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I was recently reminded of something that may help with your troubleshooting. I've been working on the fuel tank on one of my old Jeeps. I needed to move the vehicle while the tank was out of commission, so I rigged up an outboard motor fuel supply. It's one of those small portable tanks with a squeeze bulb to prime the line.
I've got this external tank feeding directly to the Jeep's engine-driven fuel pump. When I squeeze the primer bulb before starting, fuel flows through the pump check valves and up to the carb. When the carb float chamber fills up, the float valve closes and I can feel the increased resistance at the primer bulb. The flow of fuel stops and the bulb becomes more or less rigid.
If you've got a similar outboard tank handy, you could try the same thing with your carbs. They don't even need to be installed, as long as you keep them upright. I bet you will find the old carb stops the flow properly, but not the new one. You'd then have extra confirmation of the problem, which should help deciding how to proceed.
I cant say I have one of those tanks around that I know of. Could I take one off of something, or do you know where I could get one cheap?
I cant say I have one of those tanks around that I know of. Could I take one off of something, or do you know where I could get one cheap?
I'm not sure I'd bother running out to purchase an outboard motor tank. Between a new tank and the squeeze bulb, it might set you back 50 or 75 bucks. I only suggested it in case you had one handy, for confirmation of the fault. You'd be better off putting that money towards a carb kit or whatever is needed for the actual repair.
I'm not sure I'd bother running out to purchase an outboard motor tank. Between a new tank and the squeeze bulb, it might set you back 50 or 75 bucks. I only suggested it in case you had one handy, for confirmation of the fault. You'd be better off putting that money towards a carb kit or whatever is needed for the actual repair.
Makes sense. Just had a thought. This truck doesn't have a fuel return line to take excess fuel back to the tank. Could installing one help my issue? Maybe put one directly before the carb? I was told that old mopars had a fuel filter that had a line for input, to carb, and return to tank. Could I install one of those directly before the carb and let that alleviate excess pressure?
I was told that old mopars had a fuel filter that had a line for input, to carb, and return to tank. Could I install one of those directly before the carb and let that alleviate excess pressure?
Could you install one of these filters? Certainly.
Would it help? No.
Fix what is wrong, versus trying to come up with a workaround.
The pressure output from the fuel pump has already tested within specs. You've got two carbs. The float valve in the new one doesn't shut off properly, and dumps excess fuel down the intake manifold. The float valve in the old carb works properly, but the rest of the carb has still unknown problems. Pick a carb and fix it, or procure a third one. You are thisclose...
Don't get sidetracked with a Rube Goldberg system to bleed off fuel pump pressure, when that has already tested within specs. The inlet pressure is NOT your problem. FWIW, I have a gauge on my '84 351W to read fuel pressure at the carb inlet. It's a different carb, but same general idea. Specs call for 6-8 psi. My gauge, which I believe is accurate, occasionally shows as low as 4 and as high as 10 psi. The engine runs fine at all times. Your carb should be able to easily handle the pressure you have already measured. In fact, one of your carbs already does, it just has other problems.
Makes sense. Just had a thought. This truck doesn't have a fuel return line to take excess fuel back to the tank. Could installing one help my issue? Maybe put one directly before the carb? I was told that old mopars had a fuel filter that had a line for input, to carb, and return to tank. Could I install one of those directly before the carb and let that alleviate excess pressure?
AMC also used a filter with a return on their mid 70's cars, the return has to point to the top.
It was used to stop vapor locks as the system kept the fuel moving so it did not have time to set and heat up to cause the vapor lock.
Originally Posted by kr98664
Could you install one of these filters? Certainly.
Would it help? No.
Fix what is wrong, versus trying to come up with a workaround.
The pressure output from the fuel pump has already tested within specs. You've got two carbs. The float valve in the new one doesn't shut off properly, and dumps excess fuel down the intake manifold. The float valve in the old carb works properly, but the rest of the carb has still unknown problems. Pick a carb and fix it, or procure a third one. You are thisclose...
Don't get sidetracked with a Rube Goldberg system to bleed off fuel pump pressure, when that has already tested within specs. The inlet pressure is NOT your problem. FWIW, I have a gauge on my '84 351W to read fuel pressure at the carb inlet. It's a different carb, but same general idea. Specs call for 6-8 psi. My gauge, which I believe is accurate, occasionally shows as low as 4 and as high as 10 psi. The engine runs fine at all times. Your carb should be able to easily handle the pressure you have already measured. In fact, one of your carbs already does, it just has other problems.
