Tohatsu m25c3 2-stroke compression high on one cylinder

[sparkey504]

long story long i have a 2 stroke tohatsu m25c3 that ive had for at least 6 years... it sat up for several years due to broken bolt on impeller housing.... fixed it and rebuilt lower unit essentially last spring.(IM NOT TO FAMILIAR WITH OUTBOARDS or anything to deep into regular engines but i do work on extremely complicated and precise CNC machines for a living so im not completely useless)
I took it out and it ran great for 20 min, died and after several hours ended up getting a tow back.

after probably 2 months of weekends tinkering with it here and there i stopped.
i was recently layed off so im determined to get her fixed while i have the time.

i have it running pretty decent on ears and barrel but idle starts out high and gets lower the longer it ran...As much as i hate to admit it because water was coming out of the pisser i thought warter system was good... turns put thermostat was stuck closed and was likely the cause of my issue last year
NOW TO THE POINT....
Because of that i want to make sure she is in good health before i take it out...so i did a compression test with both plugs out throttle wide open... Spec is 110psi
top cylinder is 95psi
bottom cylinder is 185psi

I pulled exhaust cover off to see if there was a blockage of some type and while i didnt see any blockages is did find the tip off a fin of the water impeller and it does look like new rubber (water still pisses but going to change reguardless)
so why is the lower cyliner compression 163% than manf spec?
im about to order head gasket so i can pull that off and take a look see... anything else i need to order or look at?

 

[Sea Rider]

Can't possibly have double PSI reading between both cylinders, did you made the compression test after the motor was warmed up for min 3 minutes at idle on muffs, barrel ? If not, test again as stated. Don't squirt nothing into the cylinders...

Happy Boating​

 

[sparkey504]

the motor was not warmed up at all... ran test 4 times on both cylinders... 3rd and 4th time i added a lil bit of oil and pressure was 3or4psi higher on top cylinder than previous 2 test.
bottom cylinder had same psi all 4 times... of 180psi... i even wrote down the psi on the head and immediately texted a buddy who is good with engines so i am 100% certain that was the reading that the brand new compression tester displayed.
i did give the incorrect spec in post...
The CORRECT spec is 99.6 psi.
( I JUST NOTICED it does say to, bring up to normal operating temp at the top when double checking speck)
I will run again tomorrow but this time i will record test before and after running at idle for
3 min and post the results.

I did remove lower and checked impeller and the tip i found behind exhaust cover must have been before my time as it is still 100% intact and looks good.

THANKS

 

[Sea Rider]

Have tested many motor's cylinder compressions before and have never seen such described scenario. The Service Manual doesn't mention which compression brand and model it's used at the factory. My 18 horse factory specs calls for near 107 PSI, but after 1K run hours dials when cold tested 115 PSI on each cylinder with a new compression tester. Don' kow if there's a great PSI difference when testing cold or hot ? For initial PSI tests don't squirt oil, test with dry cylinders or will increase compression and cheat the test...

How old is the motor, been run on salt or fresh water since new ? If installing a new head gasket take advantage to decarbon the whole powerhead, exhaust chamber as well and desalt all the water passages to an immaculate clean cond. If performing the whole powerhead internal maintenance order 2 exhaust cover, base and thermo gaskets as well aside from the cylinder head gasket...

Once the power head has been reassembled order a can of CRC marine decarbonizer and once motor is running perform a piston's rings decarabon, the motor will idle and run much smoother..

Happy Boating​

 

[MattFL]

Can you see inside the cylinder with the high compression at all? Maybe borrow or buy a cheap scope? Maybe you've got some crazy carbon build up in there, or something else taking up space that is raising the compression.

 

[sparkey504]

Motor is a 2001, unknown hours. I live in Louisiana and bought it from a guy in houma so id say mainly brackish warter but im sure its been salt warter... but its on a J16 carolina skiff so its not like you can take this thing out the rigs and not have to swim back.
just ran test again and its about the same as last time...top cylinder right around 90
bottom was 163psi.
i didnt take any pictures the first time but i believe the 180psi i originally said was incorrect/rounded up as 163 seems extremely familiar/standing out to me.
the first round of test i did 2 without and 2 with oil and only top had a few psi gain.
i did order the top end oem gasket kit as it was only $25 more than just oem head gasket.
there is some carbon build up but nothing bad that i can see... i do have a usb borescope for an older phone and i tried using but i can't see the reverse side of the head and really couldn't see anything that i couldn't see with just a flashlight.

being that its running i have no plans on rebuilding top end at the moment... i dont like the higher compression on one cylinder but not enough for a teardown.
if anything ill remove just the head for a look see and remove any carbon that i can access.

i would really like a 40hp for the boat but considering i paid $2500 for the boat and everything else that goes with a fishing boat except a battery for trolling motor and fishing poles about 8 years ago so i dont see myself paying more for a motor than what i paid for everything when id like to get a slightly bigger aluminum boat.

