16' SS w/70hp fuel consumption too high?

[Bill Foote]

I have had my ss for a while now and mounted on the back is a 70hp johnson 1988 model I also have a 19p prop. I have always thought that this motor was a fuel hog but having nothing to directly compare it to I never lost sleep on the issue. The engine runs perfect otherwise, starts easily, PLENTY of power and idles well also...

Then, my friend bought the exact same boat/motor combination. He has a 16' ss with a 1986 70hp johnson also a very good running motor. The first weekend after he bought his boat we went camping with the boats. We both ran the same exact distances side by side the whole weekend (about 14miles total). At the end of the trip as we were flushing our motors out I noticed his 6 gal fuel tank had about 2.5-3 gallons still in it while mine was nearly bone dry. We have the exact same fuel tanks and both were full at the beginning of the trip. Every thing is the same even oil ratio.

So, I am sure some of you are familiar with these motors any Ideas for this problem??

Imediatley I suspect that at some point someone cleaned the carbs in my motor with an instrument that removed metal from the jets making them larger. I am also suspicious of the primer solenoid, is it possible for it to leak fuel into the carb throats during normal engine operation(obviously the manual prime lever is not engaged)? Are "jet kits" available for outboards as they are commonly for motorcycles which basically include larger jets?

Thanks in advance

 

[Bob Gilvary]

Re: 16' SS w/70hp fuel consumption too high?

You didn't mention your friends prop size.
Not likely you have a fuel leak into your manifold. It wouldn't idle good.
Is your boat and his about the same weight, loaded?
Check out your hull bottom for a concave look near the transom, called a hook in the hull.
Is the weight in the two boats distributed the same.
Check compression in the two engines and compare.

 

[ezmobee]

Re: 16' SS w/70hp fuel consumption too high?

Also possible due to your motor setup that you're plowing a bit more than he is.

 

[Bill Foote]

Re: 16' SS w/70hp fuel consumption too high?

The only difference in weight distribution and load is that I had my girlfriend with me probably an additional 110lbs maybe? Other than that his prop is a 15p prop which would indicate if anything that he would get worse fuel econ. considering that mine is 19p. As far as the hull goes both are in very good shape and are not caved in near the transom as described.

What leads me to believe my motor may be running rich is that it emits blue exhaust bubbles in the wake both at idle-ish speed as well as up on a plane. And it will do this all day long, however the motor never misses a beat...

Also, my starcraft didn't come with this 70hp. When I bought my 16 SS I swapped its original 55hp johnson with this 70hp that I had on my previous boat(starline 160). I noticed about the same fuel consumption on the starline 160 as well...?

 

[Bill Foote]

Re: 16' SS w/70hp fuel consumption too high?

Also, what is the smallest HP motor people put on these 16 ss and still be worthwhile...? In other words maybe 5 people aboard, maybe tow a skier every once in a while. I have been seing a lot of late model 48spl's on craigslist lately would that be a good combination for a 16ss. The 70hp is nice and all but fuel consumption is a drag and I have only used all 70 ponies on a handfull of ocasions...

 

[ezmobee]

Re: 16' SS w/70hp fuel consumption too high?

A 50 is the smallest I'd go.

 

[dozerII]

Re: 16' SS w/70hp fuel consumption too high?

I think you are over proped and lugging the motor. What diameter and make is your prop? The first thing I would try is borrowing your buddies 15p. What speed and wot rpm are you running at?
Glen

 

[djpeters]

Re: 16' SS w/70hp fuel consumption too high?

I bet your 19 pitch prop is overworking your motor. Swap props with him once and run them again. My 22' SS with a 140 will go exactly 22 miles on 6 gallons cruising at 25mph and engine turning 4000rpm. I think your right in that it should do better.

 

[djpeters]

Re: 16' SS w/70hp fuel consumption too high?

Ha ha Dozer. Great minds think alike!

 

[Bill Foote]

Re: 16' SS w/70hp fuel consumption too high?

Great input guys, but I don't think a prop issue...

The 15X13.75 prop on my buddy's boat is actually my old prop. I ran that prop on both my starline 160 and my 16' SS(the prop that came with his boat/motor was a 17x14 from a 140hp and it has nearly zero clearance under the cavitation plate). Remember I had that 70hp on both of my boats and the island we went to is a place I frequent. Between my last boat and the 16 SS (both outfitted with the 70hp) I can't ever remember returning to the ramp with much left in my 6 gallon tank (about 14miles, always start with a full tank).

I went with a 19x13.25 as per recomendations from this forum. People w/ 16ss and 70ish hp motors recommend 19p for skiers and 21p for joy riders... But I don't have a tach on my 16 ss.

I am relieved to here that a 22' ss w/a 140hp can go so far on 6gal.

I hate to think about swapping motors because this 70hp has 130psi on all 3 cyl's. And at the beginning of summer I replaced all electrical: stator, timerbase, powerpack, coils & wires, and rectifier.... Maybe I ought to just buy new OEM jets for this model 70hp and also verify that these carbs are indeed the original carbs...

 

[ezmobee]

Re: 16' SS w/70hp fuel consumption too high?

I would still be interested to hear what your WOT RPM is.

 

[Bob Gilvary]

Re: 16' SS w/70hp fuel consumption too high?

Looks like most everything I mentioned, and then some, has been covered, exept you getting a tach and confirming it all.

You can pull the low speed adjustment needle out of one of your carbs and compare it to your buddy's low speed adjustment needle and see if there's a difference.

I ran into that in marine tech school with those 70 hp omc engines. Some of those adjustment needles were blunt and in some years they had a sharp point. Be sure of the position of the needle before removing them by turning them in until lightly seated, and counting the turns inward, then replace them by starting with them lightly seated and back it out the turns you counted before removing it.

If you find a difference in those needles then you likely have different carbs than your buddy, and if you check further you'll likely find different jet sizes than his.

This doesn't mean that you could use his carbs on your engine either. Fuel in the cylinder combustion chamber is also a cooling process, as are the spark plugs and water flow.