1977 85hp to 115hp conversion?

[nphilbro]

I've got a thread already going on the rebuild of my 1977 85hp motor so now is a good a time as any to ask this.

My parts motor is a 1975 Johnson 115hp. All the ignition components are identical and I used those to fix the ignition problems on my 85.

These two engines were basically built on the same form factor. In fact, my service manual is for both the '77 85hp and 115hp and the two rarely differ.

Could I use the carbs from the 115 on the 85 to increase hp? My parts motor has a cracked block but all the other parts are there and are in reasonable enough shape to use.

Even the gasket sets are identical except for the head gasket. I just checked the exhaust bypass covers and they interchange.

It looks like the exhaust diameter at the thermal housing is slightly larger on the 115 but the bolts line right up - so I could add that to the 85.

I've searched all over the internet but I can't find this question answered.

Any help?
Thanks
NP

 

[emdsapmgr]

Re: 1977 85hp to 115hp conversion?

No problem. That's one of the ways OMC made the higher hp engines-carbs with larger diameter throats.They will fit and run right out of the box. Since your 85 hp engine is a smaller displacement (92 cubic inches) than the 115 (99 cubic inches) you will probably find that the 115 carbs will overfuel somewhat at top end. You will need to do some repetitive top end testing and will find that you may need to go down a main jet size or two or three to get the engine to run just right. The main jet size in an 85 carb is .045. The main jet in a 115 is .059. When I put 1 5/16"carbs on my small bore 100, it would overfuel so badly, I started the other way. I put the .045 jets in and went up in main jet sizes as I test ran the engine. In the end, the engine ran great and gained about 250 rpm's-enough to swing the next size diameter prop. The diameter of the carb throats is always cast into the face flange of each carb: 1", 1 3/16", 1 5/16", 1 3/8", etc.

 

[spherret]

Re: 1977 85hp to 115hp conversion?

No problem. That's one of the ways OMC made the higher hp engines-carbs with larger diameter throats.They will fit and run right out of the box. Since your 85 hp engine is a smaller displacement (92 cubic inches) than the 115 (99 cubic inches) you will probably find that the 115 carbs will overfuel somewhat at top end. You will need to do some repetitive top end testing and will find that you may need to go down a main jet size or two or three to get the engine to run just right. The main jet size in an 85 carb is .045. The main jet in a 115 is .059. When I put 1 5/16"carbs on my small bore 100, it would overfuel so badly, I started the other way. I put the .045 jets in and went up in main jet sizes as I test ran the engine. In the end, the engine ran great and gained about 250 rpm's-enough to swing the next size diameter prop. The diameter of the carb throats is always cast into the face flange of each carb: 1", 1 3/16", 1 5/16", 1 3/8", etc.

May I please appeal to your obvious superior knowledge on this topic?
An earlier thread mention de-rating engines to satisfy legal constraints we don't have in AU(yet!). No mention in any of the posts was made of the 'jetting' or re-jetting of carby's, and your reply to the mail ahead gave me memory of exactly how this was acomplished in 'earlier days' on car engines.Mostly in reverse though I recall. Please be kind to offer some thoughts on this as a viable alternative to a group that might benefit by your experience?
Regards,Rob. spherret.

 

[nphilbro]

Re: 1977 85hp to 115hp conversion?

Thanks, EMD - that makes sense.

Would you recommend I use the existing 115 carb with the jets from the 85 to start with?

I'm going to shop the internet to see what's available in aftermarket jets.

 

[nphilbro]

Re: 1977 85hp to 115hp conversion?

I found some .052 Edelbrock jets at Autozone. I'll have to see what the form factor of the existing jets are to find out if they're compatible.

 

[emdsapmgr]

Re: 1977 85hp to 115hp conversion?

The engine should run with the 85 jets in the large carbs. The engine will probably run well, too. Problem is that these engines run just great when they are leaned out, and that's what you'd be doing. Lots more air, no more fuel. No need to waste your time with that test, unless you just want to document the baseline perf for history. One test blast, then back to idle. I'd probably install the 115 carbs with a main jet size at least two jet sizes larger than the 85 mains for the first test run. The go up a size or two each successive test run.

