1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

[Boat2fast]

Okay I get the picture. Look for a 28HP SPL late 1980s-97. These engines had the largest carb that the small port block could reasonably use. If you have nice deep pockets, buy a new one if you can find one. Make sure you linkage is correct and the engine is timed to spec. You can cheat 1 or 2 degrees on the timing.

Prop selection really makes a difference. Check with a tach at full throttle-light load. 5500 is max(published) but keep it under 5800. Find the right pitch prop to do this. This will give the best balance of holeshot/top speed/weight carrying. If you carry a spare, make it one size smaller for real heavy loads, pulling water toys etc.

That will achieve most of what you can wring out of a 25.

 

[thill]

Thanks for that info, 2Fast.

I just made a post asking why my engine bogs down after passing 3/4 throttle and nearing full throttle. I really think it's overpropped. The engine came off a light jon boat, and this is a heavier, decked V-bottom.

It's a non-electric tiller motor, so I don't have a tach, but it sounds like it's not reaching the proper RPM's to my ear.

I'm going to go pull the prop and see what pitch it is... Will post on the other thread to keep it on-topic.

Thanks.

-TH

 

[jimdandy527]

Hello,

This thread has been super helpful in clarifying some things for me. However, I have not found anything about the effect on a jet motor. We run in fairly shallow water a lot of the time and so we have replaced the prop with a jet. Any thoughts on how the upgrade would affect the performance of the jet, and more specifically do you think the motor would experience any adverse effects?

Thanks for any insight .

 

[mphelle]

What motor do you have and what is your max rpm with the 5 7/8 impeller? I had an 83 Johnson 35/25 that would only spin at 5200.

 

[jimdandy527]

mphelle » 12 Mar 2014, 12:50[/url]"]What motor do you have and what is your max rpm with the 5 7/8 impeller? I had an 83 Johnson 35/25 that would only spin at 5200.

I'm afraid I'm not well prepared, and really not all that savvy at this point... How do I know what the max RPM would be?

I'll get the specifics off the motor tonight. I believe it is a mid '80's model but really can't remember.

 

[mphelle]

A tach is a must have if you're looking for ways to improve the performance of your jet. I've used a digital tiny tach for years with good results, maybe someone here has tried this chinese knockoff.
https://www.ebay.com/itm/Hour-meter-tachometer-jet-ski-boat-sea-doo-polaris-3d-outboard-kawasaki-js-650-/231175992391?pt=Personal_Watercraft_Parts&hash=item35d329b447&vxp=mtr#ht_772wt_758

A jet discussion may be getting a little off topic for this thread, you may want to start your own.

 

[jimdandy527]

Sweet, thanks for the link. I suppose the reason for adding my question to this thread is because I'm looking to do the conversion, but I am unsure of any negative effects a potentially undersized jet may have on the upgraded motor. In reading other posts within this thread it is apparent that with the increased HP a prop adjustment is needed to prevent over rev; unfortunately my limited knowledge led me to wonder if the same would be true of a jet as well or if it will likely have less effect since the mechanics of a jet are so different.

 

[mphelle]

The J/E 20-35 all use the same pump/impeller, there's no worry about over driving that combination.

 

[Boat2fast]

mphelle » 12 Mar 2014, 15:58[/url]"]The J/E 20-35 all use the same pump/impeller, there's no worry about over driving that combination.

A jet pump is like a fixed dynamometer. The more power you have, the more RPM it will spin. It's a water pump...more power in = more water out. That's why a 25 hp will turn it at, say 5200rpm, but a 30hp may turn the exact same pump 5500rpm.

This is the same idea behind the factory test props. These make 'loading' an engine, at full throttle, safe. Test-props just 'waste' the power. They do not make thrust, forward or backward. The mechanic doesn't have to balance on the transom at 30mph to check the timing. He could run full throttle, in-gear, right at the dock.

The factory manual specifies the right test-prop and the minimum RPM the engine should reach. 1994 OMC factory manual shows the minimum test rpm for the 25hp is 4800rpm, the minimum for the 30hp is 5400rpm. This is using the same test-prop. The stronger the engine, the more rpm it will spin the test prop. By the same reason, stronger engines spin jet pumps faster too.

 

[iowaevinrude]

hey i like your post and pics of your upgrades. a couple ?'s tho, you talked about doing a link and sync, what do you mean by that? also i cant seem to find an intake anywhere unless new, they all look like the 25hp intake on the inside, where it says its a 25 to 25hp intake in the sale heading, same with the carbs. also what is a MARS edition? a motor type or certain model? thanks
again your post is awesome

thanks
alex

 

[Johny25]

Link and sync is the timing between the carbs and the timing advance as throttle is advanced. The carb and intake can be hard to find used, others have had issues with this also. Just keep searching ebay and other sites like it is all I can say. And the MARS edition was just a military model....not much different than the regular public motors

 

[shawnfish]

iowaevinrude » 05 May 2014, 16:29[/url]"]hey i like your post and pics of your upgrades. a couple ?'s tho, you talked about doing a link and sync, what do you mean by that? also i cant seem to find an intake anywhere unless new, they all look like the 25hp intake on the inside, where it says its a 25 to 25hp intake in the sale heading, same with the carbs. also what is a MARS edition? a motor type or certain model? thanks
again your post is awesome

thanks
alex

chances are the intakes are the same and you only need the carb. I have a 35 carb im looking to sell, just the body.

 

[iowaevinrude]

this forum rules. how much do you want for the carb body?

 

[shawnfish]

40 bucks

 

[iowaevinrude]

im sorry i forgot to check for a reply from you on that, i should have left contact information, my name is Alex, feel free to call or text 319-241-3692. thank you for your time

 

[airbornemike]

I've got a 2004 30hp Johnson, will putting a 35hp carb on it give me better performance?

 

[vahunter]

I've got a 2004 30hp Johnson, will putting a 35hp carb on it give me better performance?

you'll have to cross reference all parts. Its time consuming but thats the only way you'll know for sure. Pull up the parts diagrams for your model number and the 35hp of the same year and style (primer/choke system, tiller, electric/manual start) and first compare the crankcase assembly part numbers. If they match, start comparing every part of the whole outboard.

 

[airbornemike]

Thanks, I'm going to start researching all the parts. It looks like 2001 is the closest year to mine. They didn't offer a 35hp in 2004.

vahunter » Today, 17:17[/url]"]

I've got a 2004 30hp Johnson, will putting a 35hp carb on it give me better performance?

you'll have to cross reference all parts. Its time consuming but thats the only way you'll know for sure. Pull up the parts diagrams for your model number and the 35hp of the same year and style (primer/choke system, tiller, electric/manual start) and first compare the crankcase assembly part numbers. If they match, start comparing every part of the whole outboard.

 

[Johny25]

They made a 2 cylinder and a 3 cylinder 30hp in 04' and as far as I know neither can be upgraded horsepower wise as they were both completely different motors than the 40hp motors above them. I am speaking about bolt on parts that is

 

[airbornemike]

Ok I was thinking this also, they only made 35hp motors until 2001. Thanks for the info.

Johny25 » Yesterday, 23:19[/url]"]They made a 2 cylinder and a 3 cylinder 30hp in 04' and as far as I know neither can be upgraded horsepower wise as they were both completely different motors than the 40hp motors above them. I am speaking about bolt on parts that is