1985 90hp & 1987 115 hp differences

[Derangedmadman]

I need an expert's advice here.

I have a chance to pick up a 1985 90hp for $300.00. Supposedly it has good compression and will start up fine but will not run at high RPMs.

Looking at the specifications this and my 1987 115 hp (bad powerhead) are really similar.

Both are 99.8 cu in displ.
Both are 2.875 in bore
Both are 2.546 stroke

Is the difference in HP due to the size of the port openings?

What are the differences in these motors and will the parts interchange to make one good engine?

Any help is alway appreciated!

 

[Chris1956]

Re: 1985 90hp & 1987 115 hp differences

Most of the parts will interchange. The HP difference is probably port timing. You can check part numbers and probably figure it out. Carbs may have different jets and may be a bit different in size. You might look at piston types. Some have high dome, some have low dome.

 

[Derangedmadman]

Re: 1985 90hp & 1987 115 hp differences

I my best bet for a running motor would probably be use the 90 hp granted there is no major problems with it.

The guy that has it for sale seems to think it is a stator or a power pack is bad causing it to run good at low RPM's but bad at high RPMs.

I am going to look at it tomorrow and I am taking stuff to do a compression test before I buy it. If the compression is good, what would be the most probable cause of the problem?

Just need some ideas for a starting point. I have the powerpacks and stater off of mine which I have verified at the same part number.

 

[Derangedmadman]

Re: 1985 90hp & 1987 115 hp differences

I tested the compression today and each cylinder was at 95 lbs. That is a little on the low side, should I tear it apart and put new rings in it with the compression being that low?

 

[Derangedmadman]

Re: 1985 90hp & 1987 115 hp differences

anyone?

 

[blue90HP50]

Re: 1985 90hp & 1987 115 hp differences

Madman: My 2 cents, and I think we both are in the same "backyard" so to speak, you most likely will not be happy with 90 HP at 95psi. Consider looking further for a 115 HP powerhead, more "UMPH" for your money. Just my 2 cents. Blue

 

[Chris1956]

Re: 1985 90hp & 1987 115 hp differences

If all the cylinders tested the same PSI, perhaps the gauge is inaccurate, and the motor is fine.

There is a troubleshooting procedure for that stator at CDI electronics web site. Ohm out the high speed windings.

 

[Derangedmadman]

Re: 1985 90hp & 1987 115 hp differences

I put the two blocks side by side today and they are completely identical except for the size of the porting (intake and exhaust.) If I were to port these out myself and match them to the larger ones, used my carbs from the 115, wouldn't it be the same?

 

[emckelvy]

Re: 1985 90hp & 1987 115 hp differences

Your '87 115HP has a power-ported block and pistons. The 90HP does not. You're never gonna turn that 90 into a 115 without a ton of expensive machine work. There's more to it than just intake or exhaust ports.

BTW, that's 90hp at the prop, which is equivalent to a '70's 115hp (crankshaft-rated). Your '87 115 is closer in performance to a late-70's 140HP. Very strong running motor.

I'd run the 90, if you don't like it, look around for a power-ported ADI powerhead, or fix up your 115 powerhead if it's salvageable.

BTW it's almost impossible for one of these to have all cylinders equally worn out. Invariably there will be significant difference between the good and bad ones. I'd venture it's way more likely that the low readings are from the calibration of the compression gage. The fact that they are all even is encouraging, though!

HTH & G'luck.............ed

 

[AMD Rules]

Re: 1985 90hp & 1987 115 hp differences

I agree with Ed though, you will not be able to turn it into a 115 hp prop rated motor very easily.

Did you double check the 90hp compression with a different gauge and fresh battery yet?

 

[Derangedmadman]

Re: 1985 90hp & 1987 115 hp differences

No I did not, It is a brand new gauge bought specifically to test this so I didn't think it would be off but reading some of the posts on here it seems it very well may be.

I am going to buy a Micrometer today to measure the cylinder walls to see if they are within specs.. If they are I am going to re-ring it and put it back together.

On my 115 hp i was running a stainless Quicksilver 19 pitch cupped prop on an '87 Jason 1800 fish & ski. I was underpropped. Do you think the 90 hp will turn the 19 pitch or should I start out with the aluminum 17 pitch cupped prop that came with the 90 horse.