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

[Edstrossner]

I took the intake manifold off the 25 last week with the thought of adding a pair of cone spacers to the reed assembly. After removing the reed plate I noticed that there is a large difference in the crankcase openings. Cylinder #1 has a larger opening than cylinder #2. So, I decided to tear down the power head and rework the block as follows:
Squared the ports with a 5/8" end mill. This did not square the ports that much. I just wanted to get all the ports set at the same height. They were off by .014.
I milled .062 off the exhaust port bottom to match the piston at BDC.
Milled .012 off the block deck
I did a lot of hand grinding in the crankcase area. My goal was to get the crankcase openings at the reed plate to be close to each other.
I also tapered the back of the cylinder wall as it leads up to the transfer ports and knife edged the transfer ports.
The biggest change will be the boost port. I followed the basic idea that Chrysler used in adding a 1" hole to the piston skirt on the intake side and putting a 1" hole into the cylinder wall at the transfer passage way. This should pump more fuel air mix up to the transfer ports.
I still need to match grind the exhaust tuner where it mates up to the block and also to plug the exhaust relief holes in the tuner.
I hope to get the power head re-assembled this week and try it out.
I'll post my results.

 

[Lil' Blue Rude]

You might want to get some kind of epoxy and fill any kind of voids in the crank case to raise the crankcase compression back up some from the grinding you've done. Too much volume in the crank case leads to a sluggish motor do to lower intake velocity from lower pressure.

 

[Edstrossner]

I got the motor back together and put some hours on it to break it in before I ran it at wot.
The results were disappointing. I did a lot of work on the block and gained nothing.
After the first run at wot I started to look at the exhaust area where it exits the prop. The aluminum props have a smaller area for the exhaust gases to get out as compared to the stainless. So, I drilled a few holes in the gear case to improve the flow. That netted 1 mph increase. When I can afford it I"ll be going to a stainless prop.
I also reworked my 11 pitch aluminum by milling the exhaust area out a bit plus I added a cup to it.
Next, I found a way to remove the rev limiter on the power pack. I went out again and got the the 11 pitch prop to hit 29mph by tilting the motor up. Without the cup, the prop would have trouble holding it's grip at that setting. A stainless prop with a cup should be the best as others have stated.

In review, I have found that this motor does not respond to internal changes like the bigger cross flows do. Perhaps at higher rpms the results may be different. So for those who would like to up the hp on these later model 2 cylinder cross flows my advise would be to get a carb with a 1.375 throttle bore. I tried the 1.500 bore carb but I lost a little speed with it. Mill the the head to get the compression up to the 145 - 150 psi range and open up the air flow getting into the hood. These 3 mods increase the hp very well.
There is a trick to milling the heads that I have not mentioned. You cannot do a straight mill cut to these heads to get the compression up to 150. There is not enough space between the piston dome and the spark plug. The trick is to angle mill the head.

 

[Quercus]

Awesome thread. thanks to all contributors!

 

[unclerick]

I know that this post has been thrashed to death. However, I just came across a part off of Fleabay that I had to buy, just because it made no sense as compared to the part numbers and pictures associated with this post. I paid less than $10.00 and nobody bid on it because of the way it was posted. I have done this conversion several times and many thanks to Johnnyrude for his super doccumention of this conversion. But I have also seen some things I thought could add to this subject.
I bought a manifold (part # 331518). The seller said it came off a 1976 35HP Evinrude. However, it looked from the rear exactly like the 790 in the picture. I can not get this number to come up anywhere. It is exactly like the 0396790 etc in dimensions all around. However, it doesn't have the boss @ the 4 o'clock position with the part number; instead the part# is between the ribs @ the 5 and 7 o'clock positions. It also has the small nipple. It does have the 1.5 bore, the notch around the drain nipple hole and is the 1/8th" deeper plenum @ 5 3/8" plenum width that is nearly flat across the top. The 396154 cam follower (first used in 1986) is attached and appears to have never been removed from the intake. I went back through the ebay completed search and found that this manifold has been listed 3 times without bid. Hoorah for me. My point is, for those looking at this conversion, don't pass up this manifold!!!
The other item I have found is that the carb off a 1979 55hp is exactly like the 30HP in fitment and 1 1/2" bore/venturi size. The linkage fits perfectly (off the top carb) and has the choke. I have not had a non choke 55 or 50 in the parts box yet so I can't swear to the non choke fitment. Also, the idle system is fixed jet, (like the crossflow V-4's). A little dinking around with the idle jet size and the motor idles and transitions like your momma's sewing machine. The 63D main jet seems to be the ticket @ 1600 ' altitude. These carbs are a nickle a hundred. I have seen many "30" hp carbs on EBay, and the majority are not!!! I have seen many 30/35 intakes and MOST are 792's and 25hp models.
My latest creation is a 1986 20hp CRLCDC restricted engine, (converted to el) w/120psi compression, picked up from the local college for $100.00, with the above combo, choke shaft removed and the holes plugged with a knurled 1/4 aluminum pins pressed into the holes, drilled for an enrichment nipple, a solenoid mount bowl with an enrichment solenoid off a V-4. Looks like it was made for it. On a 14' Mirrocraft tin runabout went from 21.9 to 33.6 mph (off my Humminbird) w/a 012057 Michigan. Gets on full plane in 5.6 seconds. Just a little more grist for the mill.

