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73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: slideways on January 15, 2021, 08:37:57 PM
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I'm redoing my first square body inherited from my wife's grandparents. 83 K10 with a swapped stock 350 and what i think is an aftermarket qjet. Now keep in mind before teardown the truck started and ran fine when cold so im not sure what could have changed here. I rebuilt the carb pretty meticulously with videos etc using the quadrajetparts rebuild kit. It was my first automotive carb but it seems to idle and respond nice. The welch/bowl plugs were epoxied on this carb already and like i said no issue beforehand. The issue i have is the truck will not start unless you drizzle fuel down the bowl tube in the front edge of the carb. After that it runs on that fuel until the pump seems to fill and keep it running. The choke i believe works ok because the idle will stay up until it hits temp and it will kick down properly with a stab of the throttle.
I reused the original fuel pump with no mods, truck got a new manifold/headers/RV cam/SS inlet line and new Qjet filter (the one i used says its the check valve type). I ran all new hard lines from the tank to pump. Also deleted the EVAP can and plugged the carb port for it.
Any thoughts?
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from one guy dealing with fuel issues on a 350 Q-jet to another: have you checked the fuel pressure? if it's the original pump, might be time for a replacement. can it drive?
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It might be the little "well plug" under the carb,they have a history of leaking
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Was the EVAP tube tee'd to the PVC valve? Is it still in place?
What is the average morning temp where you live?
In a well adjusted world of Qjets, you should only have to depress the pedal by a few inches and the 'choked' motor should start.
If you're in cold country, perhaps you need to move the choke plate to the maximum point of closure. Doing so when the motor is cold, of course.
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Ok, i replaced the pump and still the same issue. The evap on the carb is capped and i'm eliminating the evap line to the tank and just going to run a vented cap. Right now the tank is just open no filler neck. Choke has been adjusted all over with no change. I have it set now like i watched videos where when its warm you clock the spring to just barely start to tip the flap. I dont believe the choke has much to do with it at this point because it wont even attempt to chug unless it has a few ounces of fuel dumped in the bowl tube. The truck is still on stands and not driveable. Once it starts it warms up and sounds great.
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Dumping fuel is the equivalent of what your accelerator pump should be doing.
I would look there first. If they're peppy it sounds like your fuel is draining down from the carb.
Did the new pump have a check valve? I think bd says they came that way but I have never confirmed it either way.
Mine must be shot because I have to pump the pedal while trying to start, the fuel rises and she's on fire.
Have you tried that- pumping the pedal instead of pouring fuel down her throat?
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I didnt dissect the pump to see if it had a check valve but i assumed the old one was shot and that was the issue. Its a Carter pump so it should be ok quality wise. The truck ran and started fine before i redid the fuel system and rebuilt the carb. I have about 6" of rubber line from the tank to a seamless solid run of aluminum tube all the way to another 6" of rubber before the pump. Then a SS preformed line into the filter/carb. I tried to pump it 3-4 times before cranking and it didnt make a difference. Just seems like there is no fuel there at all until the engine starts and gets the siphon back flowing.
Is there any reason why having the tank fill just open to atmosphere would cause an issue? The return line is run the same as the supply and as i mentioned i just capped the EVAP port.
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Can't say I understand the circuit of EVAP systems, bd can pry weigh in...
As for the dry carb, pump it While you Crank, see if it starts.
If it does, we share the same problem, fuel evaporating from the float bowl, while fuel in the line is gravity feeding back to the tank
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If you prime the carb, get the engine running and then shut it off will it immediately start back up? If you haven't tried this then do not let the engine come up in temp, keep the hood open and shut if off once it's stable, then try and start it again.
Though path: to loose it's prime the fuel system needs one of two type of faults.
1) a leak that allows the vacuum pressure, of the fuel trying to flow back to the tank, to bleed off.. this could be a fitting, union,hose, pump body,pump diaphragm..etc...
2) a pressure source. For the above hypothesis, heat soak is causing the remaining fuel to pressurize and force it's way back into the tank. It wouldn't take much for this to happen, especially if your carb floats are doing their job. Verify hose routing around exhaust pipes, manifold and coolant hoses. If they are close and get warm try insulating them or shielding them in those spots with anything you have on hand. If it cures then find a more permanent solution
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Keeping with the same theory of heat and pressure...
Your gasket type for the carb can play a huge royal in heat soak. Having a simple condition of a float not fully shut, heat soak into the carb and pressure building into the fuel supply.
Just to rule out the tank and lines you just ran we can tackle it from two ends at once. Place a can on the passenger fender and connect a line to the suction of the pump... See if it keeps prime.(?) If it does do the same thing but put the can below the fuel pump height and try again.
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Great points and recommendations, but consider: If there is a check valve in the pump, how can pressure be a factor unless heat is gassing the fuel and forcing it past the float bowl valve?
And in the process emptying out the line from the pump to the carb? That's a lot of heat, seems it would vapor lock as well.
Hope to hear from the member on what he finds- hate it when we go through all this only to be left with CRICKETS... ;D
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I considered that when writing all of this but also remember the floats aren't always up and simply heating up the fuel line would have the same potential for creating enough pressure to force the fuel back. Also if heat is causing the fuel to flash off in the bowl, it wouldn't need to go out of the float needle to escape. Once low enough it can take the nearest jet.
Lastly, I've spent..let's go with wasted good money on check valves for fuel systems that never left the run stand mock-up due to leak back through the valve...was not a happy camper!
