| Re: [GMCnet] Vapor Lock (yes, again) [message #88342] |
Sun, 13 June 2010 07:07 |
Gary Casey
Messages: 448
Registered: September 2009
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Previous post:
What is the solution to the spewing problem?
Chuck
I'm not sure there is an easy answer to that question. It is caused by high vapor pressure fuel being pumped into a too-small float bowl with not enough surface area to allow the vapors to escape. One can remove the spacer in the float bowl to increase the surface area. That has been done, but I can't vouch for the effectiveness. The carburetor can be cooled somewhat by making sure the air inlet is cool. Fuel vapor pressure goes down as it is "aged" in the tank. In the old days at the GM proving ground the rule was to do vapor lock and spewing tests within 30 minutes of fueling the vehicle - otherwise enough of the light ends in the fuel will evaporate. So, the worst case for spewing is to drive fast on a hot day, pull in for fuel for 15 minutes and then do a full throttle acceleration from the stop. Lots of fresh fuel going into an overheated carb. The only real thing to do is reduce the carburetor temperature. Blocking the heat riser will do
a lot. Ventilating the engine compartment will help a little. Timing your fuel stops so you have lunch at the same time might help the most :-). Second best thing might be to change to another carburetor type that has a larger float bowl - like a Holley. Another thing - fresh fuel will be cooler but the vapor pressure will be higher, so the cool fuel prevents vapor lock, but the fresh fuel can cause it. Yet another thing - the change to lower vapor pressure summer fuel blends doesn't happen at the same time all over - vapor lock is always worse during the first warm days of summer because of that.
Remember, when it comes to fuel, there is technically no such thing as "boiling." Fuel consists of many different compounds, all with different boiling points. Fuel even contains some dissolved amount of propane and butane, which by themselves are vapors at room temp. If the fuel is heated these constituents will come out of solution, making vapor and causing vapor lock. That's why pressure will build up in a fuel tank - the fuel is not boiling, but the lights ends are coming out of solution. Point is, these light ends won't simply condense and go back into the fuel, but they have to re-dissolve in the fuel - a time-consuming process. Yes, the point about fuel "boiling" at 90 degrees is correct - the light ends will start to escape at low temperatures.
Why do some people have chronic trouble and some don't? It could be that there is a leak in the fuel system that allows air in, but doesn't let fuel leak out. Once there is a little bit of air in the system the diaphragm pump gets much less efficient and stirs up the fuel, and that could lead to enough vapor to stop fuel flow. True? I don't know.
Gary
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