 

[sparkey504]

SEA RIDER.... i saw that your the go to guy in another thread about 25 to 30hp conversions... cjances are the previous owner already did it and from i see its carb gasket and timing... the current carb gasket inside diameter is the same size as opening... so how can i confirm timing has already been adjusted? if there's already a thread where you explained it and dont mind pointing me in the direction i would greatly appreciate it. THANKS

 

[Sea Rider]

What you can do is : with left hand any finger rise the white roller fully up till stops, with right hand throttle the grip fully till the cam stops against the roller, slowly throttle the grip back and forth just a bit, if the synch is right should hear a light banging going on between the cam and roller which is spot on. Which boat/size is being powered with said motor and how loaded do you generally go boating with ?

Right now the timing seen on second pic shows stopping at 20 deg which corresponds to the 25 timing, the 30 HP timing is 25 deg. As you have witnessed it's not all about a carb gasket swap, that's just half of the equation...

There's been a lot of inquiries about the 25 to 30 HP conversion, will need to do a search. It's well expalined and highly illustrated with easy to follow pics. Let me know if it was possible to find....

Happy Boating

​

 

[pvanv]

If it's running OK, I would definitely do a decarbon spray through the carburetor and then re-check compression.

 

[sparkey504]

sea rider-
its a center console 16' Carolina skiff J16 with tohatsu 25hp (M25C3) so while i vaguely get what your saying but not a tiller handle.
i tend to be over prepared so typically when i go fishing its me one other person ice chest tackle tools and probably 100lbs of **** i wont actually need.

so do i just adjust it so its at 25⁰ at full throttle or should i search the forum for the procedure?

also i looked for that crc marine tune up and decarbon spray and most places are either $30 a can or $10 shipping for a can... will the crc gdi carbon spray work the same?
do i spray thru intake while running like gdi spray instructions? or should i position cylinder at top, spray and let sit then blow it out with air gun?

 

[sparkey504]

only 2 i found on this fourm its your replies sea rider... lol
second says adjust timing 5⁰ when at full throttle and the firat has some pics... one with measurements... my top link (4)measurement is 20.61mm between link nuts (shows 20mm) i dont have 3 as im assuming that for tiller handle.

Tohatsu 25hp to 30hp worth the trouble?

I have a 1989 Carolina Skiff 1450 with an '89 Tohatsu m25c2(tiller) and a SE 2000 Hydrofoil installed. With just me (and all the gear) at around 3/4 throttle I hit around 20mph and it sips fuel. WOT zips along pretty well, but sucks fuel. With 1 fishing partner along (2 of my 3 fishin buddies...

forums.iboats.com

Newbie with a 25hp Tohatsu 2 stroke question

I recently purchased a 25hp tohatsu from a buddy. I removed the restrictor plate and installed the appropriate 30hp open gasket. My timing is set at 25 degrees wot in gear. My question is with throttle fully backed off, what should the timing be at idle? My buddy had jacked with the wrong...

forums.iboats.com

 

[Sea Rider]

Forgot to ask you on my last posts when the center console's throttle is set to max forward, while previously geared forward, check if the white roller stops at the cam's middle indent/groove, if so, that's the 25 HP carb setting with the time plate's mark stopping at 20 deg.​

Or at the upper cam's hill as seen which corresponds to the 30 HP carb setting with timing plate's mark stopping at 25 deg.​

This conversion it's not stated in any Service Manual, Tohatsu doesn't want buyers to convert a 25 into a 30 HP for peanuts costs when there's at least 200-300 bucks price differente between both models....LOL!!

Happy Boating​

 

[sparkey504]

SEARIDER... i haven't had the chance to look into the stuff in your response... at the moment im trying to get the upper motor mount to come loose...hindsight it probably didnt need changing, but the exposed portion was dry rotten and from trying to heat the thru hole for the mount i ended up burning it (I KNOW I KNOW) on the opposite side it has come loose from the drive housing side... on the passenger side its stuck like ****.... i have heated housing slightly, sprayed freeze off... kroil oil... and countless other penerateing oils and notta... i put double nutted bolt and got it to spin... any other tricks on getting this guy loose? i was using a brass hammer at first until i noticed it was marring surface, had to use file to remove material from in front of bushing.... im about to try an air hammer with blunt tip caise i do not want to cut it... any tips would be greatly appreciated...

 

[sparkey504]

between everying i had previously done and the air hammer on the face of the bolt with 2 nuts to protect threads it came loose... will probably need to use slide hammer or spacer pipe and use bolt threads to pull from housing... thanks though... will look at timing when i get it back together...

 

[Sea Rider]

Don't worry Sparkey take your time, that horse needs to be fully operational before working on the carb/timing advance synch. That's the bolt that crosses side of the steering shaft and secures the middle leg in place, right ? Sorry can't help you on this one, never have issues with seized bolts, always grease them well with marine grease from time to time since out of the box specially boating on salted waters.

Happy Boating​