OMC 15 hp 2 stroke 7000 rpm?

[thatone123]

I changed out a Johnson 9.9 to 15 and got a hp increase, but feel it is not up to 15. Now I have another 15 hp too. The early 80's 15's went up to 7000 rpm which I understand is the highest rpm outboards ever produced? Is that true to anyone's knowledge? Can I expect a real screamer motor as I have one of these 7000 rpm motors. I live in Alaska, so most water is frozen til spring. How about this? Alaska Natives know a lot about outboards as they live on the Yukon and Tanana rivers. An old Native told me that they used to increase hp by putting a wet burlap gunny sack bag over the cowling of the outboard. The motor would run much better and faster as the carb sucked in water vapor from the wet bag into the cowling opening. A primitive form of water injection. Sounds crazy, but I know motors do run better when there is mist or rain in the air.

 

[jimmbo]

Yes 7000 rpm is the top of the Full Throttle range.

 

[racerone]

I believe they were rated 15 hp at 6000 rpm.-----At 7000 rpm the motor durability will suffer.

 

[bowfisher2636]

Order you a tiny tach or equivalent off of amazon and check I've gotten one recently from amazon for around $25 free shipping

 

[thatone123]

Motor plate states operating range 5500 to 7000. Produces 15 hp at 6250. So you have a remaining 750 rpm which is very impressive. I operate in very shallow water as shallow as a foot or less. I need to get on plane ASAP or the prop digs into the mud and weeds. I meant in the first post if anyone knows any other OMC or other non-race outboard that has this high of an operating range as I have not noticed one? Some have said to run 40-1 mix if operating at 7000. I have seen a number of these 7000 rpm motors over the years and the normal 50:1 was used to no ill effect it seems as many have good compression yet, unless they were seldom run WOT which seems to be not the case especially in salt water getting from point A to B on a weekend. I use only synthetic oil always. Yes, I will check out the tach. Thanks, been thinkin' of that.

 

[thatone123]

You can get a lazer wireless tach now from China on E-bay for less than 10 bucks. amazeing!

 

[bowfisher2636]

You can get a lazer wireless tach now from China on E-bay for less than 10 bucks. amazeing!

Mine came from amazon I got one that takes a replaceable battery and is backlit a lot of the cheaper ones you cant replace the battery and I had to have a backlight due to me running a lot at night

 

[thatone123]

Wow, that is lame. Can't replace the battery. Kind of like the new outboards...disposable. Well, I guess no one knows much about the 7000 rpm issue here.

 

[racerone]

What is the issue with 7000 RPM for you ??

 

[scout-j-m]

I also confused as to what the question or issue is here???

I have a 1992 model which is also rated for 7000 rpm at WOT. On my boat with an 8" prop and just myself onboard, it will run just a hair over 6900 rpms at WOT. It sounds fine to me at that higher rpm but I am currently running a factory 10" prop because it gets my rpms down to about 6500 with just me in it or 6200 with another person. My rpms didnt drop enough that I am lugging the motor as I'm still right around the rpms the 15hp was rated at so as a result I still have decent holeshot and I feel like my fuel economy is better. The motor is relatively new to me though so I dont have enough hours on it to really be certain.

 

[thatone123]

Ok, that is interesting! The issue is this: Wondering why they made a 7000 rpm outboard as that seems to be only non-race 7000 rpm outboard I have heard of or seen. Why did they stop making a 7000 rpm outboard? Maybe this is impossible to know without talking to an engineer of that era? Does the additional RPM's really increase useable Horse power? Ok, scout, they still made it 7000 rpm up to close to the discontinued date of this motor, did not know that. I can't wait to compare this 7000 motor with some other 15's this spring. All frozen up now. Yes, by lowering the prop size that does decrease the RPM. I have a 12" inch on my 9.9 to 15 conversion. Gets on plane instantly. I want to try the 8' prop to see what happens on that motor and also the 7000 RPM. motor. These motors are of special concern to me as they seem to be the lightest power to weight ratio outboards out there, and I go places mud motors fear to tread while on plane or step. Motors like the 2 stroke Suzuki and Yamaha weight a lot more. One after market company put out a modified race version of the 15 OMC. Racing is not my interest though.

 

[jakedaawg]

If your hitting 7k with a 12p prop you will blow it up with a 8p prop. I may have read this all wrong .

 

[thatone123]

No, I have never run this motor, but have run a 9.9 with regular 9.9 rpm with a 15 carb. I have a 15 7000 rpm motor that I have just cleaned carb on, new lower unit with new water pump, new paint and etc. BTW,I also have taken off the looney rev limit, natural gear start only as I am tired of the hard to start, hard to put recoil started on and other assorted bother. I know some will say to not do this, but I ran outboards with none of this liberal overkill and did just fine in years gone by. Pretty soon the libs will want us to wear life preservers in the shower. We may offend someone if we don't. Someone may get hurt feeling.

 

[thatone123]

I think one reason that they do not make higher rpm outboards is that then they would have more problems distinguishing between the power ranges and models that make for a good profit from the same cubic inch/cc motor. Omc had the 18/25 before this outboard model and then in 74 introduced this one and did away with the 18. This 15 7000 motor may produce more than 15 with the 7000 rpm which may not be good for the OMC company as some would not buy the next up model in 74 which was now the 20 or 25 I believe, then up to the 40.

 

[racerone]

Thanks for the insight.

 

[thatone123]

No problem, Another thing that complicates this is that the 7000 era motors were rated at the head, not the prop shaft like the later motors. So some say that the 15's were only really 11-12 hp. I do know that I was using a Jap Tohatsu/Nissan 9.8 and was amazed that the increased power of the older OMC 9.9. I then went to the 15 hp carb on this motor and even got more power. This seems to contradict the head vs. prop issue it seems. My goal is to get the most power from this 15 package, and I may have to go to another brand. I have an almost new 15 Mercury 2 stroke that I think may be a Suzuki and plan to try it out after the ice goes out in the spring. They are rated at the prop and I think have a larger bore and stroke. I guess I'm bored this winter.

 

[thatone123]

I knew there was something to the 7000 rpm business. See 15 hp OMC racing outboards at Fralickboats.com of Toronto Canada. very cool!