johnson 9.9 w/ 15 carb new question

[12guns]

1977 w/ 15 hp carb, runs great. Ran about 21 before, now runs a whopping 23...and that's with a tail wind! My question is What can I do to help it breath? I would like to clean up the existing exhaust tube by opening the throat and somehow help the intake "open" a bit. Any suggestions? Also, now it is hard to start cold. I can squirt a bit of gas in the intake and cranks right up. no problems after that. I noticed my choke butterfly is not fully closed w/ the new carb. Should it be? It also has holes all over it...should there be? I want to get all the power/speed I can out of the motor. I'm enjoying playing with it and I know, I should prolly just get a bigger rig, but not yet! Thanks for and help!

 

[Joe Reeves]

Re: johnson 9.9 w/ 15 carb new question

Yes, the choke should be fully closed when starting a cold engine.

Check that the throttle butterfly is fully open (engine in gear but not running) at full throttle.

 

[Dhadley]

Re: johnson 9.9 w/ 15 carb new question

The next flow restriction will be the reeds. I'd suggest Boyesens.

 

[steelespike]

Re: johnson 9.9 w/ 15 carb new question

I'm no expert but I'm surprised a simple carb change did much of anything.I would think reed valves and maybe some sort of porting would be necessary.
What kind of boat are you running?
A very careful syncronizing of carb/ignition may help also very careful setting of the points.Could help.Probably not enough for you to feel it but you may see it on a gps.Indexing plugs can help but it amounts to maybe 1% difference in hp.About 1/10th of a hp.And who knows what the proper indexed position would be.
You may be able to raise the motor some.Probably not much but an inch could give about 1/2 to 1 mph.Try 1/4 inch at a time using little sticks to set the height.
If your looking only for top speed you might live with touchy turning just for your test run.
Trim is important too.If 2 notches seems enough but 3 seems too much you could make wedges to go between
the clamp and transom to give you a 1/2 notch adjustment or maybe 1/4 notch.If your in a search for all out speed remove as much weight as possible from the boat. for your speed runs remove the oars and use what we used to call a 100 mph paddle (about 30 inches long)Probably a 3 gal plastic tank is lighter than a steel one.and just enough gas to do your test run.Usually most of the weight needs to be in the stern.Even if you have to carefully lean over the middle seat to plane.
Of course you should wear a life jacket and have your kill lanyard hooked up.Of course you can try to find a prop that will run higher and run more efficiently.Probably a tough find for a 9.9.Perhaps a prop for a 15 would help.It might make planing a little
harder to do but run faster once planed out.
By the way as I recall 21 was the max rated speed for
the old 10 hp. motors and I would think 23 to 25 would be close to the rating for a 15.

 

[12guns]

Re: johnson 9.9 w/ 15 carb new question

great info...I fixed the choke flapper...the pull lever was on upside down. LOL! I would like to know more about the reeds and exhaust tube of this motor. I'm not planning on doing anything else as this motor has very few hours on it, and was my grandpas. I plan on keeping it in good shape for a long time before trying any serious mods. From what I understand there are some very thin reeds behind the valves?? How hard to change those? I assume it involves pulling the whole head, which won't happen, but still curious.
*********
steelespike, I did set the trim all the way down, and lost a little speed, then on the second setting, was the best. In the third I did get .5 mph or so, but w/ only my butt in the boat, the front tended to jump up and down when it ran full out. I'll prolly run on the second setting like I always have. FYI I'm running an old duracraft "fisherman" 1436 I believe. I re built the transom w/ treated 3/4" plywood and a piece of angle iron for reinforcement. It's a good rig for the few times a year that I use it duck hunting and fishing, I'm just having fun learning about the motor, something I've never messed w/ before. Thanks for all the help guys!

 

[OBJ]

Re: johnson 9.9 w/ 15 carb new question

Starting in 79', there was a thin shim installed between the reed stop and reeds. Gave a tad more opening on the reeds. In 81', a "tuned exhaust" was added to the 15's....is was an hour glass shaped tube instead of the square exhaust tube used on the 9.9's.

As Spike posted, there are a couple things to do to help garner some more performance out of the old girl.

One....get the anti cav plate about 1" above the keel of the boat.

Two....put a 91/4 X 8 prop on it. These are common and can still be gotten through darn near any dealership.

You can turn 6K on these engines and it won't hurt them. If you can get a timing light with a tach funtion or a tiny tach, get a reading at WOT and let us know what it is.

 

[12guns]

Re: johnson 9.9 w/ 15 carb new question

Starting in 79', there was a thin shim installed between the reed stop and reeds. Gave a tad more opening on the reeds. In 81', a "tuned exhaust" was added to the 15's....is was an hour glass shaped tube instead of the square exhaust tube used on the 9.9's.

As Spike posted, there are a couple things to do to help garner some more performance out of the old girl.

One....get the anti cav plate about 1" above the keel of the boat.

Two....put a 91/4 X 8 prop on it. These are common and can still be gotten through darn near any dealership.

You can turn 6K on these engines and it won't hurt them. If you can get a timing light with a tach funtion or a tiny tach, get a reading at WOT and let us know what it is.

thanks for the help. How hard is it to add those shims? if i raise the motor like you suggest, will I loose turning capability? How about the prop, If I understand right, top-end speed will increase, but acceleration will decrease. Is that right? I'll probably leave it alone, I would however like to clean up the exhause tube. From the pics I've seen I think I can grind out a bit and smooth up to increase air flow. Can it be done?