QuestionQUESTION: My son and I rebuilt his engine. When we start it, it back fires thru the carb when running. We assumed that we got the timing one tooth off. With the head off and rotating the crank the intake valve starts opening before the piston hits tdc. We pulled the cover off the bottom to check the dots on the gears. They appear to line up. If we jump the cam one tooth back, the intake valve starts to open at tdc. Are we headed in the right direction. it would be nice to find out whats wrong before we buy another gasket set. The engine broke a rod and that was the reason for the rebuild. The crank was turned by our local NAPA shop for an oversize rod bearing and checked for trueness.
ANSWER: Did you check the flywheel key? What is the engine type number.
Normally, lining up the dots is all the mechanical timing. Did you set the valve clearance?
The intake may open just slightly before TDC. You may be seeing the compression release. When the engine starts the RPM basically negates the valve opening. As you follow the strokes, cna you see or feel valve overlap. This is when both valves are just slightly open at TDC. It is hard to see the over lap but you can rotate both valves by hand at the overlap point.
Let me know about the flywheel key and valve gap.
Eric
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QUESTION: The engine type is 1001-E1. We haven't pulled the flywheel yet but we're confident that the key was in good shape when it was assembled. The valve clearance was not set. With the mechanical timing intentionally set 1 tooth off, I can rotate both valves with the piston at tdc (exhaust stroke). The timing mark on the crank is dead center of the tooth. The timing mark on the cam is half way between the tooth and valley. Is it possible that it got miss marked during manufacturing? When viewing the marks rotated together they appear to be about off about a tooth and a half. With the marks lined up, the exhaust valve is closed with the piston still down in the bore about 3/8" and the intake starts to open with the piston still 1/4" from the top of the stroke and is open a minimum of a 1/16" at tdc.
AnswerIt is unlikey the timeing marks are off...Briggs has a very good quality control process and if the engine ran before then the marks are right.
The only time I saw mis-marks was intentially set at one of the factory courses. Normally, the cam mark appear between a couple of teeth.
Do you have a manual or do you know how to check the valve gap? Is there a washer or starter cup under the flywheel nut.
To check the valves, assemlbe the bottom end of the engine, rotate the crank to TDC on the COMPRESSION stroke, then rotate 1/4 inch past TDC on the COMPRESSION stroke. Use a feeler gauge to check the valve stem to valve tapped clearance.
The valve gap should be:
Intake: .005-.007
Exhause: .009-.011
Check the flywheel key again. I have seen several keys shear after rebuilding.
Let me know about the valves.
Eric
