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I believe it’s because it can potentially bleed off too much compression and actually make it more difficult to start
+1
I bored out the outlet of a decomp for easier starting for @bradb123 562xp I built a number of years ago and he even had cold start issues at times. I put a brand new unadulterated decomp valve in its place and cured the issue. So there is credence to that theory as fact I believe.
Otherwise I can only offer folks my 5 series experience...
I also restart my 550 with using the primer and up on high idle without the decomp in the summer time when it's warm and it does the trick for me.
(You can't flood a 5 series by using the primer it just flows fuel through the carb and goes back to the tank it came from.)
I also notice the new mkII's are void of any decomp valve whatsoever...