New boat and mint 2 stroke Yamaha engine

Blankton · 15 June 2021, 21:01

Following a post that came up on facebook, I have collected a new setup today. A yamaha 9.9 2 stroke and an Avon rover SIB.

It was a bit of an impulse purchase, however I had been considering something lighter for some time and I knew that the engine wouldn't hang around.

One of the main reasons for not doing it sooner was that I thought I'd loose a lot of money on my current 4 stroke 15hp yamaha engine, but looking at second hand prices I might even make a small profit!

Boat wise, I think I'll set my yam and the Avon up next to each other and decide which is best (they are both 3.1m). The yam has got launch wheels and a few fishing modifications, which I'd need to fit to the Avon, but the Avon feels like it will be lighter, so I think I might be leaning towards the Avon boat. The seller seemed to think the tubes would be hypalon, but I wasn't sure on this model? I guess hypalon tubes would also be a reason to plump for that boat.

Oldman2 · 15 June 2021, 21:08

I think 5 mins with a spanner and you will have yourself a Yamaha 15HP.

Blankton · 15 June 2021, 21:10

Quote:

Originally Posted by Oldman2

I think 5 mins with a spanner and you will have yourself a Yamaha 15HP.

Just reading some threads on that. i'm just trying to work out if its safer to buy a 15hp reed block, or just try and adjust the one I have.

Fenlander · 15 June 2021, 21:15

Easy to do... the measurements are in the manual Oldman has... and perhaps elsewhere online.

Ahh just found pics in my folder.

As you can see the stops are easy to remove and get to the correct dimension in a vice without bending the rest of the assy.

Blankton · 15 June 2021, 21:22

Quote:

Originally Posted by Fenlander

Easy to do... the measurements are in the manual Oldman has... and perhaps elsewhere online.

Ahh just found a pic in my folder.

Some guy called Fenlander posted them in this thread

https://www.rib.net/forum/f36/9-9-to-15hp-83382.html

So you just bend out the flaps to the correct gap, a bit like setting spark plug gap clearances?

Blankton · 15 June 2021, 21:38

Quote:

Originally Posted by Fenlander

Easy to do... the measurements are in the manual Oldman has... and perhaps elsewhere online.

Ahh just found pics in my folder.

As you can see the stops are easy to remove and get to the correct dimension in a vice without bending the rest of the assy.

Thanks for that, I think our posts crossed in the ether

Blankton · 26 June 2021, 15:58

I have fired up the outboard today and given it a service - new plugs, impellor and gear oil. To be fair, it started first pull of the chord even beforehand, so I haven't made much of a difference.

The plugs were slightly oily. But the guy that sold it told me he used to run the oil a bit rich to be safe.

I am going to order a thermostat and gasket as well, just so I can break into the water jacket and see how that looks.

The only issues were -
Prop split pin needs replacing as it had been well used. Not much grease behind the prop, so the spline was a bit oxidised from the prop. So I need some grease for that.
Two of the bolts that hold the impellor housing into the bottom leg were corroding - I thought it was a bit odd that these were stainless. Stainless into the aluminium casting was leading to corroding, whereas the bolts that hold the bottom of the leg on were steel with zinc flake and there was virtually no corrosion product.

I also did a compression test and both cylinders were identical at 85psi. I thought the compressing might be a tough higher, but both were exactly the same, which is the main thing.

Oldman2 · 26 June 2021, 16:44

Looking very nice Blankton, the compressions are a little low, Fenlanders 15hp Yamaha was 120 on each when he tested his.
I noticed the fuel line is the quicksilver one, both Fenlander and I have had this silver fuel line degrade internally and separate, so watch out for that if its an old line.

Blankton · 26 June 2021, 17:14

I did the compression test after firing the engine up, so it was warm. I am sure this will have an effect, but I'm not sure which way. I might try again cold.

I've also checked Fenlanders thread and see that he brought new bolts for the water pump housing and that these had loctite on. So the white stuff I thought might be corrosion, could have been loctite......

TmMorris · 26 June 2021, 17:46

Looking good. I'd definitely check that compression again when cold. 85 is a bit low, you'd expect at least 120 on a used engine. As you say, the fact that they are equal is arguably the most important element.

You could also whip a plug out of your car and check that to see if your gauge is under reading?

Blankton · 26 June 2021, 18:02

It's a very cheap gauge, so I don't have a great deal of confidence in it. I've got a diesel, it would probably make the gauge explode!

I'll repeat when cold. Other than the piston rings and bores, I'm not sure what can be wrong with a 2 stroke, there aren't valves are there?

I used to work in a factory where we made engines. When the cylinders were honed, we did a Deep "primary" hone and a shallow secondary "plateau" hone. The idea being that the plateau hone only lasted a few hours, but held oil for running in. After a few hours only the deeper primary hone was remaining. So it is feasible that the compression improves after only a few hours running, as the plateau or secondary honing is polished out. Then eventually decreases over hundreds of hours as the bores wear.

TmMorris · 26 June 2021, 18:22

I'm not sure Yamaha went to the ultimate lengths for these.

I think that matching results tend to be the big indicator.

You could always go down to the supermarket with the gauge and a clipboard and tell someone you're offering free compression tests? Guaranteed someone will demand two and some to take home.

Incidentally, when I was collecting my used Yam/Mariner 15 the chap had a knackered 15 on a pallet that he'd bought for spares. If he still had it then it might have the carb etc you'd need to give your 9.9 the life it deserves?

Blankton · 26 June 2021, 18:29

Quote:

Originally Posted by TmMorris

Incidentally, when I was collecting my used Yam/Mariner 15 the chap had a knackered 15 on a pallet that he'd bought for spares. If he still had it then it might have the carb etc you'd need to give your 9.9 the life it deserves?

I believe all i need are Reed valves and to remove the little bit of steel that acts as a rev limiter. Furthermore, I think the existing Reed valves can be adjusted, rather than buying new.

TmMorris · 26 June 2021, 18:35

Quote:

Originally Posted by Blankton

I believe all i need are Reed valves and to remove the little bit of steel that acts as a rev limiter. Furthermore, I think the existing Reed valves can be adjusted, rather than buying new.

That would make sense. It seems illogical that there would actually be different major components given they sell/sold these to make as much profit as possible.

It seems logical that the Reed valves can be adjusted and any any air flow restrictive removed (if 15 was the largest for that carb unit then its likely its the one with no restrictive and the smaller outputs had them added.

Is there a needle change on the carb and a bit of a timing change needed? I've not looked on mine but the timing degrees are normally marked out conveniently in the rush steps for the right BHP.

Fenlander · 26 June 2021, 19:00

Just get those reed stops bent back to the 15 spec and check the throttle can open fully... it'll be fine. No new carb needed.

Re the compressions not many folks have a gauge that is tested to a pro standard so my gauge might have read 120 on your motor. As you say the fact they are even is the most important thing. If it starts OK hot and cold and you see the sort of speeds we'd expect don't give the actual figures a second thought.

Blankton · 09 July 2021, 15:26

I took the new engine out on my SIB today (with the 15hp conversion). It performed great. Popped the boat up on the plane every time. It's only 14kg lighter, but that small amount seems to have made a huge difference. The revs seem to build up quicker than the 4 stroke as well.

Top speed was 20.6knts, which was as good, if not better, than I ever had from the 15hp 4 stroke engine. This was in a bit of chop, flat calm I reckon I might get a touch more, so It appears that I carried out the modification correctly

Fenlander · 09 July 2021, 17:57

As you will have read our recent 10kg outboard weight reduction makes a massive difference.

Looks like you’ve ended up with a really nice motor.