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Mercury 4hp Gnat

R

Ready4It

Contributing Member
Friends dad gave me this mercury 40 4hp gnat. It's the 1978 model he said. It's been sitting for 4 years he said. He said it needed 2 br6hs spark plugs and maybe a little touch up.

So I bought the plugs, put them in, added gas, open gas lid thingy, open fuel, put lever on neutral, open choke, turn handle to start, and started pulling. And yes I guess you guys know why I'm here now haha.

Yeah it didn't start. If I switch it from neutral to forward and pull cord and the prop spins so I know the motor is turning over and such.
Maybe no spark? Or is the motor still to cold?


What should I do now? Thank you in advance!
 
Your friend's dad didn't do you any favors loading that clunker on you. (Got one in the shop that's been gathering dust for years--and for good reason.)

If the wimpy recoil starter hasn't self-destructed yet, pull the plugs and do a compression test and see if it has spark.

Jeff
 
kimcrwbr1 said:
That has the inline fuel filter correct? Pull the float bowl and see if fuel is getting to the carb? Do the spark plugs smell like fuel?
Click to expand...

Yes inline fuel filter.
Is that this? (View attachment)
No the spark plugs don't smell like fuel. I pulled them out after a couple more pulls and they don't smell. but I can assume they are sparking because there is a slight tint of discoloration between gap or central electrode. unlike when it was new, that part was still shiny and silver.
 
Hope it works!
 

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fastjeff said:
Your friend's dad didn't do you any favors loading that clunker on you. (Got one in the shop that's been gathering dust for years--and for good reason.)

If the wimpy recoil starter hasn't self-destructed yet, pull the plugs and do a compression test and see if it has spark.

Jeff
Click to expand...

Oh really? I didn't know that it was that bad.

But let me post some pictures and hopefully you can tell if the recoil starters are bad
 
pic set 1
 

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pic set 2
 

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When I removed the flywheel, this little piece fell out and I thought it was something broken off. But from the parts diagram its a flywheel drive key. And I don't know how to assemble it correctly back under the flywheel.

Its part #2 on diagram
 

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is this the fuel lines fuel filter?
looks like its dirty and has debris in it?
I don't think there should be that much stuff in it. if I wanted to clean it, I would have to take it off the fuel lines I assume
 

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Update:
Cleaned fuel tank, changed fuel lines, cleaned strainer, cleaned carb twice, and got ready to pull and what do ya know.

The needle that shoots fuel into the valve is broken off....
Is there a needle out there that will fit and function in the same fashion that I could install?
 
#10 on diagram.

Isn't it suppose to shoot the gas into the valve? Mines is just spitting out fuel like a fountain. Therefore no gas is getting into the chamber.
 

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Yup you bet I'm SOL.

So how big is the opening at the tip of the nozzle suppose to be? This one is big enough that a toothpick can go thru it.
oh and shouldn't the fuel be leaping out so that it can mix with the air, rather than just slowly create a overflowing sink effect?
 
Last edited:
kimcrwbr1 said:
There are two circuits high and slow when at idle it pulls the fuel through the adjustable slow needle before the butterfly and as the butterfly opens the vacuum pressure sucks the fuel up the high speed nozzle. The slow needle mixes the air fuel for a proper idle. There are videos on how to rebuild a carb but that is one of the simplest carbs built. You can get a gasket set and float but I cannot find a float needle and seat. Get a can of carb cleaner and take it apart and clean all the circuits good with carb cleaner then blast it with a high pressure hose then dry it with compressed air take a fine copper wire and clean all the real small holes first. Then put it back together you may get lucky.
Click to expand...

Yes I understand that.
I just wanna know If the tip of the nozzle, which sticks out into the venturi between the choke intake and valve intake, is suppose to be a big opening or small opening. On the one I have, I can fit tooth pick all the way through it.
 
that's why I said in post #17, its just spitting out fuel like a fountain, because the nozzle end opening in between the choke&valve is big. unless it is that big of an opening.
 
I've decided to just buy a carb that is similar to this one and adjust the shafts and fitting. Hope the valve opening lines up and matches with the chamber .
However all of those carbs I'm looking at all have that spring on the screw under the float bowl. I'm just kind of scratching my head as to what it's function is.
 
I got the carburetor and made some shaft adjustments and it lines up perfectly.
I put it on and begin to start it.

fuel on, check.
pull choke, check.
3 pulls...nothing
2 pulls at 1/2 choke...nothing
1 pull at no choke...nothing

i repeat it 3 more times to get nothing. I can say the motor feels more alive when I pull it now then before though.

I checked spark plugs and they spark good.

Whats wrong now?
 
Open the throttle and shoot it right into the motor, then choke it and pull the recoil--carefully (so it won't disintegrate, if it hasn't yet).

Jeff
 
Update:

With help of some friends we all took turns pulling this baby.
Got it to start around the 12th continuous pull. I wasn't impressed with it taking that long to start up.
so we put motor into a garbage can with water and see its spitting water real good.
However we couldn't get the motor to idle without opening the throttle up to a certain degree.

so we turn off motor and touch the screws and took 8 pulls to start. Same story again .

Turn off and touch knobs again. Same results.
Then we quit cause we tired of pulling and adjusting it
 
Little darlings, ain't they?

I suspect your carb idle adjustment is way off. Try it at 1 1/2 turns out and see if that helps.

Jeff

PS: I'm amazed that Mickey Mouse recoil hasn't exploded yet!
 
Yeah the recoil design is a big flaw on this exact motor.

so there are actually 2 screws. One on the left side (if looking straight at choke) and one under the float bowl.
The idle adjustment is the one on the left I assume and I've been told the one under the float bowl is the high speed needle. Is this correct?

So turnthem all the way in then back out 1 and a 1/2 turns?
 
It's taking too long to start up.

I started it up this morning like this without my strong friends.

1) loosened the flywheel nut (not safe I know), it allowed flywheel to spin faster n longer. Started in 2 pulls lol
 
I've had a Gnat for years, because this unit doesn't have a fuel pump it relies on gravity feed. Fill the tank up and don't go below half full. The carb is a very poor design and needs constant tweeking with the fuel mixture. Don't use the stock pull cord, remove the engine cover and wrap a long rope around the flywheel to start or after it's been sitting for a while use a socket and a cordless drill. beautiful engine, smooth and quiet for a 2 stroke but a pain usually. If anyone has managed to find a fuel pump the works with this Gnat let me know. Larry
 
[email protected] said:
I've had a Gnat for years, because this unit doesn't have a fuel pump it relies on gravity feed. Fill the tank up and don't go below half full. The carb is a very poor design and needs constant tweeking with the fuel mixture. Don't use the stock pull cord, remove the engine cover and wrap a long rope around the flywheel to start or after it's been sitting for a while use a socket and a cordless drill. beautiful engine, smooth and quiet for a 2 stroke but a pain usually. If anyone has managed to find a fuel pump the works with this Gnat let me know. Larry
Click to expand...
Anyone know of a pull start upgrade?
 
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