1992 Tohatsu M25C2 (25C2 #27906 vs 30A3 #21031 ) Carburators

[LukeFishWalker]

I picked up a 1992 Tohatsu M25C2 ( Title Says Year 1992 / Program Year 1993 ) which was owned
by an older gentleman for about 2 years and then he passed away. His son inherited it and never
used it. It sat in his garage for 10 years on a stand with the nice blue Tohatsu Cover. He still had
the original metal Tohatsu fuel tank which doesn't have a single scratch on it.
I gave $800.00 and it runs like a new motor.
I am treating it like a new motor and not planning to run it WOT yet.
Evidently the restrictor plate was removed prior to him inheriting it.

*****************************************************************************************************************
Here's my question:
I took off the silencer and the throat of the carb is not the same diameter as the gasket.
Is the throat of the 30A3 #2103 carb the same diameter as the gasket?
And will the 30A3 #21031 carb fit and work on the M25C2?
*****************************************************************************************************************

I see a couple different part numbers which come from a different web site.
2002 And Earlier M25C2 Tohatsu Outboard Carburator Diagram And Parts
25C2 #27906, 30A3 #21031
Both of the above part numbers are listed under the M25C2 parts diagram.

Thanks,
Lukus

 

[Sea Rider]

25 engines comes with a metal sheet gasket with closed center as oppossed to 30 which is a cardboard like gasket and has a larger diam center hole.

Does whatever gasket you have right now cover the entire portion of the manifold assembly lip/border evenly ? The diam of the carb throat is less then the size of the gasket. Why do you want a 30A3 carb for, to bump that 25 into a 30 ? What's the current timing set on timing plate 20-25 deg ?

25 engines comes with a metal leave gasket with closed center as oppossed to 30 which is a cardboard like gasket and has much larger diam central hole.

Happy Boating

 

[LukeFishWalker]

Sea Rider,
Currently I can see through the throat of the carb and the 25 metal plate gasket
has in fact been replaced by the 30 cardboard gasket.

With the engine off:
I can pull the choke and manually open the main butterfly and see that the restrictor plate is not present.
But, the 25 carb seem to have a reduced throat between the choke and main butterfly.
Does the 30 carb have this reduced throat just like the 25?

I am not completely certain how the timing is checked.
Please point me to a link that describes how this is checked.

Thanks,
Lukus

 

[Sea Rider]

Lukus,

Can't tell don't work with older 25C2 OB's just with newer 30A4 ones, probably same carb, those 25 & 30 have not evolved much throughout the years, are basically same, the only difference is carb and timing restrictions being both 430 CC. Don't have right now a 30A4 at shop to check throat diams, can you post a pic ?

To check, while throttle grip is fully at idle, put gear lever forward, full throttle, stand at front of crankcases's mating surface, check where timing plate marks stops at, check pic.



Happy Boating

 

[LukeFishWalker]

Yes Sir,
I thank You very much for the time and effort.
I did find a link for the service manual.
++++++++++++++++++++++++++++++++++
tecnomar.cl/pdf/pdf_servicemanual/2t/1988%20M25C2%2030A3%20003-21018-0.pdf
++++++++++++++++++++++++++++++++++

I am somewhat hesitant when it comes to clicking link from Google.
Once upon a time I used Google to search for music, I grabbed the file and BSOD.

I was able to figure out the timing and it has not been advanced to the recommended 25 BTDC
which is what the M30A requires. This could explain the lack luster performance at WOT.
I also notice that the Throttle Link Rod (Horizontial rod connecting the Throttle Cam Arm & Advancer Arm)
appears to be somewhat long. I see that the entire linkage assembly advances the timing long
before the Throttle Cam Arm makes contact with the Throttle Cam. Which, in turn allows the timing to be advanced
at least 9 - 10 degrees before the butterfly begins to open.
Is this action normal?

I'm not a 2 stroke genius, my experience in mechanics is based on gears and diesel.

Again, I Thank You For Your Time & Effort,
Lukus

 

[LukeFishWalker]

No Title

My phone is not a Smart Phone, so please bear with the less than perfect image.
Even though you cannot see completely through the throat into the intake,
I assure you that beyond the carb....It is infact the same diameter as the intake.
The only restricted area is between the choke & main butterfly

My question remains, does the 30A3 carb have a full throat?
Or, does it also have the restricted area bewteen the choke and main butterfly?

