4hp 2s outboard got me stumped

[Refueler]

That would be serious butchery, IIRC it’s fixed by a woodruff key. Not to say impossible of course.

Dunno ... never had one apart ... just wondered if flywheel was offset ??

 

[Chiara’s slave]

Dunno ... never had one apart ... just wondered if flywheel was offset ??

I have taken one to bits before. Like I say, hard, but not impossible to put it back wrong. Depends how many other places you’ve checked if it's worthwhile checking that. Look for spanner marks first, see if there’s evidence it’s been apart before.

 

[Blueboatman]

I can’t remember if there is a tiny in line fuel filter within the fuel hose at the exit from the fuel tank
Either way , it would be worth disconnecting the fuel pipe at the carburettor and visibly confirming that fuel is pouring freely through the pipe ..
Blocked air vent in tank filler cap “ maybe” is another one to verify - just run the engine with the cap off/loose
I suppose it’s worth checking that the exhaust isn’t blocked and producing back pressure
But my money is on an incorrectly reassembled set of carburetteur diaphragm gasket ‘ plates ‘
I no longer have the workshop manual but I’m sure a carb assembly diagram is available on line
Fwiw the 4/5 rebadged Mariner design is the most reliable outboard I ever owned !

 

[dankilb]

The interesting item is that many of the Tohatsu based - do have same carbs in the low HP range - but have small choke / jets fitted to adjust.

Its a long time since I did it - but you could up a 4 to a 5 by small modification to the carb ... please don't ask what it was - its so long ago and I've forgotten how.

Just re-jet. That’s it. No other difference (according to the parts diagram but I also have the 4hp with an otherwise-identical 5hp carb). Great motors they are indeed!

 

[dankilb]

I can’t remember if there is a tiny in line fuel filter within the fuel hose at the exit from the fuel tank

Yup there is and it is known to block. IIRC you can just snap it off with long pliers, to allow the pipe to flow freely.

 

[bignick]

Is it possible for the timing to go out? Not sure how they work on these, but if the advance is wrong then it will never fire properly.

Also, I can’t see how it can be the carb if it won’t even fire on something like easy-start.

 

[VicS]

Thanks for the replies.
@VicS the compression test gives a reading of 100psi which from my understanding seems acceptable. As for the ‘free air’ spark, well it arcs the plug quite happily and the choke works fine, at least from a mechanical point.
Someone mentioned that east-start, yeah we tried that as well with no luck.
We taped a latex glove over the carb throat and could see it is clearly sucking and not blowing back which suggests the ‘reed valve’ is not sticking or leaking.
If it was purely a gummed up carb, then surely fuel squirted either down the throat or even into the plug hole should show some life. In which case this would suggest no spark, but we have seen there is a spark.
Repeatedly when pulling the plug it is dry, (even after squirting fuel down the carb throat). Which I agree suggests a blocked carb. I’ll strip it again, but after having every jet, needle, cover plate and bung off of it several times already, I’m not holding my breath.
What are the cheap Chinese £40 replacement carbs like?
Chris

A compression of 100psi should be fine
If it wont fire using easy start then more likely to be an ignition system problem. In free air the spark should jump at about 7/16". How far will it jump?

Is it possible for the timing to go out? Not sure how they work on these, but if the advance is wrong then it will never fire properly.
Also, I can’t see how it can be the carb if it won’t even fire on something like easy-start.

The timing is fixed by the flywheel being keyed to the shaft .... but if the flywheel has been removed and the retaining nut correctly torqued when refitting it is possible thet the flywheel has moved and sheared the key

 

[mil1194]

I’ve had a Honda generator that would that start perfectly from cold and run for 10/15secs or so and then stop. Turned out to be a faulty plug. Of course I only found this out after cleaning and the carb and ripping god-knows what apart before realising the plug was buggered.

 

[Blueboatman]

A compression of 100psi should be fine
If it wont fire using easy start then more likely to be an ignition system problem. In free air the spark should jump at about 7/16". How far will it jump?



The timing is fixed by the flywheel being keyed to the shaft .... but if the flywheel has been removed and the retaining nut correctly torqued when refitting it is possible thet the flywheel has moved and sheared the key

I was given a 4 stroke mower where the key had sheared and the flywheel had resettled with over advanced timing - the symptom was a knackered pull start because the kick back was so vicious - and it kinda ran too! Very very badly ?

