Carburetor Percolation

Percolation can happen when your engine gets hot, then you turn the engine off and the fuel boils over, sometimes over the top of the carburetor, or more often, simply evaporates. Back in the day we called this vapor lock, which simply meant that the fuel turned to vapor before getting to the engine.

This has been a more common problem these days because our gas has a lower boiling point than it used to. Yup you guessed it, ethanol.

This makes it hard to start the next time. You have to crank and crank, or pour a bit of gas down the carburetor to get the engine started.
What can you do:
  • One of the best ways, but a lot of work, is to run a 3rd line from the fuel pump back to the gas tank. This extra line will help cool the fuel, allowing the fuel pump to pump gas instead of vapor. There are some fuel filters out there that include a return port. There are also return regulators available.
  • Put heat wrap around the fuel line in the engine compartment.
  • Put a spacer under the carburetor mount. These are hard to find so you will probably have to make one. Yup, a mill would be handy about now. And you will probably need to put longer studs in the manifold to accommodate the extra height.
  • Instead of a spacer add extra mounting gaskets. A couple of 1/32 thick gaskets isn’t going to do much. Hopefully thicker gaskets will be available for your engine.
  • Add a helper electric fuel pump. Just before startup, you run the electric pump for a few seconds, then turn it off. Use only a low pressure pump. 7 lbs max. You may end up also adding a regulator because you don’t want the pump to overrun your mechanical pump. Most 1 & 2 bbl carburetor run 4-4.5 lbs. Bigger carburetors might handle 5-6 lbs.
  • Cool the engine with a cooler thermostat, bigger fan, or bigger radiator.
  • Try different brands of gas. Different octanes won’t help you here.
  • Use non ethanol gas if you can find it.
  • Remember when we used to put wooden clothes pins on the fuel line? The clothes pins act like a heat sync, drawing heat away from the fuel line and my wife says the wooden clothes pins are still available at the dollar store.
Anyhow, the basic answer here is to cool your engine and/or fuel.

Is Ethanol Good for my Car?

Classic cars are not the best match for today’s ethanol enhanced gasoline. If you use it in many of the older models, be prepared for earlier component failures. Most automotive parts wear out over time, but ethanol speeds up the deterioration of many, especially those made of rubber or cork.

Older cars with rubber fuel lines are especially vulnerable. A failed rubber fuel line can result in an engine fire. Who needs that? But the fuel line constitutes the most easily addressed area of concern. Other components that are frequently damaged by ethanol include carburetor float valves, rubber accelerator pumps, pump diaphragms and die-cast carburetor bodies, rubber fuel pump diaphragms, galvanized fuel lines and fuel tanks, and gaskets made of either cork or rubber.

Because of its deleterious effects, ethanol rated as E85 (which is only 15 percent gasoline and 85 percent ethanol) should be avoided at all costs. But even the smaller 10 percent ethanol rated E10 will deteriorate components far faster than pure gasoline. Ethanol also attracts moisture, so use a quality fuel stabilizer to help control water retention.

Of course, if you can avoid ethanol altogether, that is your best option. Today there are not many places where you can get ethanol-free gas, but it behooves you to look for them. If you are lucky enough to have a local supplier, seize the opportunity for your car’s sake.

If ethanol cannot be avoided, then make regular fuel system inspections your new standard operating procedure. If your car does not have to retain its stock condition, it is a good idea to install an inline fuel filter so inspection of your gas is easier. Monitor this and the fuel bowl for traces of cork or rubber. If you see either, you already have trouble.

Also inspect the carburetor, both internally and externally. Look for signs of fuel leaks or residue of cork or rubber in the fuel. Check for loosened bolts from weakening gaskets. Check the needle valve. Its replacement may be required much more frequently than in the old days.

Unless the car is driven quite infrequently, regular monthly inspections of your fuel system should be performed and rubber components should be considered for replacement annually.

Of course, if your classic does not have to remain stock, your options increase dramatically. There are now stainless steel fuel tanks available, as well as stainless fuel lines, and they are completely trouble-free in relation to ethanol. There are also replacement carburetors that are impervious to ethanol problems, so if you drive a car that does not have to remain original, these replacements should solve your fuel-related problems for good.

Ethanol and classic cars are not good bedfellows. But a careful regular inspection regimen and timely replacement of key parts can make the relationship at least livable.