Fuel consumption twin 1987 yamaha 150's

[lime4x4]

Anyone have an idea how much fuel twin 1987 150 2 strokes burns? I know fuel gauges are a never accurate. But it seems everytime i take the boat to the local lake it uses a 1/4 tank of fuel according to the gauge. I generally run the boat at 4K for about 15 min then i'm putting along at 1K for another 15 min. I've tried filling the tank and refilling after using it but that doesn't seem to hold out. The other weekend i filled the boat with fuel. I stopped when fuel started spitting out the vent. Went to the local lake when i was done i filled the boat back up and it took 21 gallons of fuel. I know the older 2 strokes aren't exactly fuel effeint but using over 20 gallons of fuel in 30 minutes seems kinda high. I have 30 foot cabin cruiser with a single carbed 350 vortec engine that gets better fuel economy

 

[GreatWhite23]

I can tell you I average a little over 1 mpg with twin 250 OX66. I had twin 225 on a gulfstream the fuel was slightly better. I thought the 150 got about 2 mpg. When was the last tune up?? My sweet spot seems around 3800-4000. The oldies where good but not known for good fuel consumption

 

[lime4x4]

Both fuel/water filters were replaced. both fuel filter screens at engines were cleaned. Replaced all sparkplugs.Did all this this past spring. Engines run great. The only thing i haven't done yet is rebuild all 6 carbs

 

[seasick]

Anyone have an idea how much fuel twin 1987 150 2 strokes burns? I know fuel gauges are a never accurate. But it seems everytime i take the boat to the local lake it uses a 1/4 tank of fuel according to the gauge. I generally run the boat at 4K for about 15 min then i'm putting along at 1K for another 15 min. I've tried filling the tank and refilling after using it but that doesn't seem to hold out. The other weekend i filled the boat with fuel. I stopped when fuel started spitting out the vent. Went to the local lake when i was done i filled the boat back up and it took 21 gallons of fuel. I know the older 2 strokes aren't exactly fuel effeint but using over 20 gallons of fuel in 30 minutes seems kinda high. I have 30 foot cabin cruiser with a single carbed 350 vortec engine that gets better fuel economy

Those motors usually will use 10 to 12 GPH at those revs, so 20-25 GPH for two motors. !5 minutes should burn off about 5 gallons. Consumption at 1000 revs should be much lower.
How does your exhaust look? Black perhaps? I suspect that you are running rich on one or both motors. Are your motors carbed?
Also look at the water for a fuel sheen just in case you are dumping fuel out the exhaust.

If you are running real rich, your plugs should be fairly black with carbon after a relatively short usage period. You might want to pull the plugs and see how they look and if all look similar.
If the motor is injected ( I dont think the 150s were injected in those years) there are other things to check.

The funny thing is that it is possible that the fuel consumption is way up at low revs especially if carbed and the needle valve doesn't close or the float is bad or mis-adusted. If you troll you will burn rich but if you rev up to return you can burn off the carbon from the plugs and not know what is going on. You should see excessive smoke at low speeds if that is the case.

 

[lime4x4]

Motors are carbed no sheen noticed.I'm sure the carbs haven't been touched. At idle both engines run a little rough the port engine has a habit of stalling occ after prologed ideling but i think that's do to it idling lower then it should. Come winter i'm gona go thru all the carbs. Also for 2-strokes there is no smoke when running them. The only time they smoke is when there first started

 

[seasick]

Motors are carbed no sheen noticed.I'm sure the carbs haven't been touched. At idle both engines run a little rough the port engine has a habit of stalling occ after prologed ideling but i think that's do to it idling lower then it should. Come winter i'm gona go thru all the carbs. Also for 2-strokes there is no smoke when running them. The only time they smoke is when there first started

You might have a bad diaphragm in one or more of the of the low pressure pumps. That could explain the stalling and more importantly, depending on which diaphragm is leaking, you might be dumping fuel into the crankcase and subsequently the exhaust. If this is the case, by unbolting the pumps ( unbolt the pump from the crankcase and not the screws that hold the pumps together), pump the primer bulb hard and see if fuel leaks out the back of the pump. If it does, the pump is bad.

 

[lime4x4]

I will check them out. Getting ready to go on vacation next week. Just curious thou if the low pressure fuel pumps were leaking would't i have starting and power issues?

 

[seasick]

I will check them out. Getting ready to go on vacation next week. Just curious thou if the low pressure fuel pumps were leaking would't i have starting and power issues?

No necessarily. It depends on how much is leaking and if it only leaks under higher pressure. The pumps are plumbed in series so even if one is bad, you can still get fuel flow to the carbs, just not sufficient for higher loads.
Also check that the screws that hold the pieces of the pump together are tight. They tend to loosen up but if they do, fuel usually leaks down into the cowling and causes a sheen on the water.

You could have a bad float or needle valve that doesn't completely close when pressure and fuel flow are high. That will cause a rich mixture and normally a black exhaust smoke but at speed it is really hard to see the smoke.

 

[grady23]

I have a pair of SW Series II 150's on my Gulfstream. Fuel usage is usually about 14-15 GAl Per Hr at 3400 cruise. When I troll on one engine, it's about 1.7 - 2GPH. If you decide to rebuild the carbs, Check the floats for proper height and also make sure they are not leaking which will cause them to sink and give you very high fuel levels. When I rebuilt mine a few years ago, I discovered the floats on some were off as much as 3MM from the factory -- They had never been apart until I did them.

 

[lime4x4]

Got to work on the boat a little today. After pumping the gas balls up i could smell gas. Found the screws holding the pumps together were loose. tighten them all up and unbolted the pumps from the crank case and no fuel was coming out the hole. This winter i'm gonna rebuild all 6 carbs along with replacing all the floats and rebuild all 4 low pressure fuel pumps

 

[seasick]

Gladd you found the loose screws. As I said, that is fairly common.

I do not recommend rebuilding the LP pumps. It is a better bet to bite the bullet and replace them. I prefer the OEM pumps as opposed to the after market ones.

 

[Clockwork]

I had the same motor but 1990 as a single on a 20ft overnighter. got 2mpg no matter what. i think you can expect about half that given you have two of them.