2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

[smashed]

This is an engine I bought 2 months ago. I was told it has 20-30 hours on it. And it really looks almost new. There can be seen some oil on the motor as if the gasket is leaking.

Here is the problem - initially I was getting 5250-5300 rpm. Then after 5-6 operating hours I got 5100. After another 4-5 hours now the rpm are at 4850 at the same conditions and weight distribution in the boat. The speed also dropped from 16 mph to 15 mph. Initially the boat was getting to plane with 2 people. Now it won't.

I was afraid of compression problem. I don't have compression numbers when I bought it. But the current test gave the following:
132 and 137 psi after 5 strong pulls of the cord on a COLD engine.
120 and 125 psi after 5 strong pulls of the cord on a warmed up engine.

Are these values in the expected range for this motor? And what do you guys think the problem might be? I am hoping for a minor adjustment that will solve the issue.

Thanks in advance! Let me know if more info is needed.

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

In a service manual for the 2.5 - 40 hp Tohatsu 2-stroke engines, I find that the compression in the 8/9.8 hp must be around 55-60 psi??? The 9.9/15 is at the 115 psi. I wonder what is going on, lol. I would love to have the 9.9 and convert it to 15 hp but is this the case? On the cover it says 9.8 though, and the engine is really light, so it must be the 9.8.

 

[pvanv]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

We need your actual model (is it M9.8B?) to help much. Your problem could be a lot of things, even a leaky head gasket or poor fuel. Odds are the compression in and of itself is fine. What do the plugs look like? What is your oil ratio? No, the M9.8, already being the big brother of the M8, cannot be increased to 15 hp.

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

My mistake - it is 2001. I will attach some pics of the motor.
The model is M9.8B 3K9 Serial No 20776 The Serial No is also stamped on the head which most likely eliminates my doubts of it being a 9.9 based on the compression numbers.
I use Super 93 in an approximately 45-50 to 1 ratio. I add 80-85 ml of Pennzoil Premium Plus marine 2 stroke oil. For a perfect 50:1 I believe I need 75 ml so I am usually a bit rich in oil.

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

Here are pictures of the spark plugs. I had the motor running for 10 minutes yesterday, I guess the wetness on the plugs must be 2 cycle oil?

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

There is some oil on the head, but it was there when I bought the outboard and I don't think that the amount has increased. There is some sand dust at the base where the engine sits (top of leg, under the cover) and the base is covered with a thin film of oil.

The sand can be seen on the last image.

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

The engine is uniformly painted, seems to be original. The paint is peeling on a couple of spots though.

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

Last set of pictures. Some oil behind the head cover. And a bolt that has a thin film of black paint, that differs in appearance compared to the rest of painted components.

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

My confusion with the compression numbers comes from this chart I found in a Tohatsu Service manual (see attached image). The engine volume (169 cc) in the manual matches the volume of my motor, and the service manual is for the 2-stroke motors from 2.5 to 40 hp. Unless the compression meter I am using is faulty (Innova brand) and gives me 2x the actual compression number, I can't understand such a mismatch.

According to the chart the compression should be 392 kPa. I am getting 860 kPa, which is closest to the numbers for a 9.9 (and 15) hp engines (780 kPa).

 

[pvanv]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

All looks OK. You could run less oil, not a biggie. How old is the prop... perhaps it may be slipping in the hub?

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

The prop is (seems to be) the original aluminum white one. The paint on it is peeled halfway. If it was slipping though, shouldn't I see increase in RPM not decrease?
I will try to dismantle the carb and clean it. Might be that some of the sand that entered under the cover got inside the carb. Or could it be the higher octane gas that I am using? It is rated for 87 as seen on the label.

Anything specific to look for or be careful of while I take the carb apart? How are the throttle rods/system attached?

BTW I see some black residue on the pistons. Is this to be expected after only 40-50 hours of operation?

 

[Sea Rider]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

Smashed,

Is the engine kind of missing/misbehaving at 3/4 to wot throttle ? Or runs fine missing top rpm and speed ?

If missing check ;

-Tank engine fuel filter & carb to be immaculate clean.
-BR-7HS-10 or B-7HS-10 gapped to 1.0 mm electrode tight gapped.
-50:1 fuel/oil ratio, about 75 mm per 3.78 gallon.

Is head gasket countour with salt formations, paint has peeled off ? Perform a head torque with wrench to factory specs.

If just a slower rpm and top speed loss, hub could be on it's first slipping stage, must perform a hub slip test. Engine usually revs at higher revs when hub is kaput. Find 2 stroke 9.8 compression readings stated on service manual quite low compared to other higher HP readings, probably a typo issue when writing the manual. It's normal to see unburned fuel/oil residues & carbon formations on piston's upper head, it's a 2 strokes engine...

