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The tach needle is just a bad tach, and not related.
Popping back through the carb indicates what is called a lean fuel condition. Lean means not enough fuel, and it can mean a few different things.
1. Bad gas/old gas/water in the fuel
2. Engine is starving for fuel because of a restriction in the fuel system, or a weak fuel pump.
3. A leaking intake valve is pushing compression pressure back into the intake.
4. Carb is clogged up and now flowing enough fuel.
That being said, have you done any testing on your own yet? Run the engine off of another fuel source? How old is the gas?
Well.... if the firing order was wrong, or the plug wires were so bad to the point they were cross firing that would also do it. But other than that jumping tach would just be a fluke. You can get a spray bottle and mist down the plug wires and see if you can induce a misfire that way .
When was the last time the engine ran correctly? Has it been sitting a long time?
A sticky valve wouldn't do it.
Here is what I would do if you brought it to me.
1. I would mist the plug wires down with water and see if I could get it to misfire or not.
If no change
2. I would hook up my own tank to the engine, and run it off of that and see if it clears up. Because the problem is intermittent, that would rule out a bad pump (they either work or they don't). If the engine runs better on my tank then I know there is either a fuel issue, or there is a blockage issue in the tank.
3. I would go ahead and rebuild the carb.
Plug wires are easy enough to rule in or out. But after that, if it is intermittently, that would point towards fuel being the issue. Be it bad gas, a blockage in the tank, or a clogged carb. Those would all cause a lean fuel condition to engine, which gives you the popping. Does that all make sense to you?
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A misfire in general can cause the tach to do crazy things. But mist the plug wires and see what happens, as that would actually explain both symptoms.
Was there anything else?
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