1996 Tohatsu M40C carb replacement

mudchuck

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If I replace the entire carb, do I also need to get a replacement valve seat? [h=2]#345032400[/h]
 

Sea Rider

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Why do you need to replace a complete carb ? Having needle seat issues ?

Happy Boating
 

mudchuck

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Why do you need to replace a complete carb ? Having needle seat issues ?

Happy Boating

Running poorly, having cleaned carb plenty of times, runs good for a little while, leaves me stranded on the open lake too much, very long paddle to the launch...
Plastic fitting that is swaged onto the brass fitting for the hose to connect to is cracked and cannot buy that piece without the whole carb. Aspirating air, I tried applying RTV sealant and clamping hose onto little brass knobby stub, but unsure if it's still sucking in air.
Float & main jet sticks and I pull carb often to clean it, but tired of doing that.
If this doesn't solve my problem then I might search for another motor...
 

pvanv

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Yeah, those plastic inlets were never available separately. I have seen folks drill the carb base out and thread in brass ones, but it requires skill, patience, and a little luck to do that.
 

mudchuck

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Also bought a fuel pump & filter, going to swap them out too.
I'm not crazy about swapping out parts just for the heck of it, but I'm inclined to think I have a fuel related delivery problem, and not a power pack issue...fingers crossed.
Reason why I'm not 100% certain it's one or the other is the spark is fine on both plugs and doesn't misfire, can cold start no problem, motor warm up and get going no problem, troll all day and intermittently pick up troll plate and run to another spot, get back to trolling, and when done for the day, go to full throttle then suddenly into a few minutes running it sputters, appears to misfire and loses power, sometimes resulting in engine cutting out, then incredibly hard to start and once fired up again, will rev to almost full throttle while in neutral but then engage in forward will sputter/die/hard start.
Takes several minutes of frustration and then get to start/go into gear only to seem like full throttle run is barely 10MPH.back to the docks.
 
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Sea Rider

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Lets hope that's a carb, fuel pump related issue. Good Luck, report findings.

Happy Boating
 

mudchuck

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And to make matters worse, the lower unit seal job I did last summer failed. And I'm 100% certain I placed them correctly, I think the salt water corrosion from the PO's has pitted things and water seeping thru, now I have to deal with that again.
Boat will have to go into early winter storage...
 

Sea Rider

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Bad news, when restoration work is completed, would recommend to check the carb/timing synch of that motor, been finding lately several 30-40 HP motors running with less timing advance that what the factory recommends.

Move gear lever to forward, fully throttle the throttle grip to max, stand at middle of motor and check if the 25º timing advance seen on the upper plate stops right at middle of both mating surfaces.


Click image for larger version  Name:	Tohatsu M40 Timing.JPG Views:	1 Size:	158.6 KB ID:	10789611

If with less timing advance, remove the upper plastic retainer, yellow >>, loose the nut >> and decrease the rod distance (red line) enough for timing plate to fully advance to 25º. Secure retainer on ball, tighten the loose nut, don't over tighten it. Report if within tech specs or way off..

Happy Boating
 

mudchuck

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Good news!
I pressure tested the lower unit seals I replaced last summer and they're all fine. And no water in the gearbox oil when I drained it to test either.
The chocolate milky mix coming out of the drain hole I suppose is a result of unspent fuel in the exhaust and the shallow water at the launch had some mud in it.
I've replaced the carb, fuel pump & filter and one of the hoses that had a split in it.
Cleaned sparkplugs and see they're oily, so since I do a lot of trolling and not much for speed runs, and run the motor with no thermostat, curious if perhaps I'm not letting the motor run at decent temp to burn off fuel and allowing the plugs to foul too easily...thoughts???

Now I just need someone here help me understand the air/fuel mix settngs for the low & high set screws.
Which is idle & what is the other one?
Lower one on side is 3.5 turns out from all the way in, upper one is 2.5 turns out...are these correct?
I've not started the motor on muffs yet, really want to make sure the screw settings are close to correct.
Thoughts???

I will also look into the 25 degree timing advance.
Thanks!
 

pvanv

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You need a thermostat, or the plugs will foul prematurely. Start at about 2-1/2 turns out and adjust from there. There is only the pilot adjustment screw. The only other external adjustment is the idle stop.
 

Sea Rider

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Besides fouling plugs prematurely, motor will achieve more carbon build ups on the entire exhaust system, piston rings and cylinder head due to a much lower working temp than what the motor needs to work with. Install a thermo ASAP.

And when returning to Terra Firme once excessive trolling fishing has ended go full throttle as if stealing the combo to clean the plugs while returning.

Happy Boating
 
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mudchuck

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You need a thermostat, or the plugs will foul prematurely. Start at about 2-1/2 turns out and adjust from there. There is only the pilot adjustment screw. The only other external adjustment is the idle stop.

Which crew is which?
Thermostat ordered.
 

pvanv

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Item 1-23 in the diagram is the mixture screw. 1-21 is the idle speed stop screw.
002-21035-3_FIG04.jpg
 

mudchuck

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Great news, carb, fuel pump & fuel filter replaced, screws adjusted and the motor flies!
Timing advance exactly as the picture Sea Rider shows, no issues there.
Bad news...thermostat and gasket shipped were wrong models they're meant for 25HP or less, wont fit-wrong size.
Reordered thermostat but on backorder.
Now if the weather will just cooperate so I can get some on the water time in to actually fish.
 

Sea Rider

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That M40 is powering which type of boat, its length and with how many souls on board ? If wanting to get the max out of that boat/motor combo a prop maximization will be highly recommended, but will need an induction tach installation for that.

Happy Boating
 

mudchuck

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That M40 is powering which type of boat, its length and with how many souls on board ? If wanting to get the max out of that boat/motor combo a prop maximization will be highly recommended, but will need an induction tach installation for that.

Happy Boating

14' usually 1-2 of us on board. Not trying to max out speed...just wanting to get out to fish and the motor be reliable to get us back to the launch...
Has an 11 prop on it.
I had it up to 31MPH, quick enough for me! It'll scoot...and go slow enough to troll 2.3-2.5, have a troll plate for slower too.
 

Sea Rider

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An 11 pitch prop is about right for a light to medium load, have seen way over propped props on those and on 30 HP motors.

Happy Boating
 

mudchuck

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So update:
Replaced carb with brand new one mid-summer 2019, got it dialed in and motor runs flawless, although top speed is max out at 27MPH.

Winterized and stored, (I like seafoam!) I just pulled it out of storage March 2020, replaced gear oil before winter storage & after, clean plugs, fresh fuel & mix oil and started right up reinvigorated and ready for spring shakedown run.

Had it out on a chilly morning & she purrs, especially with new thermostat helping hold correct motor temp too.
Top end no issues, minimum throttle & idle no issues even with troll plate engaged, runs just fine with plate at .6 to 1.0MPH great for spring eyes.

Now here's where I run into some issues...when I go to troll for spring salmon, I need to get troll speed in around 1.75 to 2.25 AVG with 2.4MPH max for spring salmon, but when I try to get throttle set on anything above minimum throttle it bogs down and conks out, but starts right back up. This is a consistent occurrence.
I've taken to pulling motor cover and tuning idle speed on carb temporarily to troll at these speeds, then set it back, but not sure if this is what I really need to be doing.
Seems like the new carb has that sweet spot that the jets & needles have a hard time finding.

Any thoughts???
 
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