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08 Honda 420 timing issue?

3428 Views 9 Replies 5 Participants Last post by  shadetree
I rebuilt this bike from mud junk that didn’t run. Now it will start but blows out the throttle body. Valves are adjusted tdc on compression stoke ok and cam is set correctly under front cover with a new timing chain.
Is it safe to say the cam is bad?
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Anything is possible on a mud bike! Sometimes thinking can be your worse enemy too... :)

Its rarely productive to make unfounded assumptions, so... does it pass the compression tests, wet & dry? Is the exhaust system clear? Piston not installed backwards? Valves straight and true with good match & seal to valve seats? etc. I've seen a lot of bad cams/lifters over the years, so I can verify as fact that when lobes wear flat, you'll see standoff above the intake.

You can easily measure the valve lift (at the valves) using a dial indicator. Add a degree wheel and you can verify TDC location (check for a bent connecting rod) and whether each cam lobe is good/bad, and verify valve opening & closing event timings, along with lobe center and lobe separation angle.
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