1974 Johnson V4 Gas Hog

[scm93]

I have a 1974 Johnson - 115 hp - V4 on a 16' Glastron GT-160. I'm very pleased with both the boat on motor performance, except for the amount of gas used. If I run at full throttle (approx. 5,000 rpm) I can go through 5-6 gallons of fuel in about 30 minutes. If I spend most of my time at idle, the same 5-6 gallons of fuel will last about an hour. I am running Champion QL77JC4 plugs. The electrode bases are a nice tan color. I have not checked the High Speed Jets, but assume they are standard. My question: Is this normal gas consumption or is this excessive. If excessive, what can I do to improve it without running the risk of leaning out the motor and burning it up.
Thanks!

 

[wilde1j]

Re: 1974 Johnson V4 Gas Hog

Welcome to the world of boating. Completely normal consumption and nothing you can do other than run ~ 4000 RPM if you want better consumption. Your motor will use ~ 12gph @WOT. BTW, your motor should be setup to run ~ 5800 RPM @ WOT.

 

[ezeke]

Re: 1974 Johnson V4 Gas Hog

As stated, you are lugging the engine if 5000 RPM is the best you can do at WOT. However, you won't get much better than 11.5 gallons per hour at WOT on any carbureted 115HP outboard.

 

[billybaja]

Re: 1974 Johnson V4 Gas Hog

just curious how do you raise the rpm`s mine is doing the same thing i can only get up to 5200 rpm on my 1984 140 johnson and that is with useing the trim.. It run`s really well i have a 17 pitch prop on it now. I do have a 19 pitch and I was thinking maybe that would change the rpm`s?? Thanks Bill

 

[wilde1j]

Re: 1974 Johnson V4 Gas Hog

Raise the motor (AV plate should be uncovered with water @ WOT), lower pitch prop 13" would up RPM ~ 400.

 

[ezeke]

Re: 1974 Johnson V4 Gas Hog

All other things being equal, lowering the pitch raises the RPM. You get a better hole shot, but may loose a little top end.

Use the calculator to see where you are and what it will take to change for the better. The 1974 115 has a 2.0 gear ratio for the calculator. http://go-fast.com/Prop_Slip_Calculator.htm

Try 10% for the slip.

 

[billybaja]

Re: 1974 Johnson V4 Gas Hog

Raise the motor (AV plate should be uncovered with water @ WOT), lower pitch prop 13" would up RPM ~ 400.

Hey Thanks I checked the chart that suggests motor height on transom ,I took a straight edge on the bottom of the boat keel (it is a baja so pretty flat 3" wide surface) and went out to the motor and the staright edge is about 3/4 of an inch below the av plate on the motor. It is down all the way on the transom 20" leg on motor and is where it was placed from the factory in 1982.. Their is plenty of room to go up but is that the answer?? Thanks Bill

 

[wilde1j]

Re: 1974 Johnson V4 Gas Hog

Is the AV plate buried when running WOT? Measuring where it is relative to the hull is only a start ... there no way to accurately predict where a specific boat/motor combination will land.

If you can come up w/o excessive cavitation in hard turns and the AV plate is still under water, keep raising the motor a hole at a time.

Getting the motor height right will increase top speed, lower fuel use and increase WOT RPM's somewhat.

So, check where the AV plate is at WOT first!

 

[billybaja]

Re: 1974 Johnson V4 Gas Hog

Ok thanks, it is on the trailer today next time in water hopefully tom. I will be able to check it out at wot.. I had found that chart in one of the threads figured that would be a good starting point.. Thanks Bill

 

[ezeke]

Re: 1974 Johnson V4 Gas Hog

Even with or slightly above the bottom of the boat is a very good place to start, especially if you don't have power tilt and trim.

 

[emdsapmgr]

Re: 1974 Johnson V4 Gas Hog

Next time you are running the boat, check the position of the antiventilation plate in relation to the water streaming from under the bottom of the boat. Run it full speed and at full trim (if that engine has it.) The water should pass just under the antiventilation plate. That is a "normal" setting. Any lower mounting and the antiventilation plate will be submerged: you will have excess drag, excess water spray and poor performance/economy. A stainless prop may permit you to move the engine one notch higher than the "normal" setting. The higher the engine is mounted, the less drag of the lower unit in the water. If you do experiment with mounting the engine higher, keep your eye on the water pressure to the powerhead.

 

[kenmyfam]

Re: 1974 Johnson V4 Gas Hog

Absolutely normal fuel consumption.

 

[countvlad]

Re: 1974 Johnson V4 Gas Hog

Absolutely normal fuel consumption.

haha yep.... when you are at WOT you burned tons of fuel.... in order to improve you consumption you should start cruising at a lower speed.... you could install a fuel metering system and see what is your motor consuming in real time... you'll be amazed to find out the consumption numbers at different rpms....

btw did you just purchase this boat/motor? if yes, to get an idea wouldn't hurt to rebuild the carburetors, new water pump, spark plugs and wires, change lower unit oil....

 

[kitebuz]

Re: 1974 Johnson V4 Gas Hog

heck, I can burn 15 gph at 5600rpm on each of my '89 V4 120's, and almost 10 gph at 4500rpm...