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Marvel carb problem: 1928 Buick master on road under load ( uphill) will pop back thru dust collector ( air cleaner ) carb rebuilt . New plugs ,points,condenser, set valves, check timing . Cleaned screen in gas bowl and screen on inlet line to vacuum tank . Ran ok before changed carb .( carb float seat not working .)   Help.  Thanks.  Rich 

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I have a 1929 Master so I am familiar with your issue. Even though this condition could be many things , there are a few things one can do to try to isolate the cause.

The wonderful Marvel carburetors can be a pain. I usually do not choose the easy route in life. I do prefer the challenge of originality in this area.

My Marvel sometimes tends to run lean and pop back through the carburetor under acceleration or heavy load . When warmed up and under load , I can correct this by slightly and progressively closing the choke until full power can be made.  If this corrects the problem it proves one thing. It proves that the ignition is working well enough to make power. It also proves that fuel delivery is adequate to make power. The issues that remain causing this lean condition are probably blocked jets , carb venture sticking / being limited in travel or the venture spring not being correct or incorrectly adjusted. I still sometimes have issues getting my carburetor to work correctly. Sometimes the enrichment circuit that the plunger is lifted out of can be a problem too. If choking of the carburetor under load has no affect on performance then ignition and fuel delivery can be an issue. One last thing is to always do your tuning with fuel that is fresh as possible.

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It is most likely starving for gas, but you also did not mention if you checked or replaced the heat riser tube.

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Yes, as Mark stated the heat riser tube can cause total chaos in the tuning and performance department. The heat riser repair and elimination of exhaust gas to it , is a little story of its own.

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Rich, 

     Since you are new, I am reposting my marvel carb and heat riser information.  I hope it will help you with understanding that there is actually a fair amount of work required to correct the concerns of heat riser pinholes, exhaust valve positioning, and venturi swelling.   The engine should also read 13-14 mm hg vacuum and steady if that helps you with trouble shooting.     Hugh

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