Vapor lock

[Greg S]

Yes, was hoping to tie into the crossover line.

Thank you!

[Oldun]

Have read this thread with interest as (I think) I've had a few issues over the years esp in the desert / Nevada and Arizona; but never in NZ (simply never gets hot enough IMO).

I am gonna adopt the octane booster suggestion for sure, because.

May I ask a potentially-dumb question - are EFI / fuel injected engines NOT prone to vapor lock compared to carb'd ones? Thanks in advance.

[Greg S]

It depends upon the specific vehicle, and the fuel that is used. With the right temperature and pressure conditions, fuel will boil - fuel injected or not.

There definitely are some EFI injected engines that vapor lock. Some variables that can lead to this are: Fuel pressure within the lines, if recirculation lines are part of the system, fuel type, fuel blend, fuel RVP, location of the fuel pump (in tank/outside of tank), temperature conditions at the inlet of the fuel pump, outside air temperature, altitude density, and probably a handful other things.

I am not well versed on EFI, but I believe that most EFI systems have been designed to eliminate the vapor lock issue.

[mmaupin]

Yes, was hoping to tie into the crossover line.
Thank you!

Greg,

I do have a concern about doing the recirc thing.

IMO most of the vapor lock issues that happens to carbureted BH riders occurs between the fuel filter and the suction side of the fuel pump, as that is the lowest pressure area in the entire fuel system. If a fuel filter develops any D/P on a hot day, vaporization is likely.

Adding the recirc will increase flow through the fuel filter, increasing D/P. So, that is kind of a trade off for keeping the fuel cooler via the recirc.

On my SB, there is not allot of room to add a larger filter under the fuel tank. Next time I have the seat off, I may look to relocate the filter nearer the fuel pump.

Someone earlier had mentioned putting a pancake fan to give some cooling in the hot areas; not a bad idea.

Before I retired, I worked in industrial instrumentation and controls. So, for me, it was fun to size an orifice once again.

[Greg S]

Hi Mark,

It is very apparent that you were involved in something relating to this. Not many people converse in this manner. Remember the old 3 to 15 psi and 4 to 20 mA controls?

Back to the topic. It is the vapor lock condition #2, that leads me to believe that a recirculation line may be of some benefit. This condition was when the bike was operating at some velocity below 25 mph, while in a certain minimum temperature or above, and for a semi-extended length of time. If the velocity were increased, under the same temperature and time conditions, the bike would not vapor lock. My thoughts are, that the mass flow rate of the fuel was so low through the fuel lines, at 25 mph, that the fuel was able to heat up to the temperature that was required to boil, due to the relative proximity of the fuel line, to the top of the engine and cylinder heads. When the fuel reached the inlet of the fuel pump, the reduced pressure at this point, would initiate a vapor lock. When operated above 25 mph, this second condition was not an issue. I believe that the fuel, at these higher velocities, is not staying in one place in the fuel line, long enough to heat up and cause problems.

If the above conclusion is true, then a recirculation line may help to eliminate, or reduce, the problem. The fuel would have an increased mass flow rate through the fuel line to the pump inlet, and the heat transferred through the fuel line to the fuel, at any delta T, would not raise the temperature of the fuel as much as if it were at a lower mass flow rate. Even though the friction loss of the filter and fuel line may vary as the square of the velocity, the change in the velocity could be designed, with your orifice sizing, to be minimal, and just enough to eliminate the problem. In other words, simulating fuel flow rate for velocities over 25 mph, while actually traveling at a velocity of under 25 mph, may resolve the problem. I believe that the orifice would help with condition #1 as well, and for the same reason.

As for the total Q for the fuel system as a whole, I don't think that it will matter that much, unless the fuel tank is almost empty. I think that a high localized temperature at the inlet to the fuel pump is the culprit - at least in my specific case.

Any thoughts?

[mmaupin]

Greg,

That is some good stuff! I certainly do remember the 4-20 and 3-15 IO ranges. When I began, 10-50ma was common.

Regarding the recirc, go ahead and give it a try. As you indicate, it just may provide that little bit of extra cooling that is needed.

It is always amazing how hard it is to pinpoint exactly where the problem lies. So many things can affect vaporization; temperature/pressure/gasoline quality. Other factors include how full the tank is and pressure drop[s] in the fuel system.

On my previous bike a 99 SB, I had vapor lock issues with it. I insulated all of the fuel lines and never had another problem with it.

Keep us posted how the recirc works out.

[Ron Radulski]

It depends upon the specific vehicle, and the fuel that is used. With the right temperature and pressure conditions, fuel will boil - fuel injected or not.

There definitely are some EFI injected engines that vapor lock. Some variables that can lead to this are: Fuel pressure within the lines, if recirculation lines are part of the system, fuel type, fuel blend, fuel RVP, location of the fuel pump (in tank/outside of tank), temperature conditions at the inlet of the fuel pump, outside air temperature, altitude density, and probably a handful other things.

I am not well versed on EFI, but I believe that most EFI systems have been designed to eliminate the vapor lock issue.

Greg, that last paragraph in your tread, IMHO, sums it all up to what I have originally said. Barry Radu has had good results with small and big block conversions from carb to EFI. I know, you are old school when it comes to wanting to hear the Q-Jet secondaries open up. So with that being said I guess you and Mark can keep going on and on with theories like Sheldon Cooper in the "The Big Bang Theory" LOL. You two guys are very outstanding in your knowledge though, I will admit. Keep up the good input!