destroking engines?

[fuzznuts]

I have a 1979 140hp johnson on a 16ft jon boat can you destroke outboards to get higher rpm?

 

[levi_tsk]

Re: destroking engines?

uhh no modding an outboard is very VERY unrewarding
if your engine is running 5200-5500rpm at wide open throttle thats all youll get. itll come completely unglued and go nuclear on you if you run it above thatrpm.
if your rpm isnt in that range, you need to go to a lower pitch prop or if you have a four blade prop switch to a three how ever you hole shot will suffer because in outboards nothing is free
to get more rpm at wide open throttle the rule of thumb is 1" of pitch will either gain 200 rpm if you lower the pitch 1" or shed it if you raise the pitch 1" look on the inside hub of the prop for the pitch itll be around the prop nut

 

[Chinewalker]

Re: destroking engines?

Actually, I would aim a little higher. Those crossflow V4s can be very happy from 5500 - 6000 RPM. I propped mine to spin 5800 at WOT. Also helps prevent coking of the rings as the motor actually runs with cooler combustion temps if it's not lugged down. You can gain the extra Rs by prop & set up changes as noted above...

 

[levi_tsk]

Re: destroking engines?

my SERVICE MANUAL says 5500 MAX. id stick with it its isnt worth going nuclear over

 

[JB]

Re: destroking engines?

You have 140HP on a 16' Jon?? Those boats are 30-40HP rated.

You are already tempting fate. Not a good idea.

 

[Silvertip]

Re: destroking engines?

You need to understand how piston porting works on a two stroke. You can certainly de-stroke the engine, but you must change the porting and very likely exhaust tuning as well. Unless you know a really good two stroke engine tuner (someone who understands and has actually done port work), you risk screwing up a perfectly good engine. Once porting is done to an engine, there is no going back.

 

[levi_tsk]

Re: destroking engines?

i forgot about that little tid bit ^ all the more reason NOT to do it - if i had to guess hes got that 140 on the back of an older bass boat and if he does have it on the back of a 16' jonboat id be willing to bet hes just nuts enough to try to destroke it even after we told him not to

 

[wilde1j]

Re: destroking engines?

I'd forget about the owner's manual and shop manual when it comes to WOT RPM. 5800 to 6000 is just right for almost all V4 and V6's. This is one place the books didn't get it right.

 

[Faztbullet]

Re: destroking engines?

Its not feasable to destroke a 2 stroke as exhaust tuning would have to be totally redesigned. Johnson did make a "stroked" engine one year, the 1973 135hp. As for the stock V-4 with some machining,fuel mod's and racing mod's will turn 9500 rpm for hours and they sound like a mad bumble bee in a jar as the go by... but shortens the life of them as they wear out quicker. Thats why most are rebuilt every 3rd or 4th race..

 

[Dhadley]

Re: destroking engines?

That motor will run very well at 5800 or so. The stock steel reeds give up (flutter) somewhere around 6200 or so. Putting in fiber reeds will get you close to just under 7000 with an otherwise stock V4 crossflow with 140 port timing on the right hull. Working the intake manifold will get you closer to about 7200. And a motor like that will last a very long time.

Just how much rpm are you looking for? And no, the motor will not automatically explode if it goes over 5500.

 

[fuzznuts]

Re: destroking engines?

Yes i do have a 140hp on a 16 foot jon boat. thats normal where I'm from theres a guy thats running a moded 175hp johnson on a 16 foot jon boat and hes running around 80 mph.

 

[fuzznuts]

Re: destroking engines?

I'm looking for 6500 rpm all day long.

That motor will run very well at 5800 or so. The stock steel reeds give up (flutter) somewhere around 6200 or so. Putting in fiber reeds will get you close to just under 7000 with an otherwise stock V4 crossflow with 140 port timing on the right hull. Working the intake manifold will get you closer to about 7200. And a motor like that will last a very long time.

Just how much rpm are you looking for? And no, the motor will not automatically explode if it goes over 5500.

 

[Bob_VT]

Re: destroking engines?

Well.... speed costs money. You will not get much more performance from the older cross flow. The 140 was squeezing most of the power out of that powerhead. If you want to do things for the motor such as Boysen Reeds you will spend a great deal of money to create a frankenstein motor and possibly not have any reliability.

Your best power option is to look for a motor newer that 1984 and a 140 looper. That will give you much more of a change. Your present 140 is a powerhead rated motor and is actually giving you about 125 at the prop. A newer (1984 +) will deliver 140 at the prop.

 

[fuzznuts]

Re: destroking engines?

I would love a 1985 looper but i can only take what i can get.

 

[Dhadley]

Re: destroking engines?

6500 with Boyesen reeds should not be out of the question as long as the hull responds to your prop and prop rpm. However - as Bob mentioned - if you run into someone with the same boat and a V4 looper I wouldn't recommend betting that you'd out run them.

 

[fuzznuts]

Re: destroking engines?

so all i got to do is put boyesen reeds on and it should good to go sounds good thanks. and yes looper vs crossflow looper's got my vote

 

[jbjennings]

Re: destroking engines?

I'd love to see a pic of your john boat. It must be really strong, wide, and heavy to support a 140hp without catching air and flipping at top speed, or creeping horribly in the slightest wind. I'm curious since I've seen some Northerner's rigs like you describe--- Is this motor tiller operated? It seems like it might be a little persnickety if one hit a submerged object.
Anyway, I hope you get it like you want it.
JBJ

 

[fuzznuts]

Re: destroking engines?

when i start the build up ill post pics but right now all i got is the motor i and yes i am from the north but i any one knows a better size boat i should use let me know im just trying to go fast and have fun but it seems like a 16 foot is unsafe for this motor.

 

[Faztbullet]

Re: destroking engines?

Here is ya a recipe for a go-fast V-4 looper:
1985-87 small bore 140 looper block
Bore to big bore spec(3.685) and have cylinders finger ported
Raise exhaust ports.030 and intake ports .050
Replace "teacup" intakes with late model "angled" intakes
Replace steel reeds with a solid fiber reeds
Replace carb manifold with manifold and carbs from a 1998 225 V-6
Shorten the exhaust tuner 7/8 inch
Update water system like the 1987 70hp
Dump the OMC ignition and run the yamaha or custom MSD box ignition
With some knowledge,$$$ and time you can get this motor to about 165hp+!!!
Over on the Scream & fly site there was a article by al stoker on building a V-6 looper that put out 325hp!!!!

 

[metriccrescentwrench]

Re: destroking engines?

Here is ya a recipe for a go-fast V-4 looper:
.......
With some knowledge,$$$ and time you can get this motor to about 165hp+!!!

Sounds like serious $$$ if you don't own a machine shop.

Over on the Scream & fly site there was a article by al stoker on building a V-6 looper that put out 325hp!!!!

Now THAT sounds like some fun!!!