gmsports Posted October 4, 2013 Share Posted October 4, 2013 I have a 31 Cadillac that seems to be running out of fuel at higher RPM's and going up hills. Vacuum Fuel tank seems to be working properly. Good vacuum at idle approx 10"-13", great fuel flow to carb. Under load and higher RPM vacuum drops to 3"-4". Can drive around town shifting gears all day long but when cruising at 35-45 MPH can only go about 4 miles then cars dies (no fuel) wait 2-3 minutes and it starts up and runs great????? Any thoughts?? Have checked float in carb and thermo vent as well???Everything new/rebuilt.....Thank you for your time! Any info on "Aux vacuum pump" driven off cam? How many " of vacuum should it be producing??? Link to comment Share on other sites More sharing options...
Jim Bourque Posted October 5, 2013 Share Posted October 5, 2013 My 28 341A had a similar problem. Check the banjo bolt fuel connection where the fuel line enters the tank. You will probably have to remove your rear trunk rack and the panel that covers the fuel tank to get at it. Rust/crud collects there and reduces the flow to the vacuum fuel pump/tank. Make sure your gas cap vent is working. check for cracks in the vacuum fuel pump pot metal top, if you suspect any are there, seal with crazy glue as it is ethanol resistant. make sure the gasket under the pot metal top (fuel pump) is sealing. My engine vacuum pump pulls 6" at idle and 11" at higher RPM. You may also have a "check valve issue" in the vacuum tubing plumping. I had to install a 1/3 PSI one way check valve between the intake manifold and the vacuum junction block to get my car to climb the mountains here in Vermont. At extended full throttle climbs my fuel pump runs only on the engine vac pump. Good luck and remember, you just have to be smarter than the car!Jim Link to comment Share on other sites More sharing options...
Johan Boltendal Posted October 5, 2013 Share Posted October 5, 2013 If what you say is as it is, your vacuum tank is working ok, because if it mt's, it will never be filled standing and running at the first crank after 2-3 minutes. It's almost impossible to fill the tank cranking the engine. your battery will die first.Your problem should be vacuum tank to carb or carb related. Did you drill the holes in your rebuild carb with obviously a new valve seat? Johan Link to comment Share on other sites More sharing options...
carbking Posted October 5, 2013 Share Posted October 5, 2013 Someone called with this issue a few days ago, and I measured the fuel seat orifices at that time. The fuel seat orifice for the V-8 is SUBSTANTIALLY LARGER than the fuel orifice for the V-12 or V-16 as the 12 and 16 have 2 carburetors.If you have an 8 with the smaller fuel orifice, you probably are running the carburetor out of gas at speed.And as Johan mentioned, the seat is drilled for a "feed hole" AFTER installation in the carburetor.Jon. Link to comment Share on other sites More sharing options...
gmsports Posted October 7, 2013 Author Share Posted October 7, 2013 Thank you for all your responses! We will be investigating all possible fixes and will keep you all posted.. Link to comment Share on other sites More sharing options...
gmsports Posted December 3, 2013 Author Share Posted December 3, 2013 Thank you for all your responses! We will be investigating all possible fixes and will keep you all posted..We have found the problem to be Carb Icing. At higher speeds the throttle body/throttle plate is icing up and killing the motor. By the time you coast to the side of the road and wait a minute or so, the heat of the motor melts the carb ice and it runs fine (mostly happens going up hills). Installing the heat riser(inside LH exhaust manifold) and hooking back up the correct/factory heat plate over the carb, has helped quite a bit(80% better) but has not completely eliminating the carb icing issue.Any thoughts????Thank you. Link to comment Share on other sites More sharing options...
rb6673 Posted December 19, 2013 Share Posted December 19, 2013 That engine has it's own vacumn pump so the vac reading should not be dropping at higher rpm's.Might want to check pump and or restriction in it or from it. Link to comment Share on other sites More sharing options...
Ovalrace25 Posted December 31, 2013 Share Posted December 31, 2013 Steve, Carb ice can be eliminated by use of the heat riser as you mentioned. I THINK You are talking about the manually controlled valve in the front of the left exhaust manifold. However, out here in sunny California, you should not need to use that valve at all if the exhaust cross-over is open and all the manifolds are as they should be. ALSO, If the car is missing the carb cover that is used only on the Cadillacs and not the LaSalle, this can add to the icing just a bit.. You did not mention if the engine still has a working vacuum pump, driven off the rear of the cam as mentioned by others on this thread. I am coming in a bit late on it, and just wanted to through out some thoughts. Link to comment Share on other sites More sharing options...
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