77 Merc 115 Running troubles

MWB BigMike

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Joined
Aug 30, 2006
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16
I have a 77 Merc 115 outboard. It starts fine and idles fine. When I give her I get nothing. I can put around the lake fine but when I give it throttle it sounds like it is bogging out. I have cleaned the carbs and dist. cap. I have also changed the spark plugs and rebuilt the fuel pump with new seals and rubber belows. Also I replaced all the fuel lines.

I am losing all patients. I have had the carbs off 3 times, and no luck. I am in slinger Wi if anyone knows anything PLEASE! help me.
 

Chris1956

Supreme Mariner
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Mar 25, 2004
Messages
27,036
Re: 77 Merc 115 Running troubles

Mike, It sounds like the idle mixture for the motor may be too lean. If the mixture is set for a smooth idle, they simply will not accelerate. I recommend you set the idle mixture screws 1.5 turns our from lightly seated, and adjust them out 1/8 of a turn, trying to accelerate after each adjustment. Do one carb at a time starting with the top carb, try to accelerate, then mid carb, try to accelerate, then bottom carb and try to accelerate. The bottom carb runs a bit richer than the others, so it may not need too much richening. You will eventually find a setting where it idles reasonably well and accelerates well.
 

MWB BigMike

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Aug 30, 2006
Messages
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Re: 77 Merc 115 Running troubles

I have not tried a link and sync. Please forgive my ignorance as this is my first boat, but what is it?

Does this sound like a carb problem. The other day I took it out and it ran like crap. So on the water I took off the fuel pump screen and cleared itout. At the same time I took the dist. cap off to look in it.When I put it back together and ran it it started off bad then it cleared up and ran perfect for about 10 min. Then it just shut off while we where toolin across the lake. Wierd! After I started it back up, same stuff motor was waorking but no go.
 

Chris1956

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Mar 25, 2004
Messages
27,036
Re: 77 Merc 115 Running troubles

Mike, That last description is quite different from the initial description. I am not sure what to advise. Here is thelink&sync procedure made famous by our inline motor expert, Calms Canino. It is simply a way to make sure the ignition timing is good, and the carbs are synchronized with the timing. Good Luck

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

--------------------
C.C. Marine - Lake Hartwell S.C.
(cc-marine@earthlink.net)
 

Bob_VT

Moderator & Unofficial iBoats Historian
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May 19, 2001
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26,019
Re: 77 Merc 115 Running troubles

Be absolutely certain that timing event one is working

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.


Without the proper fuel to timing mixture you will bog badly.
 

MWB BigMike

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Aug 30, 2006
Messages
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Re: 77 Merc 115 Running troubles

so last night i took it out. it started out doggy. then for a miracle 10 min it ran great. then it just lost power and died. after this it didnt want to even idle right for like 5 min. i adjusted the idle screws with no success. i am lost
 

MWB BigMike

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Aug 30, 2006
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Re: 77 Merc 115 Running troubles

It is a really wierd problem. i am very proficient in cars and motorcycles but this is killing me. All I want to do is toss my kids on a tube and get out there
 

Plainsman

Rear Admiral
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Apr 2, 2006
Messages
4,062
Re: 77 Merc 115 Running troubles

Do you have ggod spark at all cylinders? Before my switch box went I had similar problems. Maybe that's someting to look at.
 

MWB BigMike

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Aug 30, 2006
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Re: 77 Merc 115 Running troubles

How exactly would I clarify "good spark" i nkow I have spark byt how would I tell what is enough spark.
 

Plainsman

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Apr 2, 2006
Messages
4,062
Re: 77 Merc 115 Running troubles

You can get a spark tester at an auto parts store. If it's the dial type tester, set it to 7/16 and you should get a strong blue spark.
If the tester in an enclosed type, there are no settings, you should just a strong blue spark.
Testing at dusk or dawn will be a good time. Or I guess in the dark if you want to.
 

MWB BigMike

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Aug 30, 2006
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Re: 77 Merc 115 Running troubles

When I free rev the engine I can get the full rpm range out of the motor. When it is under load I can only get about 2400 rpm. The carbs are wide open and I am getting plenty of fuel. Is the switch box a good possibility? It is 400 bux so i dont want to dump that much and still be stuck.
 

gungalunge

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Mar 5, 2006
Messages
126
Re: 77 Merc 115 Running troubles

Big Mike I had a simular incident with my engine doing all of the stuff yours does .. ..all that stuff. I found the pump bulb feeding fuel to the engine came apart and was restricting my fuel line I replaced the whole thing . my engine came alive . I later took the old fuel hose a part and some plastic peice shot out of it and i got gas in my face . then i had to retune everything on my engine that I messed with durring the incident . if you can just try a new complete fuel hose set up from your gas tank to the engine . It worked for me Even though I thaught i had a good fuel supply .
 

akita8

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Jul 15, 2006
Messages
18
Re: 77 Merc 115 Running troubles

You did'nt mention if you did a compression check. The symptoms could be indicitive of a bad cylinder.
 

MWB BigMike

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Aug 30, 2006
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Re: 77 Merc 115 Running troubles

i will certainly try the fuel line. something has got to give.
 

ricksrster

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Jun 19, 2005
Messages
2,022
Re: 77 Merc 115 Running troubles

Also try adjusting the tilt. My motor would bog down when I added more people in the boat. Adjusting the tilt made it zoom right along.
 

MWB BigMike

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Aug 30, 2006
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Re: 77 Merc 115 Running troubles

OK!! I replaced the fuel tank and line with the bulb. Also for shits and grins I tried a new set of carbs. Still the same headache. Any other ideas?
 

laker925

Petty Officer 3rd Class
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Jun 10, 2005
Messages
85
Re: 77 Merc 115 Running troubles

sounds like the pros know what they are talking about!
 

MWB BigMike

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Aug 30, 2006
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Re: 77 Merc 115 Running troubles

Well I took her out again, and no sucess. Now it runs the same as before, but it just cuts off for no reason. It only used to only do this when I was at WOT. Please I need advice.
 
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