GM 235 6 cylinder

[348tripower]

I have a 51 Chevy in the shop with this 6 in it. It has electronic ignition and the 1 barrel carb.  
The carb was changed out last summer. This will start and run but after it runs for a bit it loads up. I have to take the air cleaner off to keep it running. Then after some time of running I have to put it back on as it seems to lean out too much.
I am looking for vacuum leaks and intake leaks but haven't found any yet. The owner said that previous to the carb change he had to run with the choke partially on at all times.
Anybody have any suggestions on this?

[Fat Cat]

have you considered that there may be a fuel delivery issue that it leanning things out at times?

If this is a possible problem then if you set the carb for correct fuel delivery in a lean condition when it goes back to full delivery it would be loading up. I would look at bad fuel filters, kinked fuel lines, objects in the tank, and bad venting on the tank.

[wayne petty]

fat cat is correct ... test the fuel system for delivery..

on a cold engine...  disable the ignition system..

disconnect the fuel line to the carb..... extend the fuel line with some fuel hose and use a 2 or 3 liter clean clear plastic bottle..

have a friend or use a remote starter button to crank the engine for 15 seconds... while you observe the pulses of fuel from the end of the hose into the bottle.

each pulse should be full and equal to other pulses.. not diminishing...

sediment in the fuel system will usually foul the check valves in the fuel pump and prevent proper pumping action .. this limits the flow and can also cause things like vapor lock... as the pressure pushed out of the fuel pump does not hold .. it surges back into the pump housing and fails to allow enough pressure to force it thru the needle and seat...

two things.. add a decent fuel filter just before the fuel pump... and run this test again.  i use a inline fuel injection filters when possible.. they have finer elements to catch the smaller particles.. sometimes you can flush the sediment out with this full flow test if you have a fuel filter installed directly before the pump..

lastly.. drop some magnets into the fuel tank.. or stick a big speaker magnet to the bottom of the fuel tank.. the magnets will attract all the Red dUST to them.  (read capitals)

somebody put up some great info on the rochester model b carb...

http://forums.classictrucks.com/70/7315770/1947-1959-chevy-trucks/rebuilding-rochester-model-b-carburetors/

[phat46]

Maybe a combination of the choke not coming off fast enough, then a vac leak or a lean setting. Of course if you have a manual choke that shoots my theory all to heck.  

[Arnold]

My 66 GMC 960 with a 292 did something similar.
I suspected it was either a leaking inlet..or a binding or soggy float.
SOoo I rebuilt the carb. Same. I should have checked the pump pressure FIRST :oops:
Pressure too high. I bought a new pump..same.
Then I compared the new pump to the old one.They initially appeared the same. On closer inspection..the old pump..the tang that operated it had
a small hole in it. YA that is because on the original pump arm there was a rivetted piece THERE that controlled(limited) the travel of the arm thus limting the pump pressure.
Found the worn off rivetted piece
In the OIL PAN

[Crosley.In.AZ]

ahhhyup,

on my 235 with dual one barrels, I had to install a fuel pressure regulator to lower pressure and smoothed out the pulse from the pump too.  

This back in the 1990's ...  pressure regulaltor you ol duffers probably seen.. little round dial knob on top to set pressure.  Chromed up pot metal unit.  Worked well, looked cheap as hell

[348tripower]

My 66 GMC 960 with a 292 did something similar.
I suspected it was either a leaking inlet..or a binding or soggy float.
SOoo I rebuilt the carb. Same. I should have checked the pump pressure FIRST :oops:
Pressure too high. I bought a new pump..same.
Then I compared the new pump to the old one.They initially appeared the same. On closer inspection..the old pump..the tang that operated it had
a small hole in it. YA that is because on the original pump arm there was a rivetted piece THERE that controlled(limited) the travel of the arm thus limting the pump pressure.
Found the worn off rivetted piece
In the OIL PAN

The fuel pressure thing. I did some searches this morning and aftermarket pumps seem to be a problem. I will have this back probably next week and will start the testing process. One other problem is the fuel line runs too close to the front of the engine causing vapor lock when it gets hot. I am going to move it.

[wayne petty]

ahhhyup,

on my 235 with dual one barrels, I had to install a fuel pressure regulator to lower pressure and smoothed out the pulse from the pump too.  

This back in the 1990's ...  pressure regulaltor you ol duffers probably seen.. little round dial knob on top to set pressure.  Chromed up pot metal unit.  Worked well, looked cheap as hell

if anybody wants to install one of those cheep chrome twist knob fuel pressure regulators.. please install it in an area where if it decides to leak.. that it won't drip across any hot engine part or ignition wire/cap/distributer.. or can produce a puddle on the intake...  yea.. i know its a 235...  you want the flame jobs on the outside of the paint.. not coming out from under the hood..


these are at HF... but every parts store sells a version...if you don't have one..

http://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html

these go from 30 inches of vacuum to 15 pounds of pressure.. so you can also check the suction of the fuel pump.. to make sure it holds suction.. and holds pressure on the carb side...

i have just had and heard terrible issues with the small regulators.. usually when they are installed over the top of the engine where the HEAT SOAK after shut off bakes them till they fail....