Differences betwen a 20hp and 30 hp Johnson

[Grew cub]

Hi guys<br />I own a 1986 20 hp Johnson. It's on a small tunnel hull called a grew cub. It seats 2 peolpe. I have owned it since 1989. If the two peolpe are to heavy, you have to lean forward or even lay on the front of the boat to get it to come up on the water and then away you go. <br />I have always wanted to move up to a 30 or 35 hp.<br />This leads to my question. Does anyone know the internal differences between the 20,25,30, and 35hp motors. I have looked at the specs and it is the same piston size and displacement. So what did they do to increase the hp, and can I do it also. Or am I better off looking for a 30 hp and just selling my 20.<br />I had a good look at someones 30 hp a couple of weks ago, and the only diffrence I could see was that the carb had a much larger internal bore, I guess to allow more air into the motor.<br /><br />Thanks for any input

 

[Johnnielund]

Re: Differences betwen a 20hp and 30 hp Johnson

I have a 1990 25 hp Johnson that the previous owner had converted to a 30 hp by a very reputable marina here in northern Illinois. I have a 16’ Lund Explorer and am very happy with this boat/motor combination. According to Joe Reeves the 25 and 30 have the same block, cyl head, crankshaft, rods, pistons, leaf plate, leaf valves etc. The differences between the two are as follows:<br /><br />The throat hole venturi on the on the carb and manifold is larger on the 30 than the 25.<br /><br />30hp is hit at 5500 rpm. 25hp is hit at 5000 rpm.<br /><br />The cam, which is attached to the magneto armature plate, is different. The 30hp cam is # 326876. The 25hp cam is # 334750.<br /><br />The orifice at the extreme top of the carburetor is different. The 30hp is # 39 (.039) # 334568. The 25hp is # 30 (.030) # 317473.<br /><br />The high speed jet, located at the center bottom portion of the float chamber is different. The 30hp is #65D (.065) #327541. The 25hp is #51 (.051) #331567.<br /><br />I hope this information is helpful to you. It is my understanding that trying to increase hp is usually not a good idea, however I think in the case of this particular motor it works well.

 

[lark2004]

Re: Differences betwen a 20hp and 30 hp Johnson

possibly also the exaust, There is usually a resonator that fits inside the leg, and is tuned to help scavange the exaust out of the motor.

 

[fireman57]

Re: Differences betwen a 20hp and 30 hp Johnson

Are you sure you have it trimmed correctly?<br />Also your 20hp will probably sell at or close to the price of a 30hp used. I would opt for the 30.

 

[Grew cub]

Re: Differences betwen a 20hp and 30 hp Johnson

Yes, I have played with the trim, and it only made things worse. I moved from a 15 to a 13 pitch prop. That made take off better, but I lost some top end speed. It was a fair trade off.<br />If the first response is any indication of the depth of parts that would need to be changed, I most certainly would be better off looking for a used 30 hp. <br />The only draw back is that I know how my 20 hp has been treated all of its life. I would say there is no more that 100 hours on it, and it runs great. All I have ever had to repair on it is the neutral safety switch. O ya, and repaired the prop a couple of times. But that tends to go without saying.<br /><br />Anyone else ?<br /><br />thanks to all

 

[G DANE]

Re: Differences betwen a 20hp and 30 hp Johnson

Hi Tunnel Hull<br /><br />The 20 Hp will have a different cylinderhead and carbureator than the 25/35 HP's. If you get the carb from a 25, you will have almost 25 HP. The major difference from 25 to 35 is the midsection and thru hub exhaust, which the 20 and 25 unfortunately did not have in 86. The 35 also has another intake manifold. From 1984 a different carb too, but thats minor.

 

[G DANE]

Re: Differences betwen a 20hp and 30 hp Johnson

Tunnel Hull - watch ebay for a period, the 25 carbs show up regulary and are few $.

 

[Grew cub]

Re: Differences betwen a 20hp and 30 hp Johnson

Thanks everyone<br />I think I'll keep my eyes open for a good used 30 hp unit.