OX66 and fuel vapor pressure

[family affair]

Over the years I have experienced 2 entirely different large displacement outboards exhibit the same performance issues in the heat of the summer - lousy low rpm performance. Initially I thought it was simply the heat because in the spring and fall both engines ran significantly better at low speeds.

A few years ago I read an article going into the details of the difference between summer and winter blend fuels. The main difference is the vapor pressure that reduces the volatility of the fuel in the summer.

Back to the engine. Last year I kept close tabs on our fill-ups with respect to the change from winter to summer blend. We started off the season with a full tank. The OX66 ran like a sewing machine across the entire rpm range even when the water and air temps were up. We ran the tank almost empty and filled up with summer blend BP from a busy station (fresh fuel). Dropped the boat back in the water and within 15 minutes it idled like crap! I strongly suspect the lower fuel volatility is the issue on an EFI 2-stroke. Once that tank was replaced with winter blend, we had a happy Ox again.

Anyone out there with an OX66 that does the opposite? This engine is so fickle that I don't dare mess with what works well for half the season, but if a proven tweak to something can give me better results throughout the year, I might be willing to make some adjustments.

Any other suggestions are welcome.
Thanks!

 

[seasick]

I am curious how you can know which blend you are getting. For example, even if a gas station gets a load of 'summer' blend, their tanks could contain a fair amount of 'winter' blend.
My experience with my OX motor is quite the opposite. Summer (warm) performance at idle is much better than fall (cold) and that is often with the same fuel.
I also suspect that if your motor was running OK and then you filled up and it ran poorly, you got bad gas.
The SX motor can operate poorly for a lot of reasons especially at low idle. Weak low pressure pumps are a common cause. They tend to work better after warming up.
Poorly or defective O2 sensor, or temp sensor can cause issues. Injectors can operate poorly at low or high revs ( clogged makes high revs an issue while sticking open open can make low revs an issue).

The temperature of the fuel is a factor. Surprisingly, the temp of gas in underground tanks is fairly uniform across the country but the temp of fuel in the boat tank can vary significantly.
I am not saying that you don't pose a possible reason for the performance issues but rather I feel there are so many factors and components on those motors that can cause similar issues it is difficult to say what the cause is.

One thing you may want to make a note of is the idle speed when you motor is started and how long that speed takes to slow down for different outside temps (and water temps). If a cold motor idles low when started, there is either a choke issue or for OXs something that is misleading the ECU.
The water temp is a big factor since the rate that the motor warms up depends greatly on the cooling water temp ( and the t-stats).
AN OX motor with a clogged O2 sensor will run rich. When cold, that is OK, sort of like the 'choke' is on. As the motor warms, the mixture is too rich and poor performance will occur. Likewise if the t-stats are bad (stuck open, the motor at low speed and load may never reach optimal temps and also run poorly.
I have had a random cold weather low idle issue on my motor for 10 years and have yet to figure out what it is. Perhaps it's normal. When cold starting, I just add some extra throttle ( in neutral) and let the motor warm up a bit longer. Once warm and run a minute at load, all is perfect.

 

[family affair]

I am curious how you can know which blend you are getting. For example, even if a gas station gets a load of 'summer' blend, their tanks could contain a fair amount of 'winter' blend.

I think it's safe to say that a large busy station has turned over their fuel by the end of June. Again, we filled up more than once on summer blend with no change. Back to winter blend, all is well. After 5+ years of the same issue every year like clockwork, I think the fuel is likely to be the culprit. It's no mystery that large displacement outboards don't flow air well at low rpm's creating poorly mixed fuel/air charge. DI 2-strokes don't exhibit the same problem because the fuel is shot directly into the cylinder providing a much improved fuel charge.

I'm reasonably confident the engine doesn't have component issues. All components you have mentioned have been replaced minus the T-stats. My only other observed issue in question is that we can't get the 3.0 mpg Grady got when they did their test. We can only get 2.7 at best, but figure it is due to running E-10, fresh water, and a prop that is not factory perfect. We've had a 5-star Yamaha tech go over the engine thoroughly and all checked out. When we mentioned the rough low speed condition, he seemed to think it was typical.

The change in vapor pressure with fuels is intended to aid cold start conditions in the winter by allowing the fuel to atomize more thoroughly. This would certainly help an EFI or carbed 2-stroke. In the summer the vapor pressure is increased to prevent vapor locking carbed engines. In my mind this could hurt some engines that need all the help they can get to get air and fuel to mix well like mine. If I'm not mistaken your OX is the smaller displacement and might not behave the same as the 225/250 OX?

I wish I knew a way to decrease the vapor pressure of summer fuel, and my problems might go away. I've also wondered if ditching the steel reeds for carbon fiber might help low speed performance. I don't feel like throwing $300+ at it to find out I'm wrong!

 

[seasick]

I am curious how you can know which blend you are getting. For example, even if a gas station gets a load of 'summer' blend, their tanks could contain a fair amount of 'winter' blend.

I think it's safe to say that a large busy station has turned over their fuel by the end of June. Again, we filled up more than once on summer blend with no change. Back to winter blend, all is well. After 5+ years of the same issue every year like clockwork, I think the fuel is likely to be the culprit. It's no mystery that large displacement outboards don't flow air well at low rpm's creating poorly mixed fuel/air charge. DI 2-strokes don't exhibit the same problem because the fuel is shot directly into the cylinder providing a much improved fuel charge.

I'm reasonably confident the engine doesn't have component issues. All components you have mentioned have been replaced minus the T-stats. My only other observed issue in question is that we can't get the 3.0 mpg Grady got when they did their test. We can only get 2.7 at best, but figure it is due to running E-10, fresh water, and a prop that is not factory perfect. We've had a 5-star Yamaha tech go over the engine thoroughly and all checked out. When we mentioned the rough low speed condition, he seemed to think it was typical.

The change in vapor pressure with fuels is intended to aid cold start conditions in the winter by allowing the fuel to atomize more thoroughly. This would certainly help an EFI or carbed 2-stroke. In the summer the vapor pressure is increased to prevent vapor locking carbed engines. In my mind this could hurt some engines that need all the help they can get to get air and fuel to mix well like mine. If I'm not mistaken your OX is the smaller displacement and might not behave the same as the 225/250 OX?

I wish I knew a way to decrease the vapor pressure of summer fuel, and my problems might go away. I've also wondered if ditching the steel reeds for carbon fiber might help low speed performance. I don't feel like throwing $300+ at it to find out I'm wrong!

You seem to have done your research and are correct in the assumption that my motor is smaller. It is 150 horse. When you mentioned the reeds, I cringed. First of all that is an area I feel uncomfortable attacking. I also was not aware they there were carbon fiber replacements. I will have to do some research just for the heck of it. You got my interest up so I will also look into the vapor pressure aspects.
Thanks for the mental stimulation. Keep us posted on your findings.