# Acceleration flat spot in Merc 80 outboard



## Bob Shaw (Mar 22, 2008)

I've got a '79 Merc 80 outboard, that I recently had to overhaul the carburators due one of them flooding. Needed new needle and seat. Cleaned em real good with carburator cleaner, and now I get a flat spot in acceleration at about 1/4 throttle. Get past that and it'll throw you out of the back of the boat. It acts like it may be starving for fuel. If I don't give it more gas, it will actually stall. Does anyone have any idea what I may be running into? Could it just be the idle adjustment, or, is there a medium speed jet that could be the issue?

I did have a problem with the wiring harness, that I just replaced, and haven't had it back in the water since, but, I don't see how that could cause this problem. Sure could use some advice.

Thanks, in advance.

Sorry if this is a duplicate. I tried to edit and hit the back key.
Bob.


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## Big Al (Mar 25, 2008)

I had a '78 Merc 140 "Tower o' Power" that was doing the exact same thing. It would wind up fine until it got to about 2000 rpm and then it would fall flat on it's face. If you ever got it past that it would get up and go just fine. Finally found that my reed valves were gummed up. Try cleaning your reed valves, that was what my problem was. It ran good after that. Hope this helps.


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## Bob Shaw (Mar 25, 2008)

Big Al said:


> I had a '78 Merc 140 "Tower o' Power" that was doing the exact same thing. It would wind up fine until it got to about 2000 rpm and then it would fall flat on it's face. If you ever got it past that it would get up and go just fine. Finally found that my reed valves were gummed up. Try cleaning your reed valves, that was what my problem was. It ran good after that. Hope this helps.



Thanks Big Al, I'll chgeck it out.


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## GoldDot40 (Mar 25, 2008)

Is it equipped with any type of gas filter? How long has it been since you changed it? Also check your spark plug(s). Any time you have an engine that experiences carb problems, there's a good chance the plug got fouled. If it checks out OK, then move on to your air mixture screws OR something else like cleaning the reed valves, etc. When you finally get it running decent, run a bottle of Sea Foam through a tank of gas. I'm guessing you'll notice a nice positive difference overall.


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## Bob Shaw (Mar 25, 2008)

*Bassquatch, I hope you're right*

Bassquatch, I sure hope you're right. I'll check the plugs and the Idle adjustment (I don't think there is a fuel filter). I hope it's not the Reed valves, the only place I see Reed Valves in the Clymer manual are part of a complete overhaul. I really don't want to get into it that far. It didn't have the flat spot until I overhauled the carburators, so the fouled plugs or idle adjustment seem to make sense. I'll keep checking.  I'm going to put it in the water the first weekend in April so, we'll see how it runs. That should give me a good chance to set the idle, now that I have the tach working again. 

Thanks, again.

Bob


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## Slug-Gunner (Mar 26, 2008)

*Sea-Foam FIRST!*

Try putting Sea-Foam in the fuel FIRST, BEFORE YOU TRY DOING ANY ADJUSTMENTS. If it is the 'reed valves', it will clean them for you (especially, if you let it set over the winter with fuel in the carb bowls). Sea-Foam can be used in place of Sta-Bil when storing an engine too. 

"Idle Adjustment" will HAVE NOTHING TO DO WITH YOUR "FLAT-SPOT" PROBLEM.
This is usually related to the "transition" jetting or fuel control in a carburetor.... probably one of the functions of the "reed valves". "Reed Valves" can be easliy damaged when overhauling a carb too by "mishandling" them.

HINT:
Always disconnect the fuel line (or turn it off) and let the engine run until it quits on its own due to carbs RUNNING DRY.... you'll save a lot of fuel related problems by doing this. This also works well on lawn mowers and other small engines used around the yard - old 'stale' fuel is the cause of many small engine problems when left to set in the carbs or fuel tank for more than a couple months.


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## Bob Shaw (Mar 27, 2008)

Thanks Slug-Gunner, I bought this boat last summer and it had sat for as much as 7 years. When I got it running, it actually started and ran, the upper carb start started flooding, but I did not have a flat spot. After I over-hauled the carbs, I got the flat-spot. 

I'm definitely going to try the Sea Foam. I'm not sure what the reed valves are or what their functions are. I'll be at the lake late next week, so that will be a good oppportunity to try it out and to run some Sea Foam through it.

This is great. This is the kind of advice I needed.


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## Slug-Gunner (Mar 28, 2008)

*Quick Question ???*

Bob,

Do the float bowls on your carbs have an "adjustment" in the bottom center of the float bowls?  If so, is this "adjustment" a 'tapered needle' that goes into a jet directly above the bottom center of the float bowl?

If this is the case, then the "adjustment" of this needle can possibly affect the "flat spot" you are experiencing. Let me know and I can help you with this "adjustment" as it may control this "transition" area of the fuel control. Pull the fuel bowls off to be sure no "crud" has gotten in them from the fuel tanks too - unless you have gotten brand new fuel tanks to use.

I would still suggest using the Sea-Foam in the fuel system, especially if it had set for 7 years prior to your getting the boat. I'd also suggest putting an in-line fuel filter between your fuel tank and the inlet line to your carbs. The Sea-Foam will probably break loose a lot of "gunk" that has accumulated in your fuel tank and you don't need it fouling up your rebuilt carbs and clogging the jets and passages up. Try using one of the clear in-line filters so you can see what accumulates - probably need 5/16" or 3/8" fitting size to fit your fuel lines. 

I helped a friend who had bought a 18' flat-bottom boat with a 25 HP Mercury outboard on it who had similar problems with it that had sat for 5 years before he got it. He ended up having to have the water pump replaced  in it due to overheating before we got the "flat-spot" running worked out. His had the "metering jet" adjustment in the base of the float bowls (3 carb setup). Also had to totally rewire all electrics on the boat since either squirrells or mice had eaten the insulation off the wiring in many, many areas. Works great now, thanks to Sea-Foam and a little "knowledge" of engine mechanics.

Slug-Gunner


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## Twenty five ought six (Mar 28, 2008)

You may need to synchronize the carbs.

Look at this:

http://forums.iboats.com/showthread.php?t=168855


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## Back2class (Mar 28, 2008)

I had a 150hp Merc on a 16ft that was kind of the same. Started great, ran low rpm great with no idle problems. But woud take forvever to get on plane, almost like you were giving it 1/3 throttle even though it was pinned. Once on plane it would LAUNCH from 30mph to about 70 in seconds! So long as on plane it would throw you back in your seat. I sold the boat without ever fixing it but it soulds like what you are talking about. Wish I could be more help.


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## Bob Shaw (Apr 13, 2008)

Well, I shoulda listened. I ran a tank with Sea Foam, and backed off the idle jets 1/4 turn twice and got a great hole shot, then the lower carb started flooding. I guess the Sea Foam washed some trash out of the tank. Shoulda put in an in-line fuel filter like Slug-
Gunner suggested. Now I gotta pull the carb again. But, for what it's worth, it ran like a scalded dog for a while.


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