I am with Karl fix the issue don't try and Rube it you will be happier for it.
Dave ----
Not sure if this is the same situation but. My 81 one day stopped it turned over fine but wouldnt fire at all. I checked everything blahblah blah had spark and gas n what not but talked to a old man and here what is was and check this out for sure. There is a ground wire from the block to the firewall even if it looks fine replace it any wire will do. If your getting spark and gas but its not firing whatsoever CHECK THIS.
Originally Posted by Mack Blair
Ok so here goes. I am 16, just got my license a few days ago. Before then, I had received a non-running 1986 F150 off a friends grandparent's farm. It is rough, but it has a title and it was free. For a first truck I couldn't complain. Well I got it home and all it took to get the old 300 six running was a mechanical fuel pump. fired right up. After that I did basic tuneup items. Cap, rotor, plugs, etc. Well after driving it around the property for a few days, it suddenly stopped running. A bunch of gunk from the gas tanks had made its way up into the carb. I rebuilt the carb and put it back on (after cleaning the fuel tank) and it wouldn't run right. I kept fiddling with the carb, must have taken it apart 10 times. In the end, I got fed up with it and bought a new carb. The link to the carb I bought is: https://swperformanceparts.com/162-C...hoC1lsQAvD_BwE
While I was waiting on the carb to come in, I pulled the intake and exhaust manifolds off and replaced the gaskets, just because. I also removed and plugged the holes for the EGR tube, because it was damaged when I took off the manifolds. I put the new carb on and it has some funky issues. I dont think I have vacuum lines hooked up correctly, or something of the sort, I'm not sure. Basically what i've confirmed. The engine is getting gas, I can watch gas squirt into the throat of the carb when the throttle is opened. It is getting spark, I checked each plug individually while the motor was cranking. each had spark. I turned the motor to TDC on the compression stroke and set the distributor to plug #1. I also did a compression check and each cylinder was between 145 and 155 psi. But that truck will not start to save its life. Sometimes itll get a little off starting fluid, but very seldom. I have found that sometimes, if I disconnect the fuel line from the carb, the carb will push gas out of the inlet for a split second and then the motor will run just about perfectly until it drains the carb. I don't understand. Any help at all is appreciated, I need this truck on the road ASAP. Thanks y'all!
Some information about the truck
1986 F150
4.9L (300) L6
4-speed NP435 Tranny
Came with dual 19-gallon tanks
Has the TFI style (distributor mounted) ignition module
I don't know what else you guys might need, but let me know! I'm open to share anything about the truck if it helps me get it on the road.
P.S. The truck's name is Bull. Just thought I'd include that.
Could I install one of those directly before the carb and let that alleviate excess pressure?
Something I forgot to mention. These special filters have a small orifice in the return line. It's not just an unrestricted port, or the fuel pump could never build up any pressure at the carb inlet. The return flow is restricted. Only a small volume of flow is returned to the tank.
Thanks to this carefully designed restriction, the pump still has adequate reserve capacity to build up its rated pressure. The big spring inside the pump controls the output pressure. Even with a small percentage of fuel returning to the tank, the pressure at the carb inlet is still the same. The pump has to work a little harder, that's all. But adding a return style filter (and new return line) won't reduce the pressure.
I cant thank yall enough for all your awesome advice. The truck is ALIVE! I ended up using the new carb, but replacing and resetting the float to be what it should. Its sitting here in the driveway idling now!
Well... it WAS great news. The truck runs, but sometimes it will sit there and run for an hour, sometimes 30 seconds. Sometimes it will just suddenly die, sometimes when it dies it backires through the carb, through the tailpipe, or both. I dont even know anymore.
Well... it WAS great news. The truck runs, but sometimes it will sit there and run for an hour, sometimes 30 seconds. Sometimes it will just suddenly die, sometimes when it dies it backires through the carb, through the tailpipe, or both. I dont even know anymore.
Don't panic! This was a project dragged out of a field, right? After your most recent repairs, and you elatedly reported it was running well, how many miles have you driven the truck? I'm guessing the truck is not yet registered, and the total post-repair mileage is approximately zero.
