18HP Tohatsu - what compression is "good"

[RRitt]

I am going to go look at an 18HP 2005 Tohatsu for sale. What range of compression readings should I expect to see on a good Tohatsu four stroke 18HP engine?

My big concern is avoiding an engine that has overheated. The water around here is extremely shallow with coarse sand known for its ability to destroy impellers in a single grounding. Does anybody know what kind of psi reading to look for on the older 2 cylinder tohatsu?

 

[TOHATSU GURU]

Re: 18HP Tohatsu - what compression is "good"

You look for even rather than a particular number.

 

[RRitt]

Re: 18HP Tohatsu - what compression is "good"

If an engine overheats will the compression vary between cylinders? I am mostly worried about heat damage. Overheating from chewed up impellers is common around here.

 

[pvanv]

Re: 18HP Tohatsu - what compression is "good"

If an OB is overheated to the point of damage, the compression will often vary between cylinders, as one cyl may suffer more than the other. The crankcase oil also acts as a coolant in these, which can save an overheating motor for a minute or two. I have seen a few that had been so overheated that some of the plastic bits started to melt... yet the seals were still good, and compression was acceptably even... And they still ran fine.

 

[RRitt]

Re: 18HP Tohatsu - what compression is "good"

okay, the guy says "won't start. Probably got some bad gas and needs a carb cleaning". There is no telling how long it has been sitting or what is the history. Under the cowl looks almost clean enough to eat off with no signs of discoloration or heat. All plastic is still soft and pliable.

Compression tested 50-60-70 on three bumps and was even on both cylinders. I added tablespoon of oil and compression went 60-70-80. The same compression tester reads 120psi on my 520cc 16HP John Deer.

What do you think?

 

[JB]

Re: 18HP Tohatsu - what compression is "good"

I would forget about that one, RRitt. "Won't start" is a killer.

The compression is minimal for a rope start engine.

 

[RRitt]

Re: 18HP Tohatsu - what compression is "good"

I would forget about that one, RRitt. "Won't start" is a killer.

The compression is minimal for a rope start engine.

It is an electric start. When you say minimal I am not sure if you mean rope start Tohatsu naturally have low compression .. or .. if you mean a 70-80psi is a low reading. I assume the later. I haven't worked on engine guts in decades. So I don't really have any idea what is normal for a post gasahol engine.

It sat for a year. Should a small tohatsu fire right up after sitting? It doesn't have a choke knob and my riding mower doesn't fire after a year without some choke.
Spark is strong.

 

[Sea Rider]

Re: 18HP Tohatsu - what compression is "good"

If you boat in extremely shallow waters with coarse sand known for its ability to destroy impellers and probably liner pump case under extreme sand abrassion, go for the chrome plate pump kit, would be advisable to sit startbord and check water pee very often to avoid unwanted overheat problems. More or less the same overheating issue when boating on extreme algae/kelp environments

Happy Boating

 

[JB]

Re: 18HP Tohatsu - what compression is "good"

Hmmmmm. YABUT it is electric start. Does it have compression relief for starting??

80 psi is dreadfully low for an electric start 4 stroke, whatever the make.

Won't start is still a killer for me.

 

[RRitt]

Re: 18HP Tohatsu - what compression is "good"

Hmmmmm. YABUT it is electric start. Does it have compression relief for starting??

80 psi is dreadfully low for an electric start 4 stroke, whatever the make.

Won't start is still a killer for me.

I don't know what compression relief for starting is or how to identify. Is it something that could cause a low reading on a healthy block?

"Won't start" doesn't bother me as long as the short block is healthy. It is a very good price on an engine with no evidence of saltwater corrosion and by all appearances not too many hours.

... but ... a rebuild? nah. if I'm going to sink that many hours into an outboard then it's going to be my Johnny. Deer that is. Johnny Deer.

 

[pvanv]

Re: 18HP Tohatsu - what compression is "good"

Tohatsu 4-strokes have decompression for starting.

 

[RRitt]

Re: 18HP Tohatsu - what compression is "good"

ok, so the guy said it was running until it got bad gas then his father took the carburetor apart and it would not start after that and if you did manage to get it to run that it ran badly. He said that if the compression was not good after warm up that he would give me my money back and he knocked price down to $800. Allegedly, the engine was hardly ever used. All the internal fittings, teeth on starter, electrical connectors, and residue in pan look low-hour.

I got it home and put a couple tablespoons of seafoam down each spark plug hole and let is sit for a couple of days. Then i squirted some gas down the carb throat and it fired right up ... ran for a few seconds and died. I took off the fuel line and it had ethanol poisoning. Put new fuel lines on and now it starts up okay, runs rough, dies on low idle, and has 150psi compression on both cylinders.

I'm guessing I need a new carb. The hose fitting had a strong layer of residue from ethanoil poisoning and the accelerator pump boot is in pieces. I'm not familiar enough with Tohatsu to recognize what his dad may or may not have done.