 

[alfaguy]

Does anyone change the plastic timing cam when making the 25 to 30 conversion. Another post indicated they are different, even though the parts breakdowns show the same number for both models. I ordered the part quoted and they are indeed different, and it seems the 30 hp cam does not advance as quickly as the 25. I will look for the part number if anyone is interested.

 

[thill]

Edstrossner,
I've read your posts with interest, as far as opening up the throats to give the motor more lung capacity, and as far as shaving heads, to increase compression.

But what did you do as far as getting more fuel mixture into the increased airflow? Perhaps, it's as simple as going up a main jet size or two?

DUHHH... Just noticed where you went to a 63D main jet. I had only read the earlier post, and not that last report.

-TH

 

[bamalivin]

I have a 1989 Evenrude tracker 20 hp that I am prepping for this upgrade. I know that yours is a 1988 but are the same parts compatible. I have read conflicting information regrading this part interchangeability. I read the thread and understand that I will also potentially be in need of a intake, I just want to make dang sure before I start ordering parts. I did notice a color scheme difference between your 1988 and my 1989. Mine is just like the one in the video that I am linking to this post (except mine is a 20 hp of course). I appreciate any help and advise that can be passed in advance. ~Billy

https://www.youtube.com/watch?v=xmckQllkBc4&nohtml5=False

 

[the-outdoorsman]

I was led to this thread by way of google so this is my first post. I am not sure if anyone is following it anymore but I hope so. I have a 1990 Evinrude 20 hp (E20CRESB). From reading through a lot of these posts it seems I should be able to do this conversion. I know I can bore out the hole in the intake to make it a 25hp but I want to convert it to the 30hp. How do I go about figuring out the part numbers for the intake and the carb? I think that is the only two things I need to buy. When I look up a 1990 30 hp, there are several model numbers.

Any help would be much appreciated.

 

[nccatfisher]

Well if you googled the original question I would have thought you would have tried a search here for your second one, you would have found it has been asked numerous times with usually the same success. You will most likely have to just dig around to you find them quite honestly. There are a couple old threads here with various numbers that will work depending on model.

 

[wmk0002]

Interesting that this thread got bumped as I was just looking at it last week. Guy locally is selling a set of a 35hp carb and intake for $50 and has two sets. I was thinking about buying both so I could try this out if I ever get the correct model motor to try it on. Then maybe selling the other one on here. Anyone know what the going rate of the carb and intake manifold would be?

 

[the-outdoorsman]

Well if you googled the original question I would have thought you would have tried a search here for your second one, you would have found it has been asked numerous times with usually the same success. You will most likely have to just dig around to you find them quite honestly. There are a couple old threads here with various numbers that will work depending on model.

Well I did a search before I posted here and again after and I have not found any carb numbers or a thread that would tell me how to figure it out. I am sure it is how I am searching for it with my key words. I did find a member that did his 1990 motor and sent him a PM but his post is from 2014 so probably a long shot.

