1974 Johnson 115HP conversion to 135HP

[LOREJB]

I am interseted to see i anyone can tell me if this is feasible and
pros and cons (and what it would entail to do i.e parts and
amount of labor involved) I was told that there was a kit offered at one
time many, many years ago and it pretty much consisted of different carbs,
reeds and intakes? Any and all help is greatly appreciated as I am more than
likely looking at rebuilding the carbs and replacing the reeds anyways and
if this is a viable option to gain a little extra h.p. at the same time by
converting to the 135 h.p. by spending some more time and money
if the trade off was worth it and it is possible to do?

 

[crb478]

Re: 1974 Johnson 115HP conversion to 135HP

There is more to it than just carbs reeds and intake. The exhaust on the 135 is a bubble back, on a 115 it is flat there is also different porting and parts can be hard to find. Some of the real experts can tell you exactally which parts are different, but it is not as simple as bolting on 20hp in parts.

 

[iwombat]

Re: 1974 Johnson 115HP conversion to 135HP

There's also the high-compression heads which are a pretty rare beast to find. I'm not sure if the porting is different on the 135 from the 115. I don't think it is, but that opinion is worth exactly what you paid for it.

Anyway, the relevant parts are:

1) Carbs
2) Heads
3) Buble-back exhaust.

There may or may not be porting mods involved.

I believe intake and reeds are the same.


Why are you wanting to go up in HP? Top end, or hole shot?

 

[LOREJB]

Re: 1974 Johnson 115HP conversion to 135HP

I realize it will entail more than bolting on parts I'm sure it will require some fabricating and some searching for parts so
this is where i thought i would start with some suggestions from anybody
that has some knowledge in this.

I am wanting to gain a little of both but really more top end.
(i am open to any hp options in either direction=top end or hole shot) My main concern is I dont add
or do something that will tear up the motor as I realize the motor
is 35 years old and may not take abuse from aftermarket add on parts.
Eventually it will have to be re-built but im trying to optimize my preformance
without sacraficing the life left in the motor.

I have aftermarket CMC tilt trim unit installed, whale tail and
a new aluminum prop I also have a new s/s prop which doesnt seem
to make much of a diffrence but the gentlemen that sod me both said the
pitch he provided would optimize what i had for the application of the motor and boat (Magnum Tri-Hull 17'). Transom is new so there is little to no flex.

Does anyone know of a good source for the parts mentoned below and maybe any reference material for converting. I have the manual for the 115HP motor already.

Any and all help is greatly appreciated.

 

[iwombat]

Re: 1974 Johnson 115HP conversion to 135HP

DHadley is really the guy you want to talk to, but I think you could gain a lot by going with a bubble-back exhaust and machining your intake to be a dual intake (both carbs feed each cylinder). Again, DHadley had the original info, but I should have copies somewhere. You'd need to re-jet your carbs in the process most likely, but I think that'd give you the biggest bang for the buck. If you go that route, I'd suggest finding another intake so you can always switch back if you need to.

 

[crb478]

Re: 1974 Johnson 115HP conversion to 135HP

Between Dhadlay and Iwombat they should be able to give you all the information needed on these older crossflows. I may well be mistaken about the porting, but either of these can probably help you get what is available out of your motor.

 

[iwombat]

Re: 1974 Johnson 115HP conversion to 135HP

Don't you go putting me in the same league as Dhadley. He's probably forgotten more than I'll ever know.

 

[cdoliver]

Re: 1974 Johnson 115HP conversion to 135HP

I went from 45mph to over 60mph by doing the intake mod, boyesen reeds, changed to better heads, and LOTS of tweaking with the setup and props. Dhadley helped me with all of this. I would say getting the setup right made this biggest difference in the top end. The intake mod+boyesen reeds made a noticeable improvement. This was with a crossflow v6. Its all going to be up to how well your hull responds to small improvements.
To find the parts you are looking for, I would check ebay and craigslist. Have fun

 

[LOREJB]

Re: 1974 Johnson 115HP conversion to 135HP

Thanks to all for the advice. Does anyone know how to get in
touch with DHadley as I am new to this posting threads thing.


Just a sidenote my Tach is pegging at 6 grand. (this is the original
tach and may not be correct as sometimes it registers and
sometimes it doesnt) The manual states 4500 to 5500. My question
goes along with thie initial thread as i do not want to do the conversion
if this is only going to overload the motor. Is there a way to check
the Tach to see if it is working or do i have to replace and if so any suggestions on what kind?

 

[emdsapmgr]

Re: 1974 Johnson 115HP conversion to 135HP

OMC makes a filler block replacement kit for their later model crossflows. 392000. It's about $180. It may fit your block-you need to check the exhaust chest to see if the block has the cast bosses to mount these filler blocks. Once installed, the block will have to be decked and you will need to add the two bubble back exhaust covers. The 135's used larger carbs and they will make a great improvement. Also the 135 heads. I'd add four of the later model V4 rubber intake filler blocks (322684-NLA) and a set of composite reeds. At 6000 rpm your flatback block is beyond the optimal torque range and you are on the downside of hp curve. You are better off to prop it to run up to 5650/5750-change up one prop size. You can turn it higher after all the mods are installed.

 

[jaf65]

Re: 1974 Johnson 115HP conversion to 135HP

This is a request for DHADLEY and/or IWOMBAT for a candid reply post to the following. Don't hold back -I respect your expertise!
In looking for a 90-140 hp motor for a 24ft. Pontoon I ran across (have not seen or heard it yet) that is reported to be a 1977 110hp Johnson (I think really a 115hp and a 1978 yr). The story is the owner (he taught shop tech) had his class under his super. take this motor and do a conversion by boring cyls. to .064 over, new Sierra Pistons, rings, bearings, seals and gaskets. rebuilt carbs off of a V6 Johnson with 1.75 bore. "Bell" Exhaust and one new coil. No mention as to new or replacement heads. This man died before it was even run and his friend got it from wife and then sold it to the man who is now trying to sell it. This conversion was 6 yrs. ago and it has been upright on a rack inside since. Was started by seller in 1999 foe 2 minutes and reportedly ran strong and peed water. Reportedly T/T had worked when purchased by seller but now does not operate but makes a sound. Pics show a bubble looking back and a metal front where the 4 carb . throats are visible (no cover over front). The carb throats are two on top and two lower each set of two horizontal to each other. Neither seller nor myself are particularly adept at this level of marine mechanics. So, the questions dor yhour comment are:
1. Does this sound like a proper conversion
2. If purchased would you expect additional work/parts needed?
3. for general use on pontoon boat is this a good motor?
4. If I go to inspect motor for possible purchase, what tests/visible condition should I look for?
5. Should there be some sort of cover in front of the metal part where the carb throats appear.
6. should there be one or more power packs on motor or can electrical be converted so as to not use coils
7. Would you recommend this to a basically non-mechanic?
8. Does the resale or part-out value of motor worth the sale price of
$700.00
9. If better to discuss over phone reply here and I will give my number.

Sorry this is so long but am at a confused decision point!

 

[redjmp]

Re: 1974 Johnson 115HP conversion to 135HP

Well imho why bother with all that work and expense?
Just pick up a 135 and sell your 115.
I see them going for $500 or less sometimes and I'm sure you could get about the same or maybe even a little more for your 115.

Building yours will cost much more in parts alone.

To me it seems like the cheapest and easiest way to go unless you have a lot of extra time and cash to burn and would rather spend your time hunting for parts and wrenching than boating.