap to clean them up. I am assuming this was the wrong thing to do. If this is the problem I am dead in the water per say unless someone has a good used one. They want $100's for a new rotor. If this is not the problem then I am still stumped on what to do.

 

[jimmbo]

Your pressure is not too bad, those engines rarely get more than 6 - 7 psi at WOT,

You mention you haven't been able to get 5000 rpm. Was it like that prior to the problems? As mentioned that engines WOT rpm range is 5000 - 5500.
The Local mechanic has a point, it is a 40 yr old engine and when they suffer detonation or preignition it will get expensive fast. Myself I like the old inlines and would rebuilt it.

 

[Chris1956]

Gee, install the distributor plate, and set the timing as described. Sanding the rotor a bit is not an issue. Wipe out the dist. cap and make sure it is installed properly. It fits in a tab on the distributor base, to locate it's position. However, the tab is a bit sloppy, so recheck (or set timing) after it is installed.

If she is in time, not overheating and the spark plug wires are not arcing to ground, and she still runs crappy, suspect the carbs.

 

[ak.man1]

Ok will do. As far as the carbs I just rebuilt and when I pump the bulb fuel comes out all 3 when bulb is hard. Is there another carb check I can do?

 

[Chris1956]

If fuel drips out of the carbs, something is wrong. Usually it has to do with the floats. The orig. floats were foam and will disolve in today's fuel. The newer floats were hard plastic and can crack. Replace the foam ones, and inspect the hard plastic ones.

Did you set the float heights correctly? Did yo install new inlet needles and seats? original inlet needles and seats were brass/rubber with nylon needles. Updated inlet needles are SS with neopreme tips, and all brass seats. Some seats have a copper gasket under it.

 

[CharlieB]

These old carbs cannot take much fuel pressure. A firm handshake on the primer bulb is all that is needed, anything more can overcome the needle/seat seal and cause flooding.

Generally, if you can hold the carb upside down and blow into the fuel inlet to check the needle/seat sealing. If it holds you're good.

Far too much timing may have been one of you largest problems. Finish perfecting the tune up and keep posting your progress. You're almost done.

 

[ak.man1]

Yes I bought the rebuild kit for each one and replaced floats, needles, etc. I adjusted according to the book 13/32" and 1/4". Already ran boat with carbs rebuilt and it fired up and ran but acted funny when throttle was added. That's when I started working on the timing. So frustrating... I am going to try checking everything again this weekend.

On another note a local guy has a 1992 Suzuki 115hp 2 stroke for $1500 with harness, and controls. Any thoughts on that motor. If I can't get this running right what would the most economical used motor should I start saving and looking for? Thanks again guys.

 

[Chris1956]

If the motor bogs when accelerating, the carbs could be set too lean. if you adjusted them for best idle, they are almost certainly too lean. Richen the top carb 1/8 turn (open) and test acceleration. Repeat with middle carb and then bottom carb, testing between each adjustment.

 

[ak.man1]

Well I think it's alive again. I rechecked compression, spark, timing, and found # 3 plug fouled. Pulled #2 carb and did the needle seat test, rechecked float gaps, and all was good. Reinstalled carb and found that the connecting arm to #1 carb was out of adjustment. Butterfly on #2 was open 1/4 of the way when it should be fully closed. So readjusted and rechecked timing. Fired up on hose and made idle mix adjustments and I believe we are ok. I plan to run out to the lake in the morning and see how performance is in the water. I will report on my findings tomorrow.