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

[lucescoflathead]

I don't have a 20 Hp yet. What year would you get? I want to try to get one this summer. Thanks Todd

 

[Johny25]

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

 

[lucescoflathead]

Thanks for the info. Todd

 

[Johny25]

Update to my motor not running as well as I wished she had first run. After replacing all my fuel lines I noticed it was drawing fuel and air through the primer line to the intake and spitting fuel threw the line that attaches to the top of the carb while knob is pushed in. See through hoses are awesome Anyway I am assuming that the O rings are bad inside the primer assembly so I will be removing and rebuilding the manual primer today. This should correct the sputtering and cutting out issue I had the other day.

Ok update to this thread because I hate threads that do not finish. I received and installed my new primer assembly yesterday, got my 88' service manual also yesterday and did a correct link n sync on the motor. I also replaced the thermostat but I don't believe this was part of the issue I had with the motor. The thermostat was more of a precautionary measure. Anyway I took it out on the lake with my 3 yr old today and the motor ran like a swiss watch The only problem I had was the RPM got to high with the light load and the 10.25X11 pitch SS prop. Good thing I picked up a 10X13 the other day, so I went to shore and put the 10X13 on and no more over rev. I lost a lot of holeshot but gained top end and motor didn't rev to high. I was really surprised at the difference in holeshot from my Stainless 11 pitch to the aluminum 13 pitch! It was very noticeable. I am wondering how much it has to do with the cupping on the nice stainless prop? I am going to run my 11 pitch aluminum now and see what the difference is compared to the 11 pitch stainless. Anyway that is my update

 

[jasper60103]

Johny,
that's great news! Glad you got it figured out. =D>

jasper

 

[jasper60103]

Hey, Johny.

I was wondering if the new primer improved cold starting?
I always thought these motors were naturally cold blooded, but you may have squashed that theory.
Did your motor sneeze at cold start before replacing the primer? If so, did it sneeze after replacing primer?

jasper

 

[Johny25]

No the sneezing wasn't really noticeable until the motor got warm so I don't think it helped that much for cold starting. Although I did find out that the plugs that are recommended QL77JC4 do not work as well at cold start as the plugs that came with the motor do. The motor came with NGK buhw-2 gapless plugs in it. I pulled them out and put the recommended champions in when I got the motor home but after I fixed the primer assembly I swapped the plugs back just to see what the difference was. And the NGK's seem to work much better when starting the motor cold.

To be honest Jasper my motor doesn't seem that cold blooded. Usually even in 20 degree weather she runs pretty damn good even when cold started.

 

[jasper60103]

Thanks, Johny.
Yea, my '88 30hp Johnson has either the QL77JC4 or QL82C, not sure which at the moment.

Before cold starting, I normally give the primer a few pulls. Then startup the motor and feather the primer for the first 10 secs or so (instead of just leaving it pulled out during warm up.)

Funny thing is, my 30hp Evinrude behaves the same way.
Its probably unlikely, that I have two motors with worn out primers?
But I guess I should install the clear gas lines (like you did) to make sure fuel is flowing
from the primer during cold start.

 

[Johny25]

Well if you have a leaky prime like I had just pull the hose going from the prime to the top of the carb while it is running, if there is a lot of fuel spitting out the line while the primer is pushed all the way in then you have a problem. It will spit just a tiny bit or non at all if the primer is good. My bad primer assembly was spitting a lot of fuel into the carb when in the off position which was a big problem getting the motor to run correctly.

 

[shawnfish]

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.

that was me and i have an 25 not a 20. think the gearcase couldnt handle 10 more ponies?

 

[Johny25]

Well it may? But I won't put my name on it. My guess is there is a good reason that there are differences in the gear case between the 25 and 35

 

[jasper60103]

Well if you have a leaky prime like I had just pull the hose going from the prime to the top of the carb while it is running, if there is a lot of fuel spitting out the line while the primer is pushed all the way in then you have a problem. It will spit just a tiny bit or non at all if the primer is good. My bad primer assembly was spitting a lot of fuel into the carb when in the off position which was a big problem getting the motor to run correctly.

Johny,
thanks for the tip. I'll be sure to check it out.

jasper

 

[DIZZYSPOTS]

The carb and intake is on...last piece is to find larger diameter fuel line to fit the larger primer nipple...?how did find a suitable plastic "tee" to join the tiny lines to the single larger one? also...the dramatic difference that you illustrated in your post between the 20 hp/30hp intake and carb is nothing compared to the intake on my Johnson 20 the intake had a "restrictor" plate looking casting...the opening is 3/4" with a 1/2" vertical slot at the 6 o'clock position the new carb and intake should be a HUGE improvement...
float test Thurs PM ps the only two props I have two play with are the current 4 blade composite (4x10.5x11) and a stock OMC 4 blade alum (4x10.5x14) results to follow....stay tuned

 

[Johny25]

I found bigger plastic T's at Napa but I believe in the end I wound up using the same T and cramming a chainsaw fuel line over the nipple on the intake. It took a pair of needle nose to get it on a few times but now I can do it by hand after it stretched a little. \

Yeah I just bought and sold an 85' 20hp with the same restricted style intake. Big difference. Oh and I would go with the 10.5X14 unless you are trying to push a pontoon lol. The 11 will probably cause the motor to over rev with the increase in HP

 

[DIZZYSPOTS]

got the fuel line at Ace...used same tee..jammed on the lg nipple
test float postponed....my buddy (human ballast for front seat)..got ties up...perhaps Sun stay tuned

 

[DIZZYSPOTS]

OK only 3 weeks late, but its on the trailer...my lovely wife will act as human ballast and gps watcher...headed to Patagonia Lake in southern AZ for the test float...
G5 sliders also in place.....stay tunes for results

 

[DIZZYSPOTS]

OK only 3 weeks late, but its on the trailer...my lovely wife will act as human ballast and gps watcher...headed to Patagonia Lake in southern AZ for the test float...
G5 sliders also in place.....stay tuned for results

 

[Johny25]

will be looking forward to hearing from you this evening : ) Be safe as always

 

[DIZZYSPOTS]

WOW!...test float err...flight was awesome...to review "20" horse Johnson w/ 14 4 balde prop on a Valco U12...loaded to near gross 55lbs fuel/tank, pax 290 + 170 =515
started right up..slight mixture adjustment...out of the no-wake zone....and crank...up on plane quick....out across the lake, my wife is in front with the gps reading off speeds...she gets to 25 mph (4850rpm) and I roll off the throttle and thats way fast in a tiller-steering 12ft tin boat

Thanks Johnyrude!!!

a little over 3 weeks and we'll take it across the bay at Gonzaga in Baja...hunter for trigger, yellowtail and grouper for our 3rd annual tin boat regatta

 

[Johny25]

Awesome So did you let off the throttle or was there more to be had? 4850rpm is low for that motor, should be hitting 5500 or better after the mod. I am trying to remember what year you have and the exact mods you made? I am surprised that it didn't crank that 14pitch to a higher RPM with the little bit of weight you are pushing although hull design can also be apart of that. And yes 25mph is a good speed for a 12' and anything after that can be scary especially in rough water :lol: