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Outboard making me lose my mind


mrgnstrn

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howdy y'all.  I need some help with my sanity.

I have an older Nissan outboard that will not run without choke on. and even then only at idle.

particulars: 1996 Nissan 3.5 hp - 2 stroke with internal fuel tank.

engine is based on a Tohatsu, also same as the 3.3 hp 2-stroke Mercury.

Carb has been fully rebuilt, and what wasn't replaced was cleaned with brake cleaner.

Here is what has been cleaned: tank, fuel filter, fuel valve, fuel hose.
also replaced the hose clamps on the hose, and the o-ring between the carb and the intake.
cleaned and checked the tank vent.

The only thing in the fuel system that wasnt replaced was the main jet, which got the brake cleaner treatment, and is back to bare metal. no varnish or other gunk.

It starts reliably with choke, and will only run for ~5 seconds with choke off.
it will run roughly with choke, but it wont die, but on in idle. any higher speed and it dies.

It has all the signs of "not enough fuel" but there is no obstruction as far as I can tell. fuel drains quickly thru the valve, the fuel bowl, etc, so it seems like everything up to the main jet is good.

There is only one jet. it is not adjustable as far as I can tell.

I can get it to run in idle with choke off, but only for 5 seconds, and if I switch back to choke for 2-3 seconds, it will last another 5 seconds with choke off.

 

the main jet is clean, but doesnt act it. i am losing my mind. help!!

 

-M

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I have a 4hp 2-stroke Merc, but it's been good to me, first pull (with a can of quick-start) this year. 

How beat is the engine: piston slap, rings?  If it got cooked, it might not run right again until at least honed or more.

I've started using a bit of seafoam in my gas.  It does sound like insufficient gas, but I'd think if you can't open the throttle at all without it dying, and it won't take load, then, yeah, I don't think you are nuts, you might still have a fuel-delivery issue.  I screwed up the re-assembly of a 4-stroke generator carb once.  Mis-assembled the float valve putting on the bowl, couldn't see my mistake.  How many times have you done all that final assembly to make sure it wasn't something stupid?  Air-leak post-carb could do it too.

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cheap and easy ideas:

replaced the plug?

checked plug wire for good connector?

replaced the gas?

checked for good spark against the block?

checked float height?

(I agree it's likely a fuel issue but check easy stuff first)

Maybe:

find somebody else with same 3.3/3.5 motor and ask to swap carbs. 

expensive:

buy a new carb

 

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1 hour ago, bacq2bacq said:

I have a 4hp 2-stroke Merc, but it's been good to me, first pull (with a can of quick-start) this year. 

How beat is the engine: piston slap, rings?  If it got cooked, it might not run right again until at least honed or more.

I've started using a bit of seafoam in my gas.  It does sound like insufficient gas, but I'd think if you can't open the throttle at all without it dying, and it won't take load, then, yeah, I don't think you are nuts, you might still have a fuel-delivery issue.  I screwed up the re-assembly of a 4-stroke generator carb once.  Mis-assembled the float valve putting on the bowl, couldn't see my mistake.  How many times have you done all that final assembly to make sure it wasn't something stupid?  Air-leak post-carb could do it too.

The engine is in really good shape. and runs with choke....but never without anymore.

I have disassembled and reassembled the carb about 50 times by this point. i doubt i am so consistent to screw it up in the same way every time, so i doubt that is the cause...

but thanks for helping me narrow down what it is and isnt.

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58 minutes ago, Zonker said:

cheap and easy ideas:

replaced the plug?

checked plug wire for good connector?

replaced the gas?

checked for good spark against the block?

checked float height?

(I agree it's likely a fuel issue but check easy stuff first)

Maybe:

find somebody else with same 3.3/3.5 motor and ask to swap carbs. 

expensive:

buy a new carb

 

I have not directly checked spark, but I have run it for 5 minutes at a shot, by alternating the choke on and off. I deduce if I had shitty spark, I could not do that. I did pull the plug and didn’t notice anything really amiss. 

For fuel, I tried some of that “tru fuel” pre mixed for chainsaws and the like.  It doesn’t have any ethanol. No better performance. I’m trying a third source tomorrow. 

How does one check the float height with the bowl installed?

ive already replaced all the internals of the carb, my wife will kill me if I have to buy another freakin carb. 

Ill see if someone even has this motor to swap carbs; I don’t  hold any hope for that. 

Any other ideas?

 

m

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Just now, Alcatraz5768 said:

Im sure you have removed the main jet and poked something through it. It sure sounds like the main jet to me.

Yes. Air and then brake cleaner and more air. 

The jet is down to the metal, no hunk dirt or varnish. 

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With the motor upright take the bowl off, the float should drop allowing fuel to flow, lift the float and see if it stops the flow.

Edit

Float should be level when it's shuts off the needle valve.

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Had the same issue with an Austrian built Makita chain saw, the carby manifold was warped, you might want to check with a straight edge next time you have it off.

Sent the repair shop guy nuts till he figured it out....

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You do open up the carb and see... Never mind - you google and find the Service Manual :)

https://www.sv-zanshin.com/r/manuals/tohatsu_service_manual2005-2.pdf

(38.5 MB file)

> There is no fuel pump on this - it's gravity fed so no fuel pump diaphragm

> change the spark plug even if it looks fine. Keep the old one as a spare. It costs very little but eliminates one item.

> Leave gas cap off. Eliminates the chance of vent being partly plugged.

Did you try this?

Unscrew the gas tank valve - it might have a mesh screen in it. It could be partly clogged.