Thanks,
Lukus

 

[Sea Rider]

You are welcome Lukus,

That carb looks different than the 30A4 carb, is the shot pic still with carb attached onto manifold ? 30A4 carb has 2 bolt holes located at 3 & 9 o'clock position, your seems to be at 4 1/2 and 10 1/2 o'clock position. The butterfly portion is wider than the begining of throat. The inner small butterfly located towards manifold is same size as the throat and assume same size as the manifold intake.

The only possible way to install a 30A4 carb onto a 25C2 engine would be changing manifold for a 30A4 model, don't know if same size or if would make a perfect crankcase/manifoild match though ?

Don't finger with rods distances, will desinchronize the throttle, must play with A stopper as in pic.



Need to unlock small nut CCW some mm back, turn screw CCW some mm back and fully throttle, keep on turning screw CCW untill timing plate stops right at 25?as previously suggested, hold screw with a screwdriver while adjusting nut tight. When you advance timing plate to stop at 25? carb rod & carb cam will advance equally to achieve full 180? butterfly setting as both are synchronized. Remember, gear lever must be geared forward prior throttling.

Go for a wot spin and check if with more power & speed than before, if so, leave that carb alone...

Happy Boating

 

[LukeFishWalker]

Thank You Sir,
The picture doesn't do justice to the orientation of the mounting bolts (Carb to Intake)
They are in fact 3 o'clock & 9 o'clock.

Here's why I am under the impression the 30A3 #21031 will work on this engine.
I went to B o a t s . n e t, clicked on Tohatsu, 2002 and earlier, M25C2 and then carburetor.
Both 25C2 #27906 & 30A3 #21031 are listed as replacements for this engine.
And then if you go back a page and click Inlet, Reed Valve....There's only 1 intake manifold.
And the manifold listed for the M25C2 is the same manifold listed for the M30A3.

But, since I have never done it nor am I a seasoned veteran of 2 Strokes....Part substitutions
will always be easier if YOU ASK FIRST!

Can you recommend a tach?
I don't think I need a permanent tach, just something that will give me a baseline to go off of.
Even though my hearing is failing (Working in the keel on mains with the gen running),
I can still judge rpms after I hear the engine and see a tach.

I hope I'm not frustrating you or taking your time unnecessarily,
Lukus

Edit;
Is it possible that the carb itself is listed as having 2 different part #?
Meaning, the M25C2 and M30A3 are identical but since it fits on 2 different labeled HP engines
is has 2 different part numbers.

 

[Sea Rider]

Don't worry Lukus, that's what Iboats forums are for. Instead of buying a 30A4 carb, buy an induction Hardline or Tiny Tach, you can install it on tiller arm right next to grip, good to check max wot rpm achieved under any load condition, play with prop pitches if maximizing prop to suit better your boating applications, monitor fuel consumptions at any load/speed + keep track of the worked hours to schedule preventive maintenances. Now that know all the benefits and how to use them properly, couldn't live without one.

Right now being timed at 20? you should have white roller resting at middle groove at full grip, right ?



When timing is advanced to 25?, cam travel will advance accordingly, that means that carb's while roller will be pushed further upward wih cam's high hill and open the butterfly to achieve a 180? resting position while at full wot throttle.




Personally would not worry that much about both carb having different part numbers as both are same 430CC versions. A 25 HP is detuned by adding a restricted air intake gasket on carb and timing less the advance & carb synchro than on 30 HP version. Don't expect a 30 will shoot you to Jupiter, will achieve better hole shot at full grip, better top end speed and be able to move more load than with a plain 25.

Happy Boating

 

[LukeFishWalker]

Sea Rider,
Please accept my apologies for not replying until now.
I spent the majority of last year cast netting for shrimp and catching Red Drum until it got too cold.
This past spring I spent nearly every day in the Lacassine Reserve (Louisiana) catching bass.
The Tohatsu has been quite a pleasure to own. I did in fact get the timing to 25 BTDC at WOT (Butterfly at 180).
I was getting 5100rpm with a 12 pitch prop W/Medium Cup on the trailing edge.
The 2006 AllWeld 16' flat W/Tunnel is quite a heavy hull.
I bought a Tohatsu 10X13 SS prop and had it cupped almost like the 12 pitch.
It dropped the WOT RPM to 4700 rpm, I put the 12 pitch back on and now I am considering having my
local prop shop do some machine work to pitch the new prop closer to 12.5

I'm' sure you are scratching your head wondering why.
Even at 5100 W/12 Pitch, the best I can get is 25mph (GPS Estimate)
I have the Anti-Cavitation plate right at the top of the flat tunnel. (Engine Height)
I have a manual jack plate that puts the mount 8" from the transom.
I get a little Blow-Out at hole shot once it planes, but it grabs after the bow falls a bit.
To be perfectly honest - The hole shot is great and it can spill a full can of coke on the deck.
But, I'm always the last one to get where I'm going.
I am completely aware that there's no "Perfect" prop that blends hole shot and top end
to completely utilize HP. But, the guys are calling me the turtle.
I can get into shallow areas they can't and I can get out of the shallows when they have to troll in & out.
Can you recommend a suitable replacement prop that may suit my needs better than my current option?
I WILL NOT hold a grudge if anything you suggest is short of a success.

