How much fuel should my outboard be using?

[shark_bait_1]

I am new to the outboard but am not so new to the two stroke as in motor bikes and power tools. I just picked up a 1959 35hp Johnson (RD-21) in very good shape. It starts and appears to run ok but uses a whole bunch of fuel. I took the boat (14ft starcarft) out for its maiden voyage and every thing went fine until I ran out of gas. It really caught me off guard. Appoximately 4gal lasted less than an hour. I didn't even think to check the tank inside that time as I was expecting that this little motor should run all day few gallons of fuel. Anyhow, added fuel and it wouldn't start. Some more symptoms include fuel dribbling out the card after cranking and it being extremely hard to pull the rope start option. I tore the carb down and replaced the float, seat and valve and verifified the fuel pump is ok. Is this what I should expect for fuel usage or should I be seeing something a little better?

 

[RandyJ]

Re: How much fuel should my outboard be using?

Different opinions as to what is a small motor and what is a big one. I grew up thinking that anything over 15 hp was a pretty big motor. Our family boat was a 14 ft aluminum boat with a 30 hp johnson which I thought was a pretty big motor. Nowadays, motors are far more efficient compared to the h.p. I'd say that at WOT 4 gallons an hour would be really great efficiency. I've got a good running 1972 25 hp that runs close to 6 gallons at WOT... defiintely less than 4 mpg which is somewhere close to what a 4 cycle will get on water. At slower speeds you'll get far better mileage. I can fish for several days on a tank of gas but if I'm pulling tubes or skiiers or even riding a jet ski I'm chucking out some serious cash for gas to have fun with.

 

[lindy46]

Re: How much fuel should my outboard be using?

4 gallons per hour is about right at WOT. It'll troll all day (well, alot longer) on a tank of gas. If you have fuel dripping out the carb, you may have to adjust your float to close a little sooner.

 

[Chris1956]

Re: How much fuel should my outboard be using?

I think that fuel use is high. Normally a safe figure is 10% of the HP in Gal/hour at WOT.

So if you run a 35HP motor at full throttle, it will use about 3.5 Gal per hour. of course if you run at 1/2- 3/4 throttle, you will use significantly less fuel, say 2 gal/hour.

I would look for leaks in the fuel system. Also test the compression and inspect the spark plugs for fouling.

 

[shark_bait_1]

Re: How much fuel should my outboard be using?

Thanks to all for thier reply and time. They all make good sense and are inline wtih each other so I know I am getting the good skinny.

If I could ask for one more opinion. If the High Speed NV is open too far I assume it will run but it will run rich. Would this have any impact on the fuel consumption rate to any significance? Also, under the same settings would I expect to see a trail of rainbow as I troll around at idle.

 

[kfa4303]

Re: How much fuel should my outboard be using?

HI shark bait 1. You're right, if the hi speed needle is not correct you could run too rich (or lean) at start up and WOT. Here's a link on how to dial in your carb. While these old Big Tiwns like to drink gas, you should never see an oily "rainbow" behind your boat. If you do then you're either leaking fuel or you have a large amount of unburnt fuel coming out of the exhaust in which case you're probably only running on one cylinder. You'd be amazed how well these old motors will run on just one can. I would check the spark on both cylinders just to be sure. The spark should jump a 1/4" gap with a bright, blue, snapping spark. You probably want to use Champion J6C plugs too, if you aren't already. Here are a couple links that can help you dial in your carb and do all the routine maintenance for your Big Twin. Good luck.


http://forums.iboats.com/showthread.php?t=167352

http://www.duckworksmagazine.com/04/s/columns/max/24/index.cfm

 

[steelespike]

Re: How much fuel should my outboard be using?

Low speed valve could be be too rich too.
If the fuel pump has a hole in the diaphram it will leak gas into the crankcase
causing one cylinder to run rich.

 

[RandyJ]

Re: How much fuel should my outboard be using?

My best advice is to dig into the forums and threads and hang around... WELCOME to the forums! You will learn tons of good info here and become quite an accomplished shade tree outboard mechanic just reading the forums. The boat in my avatar is a 1983 Bayliner cuddy cabin with a volvo aq125a. I paid $200 for it on a galvanized trailer. It's still pretty ragged looking but runs great thanks to the guys on the forums and their help. I've resurrected several old outboards with the help of these guys. One in particular is an early 70's Mercury 110 (9.8 hp) that I am extremely proud of but since I've now got an electric start 25 hp johnson (1972) that runs like new (thanks to the guys here) I'm seriously contemplating selling the mercury. Several old boat motors around here need to find new homes! And I NEED SOME CASH!... so I can buy more parts to fix more old boats...

 

[shark_bait_1]

Re: How much fuel should my outboard be using?

I checked the diaphram and it was good. I didn't quite understand why/how the fule pump could cause it to run rich as it is not part of the case... or is it. The last few words of your comment cleared it up. It just kinda bellows it backup though the hose. Thank steelspike.

 

[shark_bait_1]

Re: How much fuel should my outboard be using?

One more thing... changed the plugs, dialed the HNV way down (2+ turns) and reset the float high to apply more pressure on the valve and its a new motor! Its a wow. And I found the gas leak. After it stopped spitting the fuel out the throat from the bad setup I notice the fuel bowl plug was leaking. Now the garage doesn't smell like gasoline either. A small amount of work did this motor a ton of good. Thanks to all for the help.

 

[kfa4303]

Re: How much fuel should my outboard be using?

sweet!!!!! glad you got her running. Happy boating!