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

[Matt193]

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.

Does this work for you?

 

[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.

Does this work for you?

Yes, perfect.

Thank you.


The carb I bought has a flat cut to the bottom of the butterfly and only a single hole. I was worried that it would cause the engine to idle too high, but looking at yours and my stock carb it doesnt look as bad as I thought.



It only has a 1 3/8" outlet VS the 1.5 I have read about, but the venturi looks to be the same/similar to yours. My stock carb has about a 1" venturi and 1 1/4" outlet so it should work pretty well.

 

[Matt193]

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.

Does this work for you?

Yes, perfect.

Thank you.


The carb I bought has a flat cut to the bottom of the butterfly and only a single hole. I was worried that it would cause the engine to idle too high, but looking at yours and my stock carb it doesnt look as bad as I thought.



It only has a 1 3/8" outlet VS the 1.5 I have read about, but the venturi looks to be the same/similar to yours. My stock carb has about a 1" venturi and 1 1/4" outlet so it should work pretty well.

That looks pretty close to the diameter of my old 25 HP carb. If you're upgrading from a 20, it should help quite a bit, but if you're coming from a 25, it might not be worth it. What's this off of?

 

[RaisedByWolves]

Its off of a 70.


The venturi is the real bottleneck and this one is easily 2X larger than the stock carb from my 25.










I only have $40 into it and until I find a 1.5" intake to go with the larger 1.5" carb it will do.

This being 1 3/8" matches the intake I have now.

 

[Matt193]

I read the 70 HP carbs will work, but I don't remember where. That 25 HP carb looks really tiny compared to the one I had on mine. I'm interested in finding out how well it works for you.

 

[RaisedByWolves]

Yeah it does seem small for the size engine, but other than pics I only have a carb off an 83 parts motor to compare with and those motors were from the crank rated hp days.

All in all I’m hitting a speed in the ballpark of what this motor should give me set up as is (27 loaded) so I don’t think I have the wrong stock carb.

I have played with height, the prop, added a foil and saw improvement of some type after everything. The prop is the last thing I changed as my old one was trash.

And while I gained 2-3mph loaded(me and wifey) I wound up with the same top speed with just me in the boat and the RPMs also stayed the same.

That tells me I’m running at the limits of the power I can achieve with this carb.

 

[RaisedByWolves]

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.

Spring is here and its time to start modifying boat motors again!

Does anyone know if there is a way to externally determine whether a motor has finger ports or bridge ports?

Any listing of model years that would have the bridge ports?

I just picked up a 74 35hp with a bad lower unit for dirt cheap and surprisingly after only cleaning the points it fired up and ran pretty well. I only wanted the motor for the carb and intake, but now that I know the engine is a runner my wheels are once again turning... :mrgreen:

 

[LDUBS]

"I only wanted the motor for the carb and intake, but now that I know the engine is a runner my wheels are once again turning... :mrgreen:"

And, so it begins . . . :LOL2:

 

[RaisedByWolves]

"I only wanted the motor for the carb and intake, but now that I know the engine is a runner my wheels are once again turning... :mrgreen:"

And, so it begins . . . :LOL2:

New/used lower housing should be here tomorrow. \/

Now, having dug into the two lower units I have on hand (76 35hp and a 84 25hp)I find that the 35hp has a much higher gear ratio than my 25hp lower I have from a different busted duck motor.

This means I absolutely HAVE to try this motor out to see what speed difference there will be.

The stock prop for the 35 is 11 x 9 which would be a good starting point for a larger boat, but Im going to try it with one of the many 10 x 13 or even my 10 x 15 prop to see what she does. :mrgreen:

I know the larger diameter prop would be more efficient, but until I find one......

 

[dandrews]

Just purchased a new to us rig this past week, 1984 Lund s14 dlx with a 1985 Johnson 25HP tiller, first time looking at this motor. I’m no stranger to aluminum boats, involved with hydroplane racing on the side and know enough to be dangerous. So I will beat on this topic some more and provide some more insights. Touching on the last post early 35’s (1980-83 I believe) ran a 1.93:1 gear that’s why you will see 11X9’s and 10’s on those engines. Haven’t seen any mention of total timing advance, not much talk about reed stops and finger reeds and the all important tuner pipe between the differently rated HP variations of the 31.8 block. If anyone is going to go beyond the scope of switching out parts on these engines I strongly suggest you read up on Gordon Jenning 2 Stroke Theory before you start modifying ports, heads, intakes etc. Have a clearly defined goal of what you are trying to achieve with the engine/boat setup. In my case with be looking for more low, mid and top end gains without sacrificing low speed trolling capabilities. Will achieve this through parts swaps, prop/setup and possibly some exhaust side modifications.

 

[RaisedByWolves]

. Haven’t seen any mention of total timing advance, not much talk about reed stops and finger reeds and the all important tuner pipe between the differently rated HP variations of the 31.8 block.

Hi dandrews!

Thanks for chiming in. My above project got stalled, too many irons in the fire. #-o

Most guys on this and other “tin boat” boards seem to be after bolt on mods vs tuner type mods. Simple easily repeatable stuff that will get you more top speed but still leave time in an outing to do some actual fishing, vs cursing… err… I mean tinkering. **** wildlife just does not like oily lures.

Timing advance can be tricky for most people as having a test wheel is the safest way to perform this afaik and those can cut into a budget.

Reed stops always seemed like the holy grail to me with good results having been reported. Only problem in doing this is if not done correctly (and what that is I don’t know) it can lead to reed breakage and ingestion.

I’m a toolmaker by trade and have wanted to make a jig for precisely bending the reed stops evenly along their length but again, time…

I believe this to be the key as most of what I have read about people trying this was shade tree mechanic type stuff. My theory on breakage is they wind up with a high spot or hard edge that causes the reed to get work hardened in one spot leading to failure.

 

[big_johnson87]

I own an 88' 25hp that I made into a 30hp

what's the timing on the 30hp vs 35hp? I have parts from a 79' 35hp goon an 89' 20hp and it says 34° timing is that the same for the 30hp? or are they only 30°