THIS PAPER is divided in two parts. The first part is devoted to engine tests made on gasolines having different gum contents. The tests made indicate the quantity of gum that can be tolerated in a motor fuel before it will noticeably affect engine operation. It was found that only the actually dissolved or preformed gum in a gasoline at the time of use directly affects engine operation. The gum usually collects on the hot parts of the intake system, particularly the inlet valve. Photographs showing the condition of the inlet valve and cylinder-head of the test engine are reproduced. Also, in the first part, the gum-forming tendency of fuels that are stored for some time prior to use is discussed.
The second part of the paper, consisting of the appendix, takes up the causes and methods of testing gasoline for gum. It is shown that gum is chiefly a result of oxidation, which is apparently of the autocatalytic type; and furthermore that, once a gasoline has started to form gum, it usually continues as long as air or oxygen is present.
Since no existing gum tests were found that were satisfactory, a thorough study of methods was undertaken with the object of obtaining a reliable test for gum. Two gum tests were devised, one being an oxidation test designed to show the stability of a gasoline toward gum formation, particularly when stored, and the other a modified steam-evaporation test to show actual gum in solution at the time the test is made.