Thank You For Your Time, Effort and Patience,
Lukus

 

[pvanv]

Correct WOT RPM is 4800-5500. I would lean towards the high side, so you can go with LESS pitch, say 11 pitch. SS props flex less than aluminum props, so they behave as though they have more pitch than an equivalent aluminum prop. You want to rig the height so that the surface of the water flows between the splash plate and the anti-ventilation plate, You may want to raise the motor a half inch and re-test. Likewise, make sure that your motor is trimmed to be dead square with the water at speed -- not pushing the bow down, and not forcing the transom to dig in.

 

[LukeFishWalker]

pvanv
I had considered the idea of getting an SS 11P prop also when I got the 13P.
Do you have a preference when it comes to manufacturers?
It appears that the iBoats Prop Finder hyperlink is not functioning at the moment.
I had purchased the Tohatsu 13P for right around $202.00 and did see an 11P for about the same.

I did set the manual jack plate in such a way that allows the engine to be dead square with
the bottom of the boat. Granted the plate isn't square, but I can pin the engine square for a
full load and 1 pin lower when I'm alone to prevent porpoising.

Thank You,
Lukus

 

[Sea Rider]

Hi Lukus,

Nice to hear from you, so finally have bumped that 25 into a 30 HP. Is carb's white roller max height perfectly synched with 25 deg timing when standing at front middle side of OB. Have seen 25/30 not spot on factory synchronized, that could make a boating day difference along correct propping. The correct sync procedure can be found here : Read post N? 9 and under to check timing advance.

http://forums.iboats.com/forum/engin...out-of-a-ns40c

With respect to props : assume boaters that call you a turtle are running larger OB's and combos than yours ? If your 25 powehead is 430 CC, same as of the 30 HP and now you have a converted 30 HP, don't see why you can't wot rev to max 5850 rpm range as of the 30 HP ?

Don't need SS props on portable OB's, OEM 3 blade alum props perform excellent when OB is correctly pitched and have lots in 1 increment/decrement pitches to choose from.. Going back to OEM 3 blade alum props, what's the best wot rpm achieved as usually loaded ? lets work from there dialing best prop for your application.

Been there getting the right prop for my combo, a 1/2 pitch will equal to half the rpm achieved with 1 pitch decrement. My combo with 9 pitch factory delivered achieved slight less max wot rpm range, with an eight pitch over revved lightly loaded. To be spot on max wot revs with current load needed an 8.5 pitch which was hard to get, seems is no longer fabricated by Tohatsu. Could re pitch, but not down here. Lucky you guys that have lots of prop shops in USA.

With respect to OB height, try sitting OB on transom to achieve water flow at speed to pass by skimming right under small water deflector plate, that's the sweet OB/transom height for Tohatsu OB's. Balance deck weigh evenly, trim OB to sit at 90? angle, go for a wot spin as usually loaded and pull your head out transom, check at which lower leg height is water flow passing by. If too near Cav Plate prop will have insufficient amount of water to grip spot on, will experience prop aereation issues on tight close turns and when encountering choppy water cond at straight water courses. Check transom height on flat calm, no wind water cond. Check this vid as an example.

https://www.youtube.com/watch?v=oBeZjaA76VM

To my particular taste OB needs to sit tad lower for flow to skim right under upper plate, is passing slight lower. A 1/2 inch would have been better suited. If any point is not sufficiently clear, just ask.

Happy Boating

 

[LukeFishWalker]

Sea Rider,
Yes, I can see the throttle butterfly at near perfect level and 25 BTDC.
I had to adjust both the Sync Bar and the lower throttle arm.
The Sync bar doesn't allow the extreme fine tuning, but the lower throttle arm does.
You can achieve a 180 turn with the lower throttle arm, but only a 360 turn with the sync.
With another smaller adjustment on the water it does everything I need and for that I
am very grateful.

The other guys have the older riveted boats that even I can pick up by myself, and I'm disabled.
Even the 14' riveted boats were rated for 25hp and I don't think it's fair that mine is rated at 25hp.
On the other hand....My 16' hull is the commercial model with the 10GA Aluminum and this
hull is heavy. I can barely pick up 1 end. I have the open floor plan (No Middle Bench) which
allows me to put the battery in the front compartment and the 12 gallon fuel tank in front too.
Trying to offset and balance the weight to prevent porpoising when I am alone.

