The making of a rocket!
When transporting people in our dinghy, we have numerous times received comments about how much get-up-and-go our Yamaha 9.9 HP engine has. Sometimes I’ll just smile and agree, but other times I’ll go on to explain that even though the engine’s cowling says 9.9 HP on it, the engine is really operating as a 15 HP.
A number of years ago, when we were having difficulty with a dirty carburetor, a new friend, also named Mike, came to our boat to give us a hand with it. After he had finished teaching me how to clean the carb, he asked me if I’d like for him to make our engine into a 15 HP. Once he explained how little was involved to make that happen, I said go for it. I’ve come to believe that if people realized how easy it is to make this change, everyone would do it!
The video below explains it in general terms, although it a bit too quickly goes by what you need to do to make the change.
Apparently, in a 1996 or newer Yamaha 9.9, the only difference between it and its 15 HP brother is the reed valve assembly. To make the change, you could purchase a reed valve assembly for a 15 HP model and swap them, or you could do as Mike did and simply bend the reeds until they match the specs for a 15 HP (6mm). In our case, while sitting in our dinghy, our friend used a set of channel lock pliers to bend the reeds, using a 6mm wide drill bit as a gauge to measure the distance. When the 5-minute procedure was completed and everything was put back together, the difference was HUGE. While prior to this adjustment we could only plane with 3 people on board if the third person in the dinghy was under 120 lbs., afterwards we were able to plane with 4 people in the dinghy!
Reed valve assembly
Disclaimer: For entertainment purposes only. If you try this and break your engine, your tender, or yourself, I accept no responsibility. 🙂
What dinghy do you have?
It is a 8.5′ Hypalon RIB (fiberglass) made by BoatUS, a brand that is no longer available. We love it!
Very good info. When I was 16, we could only legally ride 125 cc motorcycles. Some would by a larger bike and changed the side covers so it looked like a 125 cc
But I know you would never do that, right? 🙂
We have an 18hp Tohatsu with 9.9 on the cowling. This is because in Australia at least the tender manufacturers have to have a plate stating the maximum hp of the motor to be used. As with Yamaha, the Tohatsu 9.9, 15 & 18 are all the same (and therefore the same weight), with the only difference being as you have described.
I have heard that the European version of the Yamaha 9.9 / 15 is something like 13.5.
Must be the conversion rate… 😀
Used to be common practice in the automotive industry. The only difference between a “grocery getter” and a performance car was frequently upgrading a 2-barrel carb to a 4-barrel carb. Don’t know how they manage that today with everything computer controlled and fuel injected – different curves in the computer perhaps?
bob
In this case, it’s a pretty easy, or inexpensive fix.
Outstanding info. Definitely putting it in the to do pile.
🙂
Think.it would work with the 6hp 4 stroke yamaha?
My guess is no, sorry.
I was under the impression there were also some differences in the jets in the carburetor, and in the diameter of the exhaust output. So the reed valve changes are one part that would increase power, but the other changes would also increase it.
The risk of not changing out the exhaust output (if my information is accurate) is that you will get increased back pressure in the exhaust, which could cause more accelerated wear and tear on the engine. For example, that back pressure might increase blow by on the rings, causing other problems.
Again, my information is somewhat different than what you’ve presented, though it was something we discussed in an outboard repair & maintenance class I took at a local Vo-tech.
When buying an engine, easiest way to tell if the engines are the same is the weight. If the engines weigh exactly the same, it’s the same block and the regulations/fuel & air bits are the difference in power yield.
Of course these changes will instantly void the manufacturer warranty, which is one reason we shelled out the extra few hundred bucks when we bought a brand new 15 instead of a 9.9.
What you’ve noted is a commonly held belief, and I am not experienced enough to refute it. All I can say is this: our friend, who looked after a fleet of rental boats operating on these particular engines, told me that he compared the schematic of each engine and that the only difference is what I noted. I was told that a 9.9 and a 15, and their European counterpart (13.5?), are the same. Take that for what it’s worth.
As for purchasing a 15 straight away, that is a no brainer, if one is available. In our case, we bought one of the last remaining 2-strokes in Florida. At the time I probably would have been scared off of a 15 anyway since our tender is only rated for an 8. Now I know different.
It’s also worth pointing out that this mod was made in 2011, if I recall correctly, meaning that we’ve been running the engine this way for 5 years. I’d suspect that if it was going to cause any problems, they would have already manifested themselves.
As I said though, the post is for entertainment purposes only. 🙂
When our old Mercury 9.9 4-stroke was giving us the problem in St. Martin, we bought a Tohatsu 18hp 2-stroke from Boater’s World. It was around $200 cheaper than the 15hp version. More demand – they would have had to order the 15hp, and they had plenty of 18’s in stock. As Catherine points out, they’re the same basic engine.
The choice seems pretty obvious to me.