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Old 07-27-2020, 03:34 PM   #9
30 Closed Cab PU
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Join Date: Aug 2018
Location: Wisconsin
Posts: 2,291
Default Re: Backfire in overrun mode (load change)

Do you have a different known good carb you could swap in? Another cause of running lean is a worn throttle plate shaft/bushing. The quickest way to troubleshoot carb issues is trying another carb.



A little trick used by some is if the valves are sealing poorly, due to carbon, is to use a misting bottle with water and spray water into the carb air intake with the motor running.


If you suspect valves like you indicated using a borescope is a great idea, also might try performing a leak down/compression test.


If you suspect sticky valves try running Marvel Mystery oil in the gas. You may be able to see sticky valves by hand cranking the motor, look at each cylinder separately with a borescope. Otherwise removal of the valve access cover might be needed.


Just a few additional ideas that do not require head removal.
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