3.5 Tohatsu 2 stroke problems

davidpbo

davidpbo

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After sorting the 8hp my 3.5 is now giving grief. I thought stripping the carb and a new plug had sorted it but it is not right.

Now starts ok on full choke as always, used to have to immediately take choke off. Will now only run with partial choke. When in gear if raise the revs it cuts out.

Is likely to be a patially blocked jet? It looks clear.

I think the screw on the left is the idle adjustment. Is there a mixture adjustment screw? I rather think it is fixed but don't have a manual to hand.
 
davidpbo said:
After sorting the 8hp my 3.5 is now giving grief. I thought stripping the carb and a new plug had sorted it but it is not right.

Now starts ok on full choke as always, used to have to immediately take choke off. Will now only run with partial choke. When in gear if raise the revs it cuts out.

Is likely to be a patially blocked jet? It looks clear.

I think the screw on the left is the idle adjustment. Is there a mixture adjustment screw? I rather think it is fixed but don't have a manual to hand.
Click to expand...

Not actually from the 3.5 manual but this carb on Lakesailors website looks like basically the same one. It may help you. http://www.lakelandimages.co.uk/For...u/Section 3 - FUEL SYSTEM AND CARBURETION.pdf

The symptoms you describe do suggest a partly blocked jet or incorrect adjustment.
 
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If it's like my 4HP mairiner that I fixed recently, there's adjustment on the idle jet, but no adjustment on the main jet (not external to the carb anyway)

your symptoms sound more like fuel starvation. Check the fuel system, pipes, taps, filters etc.

If you open the float chamber drain screw do you get a good flow of fuel, or does it empty then just drip? There should be a good continuous flow of fuel with the taps open and the drain screw open.
 
I had what seemed like symptoms of fuel starvation on my To 3.5 a couple of years ago. It turned out to be corrosion on the coil earth connection to the block. A quick clean of the terminal and all was well. The give-away was a faint and intermittent ticking sound as the spark jumped the corrosion.
 
Sounds like a problem that I Have with my tohatsu 2 stroke 2.5hp.

Clean the carburettor, something like Wynns carb cleaner, as 2 strokes are notorious for leaving behind a lot of crud to gum up the works when the fuel evaporates.
 
There is a filter in the bowl under the fuel tap, is that clear? Also check for flow through the tap, I had one that was blocked and restricted the flow.
 
jac said:
2 strokes are notorious for leaving behind a lot of crud to gum up the works when the fuel evaporates.
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...so don't let it evaporate. Switch off the fuel 40 metres or so before your destination and run the carb dry periodically.

(And, talking of goo, think yourself lucky we don't use Castrol 'R' any more. Mind you, it did smell good.)
 
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We had a similar problem with our Tohatsu 3.5 at sea a few weeks ago (just when we needed it), which turned out probably to be water in the float chamber. We took the carb off, stripped it completely, cleaned it all out, including blowing through all the airways, adjusting the float hinge, fitting a new O-ring, and carefully reassembling everything. It worked perfectly from the second pull. It's not a difficult job to do, and probably only took an hour from start to finish. We also cleaned out the tank and fuel line, using new petrol/oil mix. There is a good site here: http://seaweed.thebilge.com/tohatsucarbrebuild.htm describing how to do it.

The mixture is adjusted by putting a small E-clip on different notches on the main jet needle, but there shouldn't be any need to adjust this, which is just as well, as you can't do it from externally, but have to dismantle the carb each time. The screw on the side is the idle adjustment, as you say.

I have no idea how the water got into the carb (if indeed that it what it was), as we drain it at the end of the season by running the engine until it stops, and then use fresh petrol every new season. It was even running perfectly the previous week, and I am not aware of any possibility of water having got into it in the intervening few days. One of life's mysteries, I guess.

Rob.
 
Had a similar prob with my 3.5 Johnson. Sticking float and crud in the chamber was the problem. Also had fuel running out the carb when the fuel shut off was opened.
 
macd said:
...so don't let it evaporate. Switch off the fuel 40 metres or so before your destination and run the carb dry periodically.

(And, talking of goo, think yourself lucky we don't use Castrol 'R' any more. Mind you, it did smell good.)
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I have always done that but still had something the consistency of poo foam in the bowl (Some time ago).
 
