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Old 06-10-2015, 10:21   #16
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Re: Go-No Go Mercury 3.5 Help please

Assuming it's not gas, there are many ways to check for a vacuum leak, You can use a piece of vacuum hose like a stethoscope and listen for it, or a little oil dribbled on places you think may be leaking, oil disappears you have found the leak, or use spray carburetor cleaner at leak suspect places and listen for the engine to change how it's running, if it changes, you have found the leak.

But a rubber stopper with a hole in it, is sort of asking for trouble gas wise, the new stuff seems to have an affinity for water, the alcohol in the gas will absorb water out of the air and cause problems.
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Old 06-10-2015, 11:35   #17
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Re: Go-No Go Mercury 3.5 Help please

yes gas can go bad in less than 6 months but your symptoms and the source of the gas make me think fuel filter. generally when the gas goes bad you get a gummed up carburetor.
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Old 06-10-2015, 12:00   #18
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Re: Go-No Go Mercury 3.5 Help please

I would start with fresh gas make sure you drain the carburetor so it and the lines are full of fresh fuel.if that doesn't work unscrew the drain on the bottom of the carb to make sure fuel flows freely. Of course in a well ventilated area and catch all the fuel. If you have good flow and it's a two stroke replace the reed valve with the one from the engine you got the carb off of.
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Old 06-10-2015, 12:03   #19
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Re: Go-No Go Mercury 3.5 Help please

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Originally Posted by Dave22q View Post
yes gas can go bad in less than 6 months but your symptoms and the source of the gas make me think fuel filter. generally when the gas goes bad you get a gummed up carburetor.
It seems to lose potency long before it gums things up. Still flows ok, just no energy in it.
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Old 06-10-2015, 13:56   #20
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Re: Go-No Go Mercury 3.5 Help please

I had a similar problem turned out to be bad fuel, had to drain and wash out the tank and carburettor.
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Old 06-10-2015, 14:54   #21
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Re: Go-No Go Mercury 3.5 Help please

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Originally Posted by belizesailor View Post
That or a fuel pump issue. Dont know this specific motor, does it have a diaphram pump like many OB's? A pin hole in the diaphram or leaky pump gasket can cause this.

Or, a dirty fuel filter.
I tend to agree on fuel pump possibility. I also had a problem once with my antique 7.5 HP Kiekhaffer Merc. The fuel hose between the tank and the carb de-laminated so that fuel flow was restricted. Very hard to find. Both pump & hose problems could be caused by ethanol.


One other problem I found once was a poor seal from the carb to the block. Loss of vacuum starved the engine. The bolts were loose but a bad gasket - same thing.
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Old 06-10-2015, 16:53   #22
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Re: Go-No Go Mercury 3.5 Help please

OP is using the tank on the engine so it will gravity feed. So that eliminates the fuel pump. The carb was also replaced I doubt it was put on loosely but you're right a loose carb seal will cause those symptoms
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Old 06-10-2015, 17:05   #23
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Re: Go-No Go Mercury 3.5 Help please

I have a 2.5 HP Merc, 2 stroke. When it acts like that, I drain the fuel bowl and put in a new filter (small, clear inline between tank and carb). If you are going to put in a filter, the length of hose is tricky. Be sure to cut out enough of the old one so that the total length with filter is the same. Otherwise the extra lower hanging loop will cause an embolism.
Get rid of all the gas, start with fresh stuff. And add stabilizer.
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Old 06-10-2015, 22:26   #24
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Re: Go-No Go Mercury 3.5 Help please

I had a problem like this and the gas cap was designed to be vented so that the fuel pump could work without creating a vacume in the fuel tank. Check the cap and or try another cap.
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Old 06-10-2015, 23:31   #25
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Re: Go-No Go Mercury 3.5 Help please

I had a problem like that on a 2.3 Honda. Turned out the ethanol had caused a small rubber fitting in the carb to slough. These got caught in the jets. Since you replaced the carb is it possible there are particles in your fuel tank? Also agree with cleaning out the fuel tank. At the end of the season I have taken to removing the tank turning it over and getting every last drop of gas out of it. Then I clean the tank with a little acetone. It takes about 20 minutes but considering I spent hours last season cleaning the carb and the fuel lines etc. It seems worth the trouble.
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Old 07-10-2015, 04:01   #26
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Re: Go-No Go Mercury 3.5 Help please

I have a Yamaha 4hp 4-stroke which was having all sorts of running problems, hard to start, wouldn't run steadily, stalled under load etc. After a great deal of expensive investigation, it turned out to be that the spacers at the end of the carburettor had been replaced the wrong way round, - they are different sizes! This meant the butterfly valve attached to the choke couldn't open or shut properly. Constantly running with some but not all choke so it didn't want to start, wouldn't produce full power and died when warmed up. It may not be your problem, but it's worth checking!
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Old 07-10-2015, 13:58   #27
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Re: Go-No Go Mercury 3.5 Help please

Thanks everyone, I've tried the new fuel, went for about half a minute then went back to same problem, I have an ace engine mechanic here in PR whos advice is to return it to the dealer in San Juan under it's warranty as its foxed him. so that may be all we can do. but thanks again
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Old 07-10-2015, 16:59   #28
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Re: Go-No Go Mercury 3.5 Help please

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Originally Posted by sparrowhawk1 View Post
OP is using the tank on the engine so it will gravity feed. So that eliminates the fuel pump. The carb was also replaced I doubt it was put on loosely but you're right a loose carb seal will cause those symptoms
As I noted, the fuel line delamination was on an antique merc. No fuel pump and the tank was around the flywheel above the carb. A bad fuel hose will do this.
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Old 07-10-2015, 17:09   #29
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Re: Go-No Go Mercury 3.5 Help please

Guys, it runs although poorly at full throttle when fuel demand is highest, and a new known good carb and fresh fuel didn't fix it.
So, anything that would reduce fuel flow, bad fuel pumps, clogged hoses, clogged vent caps and the carb are not the problem.
If you have a vacuum leak, engine will run although poorly at full throttle as there is almost no vacuum, so almost no leak, but partial throttle when vacuum is high, leak is large and engine won't run.

I don't remember but did we find out if it's a two or a four stroke? Two stroke may have reed valves and possibly even crankcase seals leaking, four stroke could be a stuck or bent valve as well as vacuum leak.

When you find out what it is, will you post the answer?


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Old 07-10-2015, 20:33   #30
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Re: Go-No Go Mercury 3.5 Help please

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Originally Posted by a64pilot View Post
Guys, it runs although poorly at full throttle when fuel demand is highest, and a new known good carb and fresh fuel didn't fix it.
So, anything that would reduce fuel flow, bad fuel pumps, clogged hoses, clogged vent caps and the carb are not the problem.
If you have a vacuum leak, engine will run although poorly at full throttle as there is almost no vacuum, so almost no leak, but partial throttle when vacuum is high, leak is large and engine won't run.

I don't remember but did we find out if it's a two or a four stroke? Two stroke may have reed valves and possibly even crankcase seals leaking, four stroke could be a stuck or bent valve as well as vacuum leak.

When you find out what it is, will you post the answer?


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Again, My old OB would always start & idle, or run slowly. As throttle increased, the delaminated hose would collapse & choke the flow. It will cost only a couple dollars to eliminate this from your list. The hose I replaced even looked fine when inspected. I happened to notice the liner loose near the barbed fitting. On inspection, the lining was a limp tube. Fuel could get behind it and flow was nearly blocked. If you idled, the carb would re-fill. Good then for a quick run and then quit.

I've also had motors including a car where I replaced plugs & found new plugs to be bad.
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