QuestionQUESTION: my 1965 spit will back fire through the carbs. it is running very rich. burns excessive fuel. just done.thoughts?
ANSWER: Hi Dan,
How did you determine that it is very rich and under what circumstance does it back fire into the carbs?
Howard
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QUESTION: Howard,
it burns allot of fuel, the exhaust smells of fuel. when it backfires from the carbs, the backfire is fuel and air. it will backfire when the throttle is pushed. it will also back fire from the carbs when moving and a small amount of throttle is applied. i adjusted the points gap and it seems to help. the problem is still happening. the point gap will change, is this the bushing?
Thanks
ANSWER: Dan, Burning a lot of fuel does not mean it is rich and neither does the smell of fuel in the exhaust. Backfire into the carbs on a throttle opening is a sign of a lean mixture. A rich mixture will not make an engine backfire into the carbs.
Rather than trying to think what might be wrong you need to test to arrive at what is really wrong. These tests are a MUST and the first is a compression test. Remove all the plugs and be sure the throttle is held open and that you make at least 4 or 5 revolutions on each cylinder. You should see 135 to 180 PSI on each cylinder with little difference between cylinders. (Compression gauges are not expensive and can be purchased at most any auto parts stores even at Wall Mart.)
Points gap needs to be .015" on a high point of the cam lobe. You can wiggle the shaft to see if the bearings are loose. Check the gap on each of the four lobes to see if the shaft is true and not bent. Then set the ignition timing exactly to factory specs.
What kind of oil are you using in the top pot of the carburetors? I recommend engine oil. Unscrew the cap and lift it up and push it back down and if you feel hydraulic resistance, there is enough oil in there. If it is empty fill it to about 1/4" below the top of the inner tube not the outer rim.
Then warm the engine up to operating temp and set the idle at specs or just a little under 900 RPM and lift the spring loaded pin under the lip of the pot and note the reaction. If the engine speeds up and stays fast the other carb is too rich. If the engine dies, either the butterflies are not in sync or the other carb is too lean. Check both carbs. When they are correct, the engine will slow down and sound like an old two cylinder engine running slow.
Howard
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QUESTION: Howard, the compression test shows each cylinder has between 155 and 165 lbs per. The timing is correct and the points gap is also set to spec. i am using the SU oil from Victoria British. the spark plugs were corroded with a thick hard white material that was somewhat blistered. pushing the pin under the carb caused the engine to stall.
AnswerThe plugs, the back fire into the carbs and the pin lift all indicate a lean mixture. If both carbs cause the engine to die when the pins are lifted you need to richen the jets by turning the adjusting nuts down by three flats at a time until you get an RPM increase when the pin is lifted, then lean the adjusting nut back up until it will respond correctly. It is always best to go too rich and inch your way back to correct. A lean engine will at high RPM burn a hole in a piston quicker than a NY minute. It will also cause high RPM detonation which also burns holes in pistons. The deposits on your plugs is probably melted aluminum. You should clean or replace the plugs before adjusting the carbs.
Let me know,
Howard
