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Tohatsu 9.8hp replacement for Yamaha

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Joined: 20 Feb 2007
Posts: 15
Location: UK,Chester

Tohatsu 9.8hp replacement for Yamaha   Posted: Sun 03 May 15, 20:14    Reply with quote

For those with older 920's that came with the Yamaha engine there is a problem when it comes to engine replacement as Yamaha no longer make the 9.9 in extra long leg and the Honda needs hull modifications. However, there is now an easier solution!

Tohatsu now produce at 9.8 in ultra long which has the options of a sail boat propellor, electric start and charging output. Some modifications are still required, but less than for the Honda. Here's what's needed to make it fit...

The holes for the mounting bolts are in a different place, so the old bolt holes need filling with wood filler on the outside and gelcoat filler on the inside.

The wooden engine mount block needs thinning by 3mm at the front. This takes seconds with a sander attached to an angle grinder. It's not necessary to go vertically down as far as the fibreglass, just enough to allow the clamps to go on.

New mounting bolt holes need drilling. These are higher up and through the wood block only now. You need a long 8mm wood drill.

The tiller link needs shortening by about 2cm and the end fitting reattached with 3mm rivets. The Tohatsu tiller is no longer parallel with the main tiller vertically so the angle of the end fitting needs adjusting by a few degrees. It's easy to see when the new engine is in place.

The tiller link fitting on the engine needs attaching to the engine's tiller. It's a bit of a fiddle to get the nuts in on the inside but there's room.

The fuel connector is different and the new fuel line that comes with the engine needs attaching to the old fuel line at the priming bulb. My engine supplier did this as he had the crimps.

The electrical plug needs reconnecting on the new power lines from the new engine - not difficult.

In all it took about 2 hours to do - much easier than GRP modifications needed for the Honda. I can post a picture of the new engine in place if anyone is interested.
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Joined: 01 Nov 2006
Posts: 191
Location: USA, Long Island Sound

   Posted: Mon 04 May 15, 15:24    Reply with quote

I replaced my Honda 10 with Tohatsu 9.8 last year (I completely rebuilt my Honda so I have a spare engine...).

I had to file the wood mounting bracket. I used a chisel to cut a slot on the front side where the mount is tightened. I also had to drill new holes for the bolts.

I solved the linkage issue by attaching an aluminum collar to the new tiller and attaching the linkage fitting to the collar. So I didn't need to rework the linkage bar.

I put brand new fuel line and fuel tank that came with the engine and saved the old tank and fuel line for the old engine. I also installed water separating filter between the priming bulb and the engine and mounted it on aft wall of the cockpit.

I attached the old male electrical plug to the new engine - very easy but need to ensure correct polarity!

Here are my observations:
Tohatsu tiller catches on the side of the boat near rear cleat when the tiller is all the way to port. Minor inconvenience.

The alternator is much smaller than the one on Honda. I would need to run the engine an hour or more to charge the battery where Honda only needed 15-20 minutes.

The engine is a little bit noisier than Honda and has more vibration. It is also 25-30% less economical in fuel consumption. In the US, the cost of fuel is insignificant, but I would need to carry extra containers when on a cruise.

At full open throttle I can only get about 3300 rpm with the standard prop. I don't know what the rpm were on Honda as I just got the tachometer with the new engine. I feel that the top speed is .1-.2 kn less than with Honda, but it is difficult to prove.

Each engine has its own quirks. My Honda did not like to run in gear at idle, especially when just shifting. I always had a to give it extra gas - a lot of extra gas. Then I could turn it down to idle. Tohatsu does not always engage the lock when put in reverse after moving forward. I had 2 very close calls because of it. I need to figure out how to adjust the locking mechanism.

Honda has electric choke and sometimes did not want to start when warm - would need 2-3 attempts.

Tohatsu has manual choke. I can never figure out whether it is warm or cold. So if it does not start with choke, I try again without and it starts immediately.

Tohatsu cover is much flimsier than Honda. It also fits poorly.

