During the course of the experimental work included in this investigation, it was found that certain adjustments and conditions had a major effect on the performance of the tested engine, whereas certain others had a very minor effect. In order of decreasing importance, the conditions which affected the engine's operation were: (1) restriction of the air supply to the carburetor; (2) restriction of the flow of exhaust products from the engine; (3) a change in the carburetor float level; (4) a variation of the ignition timing within five degrees of that recommended; (5) the use of spark plugs which had been run a larger mileage than that recommended by the manufacturer; and (6) a change of the distributor breaker point gap within 0.007" limits of that recommended by the manufacturer. Restricting the air supply to the engine had a very large adverse effect on the economy of the engine, and a smaller effect on the power. Restriction of the exhaust line had approximately the same effect on the power and economy of the engine. Raising the carburetor float level decreased economy characteristics markedly, and power characteristics to a much smaller extent. Lowering the carburetor float level increased both the power and economy of the engine. A five degree advance in the ignition timing resulted in a slight increase in power and economy in the low speed range, this increase being at the expense of violent detonation. The performance in the high speed range was adversely affected. Retarding the spark timing five degrees had a detrimental effect on both power and economy, although this effect was comparatively small. The use of spark plugs beyond the recommended mileage limit showed a decrease in power and economy when this effect was not counteracted by the increase in the size of the gap due to normal usage. Spark plugs which had been cleaned at frequent intervals showed up much better on test than did those which had not. A change in the spacing of the distributor breaker points through a 0.014” range showed an effect of less than three percent when the natural effect of this change on the ignition timing was corrected. Things such as air cleaner and muffler restrictions, changes in the carburetor float level, and fouled spark plugs caused enough effect on the engine's performance to indicate the advisability of checking these things often. Other conditions which would normally be checked in a minor engine tune-up do not have an appreciable effect on the performance of the engine, providing that their effect is not sufficient to make the engine miss. / M.S.