More HP through carburetor upgrade?

[wmbreck]

I have an underpowered boat now and can't afford an adequate motor.
It was used for fishing and for that it was fine.
It is a Johnson 28 SPL. on a 15' fiberglass. does 10-11 with 2 people and maybe 18-19 mph with one passenger according to my gps.

I've read several times (and even today on craigslist of people upgrading motors mainly through carb changes. The one I saw said he took a 25 hp to 40) I've seen several say they took a 9.9 to 15hp because the parts are interchangable and it is basically the same motor otherwise?

My question is simple. Is that difficult, expensive, or dangerous providing the right carb could be located. I prop change might help too if this could be done 10-15 more hp and 5 more mph would make it so we could live with this setup until we can get a 50 or 60+. It might be worth it if this could be done for $300-400 vs $3-4 grand for a fairly new 50 or 60hp.

Thanks.

 

[Chris1956]

Re: More HP through carburetor upgrade?

In rare cases a motor manufacturer will intentionally throttle the motor back to a lower HP. This happens on some 15HP motors that are throttled back to 9.9HP to allow use on HP restricted lakes.

You need to see if this applies to your motor. I doub't it does however. Match up the part numbers for the block, pistons, heads and other parts that generate the power. if they are the same, and the only difference is carbs, you may have a shot at it.

BTW - most of the HP differences in models that share displacements, is the port timing of the block, leading to improved exhaust scavanging action.

 

[Fl_Richard]

Re: More HP through carburetor upgrade?

IS it doing all she can?Most people dont have tachs in that size catagory. Perhaps a prop change is in order?

You might get 17 all the time with 2 people on board. That would probably satisfy you no?

 

[Rancherlee]

Re: More HP through carburetor upgrade?

the issue is that your 28spl is twards the upper end of that engine block, I believe the 20/25/28spl/30hp engines all run the same block. From my research when I had a 28spl is that it was a 30hp engine with an older/cheaper style non thru hub exhaust lower unit on it. They did make a 35hp for a short time built on that block but that went away when the engines got "prop rated" in the mid 80's. The good news is that you should be able to sell your 28spl for what you can buy a similar year 50hp engine for. I managed to sell my clean but faded 88' 28spl with controls for 600$ last fall. Craigslist is usually loaded with 50-60hp twin and 60-70hp triple Johnson/Evinrude motors for a good price.

 

[JB]

Re: More HP through carburetor upgrade?

Unless your "15' fiberglass" is a real garbage barge that 28SPL ought to do 25-28mph with a couple of passengers.

Investigate: bottom cleanliness, AV plate height, trim, balance and prop performance.

Tell us more about the boat and what HP it is rated for.

Fine tuning the rigging is always a better performance improver than hopping up the outboard.

 

[kenmyfam]

Re: More HP through carburetor upgrade?

I have to agree with JB.
Simple stuff first !!!

 

[wmbreck]

Re: More HP through carburetor upgrade?

Here is the exact boat from the original ads and stats in black and white. A real classic - but I do like it the old look. Everything on it works. It is either the 142 or 152 model.

http://www.larsonboats.com/assets/pdf/archives/1965.pdf

Here are some live shots at a site I just made an hour ago. I'm in Jersey now and the boat lives at our place on lake Ontario. Going back in 2 weeks.

http://johnsonlarson.homestead.com/index.html

Wondering if the prop is right because someone did mention that it was switched out for one for fishing by the former owner. Might be small part of the top end problem and why the 10 mph top speed with 2 aboard which is like watching paint dry - oil based If I thought a new prop would give me even 3-4 more mph (like watching latex paint dry)

I could live with that until next summer until I can find a 50-70 hp. I would be ok with 14-15 mph wih 2 people and 22-23 with one. Would be nice to take a better prop back there with me, but which one? No sense looking unless this one in the photos above really does not look like the stock prop. Can't find the exact year as the no's I was given over the phone were wrong, but they only made the 28spl for a certain period - I think through 1996? Tough calls all around. Nothing that can't be solved fast by winning the lottery. but my wife wants the kids fed and clothed. How shallow is that?

Thanks again for your time and wisdom.

 

[lmuss53]

Re: More HP through carburetor upgrade?