Happy Boating

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

Thanks for the thorough answer Sea Rider. The motor is not missing, works well imo.
However I think and hope that you are right about the slippage. But not a hub slippage as I have decreasing rpm. I am thinking that probably the shape of the blades is changing. I am not sure if the propeller is original, there is no Tohatsu stamp on it, just a 7.5 number. I am hoping that the blades are twisting slowly from their optimal position. And when the propeller turns the blade hits the water instead of biting into it. This of course may be a nonsense. I just ordered a 7 pitch propeller, will perform the tests at the end of the week. If I get 16-17 mph with my 14' Saturn, then the problem will be solved. If not, I will start digging into the carburetor.
BTW, I hit some rocks in the Delaware river which slightly warped the propeller (tip of 1 or 2 blades). After this the performance suddenly dropped to 4850 from 5050-5100 rpm on the next outing. When I hit the rocks I couldn't evaluate the damage, because I had 6 people in the boat and it was not planing at any time on that journey. I discovered the decreased performance on the next outing. Before the rocks I believe the propeller had come in contact only with sand and am hoping that this is when the blades were twisted. Just a new propeller would be an easy fix. I was due for a new one. Hopefully with the new one I can get my RPM to the 5500-5600 mark. That would be perfect!

The head gasket has almost unnoticeable white formations, I thought this is the gasket. Will retighten it at a later point if it worsens.

I am happy to find that the compression is good at least. As this is a used engine, I had my concerns that it wasn't properly broken in and that it is slowly failing. I will continue to pay attention to the compression numbers in the future.

 

[MattFL]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

A mangled prop can really hurt performance, so if you noticed everything going bad after hitting the rocks then there's a good chance that the mangled prop is the problem. Just be sure you're replacing it with the same prop. A different prop design can behave drastically different, and that's a whole new set of variables itself. Chasing 2 problems at once can be difficult.

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

I took a look at the prop now, and the blades are thicker than I thought. I am a bit skeptical the blades got twisted. And actually the initial performance drop was before I hit the rocks and sand. Anyway, will see what the new prop does. I got the 7 instead of 7.5 pitch. And it is 3 blade one same as what I have now.
I totally agree with you that changing the prop variable throws things in another direction. The possibility of a faulty prop will be eliminated, so I could search for a problem elsewhere if the problem persists and worsens. The new prop is original and optimized, and if the engine turns out to be fine, the new prop will increase the speed of the boat, I am sure of it.

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

In the service manual I read that carb cleaning solution should not be used to soak the carb. Is this correct? They recommend just isopropanol and soapy water.
If I shall use carb cleaner, please point me to a specific brand/product.

 

[pvanv]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

The gasoline in Japan is much better than in the US, so they don't see the varnishing and water issues that we do. That and the possibility of eroding metals if a very aggressive cleaner is used for too long is why the manual is written that way.

In the US, we use real carb dip (such as Tyme or Gunk brands), available in 1-gallon cans at auto parts houses to clean carbs. Complete disassembly (including jets and emulsion tube nozzle), then a 4-hour room-temperature soak of all non-rubber parts, followed by a blow-out with generic carb spray... then a careful reassembly does the trick.

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

Thank you very much for the detailed explanation!
If I find difficulties taking the carb apart, I will post them here.

 

[smashed]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

I tried the new propeller - 7 pitch, 3 blades. On the tachometer I get 6000-6100 rpm when I am alone on the boat. Should be fine for when 2 adults are on boat (will keep rpm at 5300-5500 rpm).
What propeller pitch should I get to achieve 5400-5600 rpm at WOT - an 8 or 9? How fast does the maximum hp drop after 6000 rpm? Maybe I can get a bit higher speed at 5500. The motor is rated for 5000-6000 rpm at wot.

I find some improvement with the new propeller - there is more thrust before getting on plane, and the speed is improved very slightly - to 16 mph (from 15 mph with the supposedly mangled, deformed propeller).

 

[MattFL]

Re: 2003 Tohatsu 9.8 2-stroke engine - decreasing performance, compression numbers

If you are going to have 2 people in the boat often then I would keep that prop. If you need a motor to run for long periods near it's peak output, then it's better for the motor to be higher in it's allowed rev range. This generates less heat and is actually less stressful on the parts. If you're boating alone most often and you just have to have the highest top speed possible then try adding an inch to the pitch, but if it were my boat and I expected to have 2 people in it often then I would keep the same prop you have now.