As far as OB Height.....I've tried several positions and found the best height.
The Manual Jack Plate is nothing but two 6' aluminum angles 18' long bolted to the stern and
an adjustable vertical transom. I've found that 5' up and 6' back from the boat transom is perfect
and allows water to flow 1 to 2 inches above the anti-cavitation plate. That way, I can still get
into the shallow water environments but yet still have enough of the foot in the water to prevent
almost all blow-out until WOT and Plane. Transom Height is 15' from tunnel to top of transom.


We have an older gentleman who has been servicing props for over 50 years and has quite a
reputation here. He's affordable and knows what he's doing, other wise he wouldn't have lasted 50 years.
He stands by his recommendation of the 12P and I had to see it for myself. That's why when I
put the 13P and lost nearly 600rpms I knew I was wrong. I am going to give the 11P a try next
and have him cup it, if the rpms are too high I will have him cup it to double. He charges $35.00
to cup each time and his turn around is normally 1 day.





Thank You And Everyone Else Who Has Taken The Time To Respond,
Lukus

 

[Sea Rider]

Lukus,

Try lowering OB for water flow to skim right under upper plate, that's only one inch or tad more leg under water, you could use the shallow water tilt adjustment when boating close to shore. Are you testing OEM props ? those are all cupped.. Dial a prop that will let your OB rev around full wot 5850 revs as usually loaded. So you have lost 600 wot revs when going for a 13 pitch. I gained 500 + revs going from a niner to an eight as currently loaded. Are you testing each prop with tach ?

Happy Boating

 

[LukeFishWalker]

Sea Rider,
If you open the attachment, you will see a scratch on the mobile part of the jack plate.
That's where I started just above the 2 small sharpie marks.
I know that putting the plate on the scratch mark does not allow any blow-out/cavitation at all.
I used that as a "Go-Back" reference mark and if needed I can return to that mark.

This 25 came to me with an after market SS 12P with a slight cup. No idea what brand.
I then bought a "Parts" motor that has a damaged fire deck on the block and it came with an
after market SS 13P with no cup. There's signs of nicks and thinning of the blades on this prop.
The 13P I had cupped is a OEM SS Tohatsu 10X13 and is quite heavy and thick.
If I do get an 11P, I will likely get an OEM SS Tohatsu 10X11 unless you can recommend an
aftermarket prop that you'd buy for yourself.

Also in the picture - There's a digital Kedsum Tach that I started using and so far it's pretty dang close.
I mounted on my buddies boat that has a factory tach and this Kedsum is within 20rpms.

When I lost 600 rpms, I went from the aftermarket SS 12P W/Moderate Cup to the SS Tohatsu 10X13 W/Moderate
Cup. So, if the same occurs by going from the SS 12P W/Moderate Cup to a SS Tohatsu 10X11 W/Moderate cup,
I would be real close to the target rpm of 5800. The reason for using a cupped prop in my case.....
Is the shallow water environment full of weeds and grass where I need the hole shot.
Get up, dump the wheel, keep going, dump the wheel, keep going and dump the wheel.
Dump the wheel = Decelerate and pull the foot out of the water to dump the debris off the skeg,
hit the throttle, dump, hit the throttle, dump and do this until I get out of the grass and weeds.
I can do this and keep the boat up on plane with the cupped prop because of the tremendous thrust.
And that's another reason for attempting to keep the OB as high as possible without sacrificing
blow-out and cavitation. Even though the water is actually 2 or 3 feet deep, the debris clutters all the
way to the surface. And, that's where the bass like to hang out is in the thick cover.

I hope this kind of explains my goofy way of thinking in line with your questions above.
Thanks,
Lukus

 

[Sea Rider]

Lukus,

Personally like OEM 3 blade alum props to play with, available in 1 increment/decrement sizes. With one less pitch can achieve around 400-500 + wot rpm when testing 18-30-40 HP Tohatsu OB's with induction tachs on light combos. Depending on the max wot rpm achieved with combo, usually dial next lower pitch to achieve top wot rpm for any of the 3 mentioned models. Tohatsu OB's sent with installed 3 blade props are usually medium pitched, soon after break in period ends must dial a pitch that revs around full wot revs for that combo as usually loaded.

Buying and testing SS props, 4 blade, other brand props will be dumping money into the blue, besides are not available down where I live and can't recommend props that have not tested myself.


Happy Boating