Honda mounting/tilting mechanism is MUCH-MUCH better. On Tohatsu I have to stand on the swim platform and hold the lever while tilting the engine in either direction. On Honda, you just push or pull the lever, and then tilt the engine from the cockpit.

Tohatsu is about 15 kg lighter!

Tohatsu does not have oil filter. Changing the oil is inconvenient.

I put an after-market fuel tank of 6.6 gallon (25 l) capacity early on. A few years ago US changed fuel composition to include ethanol. This created a lot of problems with fuel as ethanol is hygroscopic. This fuel only has about 6 weeks shelf life. Since I use very little fuel, my tank was mostly empty and a lot of water was absorbed into fuel which ultimately caused problems with the carburetor (which ultimately led me to a complete engine rebuild). It was recommended to me to use the smallest tank and keep it as full as possible. So now I put a new tank of about 3.2 gl (12 l). Since this much fuel will last me 2 months, I always add fuel stabilizer. I also added water separating filter. Now I only use my large tank when going on a cruise.

Overall, I am happy with the new engine. But I MUST solve the issue with reverse lock.

BTW, Tohatsu is about 35% cheaper than a Honda.

My rebuilt Honda starts and runs beautifully. Looks like the idle speed problem was also solved. After 12 years with Honda, I would recommend the following:

remove and clean thermostat every two years to avoid corrosion freeze

remove and clean carburetor every 2-3 years. otherwise the bolts and nuts freeze and impossible to remove when needed!

drain carburetor at end of season. i used to just run it dry, but fuel is left at the bottom of the bowl.

remove the lower unit and lubricate bolts and mating surfaces every two years to avoid corrosion freeze even if you are not changing the impeller
(the impeller looked perfect after 10 seasons)
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Joined: 23 Aug 2015
Posts: 1
Location: England South Coast

Tohatsu 9.8hp   Posted: Mon 24 Aug 15, 20:16    Reply with quote

After reading the above posts I also replaced my Yamaha 9.9 with the Tohatsu - I've been avoiding making the change to the Honda due to the work involved in modifying the transom.

The advice on the forum was really useful & the change was very easy. I also used a chisel to cut the wooden transom down.

To connect the tiller bar I attached an aluminum plate underneath the engines tiller (using a couple of existing screw holes) & mounted the bar attachment alongside the engine tiller. This seems a bit neater than bolting it on top of the engine tiller.

Also to avoid the engine tiller catching on the cleat I set a small aluminum block into epoxy & adhered this to the underside of the engines tiller to raise it about 25 degrees (this was something I had to also do on the previous engine when I replaced a tiller).

I've just finished running the engine in & so far all works fine - I think it is quieter & less vibration than the Yamaha at idle / low revs but slightly noisier at full speed - when it seems to rev higher than the Yamaha. I have high trust props on both. I'd agree though that it is probably slightly slower than the Yamaha.

I've had no problems with the tilt mechanism - very similar to the Yamaha no problems with it engaging in reverse.

A couple of clear advantages - cost about 1/3 less than a Honda & as has been said the lower weight is a real plus- getting it on & off the boat is so much easier. I also like having a manual start in addition to the electric - although the Yamaha never failed to start due to electrical reasons it is reassuring to know that should this happen the engine can be quickly started without having to semi dismantle it.

One possible disadvantage is the position of the gear change on the front of the engine - it takes a bit more of a stretch than on the Yamaha

Please click on any of the photos to see them enlarged!

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Joined: 02 Nov 2006
Posts: 83
Location: Wales

Tohatsu 9.8   Posted: Sun 20 Sep 15, 0:19    Reply with quote

Is this a 4 stroke engine, or 2 stroke? I used to have the 2 stroke on my F27.
DF920 Pelican, Swansea
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Joined: 01 Nov 2006
Posts: 191
Location: USA, Long Island Sound

   Posted: Sun 20 Sep 15, 20:41    Reply with quote

4 stroke.
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