Look around for a prop selection tool like the one at Turning Point props and enter the data for your boat and motor combo. I tried but I don't know all you details. The prop selector will let you get an idea of what the prop pitch should be. You'd need to know what you have on it then to decide if you want to reprop. If you don't have a tach look into one of those "tinytachs" that you can hook to any motor by wrapping a wire around a plug wire. You want to get your motor running at or near the max recommended RPM at Wide Open Throttle with a normal load. The recommended RPM should be on a plate on the port side at the transom clamp, with the model and serial numbers.

http://www.tinytach.com/tinytach/index.php

Just by what your are saying, a first guess would be that if the motor is working right it has a prop that is pitched too high right now. But that is assuming that a lot of other things are OK.

Does the boat roll up on plane with one person? ...2 or 3 people? If it will plane out with a lighter load but not with the bigger load that is another indication that it needs a lower pitch prop.

 

[ondarvr]

Re: More HP through carburetor upgrade?

As I posted in your other thread on this subject, I don't think you're going to be able to improve performance much after seeing the boat its on. This boat is rated at 75HP, a 28 isn't even half that, 70% of the rated HP is considered about the lowest you can go and expect it to perform adequately.

 

[G DANE]

Re: More HP through carburetor upgrade?

Unless you take everything apart and do highengineering work, there is litterally nothing you can do to gain more horsepower from that motor.

I have a 1998 30hp on a 16' dighny, 6 ft. beam . It is a Swedish produced Oernvik weighing 480 lbs dry, without motor. It does exactly 34 miles with one person GPS, and around 25-28 with 5 persons. Also kind of "million bubble hull". It can pull kneeboard, waterski - though not single slalom ski. Mine is with proprated horsepower, since 1998 model, the former 35HP, same powerhead.
My guess would be you should go around 25 in that boat, with at least 2 persons in it. Does the motor run as it should ? Have you done a sync'n linck. Have you veryfied the throttle opens all the way, as I can see you have remote.

How heavy is the boat, those hulls are foam filled, and now and then the flotation foam can be waterlogged, but boat will be heavy and sit low in water, then. VERY nice and clean looking boat and motor, by the way, looks like a nice combo. A 28 will never make it a rocket, but it should be able to run very fine with it, as I said at least 25 mhp with two persons, and like 16 - 18 with 4.

 

[coolguy147]

Re: More HP through carburetor upgrade?

from the pictures it looks like the motor is trimmed down wayyyy to low

trim it up some. move it 2-3 holes out or whatever give you the best speed without venting.

looks like you're plowing the entire time. no plane at all.

 

[wmbreck]

Re: More HP through carburetor upgrade?

You asked:
"Have you veryfied the throttle opens all the way, as I can see you have remote."

Not sure what you mean exactly (new to all of this. Just a metal boat with 9.9 before this) But when I push the throttle fully forward it always seems like it should go more, or at least rev higher than it does. Maybe changing the tilt will help as someone just said. The water logged foam is also an interesting thought because, when I pulled it out 2 weeks ago I jacked the front end up a high as I could and pulled the drain plug. It was just meaning to get 2" of rain water out of the small 12x12" hole there. But instead water began trickling from underbeath the floor too? So maybe that was an ingredient too as extra weight kills. I know that much from our 13" metal Duranautic 9.9. (blue boat in the one photo) 14-15 mph with 2. Dead with 3!
Last year I put a 25 tiller on that one day and it almost flew out of the water with 3 aboard

My goal now is to do a little of all the things you guys are suggesting - at least those I understand and can afford and try myself. Loacl marian is $90 an hour for labor alone. NOT! I'm stunned at the wisdom here. My only area of being confused is the spectrum of what I'm told I should be able to get out of it - all the way from what I get now to 10-11 to 24-25mph with 2 aboard. Should be fun experimenting with a few of these hints. Goal someday is to pull a tube for the kids but that will mean 60+ hp - not now in the budget after buying the craft. I'm a pastor and its nasty being poor when it comes to something like this So even a new prop is a big investment!

Sorry these posts are so long but I'm starting at ground zero. Thanks again.
Bill

 

[G DANE]

Re: More HP through carburetor upgrade?