If all you've done is run at idle, or maybe zipped up and down the driveway a few times, that poor engine is just itching to work hard and earn its keep. Those forlorn spark plugs might be fouled with carbon. Some of the piston rings may be stuck in their grooves. You may have a valve or two sticking. The simple and surprisingly effective cure?:
The (Insert name of ethnic group) Tune-Up!
Work that engine! Take her out on the highway! Get those piston rings seated! Run a Prius off the road! Blow out any carbon! Impress some girls and do donuts in the Dairy Queen parking lot.
Do those things first and then see if the engine now behaves itself. Long periods of idling are tough on an engine. If the truck isn't yet roadworthy for other reasons, put the engine condition on the back burner until you can give her a good (Insert name of ethnic group) tune-up. You know the engine is now capable of running well, at least some of the time. That's huge progress. Don't tell yourself otherwise.
Don't panic! This was a project dragged out of a field, right? After your most recent repairs, and you elatedly reported it was running well, how many miles have you driven the truck? I'm guessing the truck is not yet registered, and the total post-repair mileage is approximately zero.
If all you've done is run at idle, or maybe zipped up and down the driveway a few times, that poor engine is just itching to work hard and earn its keep. Those forlorn spark plugs might be fouled with carbon. Some of the piston rings may be stuck in their grooves. You may have a valve or two sticking. The simple and surprisingly effective cure?:
The (Insert name of ethnic group) Tune-Up!
Work that engine! Take her out on the highway! Get those piston rings seated! Run a Prius off the road! Blow out any carbon! Impress some girls and do donuts in the Dairy Queen parking lot.
Do those things first and then see if the engine now behaves itself. Long periods of idling are tough on an engine. If the truck isn't yet roadworthy for other reasons, put the engine condition on the back burner until you can give her a good (Insert name of ethnic group) tune-up. You know the engine is now capable of running well, at least some of the time. That's huge progress. Don't tell yourself otherwise.
I have already done spark plugs, cap, rotor, and a couple other things. I completely agree with you, though. It just needs ran. I have to rebush the pedals, because at the moment, if you push down the clutch, the brake tries to go with it. Other than that, Im ready to run it around! Needs a good cleaning though. And a door alignment lol.
I have already done spark plugs, cap, rotor, and a couple other things. I completely agree with you, though. It just needs ran. I have to rebush the pedals, because at the moment, if you push down the clutch, the brake tries to go with it. Other than that, Im ready to run it around! Needs a good cleaning though. And a door alignment lol.
if you changed the plugs BEFORE it was running, they could have gotten fouled during the rest of the process. Its easy enough to remove a couple (one at a time) and check them out. After I was able to get my truck running, I did run seafoam through the fuel tank and that did clean out a bunch of carbon. Its about $8 a can, I thought it was worth it.
if you changed the plugs BEFORE it was running, they could have gotten fouled during the rest of the process. Its easy enough to remove a couple (one at a time) and check them out. After I was able to get my truck running, I did run seafoam through the fuel tank and that did clean out a bunch of carbon. Its about $8 a can, I thought it was worth it.
I should clean the plugs. Ive had one out while I was tinkering, and I noticed that they were pretty carboned up from running rich while the carb wasnt right. I'll take some sandpaper and clean them up. Would seafoam hurt a carburetor at all?
I should clean the plugs. Ive had one out while I was tinkering, and I noticed that they were pretty carboned up from running rich while the carb wasnt right. I'll take some sandpaper and clean them up.
Put the sandpaper on the ground and take a step backwards, hands in the air where we can see them.
The sandpaper won't help. Once carbon is established on the plugs, it creates an electrically conductive trail. Instead of the spark jumping the gap, those two-timing electrons will take the easy way to ground.
It used to be common to clean up spark plugs with a special sandblaster. But even that requires using a specific media that is non-conductive. Most sandblast material will leave behind a conductive film, so not much improvement over the carbon itself.
If the carbon is very light, sometimes you can burn 'em clean with a good [insert name of ethnic group] tune-up after fixing the rich condition. But the easiest method is to spend the $8 and just replace them all. And quit running the truck until you get her back on the road, as you're likely to foul the new ones if all you're doing is running at idle or putzing in the driveway.
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