75HP vs 90HP Mercury carb engines

[Jacob]

What are the major differences??? How can I get more power out of a 75HP??? Thanks guys.

[4JawChuck]

What year?

My 2003 75HP 3cyl Merc needed the 90HP Carbs and initial timing changes to make it a 90HP...otherwise the engines are identical except for a raise in max rpm from 5250-5500rpm.

[JP Love]

Hajhhahahhahha

[Jacob]

It's a 2003......AWESOME!!!!!!! THANKS!!!!!!

[Boz]

how about a 1997?

[Jacob]

how about a 1997?

I would think your 1997 would be the same thing. i don't think they changed these engine very much trhough the years. I'm probably wrong but....... more than likely right...........

[4JawChuck]

I never researched the earlier models which might be the 4 cyl versions, only the 3 cylinders.

My Merc Manual covers years from 1994 and newer serial number 0D283222 and above. I researched all the part numbers between the two 2003 engines and all the part numbers are identical except for the carbs and timing, the carbs are WME-87's for the 90Hp and WME 84's for the 75HP. I sourced the 2003-90HP carbs locally and set them up myself. There was a significant increase in power which was noticable especially above 5000 rpm, below that speed I noticed an increased sensitivity to syncronization causing sneezing at idle with the larger carbs which required careful setting of the throttles so they opened identically and closed properly at idle. The blocks are integral so the power head is the same part number from 75-90HP in 2003. The power falls off above 5700 rpm with the new carbs so prop accordingly.

Setting the timing accurately to the new 90 HP spec from 18BTDC to 20BTDC made a huge difference in idle quality as well. I found setting the timing at WOT more important than the cranking timing technique as it gives you an idea what the engine is seeing..I found out my plastic ball connectors connecting the throttle to the timing lever were worn out on my engine and replacing them stopped the timing from jumping all over the place at WOT and idle. I recommend changing all of the plastic ball connectors...they are a pi$$ poor design and wear quickly...a squeeze of heavy silicone grease prevents vibration from varying the timing when there is a little clearance on these components as well.

There are a ton of differences in jetting for the different year carbs so doing the swap requires knowing which carbs had what jets although they are all similar. If you are going to run premix instead of injection there is a difference in main jet size also from 0.060" for the injection models to 0.062" for the premix models. I recommend getting the factory Merc Manual, its part #90-830234R04 for the english version...well worth the money.

One last note, Merc specs the idle air mixture at 1/2 turn open on carb #1 and 1 1/4 turns open for the other two to make starting easier,I found setting them all the same better for starting and the idle is smoother also, I fine tuned them by doing cold starts with the starter cord...if it starts cold with the starter cord easily, it lights off instantly with the electric start. I think all my carbs are set at 1 3/16 turns open or around there, the engine starts easily even in subzero weather with a little choking with the enrichment on the key and settles into a nice 850 rpm idle in short order in all kinds of weather.

Having a healthy engine to start with is important since if one cylinder has less compression than the others carb tuning becomes a nightmare as it is with any two stroke, check the compression before doing the swap to avoid the hassle of wasting time and money on a worn out engine.

[Jacob]

OK, so how do I modify the rev limiter????

[WillFish4Fun]

Anyone ever swap a 75 powerhead for a 90 powerhead? A thrown rod punched holes in both sides of my 75 block, so now am looking for replacement powerhead (rebuilt @ $2800+) or complete used powerhead assembly with carbs (drop in place) for same price or less. There are some 90's available and wondered if this is something Harry homeowner (me) could pull off successfully in my driveway? Is it worth upgrading or just get a drop in 75 and be done?
Thanks, Will