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what year is this Mercury

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Beagleman

Regular Contributor
Good day guys. A computer illiterate friend of mine asked me if I could identify his 90 HP Mercury, 6 cylinder motor, serial number 618 7554. As well, what carburetor fits it? Also, is there any way this can be modified to burn less gas? After all, she's a gas guzzler.

All the best.

Robby
 
It appears to be an 82 model.---Boats do need a lot of fuel to move.--A 6 cylinder motor does have a lot of " internal drag " and that 90 hp is not efficient.-In good condition it should be a smooth runner and outlast many newer motors !-------Check the fuel pump and carburetors for leaks.
 
Burn less gas? Yep, keep the throttle at no more than 2/3's-3/4's when you are trying to get somewhere in a hurry.

A 90 horse at full throttle, whether it's an old one like this, a brand new 4 stroke or a brand new direct injected 2 stroke will burn about 9 gallons per hour.

There is a "sweet spot" on all motors where they have reached maximum timing advance but there is still throttle left. At that point they burn about 50-60% of max fuel with minimal reduction in speed compared to their full capability.

Spending about a few minutes of your time you can find that sweet spot without the motor running - can be done in the driveway.

The process is easy. Remove the hood (cowl) of the motor and have someone operate the control throttle/shifter. As they advance the throttle you observe the link rod that is connected to the trigger (under the flywheel). At a point you will notice that it no longer moves when you hit a certain point advancing the throttle (but the linkage on the carbs continues to open the butterflies further). If you put a mark on the controller, opposite the lever when the trigger link stops advancing, that is your maximum timing advance.

Pushing the throttle past that mark does give you a little more speed, but also puts the motor in "gas guzzling mode".

Other ways to save gas are to install a "kicker" if you do alot of trolling or very slow speed maneuvers (like in canals etc that have no wake zones, speed restrictions, whatever).

A 9.9 pushing a big rig, even at full throttle making 7 or 8 mph will always burn less gas than a 90 (or higher) horse throttled way back.
 
I had a 115 of the last year produced/sold.....88/89. It had the ported piston and whatever else went with Mercs Direct Charge engine. It was on a 17+ foot Ranger Fisherman which is a solidly built (pretty heavy) bass boat pad hull with a walleye layout. I had a 10 gallon built in tank in that boat and never went out and ran out of gas. I marveled at how fuel efficient that engine was. I'd play at WOT from time to time, but usually I was running around 2/3 throttle. That boat, on the pad could do 35 easily with low hull drag at that point.

I always suspected that part of the reason was the way I had it propped. At WOT trimmed correctly, the engine ran a solid 6000 rpm. Ran it for 7 years and sold it to my SIL. Being 500rpm over upper limit told me that I had the 2 stroker lightly loaded and the DC porting didn't hurt as Merc came out with that to compete with the OMC loopers while the OMC patent was still in effect and we all know how fuel efficient loopers are as compared to cross flow engines. I don't know when they DC'd those towers. Not sure on the 82 year model. But I am convinced that lightly loading a 2 stroker gets you better engine hygiene, better all around performance, and less fuel consumption for a given set of conditions.

So, I'd look for something to change that could improve your economy and letting the ponies out of that engine to me would be the first place to start....unload it somewhat from where you are running. What's your WOT rpms?

Totally agree with galamb and running at cruising speed where he mentioned, rather than WOT....if your boat stays well into the plane position.....minimum hull drag. If cutting back there puts you down in the water where you make a large wake and all, you will offset any fuel improvement in the engine, by boat drag. Years ago Merc used the term "Back Drag Carburetion". You pushed the throttle to the wide open position, let the boat come up to speed, then cut the throttle 1/3 which put the setup right where galamb said. Just a bunch of sales hype on naming it (opinion) but on performance it worked. Nothing different or special about the carbs (opinion).....like what kind of tricks can you play with a venturi, bowl of gas, and a butterfly?

HTH,
Mark
 
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