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Car Forum / Chevrolet / Chevrolet Trucks / November 20041985 Chevy Pickup with odd running problem
Hi, I have an 85 pickup (V8) that has had new plugs, wires, distributor cap, rebuilt carb, new fuel filters and pump and still has the same problem. Problem- You press down on the gas and truck can not get out of it's own way until the 4 barrel kicks in. Ok...now here is the weird part. Truck can run great today and tomorrow....but yesterday it had the no power problem and tomorrow it may run great or it may not. It is a little scary when wanting to pull out in from intersections not knowing if I have any power. I am guessing that it is something simple....but just don't know what to do next. My gut feeling is that I have some simple electrical sensor that keeps the carb from filling with fuel even when needed.....sensor works sometimes and sometimes not. Truck has 180,000 miles and when she runs good...she runs real good. Help please Marshall > Hi, > [quoted text clipped - 18 lines] > > Marshall Your carb doesn't have any sensors; it's completely mechanical. Sounds like your accellerator pump isn't working all the time. The AP squirts gas into the carb when you hit the pedal to make up for the throttle plates opening quickly, allowing the engine to keep running until the vacuum pull on the jets gets it the fuel it needs with the throttle plate position change. On EFI vehicles as soon as the throttle plates move the ECM sees this via the TPS and gives it more gas thru the injectors. With carbed vehicles it takes a second or so for the engine vacuum to draw the extra gas it needs thru the jets, hence the need for an accelerator pump. Check the mechanical linkage to the AP and see if it's sticking. If you have the stock Rochester Q-Jet the AP is at the top-front of the carb, on the driver's side. The plunger sticks strait up and there is an arm attached to the throttle linkage the depresses the plunger with pedal action. Doc Hi Doc, We have had the accellerator pump out twice and made adjustments to permit more fuel....no success so far. Of course the problem is that it varies from one day to the next. Are there any electrical or computer issues that can cause this? The truck had the same identical problem with the old carb as with the brand new which causes us to think that the problem is somewhere else. We put new hoses on. I wonder if the vacuum is not working. thanks for your help Marshall > > Hi, > > [quoted text clipped - 37 lines] > > Doc > Hi Doc, > [quoted text clipped - 14 lines] > > Marshall As I said before, no computer, no sensors, no electronics. There is absolutely nothing electronic involved with your fuel delivery or spark advance. Doc >> > Hi, >> > [quoted text clipped - 47 lines] >> >> Doc > As I said before, no computer, no sensors, no electronics. There is > absolutely nothing electronic involved with your fuel delivery or spark > advance. > > Doc If it's a 305, it will have an ESC module and a knock sensor. The ESC can be by-passed in accordance with the [numerous] TSBs that were issued in the mid 80s for symptoms very similar to what the OP is describing. The OP should; Verify base ignition timing Verify the the mechanical ignition advance is working (not rust bound) Verify that the vacuum advance is working correctly Verify correct fuel pressure -and- volume Verify that the choke is opening fully Verify exhaust back pressure Verify that there aren't any worn camshaft lobes |
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