What is my best option for getting a good tohatsu carb? are the small outboards like small tractor engines where just about any carb fits just about any engine?

 

[levi_tsk]

Re: 18HP Tohatsu - what compression is "good"

id look for an exploded view of the carb and tear it down to see if anything is missing if not id put it in some carb dip and rebuild it before i went buying a new carb -he may have just dont a crappy job rebuilding it..... and it may just have a bit of rubber from the dedegraded hoses in one of the jets or something -adip and some air will blow that stuff out most of the time...

 

[pvanv]

Re: 18HP Tohatsu - what compression is "good"

Right. Whip out your Factory service manual (don't bother with the aftermarket manuals), and go through the carb, including a soak in real carb dip, and possibly a new kit 3R3871221M $52.67. If you find white aluminum oxide inside the carb from corrosive ethanol, get a new carb 3V9035002M $187.98.

 

[RRitt]

Re: 18HP Tohatsu - what compression is "good"

Right. Whip out your Factory service manual (don't bother with the aftermarket manuals), and go through the carb, including a soak in real carb dip, and possibly a new kit 3R3871221M $52.67. If you find white aluminum oxide inside the carb from corrosive ethanol, get a new carb 3V9035002M $187.98.

Where is best place to order Tohatsu parts?
if a new carb lists at $190 then maybe I can mail order a new one for $90-$100ish. When I search internet for tohatsu parts nobody is listing them. A bunch of places say "email for quote". nah, not today. I don't want to be on ten different mailing lists.

 

[pvanv]

Re: 18HP Tohatsu - what compression is "good"

To find your nearest dealer, browse to http://www.tohatsu.com/dealersearch/index.html
Most (myself included) will ship wherever you like. I doubt you will find that pricing on a new carb.

 

[RRitt]

Re: 18HP Tohatsu - what compression is "good"

To find your nearest dealer, browse to http://www.tohatsu.com/dealersearch/index.html
Most (myself included) will ship wherever you like. I doubt you will find that pricing on a new carb.

well, i'll shop around a little bit. if it's gonna be $180 then I'll just make a mounting adapter and use a john deer carb. they sell tons of jets cause the same carb body is used on almost every generator, lawn mower, snow blower, tractor, and toaster oven made.

 

[pvanv]

Re: 18HP Tohatsu - what compression is "good"

If you haven't even pulled the carb and looked it over yet, why jump to conclusions? It may be as simple as a proper cleaning and correct reassembly/adjustment, with the aid of the Factory service manual. An often overlooked item in that carb is the rubber block-off plug. Making a Frankenstein seldom results in good running, because matching velocities, mixtures, bell crank travel, and such is a fairly complex science. For example, bolting a carb from a 9.9 on a 9.8 (even though they look similar) will make the 9.8 run like crap. Yes, if stuck on Gilligan's Island, drastic measures are called for; but given what your time is worth (our shop is on the low end, charging $75 per hour labor), deliberately using wrong parts is not the way to go.

 

[RRitt]

Re: 18HP Tohatsu - what compression is "good"

If you haven't even pulled the carb and looked it over yet, why jump to conclusions? It may be as simple as a proper cleaning and correct reassembly/adjustment, with the aid of the Factory service manual. An often overlooked item in that carb is the rubber block-off plug. Making a Frankenstein seldom results in good running, because matching velocities, mixtures, bell crank travel, and such is a fairly complex science. For example, bolting a carb from a 9.9 on a 9.8 (even though they look similar) will make the 9.8 run like crap. Yes, if stuck on Gilligan's Island, drastic measures are called for; but given what your time is worth (our shop is on the low end, charging $75 per hour labor), deliberately using wrong parts is not the way to go.

I already took it apart, cleaned it and put it back together. I'm not familiar with it so I just put it back together exactly as I found it except with new gaskets. maybe it is the plug. it runs like an air passage is plugged. If the briggs doesn't work then I'll look at it next weekend. I like the briggs carb better. It's a more durable design. All things equal I'll take it.

It is surprisingly easy to put a briggs carb onto the tohatsu. the through bolts line up like it was meant to be and the throat is 1/16" larger. It only took about 20 minutes to drill out a small slab of 1/2" aluminum. I already have a spare 13hp 420cc briggs carb laying around. I have not bolted it together yet. I still need to bend some small rod to make a throttle lever.

 

[levi_tsk]

Re: 18HP Tohatsu - what compression is "good"

i agree with pvanv jerry rigging has no place on an outboard, or in the marine environment at all EVER!! and should only be a last ditch effort to get you home in a bind and should be corrected PROMTLY after it does what it needed to do....not safe.... not smart and if someone were to get hurt by this thing after you rigged it up youd feel pretty bad id like to think ....especially if it was your kid or your wife