I do hope he has figured it out by now.
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Where did you by the carb kit?
Some kits might not be ethanol compatible.
Did you replace the accelerator pump?
Did you reinstall the check ball for the accelerator pump?
What did you do and what changed..?
Carburetors require a pump shot or 2 to introduce enough fuel to start the engine as well as set the choke closed after pressing the pedal.
The carb bowl will evaporate quickly with ethanol fuels available today.
Drain back shouldn’t occur past the needle until the fuel evaporates and the float drops.
Should not take More than a few seconds and a couple of pumps to start even after just one day of it not being started.
Do you have a positive steam of fuel from the accelerator pump after cranking for a few seconds and then pumping the throttle?
If not this is a problem.
You have induced this problem have you not?
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I havent had time to continue troubleshooting this but here are some more details to help answer questions. I rebuilt the carb with a complete kit from qjetparts.com. Had ethanol compliant stuff i guess as its like the best kit on the market apparently. I had never done one before but watched as many videos as i could and was very thorough. I dont think any fuel shoots out the accelerator when its totally dry.
When i was first starting out testing and breaking in the cam etc i had just a rubber hose sucking into the pump from a 5ga can on the floor. This also had the same issue. Carb needed priming to fire. I figured just because it was a short steep drop that was why it ran out of the system.
One time i vise gripped the rubber line off at the return and supply and shut the truck off and started it easily the next day. My next attempt at that will be to vise grip it off at the tank rubber line to see if i can keep fuel in the hard lines for a time and restart it in a few days.
There is nothing heat related near any fuel line except the factory hard line that goes from pump to carb. Up until recently i just ran flex pipe from my collectors into a semi truck muffler to run the engine.
The engine will refire when warm with little issue. I haven't done this much at all though. Since the truck is immobile i usually run it long enough for the choke to open and the idle to set itself down while looking for leaks etc.
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I didnt dissect the pump to see if it had a check valve but i assumed the old one was shot and that was the issue. Its a Carter pump so it should be ok quality wise. The truck ran and started fine before i redid the fuel system and rebuilt the carb. I have about 6" of rubber line from the tank to a seamless solid run of aluminum tube all the way to another 6" of rubber before the pump. Then a SS preformed line into the filter/carb. I tried to pump it 3-4 times before Here is the question cranking and it didnt make a difference. It won't make a difference because the bowl is DRY Just seems like there is no fuel there at all until the engine starts and gets the siphon back flowing. There is no siphoning in a pressurized system.
Is there any reason why having the tank fill just open to atmosphere would cause an issue? The return line is run the same as the supply and as i mentioned i just capped the EVAP port.
What happens when you pump the throttle while cranking the engine? Have you confirmed that the accelerator pump is spraying two streams down the barrel (engine off) when you push the linkage?
Okay, sorry if I sound pushy, but this is maddening. >:(
Next time, before pouring any gas down her throat try this:
1. Get a bright light, shine down the carb primary (front) barrels, pump linkage. Do you see any streams of gas? Yes or no?
If no, your fuel is leaking back down, and your carb bowl is dry. Check if your fuel pump Model number cites a "check valve".
If yes, your ignition is suspect. Try advancing the timing.
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I had the truck running yesterday. There is zero fuel coming from the pump into the throat when pumping. The system is dry. Carter fuel pump M6626 is what i installed awhile back. I cant find any info about it having a check valve or not.
Will not having a filler neck and fuel cap on cause this to gravity bleed back into the tank? Is there some pressure retained in the system with a gas cap on?
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I found that unless it sits for 2+ weeks at a time, if having this problem it tends to be the fuel pump. I had lots of problems with all different brands of the stock fuel pump even carters... I ended up buying an edelbrock 1721 pump for mine even though its stock engine but it sits a lot. In the two years since I installed it I haven't had that problem again. I think the stock replacement pumps are being built to cheaply they let the fuel siphon back to the tank. The last two I bought broke trying to tighten the fuel line up to it using a backing wrench and all. IMO all stock replacement SBC fuel pumps are garbage.
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Mine would do the same thing. Even after sitting a day or 2 between starts, I had to feed it some juice down the carb to get it started. When I had my sending unit replaced, I also replaced the 40+ year old hoses (filler and vent hose). After having that done, starts like it should. The hoses looked pretty bad after 40+ years. Only thing I can figure is that maybe there was a crack that was causing vacuum to be lost, or maybe it was something to do with the sending unit. Anyway, it worked.
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Updating this, i bought and installed a cheap amazon check valve on the supply side back at the tank. The same thing still happens after a couple days. No fuel in the accelerator pump. The motor will fire up after about 20 seconds of cranking and run fine which means to me it literally has zero fuel in the supply line the whole way back to the tank or at least not enough to fill the bowl quickly to get a start. I figured the check valve would help this at least keep fuel in the hard line to the pump but apparently not. So my two next guesses are to buy an expensive fuel pump that will hopefully stop the backflow and/or i have never gotten confirmation that doing all of this with no filler/vent hose on the fill hole causes some type of atmospheric reaction to siphon fuel back. I'd have to mock up a hose and a cap on as i still dont have a bed on during the resto.
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Move the check valve to the rubber hose going INTO the fuel pump.
In theory, it will act like the check valve that is supposed to be in your pump.
Make sure you get the direction right, speaking from experience... ::)