 

[nphilbro]

Re: 1977 85hp to 115hp conversion?

What about excessive heat buildup by running it lean/too lean? From what I understand the cylinder will melt down before the temp gauge tells me anything is wrong. Would changing the fuel/oil mix to 45:1 reduce the friction/heat? Would the excess oil burn off just the same or would it build up?

This leads me to my next question - if running too lean can cause excessive heat, would increasing the size of the jet (as you recommend), or running the .045 jets wide open decrease the combustion temperature while increasing hp?

I know this is a bit of an art form and an extreme balancing act - I'm just trying to understand better the cause/effect of the basic elements: oil, gas, air, heat within the only constant- actual displacement.

I have the feeling your recommendations likely take my concerns into account but the engineer's brain in me needs to know how so I can effectively adjust each variable and more accurately interpret the results and predict the effects. I apologize for being a noob but I'm a quick learner and I can tell you have a good deal of experience.

 

[emdsapmgr]

Re: 1977 85hp to 115hp conversion?

I sent you a PM.

 

[83Baja16ss]

Re: 1977 85hp to 115hp conversion?

Hey, everyone, I am definitely looking for a bit more information on this, looking to do just about the same thing, I have a 1977 or so evinrude johnson 85 hp that I would like to get some more power out of, I have access to a blown up 115hp similar year that I may buy the carbs off of, what else should I grab? Thanks in advance, long time reader first time poster

 

[emdsapmgr]

Re: 1977 85hp to 115hp conversion?

You may get better visibility for your thread if you start a new one in your name instead of tagging along at the end or this thread. You can't much off the 115 which will help that 85 other than the carbs. The heads won't fit and the exhaust system is the same. Some of these early 115's had the (4) rubber intake filler blocks. They would be worth transferring to your 85. You'll get 3-5 hp from them. If you do salvage them and take the reed boxes out, you might consider adding some composite reeds at the same time-they help with idling and midrange pickup.

 

[83Baja16ss]

Re: 1977 85hp to 115hp conversion?

You may get better visibility for your thread if you start a new one in your name instead of tagging along at the end or this thread. You can't much off the 115 which will help that 85 other than the carbs. The heads won't fit and the exhaust system is the same. Some of these early 115's had the (4) rubber intake filler blocks. They would be worth transferring to your 85. You'll get 3-5 hp from them. If you do salvage them and take the reed boxes out, you might consider adding some composite reeds at the same time-they help with idling and midrange pickup.

Hey thanks for the advice, I'll post a thread tonight about this. I'm picking up some boyeson carbon fiber reeds for it tomorrow, is there a difference between the 85hp reeds/reed boxes and the 115hp reed/ reed boxes?

 

[Dhadley]

Re: 1977 85hp to 115hp conversion?

The reed boxes are the same. The carbs are jetted for the air flow that goes thru that particular throat size. You not only have less cubic inches in the 85 compared to the 115 but your ports are smaller too (less port area and lower port timing). Your restriction on hp will be how much air you can pump in and get out efficiently (porting, cubic inch & cumbistion chamber size / shape). If you have the motor all apart it's pretty easy to convert the 85 ports to the 115 configuration. You'll still have the small cubic inch so it may not produce a whole lot. In 1977 the motors were rated at the crankshaft, later they were rated at the propshaft.

 

[nphilbro]

Re: 1977 85hp to 115hp conversion?

I installed the 115 carbs on mine ultimately. The hardest part was adjusting the timing. I've since sold the motor since I actually upgraded to a 140 but I noticed cavitation with the factory prop since it seemed to kick out quit a bit more about 1/2 way through the hole shot if I put too much on it. Running a smaller top end jet would have really made a difference - somewhere between the stock 85 and 115.

I have to admit - it had a beautiful throaty growl after I made the change. It sounded a lot better than my '80 140 does.

If anyone would like a link to a video of it in a tank with muffs on PM me and I'll send you the link.