If no "coffee grounds" in the timing port area, I would check a few other things before tearing it off.
Bubbles obviously = air. UNLESS the IP shaft seals are bad (which would lead to diesel fuel in the crankcase), there's gotta be another explanation for air getting in.
How full is the fuel tank? Obviously if it is near full, the fuel level is above the lift pump and the fuel filters and any leak that would admit air SHOULD be leaking after shutdown, either externally, or internally (fuel lift pump leaking into crankcase).
That being said, I have a 4020 I don't put many hours on a year, so there's not much fuel in the tank because I try to use up the summer fuel, then put #1 in for the winter, and the other way around.
For YEARS, if it sat a while, I'd have to use the manual primer, and bleed air out at the filters.
A couple of years ago, I finally replaced the lift pump (even though there was no evidence of diesel fuel leaking into the crankcase).
Problem solved.
I would take the return fitting/valve out of the pump cover, and check it for debris (there's a glass check ball in there), then temporarily install a length of clear tubing between the fitting and the return line and start it up and check for bubbles while it's running, indicating air is getting in the primary fuel system.
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Today's Featured Article - Self-Starting Tractor - McCormick 10-20 - by Francis J. Robinson. I remember it very clearly even though it was almost 50 years ago and I was only about seven years old. My Dad had purchased a new Ford-Ferguson tractor and basic set of implements when he started farming just about the beginning of W.W.II. It was considered a pretty nice outfit for their 70 acre central Indiana grain and livestock farm. Dad spent several years during the war testing aircraft engines at Allison Engineering in Indianapolis working seven days a week on a twelve hour night sh
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