Tohatsu M40D2 Rough Idle/Not Reaching WOT

[LukeFishWalker]

My 2002 M40D2 that I bought 2nd hand has a rough idle and what I'd call a misfire/backfire
when starting cold. I was just so happy that it ran well that I really had no idea it was doing it
until a guy at the launch told me, "Does it miss like that all the time?"
And of course I had to reply, "Well, it's a used engine and I'm not used to it yet."

I bought a Tiny Tach TT2A and the 120 setting gave me an odd reading at WOT,
so I put it on the 360 setting and the best it does is 4650.
The boat is a 16' Alweld .10 Gauge and is quite heavy.
I have a Factory Tohatsu 14p prop.
Any ideas as to why 4650 is the best rpm at WOT?

Tomorrow after I clean out the ice chest, I am going to take each carb off and
inspect each one individually.

Thank You For Your Time And Patience,
Lukus

 

[pvanv]

It would be nice to get 5000 to 5700. 4700 is a little on the lugging side.
When checking/cleaning those carbs, be very careful that you don't snap off the plastic fuel inlets.
Then verify link and sync, (timing, etc.) and re-check tach at WOT.
Check your transom height (a half inch up or down makes a big difference), and re-check tach... and then maybe go down 1 or 2 inches pitch if needed.

 

[Sea Rider]

LF,

Is that 40D2 with ignition timing set at 18? and cabs fully opened at full throtttle ? Re adjust if needed. The issue about having a 3 carbed OB is that will need to have a musician ear or count with a digital vacuum gauges to adjust properly all 3 carbs when at idle.

At 4650 wot revs, Ob is 1,050 revs from max factory wot range. If wanting to rev there, will need to go down 2 less size pitches to a 12 pitch. Depending if OB sits and revs at the sweeet transom height along 90? trimmed while at plane could have 300 to 500 + wot revs per one less pitch. This is not rocket science, will need to go for a wot spin as usually loaded to check that.

Do find when installing 2 strokes Tohatsu OB's that some of their models are being fatory deliverd with the wrong medium pitch size, need to go 1 or 2 less in pitch to pull revs at near max wot revs for current application and as usually loaded.

Happy Boating

 

[LukeFishWalker]

Thank You For The Replies,
I did notice that the carbs did not open to 180.
I had to " Bend " the stopper to allow full open butterflies.
All 3 carbs were what I'd call spotless.

Now, I do have to run with the engine trimmed down (Below 90) to prevent the bow from hopping.
Even with the 12 gallon gas tank, the battery and what ever other gear I have on board
all the way to the front I still have to set it 2 notches below 90.
When I run with 2 people, I can set it to 90 because the 2nd person sits in the middle.
I have not checked the rpms with 2 people --> That's next.

I have this long shaft OB set up pretty high with the jack plate.
When running. the water line is about 2 inches above the cavitation plate.
From what I've been told, the tunnel hull design of this 2006 Alweld causes
the stern to be " Sucked " down and that's likely the reason for my issues.

Can you recommend a reasonably priced Multi Vacuum Carburetor Synchronizer?
Amazon has a few in the US $62.00 range

Thank You For Your Time And Patience,
Lukus

 

[Sea Rider]

Check when 2 up and at plane with OB trimmed to 90? if water flow passes under, way under small water deflector plate towards anticav plate height, forget by the moment all what's mentioned about anticav plate height. Having a jack plate to play with is a plus. This is the sweet OB/transom height for any Tohatsu OB.



So OB is now at 18? timing advance with all 3 carbs fully opened, right ? Before maximizing a new prop would visually check OB/transom height, adjust if needed, tach OB with at least 2 up to know how much over propped that OB is.

With respect to diigital vacuum gauges, don't use them as only work by the moment with 2 cylinder single carbed OB's. Paul should know them well and suggest a good brand.

