20hp to 35hp results and Q.

[Thompsoncustom]

This sound right (RPM'S)

So had my boat out today for the first time after getting it fully converted from a johnson 1990 20hp to a 35hp. So I didn't use my tech before on the stock motor but it did 25mph and with the intake manifold change it did 29mph, now with the new carb is only does 32mph but my tech reads 9500RPM at WOT. It does seem like it has more power than it needs at this point and planes pretty much instantly. Boat, motor, and me might be 400-500 pounds.

So do you guys think that reading is actually right? I didn't run the motor much as I don't want to grenade it but I'm running a 10x15pitch prop right now I'm not sure if I could even change the prop enough to bring it down that far.

 

[flyingscott]

I would check your tach pretty sure the motor would have exploded before 9500.

 

[jakedaawg]

I am going to venture a guess here....your tach isn't working properly. I would be real surprised if you hit 9500 rpm and it held together. Do you know any techs that might lend you an analog shop tach like a Stevens or snap-on model? That would allow you to get a proper reading and determine the prop you need. It is so much easier to get a useable reading with an analog as opposed to a digital.

 

[Scott Danforth]

most snomobile 2-strokes are about 8000-8500 RPM, it is plausible that your changes did simply push your RPM's up to 9000. did you change props with the manifold and carb change? if you didnt, I would highly recommend you do. you added 75% to the power, which gained a 30% increase in speed, if you didnt change the prop, your motor gained about a 35% increase in RPM.

I would also verify your tach against another tach.

 

[Thompsoncustom]

I did change the prop to what the stock 35hp comes with I believe. I think I figured it out looking around on the internet I found that this tach has settings for two or four stoke motors and mine was on the four stroke so the number above are just a false reading.

 

[Thompsoncustom]

Well now that I got my johnson 20hp running good I decided a week or so ago it was time to start upgrading it. The first thing I did was swap intake manifolds and than test it and lastly out the 35hp carb on it.

The stock 20hp configuration would push my boat with just me 25mph.

The intake manifold swap pushed me 29mph

and lastly the new 35hp carb got me to 32 mph.

Now my question is did I miss anything? Didn't seem to gain that much with the carb swap. I'm running a 10x15 prop which is one of the options the 35hp came with. I have had it out with the RPM tach but had it on the wrong setting so the result from that were useless. I guess I excepted to be around 36-39mph with the carb swap.

 

[jakedaawg]

The prop is not determined by an "option". It is determined by the boat and motor combo as it relates to max rpm.

Basically what you do is take the boat out with out all the fishing stuff and coolers and whatnot. Have a half tank of gas or so. Just you in the boat. Just make it really light. Get a reliable tach, go full speed or up to max rated RPM whichever comes first. If you are only spinning say 4500 at full throttle you decrease pitch to bring it up to max RPM. If you have throttle left you increase pitch until full throttle is max RPM. Diameter can also be changed but that is more for high performance type situations.

 

[jbcurt00]

Merged the 2 similar topics and questions

 

[Thompsoncustom]

I understand RPM's determines what prop is needed but when I got the motor the stock prop was trashed so I took the best guess of what I thought it would need after being upgraded. I need to get back on the water with my tech set right and see where is comes in at. But before I start switching props I need to make sure I'm getting 100% out of the motor and upgrades so the max RPM is static.

 

[jakedaawg]

Perfect, sounds like you have done something cool. A question for you, Would you do it again? Do you think you saved enough money to make it worth it as opposed to having sold your original and putting that money towards say a used 40HP or thereabouts?

 

[jasper60103]

I understand RPM's determines what prop is needed but when I got the motor the stock prop was trashed so I took the best guess of what I thought it would need after being upgraded. I need to get back on the water with my tech set right and see where is comes in at. But before I start switching props I need to make sure I'm getting 100% out of the motor and upgrades so the max RPM is static.

Johny25 did a pretty good writeup on this kind of upgrade. Unfortunately, he doesn't hang out here anymore, but you can do a search for his thread. Good luck.

 

[Thompsoncustom]

Hmmm total project cost is probably 450-500 bucks for the motor, parts to get it running 100%, and upgrades.

Would I do it again? That's a good question and it would probably come down to what I found a better deal on. Got the 20hp for 200bucks so couldn't pass it up. If I found a 50hp in good shape for 500 bucks I would have taken that over the 20.