 

[durco]

Well lucesco I know some of those years you are talking about had different gear cases which could cause a problem when putting 10 to 15 more HP to them. You would have to cross reference to see what the differences are for what year motor you are talking about. I know I was talking to someone with an 80' or 81' and the 20hp of that year has the smaller split gear case than the 35hp. Would hate to see people doing this mod and ripping their gear cases apart so look into it. Or ask and maybe I or another can help you.

1984 was the last year of split gear case on 20-25.

 

[durco]

I did a quick cross reference and the intakes are the same. Different carburetor and the cylinder and crankcase assembly has different part numbers #-o I couldn't really tell what the difference was other than the starter bracket but there must be something else I believe. This will be a big road block in doing this conversion if the crankcase is different on the 30 than the 25. But it was just a quick look at them.

This shows the 25 and the 30hp crankcases and the different part numbers.
https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=2003&hp=30&model=J30RSTB&manufacturer=Johnson&section=Cylinder+%26amp%3B+Crankcase

The 9.9 to 15 is the only true swap with just the carburetor change. On the 25-30-35 there is a crankcase change, to wit: the 20, 25 and some 30's have "finger" ports on the chamber intake ports. 35's have "bridge" ports. True 35's will have maybe a little rougher idle and a rougher transition (from low rpm carb circuit to high carb circuit) than a jacked up 20, 25 or 30. Not saying you won't gain from the intake and carb swap; you just won't quite get the full "monty" with it.

 

[durco]

I have a 1988 25hp evinrude, what would it take to boost it to a 35hp evenrude? If you already answered this please forgive me.

I made this whole thread from the exact year and HP motor you have Jay. Start at the beginning of the thread where I layout and have pics of your exact motor converting into a 30hp. It will never be a 35hp since the 35 was re-badged to a 30hp in 1985 when OMC prop rated the HP on there motors. The old 35's were only cranking out 30hp at the prop hence the reason it was labeled the 30hp after 1984. Read the first page of the thread and let me know if you have any questions

I have a 1984 35 and a 1988 30. You can really feel the difference between the two; the 35 being stronger. When I change motors on the same hull I have to move the tilt pin because the 35 will porpoise whereas the 30 will not. Same pitch prop (13). Did some research in the parts manual. 1984 35 has a different crankcase than a 1985 30. So, when I went to OMC service school in 2000, I asked my instructor what the difference was. He said, "finger ports" vs. "bridge ports". Bridge ports make the true "hot-rod" 35.

 

[durco]

After doing a fair amount of part number cross referencing, it appears the 30 and 35 hp motors in 1984 used identical intakes, reeds, carbs, low speed needles, and high speed orifices. Even identical pistons, rods, and cranks. So how did they manage to get another 5 hp from one to the other?

By the difference in the shape of the transfer ports in the block. 30 and 35 have different blocks (machining)

 

[durco]

1984 they did not make a 30hp......was a 35hp.

Yes, they did. It was the first year of the 30hp. However, they also made the 35 - it was the last year for the 35 in the U.S. (except for military and the later 3-cyl, etc.).

 

[1648grizz]

Great thread, I bought a new 25hp 2 stroke Johnson in 2005. I believe this was the last year for the 25hp 2 stroke . On my 1648 Grizzly I would top out at 29 - 30 mph with the 10x13 prop that came with the motor. That was only me in the boat at 225lbs, I think the boat is around 400lbs.
Been looking into the carb swap and it looks like all I need is the 30hp carb off the 2005 30hp , same intake manifold .

 

[1648grizz]

Another thing I forgot to mention was that when I had the 25hp intake on the motor and I took a flashlight and looked through the intake hole with the carburetor off I could see that the intake actually blocked part of the reed plate. This made it more difficult for the reeds to actually open up in my opinion, and caused air flow restrictions.

Yeah Bigwave if you decide to do the mod let me know your make and model and I would be happy to help cross reference to make sure you can do the mod with your year Johnson. I am not entirely sure you can do it to newer Johnsons but it would be interesting to find out if you could

Great thread Johny25, it looks like the only differences between my 2005 J25TESOM and the 2005 J30RSOD is the carb, same intake manifold .

 

[RaisedByWolves]

Can someone who has the 30/35hp carb take a pic down the throat of carb so I can see the butterfly in the closed position?

I need to see what the holes/cutouts on the butterfly look like to compare to a carb I want to try out.