Happy Boating

 

[LukeFishWalker]

Sea Rider,
Thank you for taking the time to not only respond but provide the helpful photo.
Based on the fact that my cavitation plate is below the " Plane " of the water with the flat design tunnel hull
when compared to the photo.....I am going to raise it another inch and re-check.
The lower unit has 2 " Plates " : One is the cavitation plate just above the prop and another somewhat shorter.
The " Plane " of the water is just barely below the shorter plate which puts the cavitation plate about 2 inches below water.

I ran into a problem with the boat itself.
I was throwing the cast net for shrimp and noticed an unusual amount of water inside the boat.
I bailed it out and ran without the plug to let the rest out, put the plug back in and when I got to the
landing.....There was more water. When I got home I pulled up the 3/4 treated plywood and found
something very disturbing. 4 of the ribs showed signs of stress cracks and 2 were cracked.
Which allowed the bottom to flex causing the welded seam in the center to tear.
When I bought the boat used...I did see signs of minor stress cracks, but I thought nothing of it.
I was able to get 2" wide 36" long and 1/4" thick flat bar aluminum and the wife's cousin welded
these bars solid all the way around and repair the torn center seam. It's really nice to have a large
family with a wide range of skills, he is a welder by trade and really made it look simple.
I asked if I could try my hand at it....I had some scraps of aluminum and I'm going to admit something:
Welding aluminum is not something I'm good at. A simple flat weld less than 3" took me 10 minutes and the weld failed.
I felt so out of place when I had to admit to a failure, we all pride ourselves in the ability to learn - I failed.

I can only guess that the rough water I encountered and the fact there was already stress damage is
what caused the metal to fail. There were times that I would be forced to run in 2 - 3 ft waves W/20mph winds.
I of course would not be able to run WOT, but the boat would slam into about 5% of the waves pretty hard.
The flat bottom doesn't like rough water and neither does my lower back.

I will post an update when I heal up and hair over from my last trip.

Thank You For Your Time And Patience,
Lukus

 

[Sea Rider]

Lukus,

Forget all about what you have read about anticav plate even or around lower keel height, only works good when going on a straight flat, non windy water course. Test with 1-2 holes down till water flow at speed passes skimming as close to small upper water deflector plate. Once dialed can play with prop pitches all you like.

Runnig at speed on choppy water cond will only make hull to slam badly, which is not good for the hull, passengers nor your lower back as you have found out, try to maintain minimum plane at reduced throttle when encountering those harsh boating conditions.

Happy Boating

 

[LukeFishWalker]

Sea Rider,
I now have the OB as far as it can go up and out of the water and still maintain water to
the inlet. And I do have to admit, it did get the RPM's to 4800.
Once I get a few coins saved up, I will get a OEM 13P Aluminum prop and see how
that affects the RPM's.
Again, Thank You For Your Time And Patience
Lukus

 

[Sea Rider]

Lukus,

Raising OB will only achieve slight more wot rpm but combo will perform awfull on choppy, wavy, windy water cond with excessive prop aereation on tight close turns. Know it's hard to dial the sweet OB/transom height with fixed holes as opposed to clamp OB's.

Lowerl a hole for water flow at speed to skim right under or closest possible to small upper water deflector plate, as said before forget all about anticav plate height, was not invited to the party. LOL!! If flow at speed passes right over small water deflecor plate will achieve back water splashes or over transom water splashes.

At 4800 wot rpm, OB is still 1K under max wot rpm factory range, to run at max or near max 5850 rpm, will need to go at least 2 less in pitch from current prop pitch which assume is a OEM one, right ? As this is not rocket science, new OEM prop will need to be water tested to check that out.

Happy Boating

 

[Sea Rider]

Lukus,

The lower unit has 2 " Plates " : The lower one is the cavitation plate just above the prop and a shorter one which is a small "horizontal water deflector"

The " Plane " of the water is just barely below the shorter plate, lower OB one hole down and check if flow skims right under it, if over it will have water splashes out transom, if so, leave at the hole it was previously reaised.

From there can go for a prop maximization...

Happy Boating