QuestionQUESTION: Tractor is only running on three cylinders. Number four cylinder is no firing. Compression check all cylinders have between 110 to 115 psi. I am getting spark checked with in line tester. Pulled valve cover off and all the valves are working. Changed spark plugs,points,condenser,cap,rotor. Checked timing according to the book it is all right. When running i tried to move it to see if it made any difference,but it did not. The only thing i noticed is that there is a gap between the starter and bell housing. All the bolts are tight. If you turn it over by hand you can hear a slight rubbing sound from the bell housing area. Some one said that the motor was rebuilt 5 years ago and it ran fine last fall. I am puzzled it has spark and compression Thanks Ron
ANSWER: I am not sure what type of inline spark tester you are using, but if it has a neon light that flashes, there still could be no spark at the spark plug. Try a different spark plug wire just to verify. I have had neon spark testers that lied to me. If it checks out OK and you have spark, comression, and fuel, then the intake manifold might have a vacuum leak by the number 4 cylinder. Try squirting some carburetor cleaner around the manifold gasket. If it starts firing on that cylinder, you have a vacuum leak. Not sure what to say about the starter. Either someone put it on wrong or it's the wrong starter?
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QUESTION: I had all ready tried another wire but did it again anyway. Sprayed carburetor cleaner around the intake and no difference at all. Put spark plug on wire and grounded it and I have spark. Switched plugs from 3 to 4 and no difference. Thanks Ron
Answer Spraying carburetor cleaner or gas around the intake manifold will work if there is a small leak and you can hear the engine start to fire on the dead cylinder as it sucks in the liquid while you are spraying it. If there is a large leak like a chunk broken out of the manifold gasket or a cracked manifold, then maybe it won't work. If the compression is almost the same on all 4 cylinders, you know the valves, cylinders and rings are good. You have verified that there is spark. The manifold distributes the fuel mixture to all the cylinders so that cylinder has to be getting fuel. It takes 4 things to make the engine fire. Fuel, air, compression, and spark. Since you know that it has all these things, the only logical explanation is that the fuel and air mixture is not correct for that cylinder. A vacuum leak will make the mixture too lean to burn. Possibly the leak is not at the gasket area. A crack in the manifold somewhere would cause the same problem. Inspect the manifold for cracks and use a mirror to check the hidden areas if necessary. Make sure all the manifold bolts or nuts are tight.
