two stroke carb tuning

A two stroke engine is not difficult to tune. As with four stroke engine you should first check your carburetors. First let the vehicle warm up and while sitting at idle slowly twist the throttle increasing the rpm’s. Come back to idle and repeat the process a little faster each time. If you are having problems with your carbs it should eventually act up. Sometimes you have to operate it and try the accelerating to put some load on the vehicle. Eventually it should become unresponsive with a slow flat “buuuwwwaaa” sound and then take off. This would be a caused by a lean condition and needs more fuel. If the vehicle stumbles, burbles and stutters when you gas the throttle it is rich and needs less fuel. Always be careful when tuning a two stroke engine to make sure it doesn’t operate in a lean condition or damage can occur to the under lubricated engine. Although this is not the best method for checking carbs, it may help you out. You can check the spark plugs by instantly stopping the motor during the experienced poor running. Remove the spark plugs. First check to see that the plugs look the same on the electrode. If one plug is completely different than the other, check for air leaks or one carburetor not working correctly.

 

Also check for an ignition problem if one spark plug is black and fouled showing that it was not firing. If the plugs look even or if it is a single cylinder engine, check the coloring on the electrode. If it’s black the mixture is too rich or your plugs are fouled out and may be the problem. If the spark plugs look white or light grey the mixture is lean. The ideal color is a light brown or mid grey color. The absolute best way to check the mixture is by looking on the top of the piston through the spark plug hole. This will require a light, white l.e.d. penlights work great. Rotate the piston to the bottom of the bore you are looking down. Look on the sides of the piston that are in line with the crankshaft. The top of the piston should be black. The sides of the piston crown should have little “pockets” on them. You will be able to see the shiny aluminum in the pockets. These pockets are caused from cool gas flowing through the transfer ports and across that spot on the piston. The pockets should extend inward about a quarter of an inch and be appx. half an inch wide, depending on bore size. This will be an ideal fuel mixture and is very accurate. The farther the pocket extends inward the richer the fuel mixture is. If the pocket is very small it’s lean. If you see a grey ring about a quarter of the way in from the edge all the way around, be worried. The grey ring (of death) means the mixture is extremely lean and is about to burn a hole in your piston if not corrected. Depending when a hesitation is felt right away or higher rpm’s will matter. Low range is controlled by a pilot or low end jet, usually in the bottom of the carb. Some carbs that have a throttle slide will have a mid range needle that extends through the center hole of the carb. When you snap the throttle quickly on of three things happen. In a diaphragm carb there is no mid range needle, so the diaphragm’s fuel needle’s spring controls the acceleration fuel. Stiffer springs mean less fuel dumped in at acceleration time and vise-versa. Variable venturi carbs use a needle for progression and sometimes a second slide to control how fast the air enters. The best system has a separate accelerator pump on the side of the carb. Use of jets, plungers, cams and rods can change the amount of fuel delivered by these. The high rpm’s are controlled by the main jet. The main jet is almost always located in the bottom center of the carburetor. It will usually be close to the pilot jet but will be twice the size. This is the most important jet because it is used at full power. Be careful to keep this rich enough! Jets are small round brass pieces that are numbered. The higher the number the larger the hole, and the more gas it will deliver. Two stroke engines can last a long time if properly tuned, and are not that difficult to work on.