Mercury Outboard(s) various questions!

[pyro225]

Hi Guys,

I'm new to this forum and have a few questions!

First of all let me say what I have,

Currently i own 2 Glastron ssv-151s - 1 is the standard mass produced metallic rust one from the Us - the other a much rarer blue one built here in the uk.

The blue one has a 90hp 6cylinder merc brown band (84 i think) tower of power

The orange one has an 80hp 4 cylinder grey band - last of the 4cylinder 80s. (10 and 15 inch prop one for lake one for river)

I also have a 50hp 4cylinder blueband merc as well 1980.

Right so this is what i'm trying to do - i live on a river. Most sections are between 4-7mph and most people only have 25hp 4strokes on my stretch. However go a bit further and you can open it up hence why i want the power.

I will be doing the 4-7mph about 65% of the time and flat out 35% of the time.

I am trying to work out which of my ob's is going to be the best for economy when going 4-7mph as that will be on my everyday boat (assuming its the 50 or the 80 as they are both 4cyls but thinking the 80 would have less load maybe?)

Next question is about the obs themselves as the 80 and 90 have a couple of problems.

Starting easily with the 80 - she misses when shes cold and below 2000rpm - plugs and compression seem good. Over 2000rpm shes as smooth as glass all the way... any ideas?

Now my big headache, the 90... so she is about 10x harder to start than the 80, she idles like a bag of nails, when you bring her down from speed to idle she just dies and she has a misfire. I have done the compression test all 6 are good i took the plugs out and cylinders 1,2,4,5,6 lookas expected but cylinder 3 is wet with water - does this sound like headgasket/water jacket and is this likely my problem?

Ok end of questions and thanks in advance!

 

[Bob_VT]

The inline 6 motors lover to run at wide open throttle I would opt for a lower power (lower maintenance)motor if one was available. The 80 probably needs a good carb cleaning and the low speed passages are probably blocked a bit. That causes rough running below 2000 rpm. Clean the carbs completely

 

[pyro225]

Cheers Bob - so the i6 on my fast boat but which of the other two are the lesser of the two evil son fuel or will it be negligible?

 

[Chris1956]

Pyro, The IL6 motors should idle much better than the IL4 motors. Something is therefore wrong with your IL6.

On another subject, if you put the 50HP motor on one of those boats, how happy will you be at the 35% WOT that you are expecting? The difference in fuel usage between the IL6 and IL4 motors, at low speed is minimal. Maybe you save a gallon or two over the course of a year?

My advice, leave the motors as mounted, and clean the carbs on the IL6, and do a link and synch. If you are getting water on #3 only, it is likely a inner water jacket, or exhaust baffle issue. If the motor is a fresh water motor, it should be fixable, as the bolts will usually be removeable.

 

[Silvertip]

The 50 would seem to be the obvious choice for the 4 - 7 MPH speed range as that is hull speed and just a slight bit of throttle will produce that speed.

 

[pyro225]

Ok - thanks for all your responses I'm very impressed! so with regard to fuel is the hp (50/80) irrelevant with the 4 vs 6? are all 2 stroke 80/90s this thirsty - i do about $16 going about a 5-6mile round trip (although over here its equivalent $8 per us gallon) - is that about right for the 80 at 2-3000rpm?
I have found the 6 will use a couple of $ more as it is are you saying it shouldnt?
Also i'm using a smaller prop 10" on the 80 to obtain river speeds where as the 50 can use its full size prop - so they do about the same speed the 80 just has more grunt - is one of these set-ups particularly better/worse than the other?

Again sorry for being the question man!

Cheers

 

[Chris1956]

So you are getting 2.5-3.0 MPG. That is not bad IMHO. Obviously the 50HP motor will use a bit less fuel than the 80HP motor, and the 80HP motor less than the 90HP motor, but I would think the difference will be small.

I am not sure why you are underpropping the 80HP motor. If you have to do that to be able to travel at the speeds you want, I would guess the 50HP motor makes the most sense. You will obviously give up and real speed and performance, with the 50HP motor.

I guess what combo you choose should be based upon what you like more. 2X the cruising at 7 MPH vs 1X the cruising at 35 MPH. From your description, it doesn't appear that you can have both, with a single engine/prop combo.

 

[pyro225]

cheers chris - it came underpropped - with two props one for speed and one fro river. to navigate into locks it needs to be able to go as slow as 2mph without bashing around the big prop couldnt do that hence why its underpropped. i may have imagined this but i thought i read that between 3-4000rpm they are more economical than between 1-3000rpm so economy may be better with the smaller prop...or have i got that all wrong?

 

[pyro225]

also between the 50 and 90 are we talking 1 or 2 mpg or 0.1-0.2 mpg would you think>?

 

[wired247]

It takes X amount of power to push Y amount of water out of the way using Z amount of gas and air no matter how fast the motor is spinning. The shape of the hull and weight matter more than what you are pushing it with. It just happens that outboard 2 stroke motors live longer if run in their target RPM range possibly due to better fuel and oil distribution at higher air/fuel flow rates. In any case for slow speed its probably better on the motor to be twisting the motor faster than slower. Once you get on plane all calculations of fuel use vs speed are off. Between the 50 and the 90 its going to be tenths of a MPG difference at slow speeds.

 

[Silvertip]

Engines (any engine) gets best MPG at just off idle. Between off idle and about 2500 rpm the boat is still off-plane and the engine is beginning to require more throttle so economy is not good at that rpm. Once the boat gets on plane, economy gets better again because speed picks up with very little additional throttle. If you check Performance Bulletins on the Yamaha web site you can see how rpm and hull speed affect fuel economy.

 

[pyro225]

Ok so your saying between the underpropped 80 and the correctly propped 50 the fuel usage difference wont be a great deal different?

Final question (i hope) on this matter before i go on to fixing them...

My friend had a 9.9 hp evinrude 2 stroke on his bow rider while his ob was in the shop - he was easily getting 6-7mpg while going 7mph (yea it did top out at about 10mph) so he's getting well over double what i am at the same speed - at what hp is the crossover to get that much more economy kind of thing?

Thanks

 

[pyro225]

Ok - (strangely) I hadn't ever checked the prop pitch on the 90.. its a 21P which (corrdct me if im wrong) is probably way out for what im doing. I have some how acquired a range of props in good condition as they are interchangeable between 80/90 - I have a 10P 15P 19P and 21P. For what im doing would the 15P be enough on the 90 would you think? might a 21P be a reason its stalling at low rpm?

 

[wired247]

My boat had a 90 Johnson on it for most of its life running a 19 pitch stainless OMC prop. It rarely went over 40 MPH although it would go 50 when it needed to. It ran great at right off idle .Dragged this thing around for 30 years like that.

If I were you I'd put a cheap Solas Titan 4 17 pitch 4 blade stainless prop on it and never look at it again.

 

[Rubixone]

So I noticed that no one spoke about your cylinder with water in it, best test is to scope it with a camera and see what side of the piston is washing, most likely your exhaust if so you probably need two or three gaskets to reseal your exhaust baffles.

 

[pyro225]

ok wired247 - are non of my existing props ok like the 15 or maybe the 19?

Also will get it off and strip it down.

 

[pyro225]

Sorry to dredge this up - but i have been chatting to a few people who claim they know about outboards, and done some googling... they reckon that a evinrude v4 90 will get signifactly better gph than my merc i6 90 - any evidence to prove/disprove as id be open to swapping!