But the Op says in this case the engine runs normally if directly fed petrol through the air intake …
2 strokes are so simple - until they ain’t !

Has a new spark plug been fitted yet ?

 

[Refueler]

I was given a 4 stroke mower where the key had sheared and the flywheel had resettled with over advanced timing - the symptom was a knackered pull start because the kick back was so vicious - and it kinda ran too! Very very badly ?

But the Op says in this case the engine runs normally if directly fed petrol through the air intake …
2 strokes are so simple - until they ain’t !

Has a new spark plug been fitted yet ?

No he doesn't say that .... he said it fired when fuel was put direct in plug hole ... but not when via carb intake - in fact he expressly says plug is dry.

 

[Blueboatman]

No he doesn't say that .... he said it fired when fuel was put direct in plug hole ... but not when via carb intake - in fact he expressly says plug is dry.

My mistake
And it ran for several minutes and then petered out …
I hope the OP reports back on progress and fix .

 

[oldharry]

Old 2 strokes can be diabolically tempramental, as anyone who spent his misspent youth on BSA Bantam or similar will tell you! Considering there are only 5 moving parts its amazing how difficult it can be to find out why the *****y thing wont start!.

The one thing not checked, assuming OP HAS tried a new plug, plug cap and HT Lead is the crankcase seals each side of the crankshaft. Leakage there will kill a 2T stone dead, even though its apparently sucking properly. Even a small leak can upset the fuel air mix enough to leave it stone dead.

 

[Blueboatman]

Old 2 strokes can be diabolically tempramental, as anyone who spent his misspent youth on BSA Bantam or similar will tell you! Considering there are only 5 moving parts its amazing how difficult it can be to find out why the *****y thing wont start!.

The one thing not checked, assuming OP HAS tried a new plug, plug cap and HT Lead is the crankcase seals each side of the crankshaft. Leakage there will kill a 2T stone dead, even though its apparently sucking properly. Even a small leak can upset the fuel air mix enough to leave it stone dead.

Gosh I had forgotten that
With Stuart Turners

 

[oldharry]

Gosh I had forgotten that
With Stuart Turners

Yes, them too! Seagulls, another notorious non starting 2T got round it with their 10:1 oil mix which fouled the plug at the slightest provocation! I had a brand new 40+ which just would not start. After the holiday it went back to the supplier, who also couldn't start it. It was returned to the Factory, and a new one which started first pull every time was supplied. Never did hear back from Seagull. DQ Boat Yard who used to build wood and plywood boats had Stuarts by the pallet load, 10 at a time. Apparently they could be sure that one of each batch would simply refuse to start once installed. It would be replaced every time without any query from the works. DQ reckoned it was faulty seals every time though they never stripped one.

 

[chris-s]

Thanks for the continued replies!
To recap, we have managed to start it twice, only after some serious manual priming in the plug hole. But once running it ran for what seemed like several minutes at good revs quite happily until petering out and then unable to restart. Loosening the carb bowl drain showed fuel still in it.
Fuel flows to the carb quite freely and we fitted a new filter anyhow.
I considered swapping the ignition and/or carb with those on one of the other 2s engines in the shed but they all look too dissimilar. So I think it will be one last thorough carb clean and then if still no joy, off to the service guy.

 

[Refueler]

Old 2 strokes can be diabolically tempramental, as anyone who spent his misspent youth on BSA Bantam or similar will tell you! Considering there are only 5 moving parts its amazing how difficult it can be to find out why the *****y thing wont start!.

The one thing not checked, assuming OP HAS tried a new plug, plug cap and HT Lead is the crankcase seals each side of the crankshaft. Leakage there will kill a 2T stone dead, even though its apparently sucking properly. Even a small leak can upset the fuel air mix enough to leave it stone dead.

Exactly what I said about my Vire ..... now there's a temperamental engine !!

 

[mickywillis]

Could be that the crank case seals have gone? Either end of the crankshaft are probably lip seals? Or the metal to metal interface of the 2 x crank shaft casings are leaking? The other possibility is that the piston is fitted wrong way round? If the transfer ports on the cylinder walls and the ones on the piston arent correctly aligned and 180 degrees out, it wont run!