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A friend of mine has a 89 Big Boss 4x6 with a 250 2-stroke that had the threads that hold the flywheel nut on striiped. I re-threaded the crank and installed everything back together, but when I tried to start it nothing would happen. I moved the timing plate and it fired up easily. I statred it a couple of times with no problem. Abouth an hour later I tried to stat it and nothing happened. I checked the spark and its getting a good, strong spark. Next 1 switched ignition coils and still would not start. Next I peplace the excitor coil, no luck. Next I replaced the CDI unit, still no luck. Next I disconnnected the black wire from the CDI, still nothing, but still getting a good spark. Next I checked, the neutral switch and the throttle position switch, both checked ok. Next I switched carbs just to make sure, still good spark, no start. Battery is good. Next I checked the timing with a timing light, and could not see the timing makes through the hole in the case. I also replaced the flywheel key, no luck. Using the timing light, the tiimng marks are way off. If I move the timing plate, no matter where I turn the tiimng plate, the timing still stays way off. Next for the hell of it, I removed the flywhell key and moved the flywheel about 180 degrees off where it should be and tightened it down good and and checked the timing again. Still the same, no matter where I move the flyweel, the tiimng stays way off. I have checked the wiring for any breaks and can't find any. The only thing I haven't changed yet is the flywheel itself, it looks good with no broken magnets (that I can see). Is there anyway to check the flywheel itself, I don't want to spend about $150 if that isn't the problem. Is there anything that I am missing?
 

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First off, welcome to the board!
 

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I have the exact same problem but can’t get any advice on where to go with it or what to look for????
 

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First - verify the flywheel timing

Use a soda straw inserted in the spark plug hole and rotate the engine to TDC - the straw gives you a visual indication of the position of the piston. I myself use a dial indicator in a spark plug hole adapter to find true TDC.

With the piston at TDC, check the alignment of the TDC mark - as long as the spark occurs near TDC the engine can start and run. The actual correct spark timing is about 25 degrees before TDC at 3000 RPM, but the advance is accomplished electronically. Static timing is about 5 degrees BTDC.

Depending on your engine and model year, this page specifies the timing.
136880

Start with the basics - how much compression does it have?

If you have sufficient compression, spark, spark at the right time, fuel and air in the right amounts then it will run - the law of physics govern what happens.
 

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Do you have to remove muffler before checking timing with timing light? I can not see into the indicator hole or able to get a straight on shot with timing light with muffler on.....
 

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Why do you want to check the timing with a light? It's not that far off if it runs and can only be adjusted a few degrees. I never checked the timing with a light. I just align the marks during assembly and forget about it - just like the factory does. Otherwise, yes remove the exhaust to check the marks using a light.
 

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I was just curious... trying to absorb new info. thanks :cool:
 

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Now, if you are building the engine for performance, then timing is a bit more critical to extract the most performance the engine is capable of, but for utility use, the timing is not critical and timing advance as engine speed increases is electronic. The CDI sets the advance for engine RPM.

Retarded spark timing (after top dead center) provides easier starting and advanced timing (before top dead center) provides better power and economy. How much advance an engine design may tolerate or need depends on engine design and fuel available. High octane fuel will allow for more advance while low octane mandates less advance. The slower burning high octane fuel needs to start burning before the piston reaches TDC in order for the pressure from combustion to reach optimum pressure as the piston starts moving away from TDC. Lower octane fuel cannot be ignited as early and pressure builds faster, so the engine typically has a shorter stroke. Ideally pressure will increase until about 160* ATDC (after TDC) then as combustion nears completion, pressure will start to decline. Combustion should be complete between 170 and 190* ATDC as the exhaust valve starts to open at about 160* BTDC. The degrees of crankshaft rotation in relation to valve opening and closing is dependent on the purpose the engine is designed for. Performance engines will open and close the valves earlier and later (stay open longer) and the lift of the valve plays a part too. Utility engines will have more moderate valve timing so the engine will produce power over a wider range of RPM. Some special purpose engines will have very conservative valve lift and timing and may run within a very narrow range of RPM for long periods of running at a single speed. Certain pump engines may run at a single RPM and be governed to operate plus or minus 100 RPM of the set speed.

Most ATV engines are designed as utility engines offering usable power over a wide RPM range. Certain ATV's may have high performance engines designed to produce power rapidly over a narrow RPM range. High performance engines do not idle well, can be hard to start and may require special fuel. Typically the high perf engine will require frequent maintenance and part replacement. In part due to high crankshaft speed and high temperature operation. The utility engine will typically run for many years with minimal maintenance and relatively few routine part replacements.

I digress..........

If your engine is hard to start or has spark knock under normal operating conditions using the recommend fuel, adjusting the timing may alleviate the undesirable characteristic.
 
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