Silly question - have you opened the breather on the fuel tank lid? Is it blocked?

Clever me, serviced the whole thing, running like a dream. Decided to be clever and make a new gasket for the fuel tank lid. fitted said gasket, b]put o/b back on boat and went off to Scilly.

Put dinghy in the water, added the o/b and had similar problems with getting the thing to run. Eventually rowed ashore and then back to the boat.

Looked at everything and found that my new gasket meant no air was getting into the tank and hence fuel was not allowed to be drawn into the carb etc.

Oh and do run the engine dry and not leave any fuel in the carb.
 
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It does feel like fuel flow is restricted. Is it possible to remove the fuel filter from the tank to clean?

Fuel seemed to be running from the fuel pipe but I will check it again.

How do I know if I have a capacitor discharge ignition for sure? I think it is. What year did they start doing them?

I am not sure the spark is as strong as it might be. I have an old spark tester on board which is basically a variable gap. Best not used with fuel vapour around. My 8hp Mercury produced a spark that would jump app. 10mm
This one seems feeble in comparison but it was hard to tell as it was a bright sunny day. It did hurt when I got fingers in the wrong place but not as much as I have felt before.

Is there anything else I can test on the circuitry? I have a basic multimeter on board.

Can anyone provide a circuit diagram please?

I will check the earth point. Is it obvious or under the recoil ?

How does the engine generate the initial voltage? Presumably there are no points? Is there a magneto?
 
davidpbo said:
It does feel like fuel flow is restricted. Is it possible to remove the fuel filter from the tank to clean?

Fuel seemed to be running from the fuel pipe but I will check it again.

How do I know if I have a capacitor discharge ignition for sure? I think it is. What year did they start doing them?

I am not sure the spark is as strong as it might be. I have an old spark tester on board which is basically a variable gap. Best not used with fuel vapour around. My 8hp Mercury produced a spark that would jump app. 10mm
This one seems feeble in comparison but it was hard to tell as it was a bright sunny day. It did hurt when I got fingers in the wrong place but not as much as I have felt before.

Is there anything else I can test on the circuitry? I have a basic multimeter on board.

Can anyone provide a circuit diagram please?

I will check the earth point. Is it obvious or under the recoil ?

How does the engine generate the initial voltage? Presumably there are no points? Is there a magneto?
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Changed from points ignition to CDI about 1996 I think but you should be able to recognize the difference by referring to the parts books http://www.tohatsuoutboardparts.com/Parts-Books.html

Hardy enough circuit to need a diagram. Not a lot you can check with a multimeter unless you know the correct coil resistances.
You need a DVA adapter to read peak volts to check a CDI.
 
Not certain your problem with your 8hp but I had a similar issue for whilst my 6hp 4 stroke mariner played up then the 3.5 Tomatsu.

Fresh fuel was one, I started using a fuel additive as fuel could be stored for whilst. Constantly cleaning carb, it got to the point I can strip the carb in a dinghy no probs.

Final solution? I changed the garage I bought the petrol from.

Also try a new spark plug, when I was having problems with fuel it was a new spark plug every 6 months...

You can check if fuel is getting to the carb freely by taking the drain screw out of the bottom of the carb opening the fuel valve and letting some drain into a glass. Then you can let it settle and see if there is much dirt and water in it.
 
I'll second this!

jac said:
Sounds like a problem that I Have with my tohatsu 2 stroke 2.5hp.

Clean the carburettor, something like Wynns carb cleaner, as 2 strokes are notorious for leaving behind a lot of crud to gum up the works when the fuel evaporates.
Click to expand...

Had just the same, and same solution, with my 3.5
 
davidpbo said:
Got up this morning to take dog in. Engine started and ran fine and is still doing so.

Don't know why when it has not been right for days.
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You have probably sucked the rubbish out of the carb needles/ jets and burned it. If the stuff can be dissolved into petrol it can get dissolved back into it in time..

2 Strokes can run on pretty crud fuel as long as it does not block the carb...

Also on Tomahatsu 3.5's make sure the spark plug cover is on or it gets wet and stops the engine. Have had that issue before, often wonder if it was my own cover I found on the beach a mile away afew weeks later?
 
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