To check the remote cables. remove the motor hood and take a clouse look at the carbureator, right side seen from back.. Place remote handle in neutral. On the trottle shaft that runs horisontal thru the carb bore nearest to the engine block, there is a small spring pin thru. Make a helper move the throttle lever on remote against full speed - ENGINE STOPPED - OFF. It can be neccesary to pull starter rope a bit slowly, to enable the gear to engage forward, just do that. Dont force anything. When remote lever is at full throttle, the little cutter pin should almost rest against a casted stop in carb body. If it doesnt, post back and we will see how to adress it. Water under floor must come out. When you do not use boat leave it with bow superelevated and plug out. We pull tube with our 30HP.

 

[wmbreck]

Re: More HP through carburetor upgrade?

Thanks. Will look when I get there!!!

 

[ondarvr]

Re: More HP through carburetor upgrade?

G Dane

In the other thread he indicates the weight with two people to be around 1,500 lbs, that weight will kill the performance with that size motor.

 

[Rancherlee]

Re: More HP through carburetor upgrade?

G Dane

In the other thread he indicates the weight with two people to be around 1,500 lbs, that weight will kill the performance with that size motor.

I'd have to agree, the 28spl I had I DID try out on my 16' Lund that is 750# and rated for a 75hp and it did put me on plane with 3 people but it took forever. I just had to try it before I sold it as I took the motor off my pontoon boat and the lund is just sitting around. It managed to push my Lund at 24mph with just me in the boat. Also every 28spl I've seen comes with a 9x12 prop which is a 30+ mph prop for that engine. Props are a little harder to fine for this engine but are available in 9x9 9x10 9x11 at a BRP dealer

 

[wmbreck]

Re: More HP through carburetor upgrade?

Thanks to all who have written. Any adition advice is still more than welcomed.

I know we won't ever be pulling tubes on this dog, but I have to impliment some of this wisdom here. I'll be at the lake in 2 weeks and will be there for 2 weeks. One mentioned from a photo link above that the beast is not planing with 2 aboard at all. So if I can correct that and maybe pick up a few mph I'd be thrilled - for this year

Here is my simple plan. Add to if if I missed something.

1. Change motor trim position = tilt "outward" 2 holes???
2. Change prop = Lower pitch????
On this site they have Johnson 28hp props with pitches
from 9-12. - http://tinyurl.com/23vjlas

Which would be best for my stated need?

3. Put hydofoil on. already have one that should fit from Cabelas
4. make sure there's no water under floor in foam.
5. check to see if full throttle really is full as one post mentioned.

Again, if any of this above is backwards or needs refining, let me know and thanks much! My 17-year old son's vacation happiness (and therefore mine) is at stake here

 

[wmbreck]

Re: More HP through carburetor upgrade?

For those who posted to help my underpowered boat issue here, I have some decent news. I did all that was suggested, differerent prop with fatter baldes, trimmed motor out, and even added hydrofoil. checked compression and it was good.

But it was something else that made all these of some use. There was a lever next to the throttle that I did not touch as I had no manual or knowledge of what it was. A neighbor came by and said what it was. Turns out it was the 'spark advance' . He had onew on a previous boat. Pulling it up in neutral and then starting the boat raised the rpms dramatically. I had never heard it rev like that before. Still think it is a bit off and might need some adjustments but I was able to get the boat going from 8-10 to 18-19mph with 2 aboard. My son hit 22mph on the GPS alone. It still takes a couple of minutes to get on plane but it does. I can hear the rpms rising slowly.

Not sure if there is anything else I can tweak it a little more until we can find a larger motor but at least it is just enough to enjoy. I think even a 40hp would do what we need based on what the 28 now does.

Thanks again for the wisdom and help.

Bill

 

[ondarvr]

Re: More HP through carburetor upgrade?

The spark advance should be connected the throttle linkage and move at the correct time and the correct amount. If it moves more than its supposed to, or at the wrong time, very bad things can happen, like burning holes in the pistons.

Was it not connected before, or did you just move it more?

 

[jay_merrill]

Re: More HP through carburetor upgrade?

I doubt that the lever is a "spark advance." Its probably a "warm up lever" and it shouldn't make a bit of difference in regard to top speed. If it does, I'd hazard a guess that there is something wrong with your remote control and/or throttle cable.



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