Onan 4000 Backfiring through Carb

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hockeymonkey

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I am having a problem getting my Onan 4000 started. I have checked fuel pressure and the solenoid. They seem to be working fine. I have pulled the carb. out and it looks clean. I have checked the spark and its good. my next step is too change all the filters and move on. However, is there anything else I can check to see if I can get this running? the generator turns over fine for a couple of tries and then seems to get restricted somehow and backfires exhaust out through the carb. This is gas generator on a winnebego chalet. any ideas sure would help. I don't want to pull the generator out unless I have to.
 
First thing that comes to mind  is a partially stuck open intake valve. I would pull valve cover and make sure valves are free. Pull sparkplug and check compression ( thumb over the hole is adequate). Second thing is a sheared key on flywheel causing spark when intake valve is open. Very rare on generators IMO. Usually reserved for mowers and such.
A stuck valve can be freed usually fairly easily.
 
Assuming everything else has been checked out..take a good look at the little switch that is adjustable for Altitude..it has markings for sea level all the way up to the top of the Rockies!..you may have inadvertently moved this if you have been cleaning or servicing the Gen Set..it's easy done and you could be running so rich it is choking itself..  good luck
 
Backfiring thru the carb is often from a lean mixture.  Pull the carb again and be sure that all passages are clean.  Some have a welch plug that may have debris behind it.  Remove the main jet and any mid-range removable jets and carefully examine them for dirt.  I use a can of spray carb cleaner, that has a straw, and blast thru all openings.  If you have a welch plug, measure it and then find a new one at an auto parts store before drilling and prying out the old one.

In addition to the carb, a lean condition can come from vacuum leaks at the intake manifold or faulty hoses.
 
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