"For the 2.5, 3.5 and 5B engines, the in-tank pickup filter is the only fuel filter used. This filter is part of the fuel cock, and if clogged or damaged, it must be replaced with a new fuel cock assembly."  Is this the filter you cleaned or replaced?

1 hour ago, mrgnstrn said:

"tru fuel” pre mixed for chainsaws and the like

Ooops. Maybe not a good idea. Their website says "TruFuel can be used in all air-cooled, 2-cycle equipment. Leaf blowers, line trimmers and chainsaws are the most common, but snow blowers, small generators and air-cooled outboard engines"

I would use a marine TCW3 2 stroke oil with at least a mid grade gas. If it's fresh having ethanol doesn't matter. If the gas is several weeks old the ethanol sucks.

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Sure sounds like a full or partial blockage in the main jet system to me.

Take the carb off and blow through the main jet while it is in the carb but use a high pressure compressed air line.

A known issue if you have been running fuel with an ethanol $ is the fuel line from tank to carb degrading. Worth changing just in case. 

 

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Sometimes when they  rebuild carbs they get some gasket-sealing compound or dirt loose in  the bowl which then stuffs up the jets.  Only solution is another cleaning.  Your symptoms sound a lot like carb.

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I have had good luck cleaning carbs using an ultrasonic cleaner. It can break up clogs in the tiny passages better than solvents or compressed air. Most good lawnmower repair shops will have an ultrasonic cleaner and will clean your carb for a modest fee

 

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Had a similar issue with a Honda. Actually bought a new carb. Since i'm at altitude, I checked to see what main jet I had been using, found it to be marked the same as the new one but had some sort of non-dissolvable deposit neatly within the bore. Put the new jet in the original carb. Motor works like new.

Buy a new main jet !

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It may have already been suggested but while the spark plug is out, check compression . Look at exhaust ports if there is a cover, they can get coked . Overall it sounds like another

ethanol induced casualty.

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Take the engine cover off....get the motor running and spray the carb and and head area  it could be a vacuum leak in between the carb and intake port

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15 hours ago, Zonker said:

Ooops. Maybe not a good idea. Their website says "TruFuel can be used in all air-cooled, 2-cycle equipment. Leaf blowers, line trimmers and chainsaws are the most common, but snow blowers, small generators and air-cooled outboard engines"

I would use a marine TCW3 2 stroke oil with at least a mid grade gas. If it's fresh having ethanol doesn't matter. If the gas is several weeks old the ethanol sucks.

Tru Fuel comes in several different flavors!  I mix 50:1 and Straight to get 100:1 for my 2 stroke Yamaha. Purrs like a kitten.  Idles noticeably lower rpms vs pump gas. But I’m guessing the problem is in the jet.  Replace carb, keep trying!  

6004E2AB-15C0-4C56-A013-C51B17E77465.jpeg

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Thanks all for the ideas

heres what I tried to get it running

new spark; no change 

re cleaned fuel filter; no change 

checked float level by letting the drain valve flow into a plastic cup and submerging the entire carb to see what level the fuel came up to; it clearly covered over the main jet

ran compressed air backwards from carb thru hose thru valve thru filter screen ; good flow no obstructions

re cleaned and assembled carb; no change

about this time my neighbor remarked that it was like I had a smoke machine every time I ran it.  Ha ha, thanks for making light of my struggle.

Sidenote: up til now I’d only been filling the tank with 1-2 cups of fuel since it makes draining for other diagnostics easy. But I had been burning a surprising amount in my diagnostics, especially with the choke on.

then I thought that maybe the problem was lack of fuel in the tank. That maybe I needed more than to just cover the little filter in the tank. Maybe I needed an inch or so covering the filter.

I was going to use the premixed “trufuel” but I remembered Zonkers warnings so I mixed a fresh gallon with TCW3 oil and put it in my tank. That’s when the crack in the tank became evident. So as I’m collecting all this fuel dripping everywhere I notice that it’s much lighter color than the stuff I’d been using up til now.

So I drain the tank below the crack and start her up and.... she runs normally again!

so I think what happened is that I had mixed my fuel way too oil heavy at some point. Like 10:1 or 20:1 instead of 50:1.

and for it to run at all it needed to suck a lot more fuel to get to the gas part of the oil mixture. When the choke was off, it was pulling “fuel “ but that fuel didn’t have enough gas so it starved. Plus the oil doesn’t atomize like gas does.

And my neighbors joke about a smoke machine makes sense since I was burning oil like crazy. And now that I think of it, there was oil on the plug, but I didn’t recognize that even a 2stroke shouldn’t have that much on the plug. 

I think the prize indirectly goes to Zonker, but thank you all for making me check everything. 

-M

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On 6/28/2019 at 7:24 PM, Zonker said:

I would use a marine TCW3 2 stroke oil with at least a mid grade gas. If it's fresh having ethanol doesn't matter. If the gas is several weeks old the ethanol sucks.:)

Zonker, Octane has nothing to do with the quality of the fuel!  100 Octane won’t run any better or worse than 87 Octane if everything else is the same.  Octane is the knock resistance of the fuel.  Knock is very uncommon in small two stroke engines, as there is rarely enough compression for pre-detonation. It could possibly happen in a very old engine with carbon deposits after it had been running for a while, but not likely as the rings would most likely be shot.  

 

mrgnstrn, I wouldn’t worry about the Tru Fuel. The only times my Yamaha has acted up was from letting pump gas sit too long in the tank. That being said, I am headed off to the gas station to mix up a gallon of 87 Octane with TC-W3 oil, and a dose of SeaFoam, but it’ll get used up in the next two weeks!

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