340 engine isn’t getting enough gas, why?

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Dakota Smith

Dakota Smith

1976 340 Feather Duster
Joined
Feb 17, 2018
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Location
Georgia
In the last couple of weeks my 76 340 Plymouth Duster has been severely lacking in power once I get to 3rd gear. I guess there is something preventing enough gas into the carburetor, but I am not sure what it is. Everything is brand new from the fuel tank all the way to the carb, but the carb isn’t new. When spraying brake fluid around the carb while the engine was running, I noticed air is escaping from somewhere around the carb. Could changing the carb gasket be the problem or do you guys have any other suggestions? Also, the fuel vent line isn’t attached to anything at the moment. It is just open under the hood. I plan on getting a charcoal canister soon.
 
Dakota Smith said:
In the last couple of weeks my 76 340 Plymouth Duster has been severely lacking in power once I get to 3rd gear. I guess there is something preventing enough gas into the carburetor, but I am not sure what it is. Everything is brand new from the fuel tank all the way to the carb, but the carb isn’t new. When spraying brake fluid around the carb while the engine was running, I noticed air is escaping from somewhere around the carb. Could changing the carb gasket be the problem or do you guys have any other suggestions? Also, the fuel vent line isn’t attached to anything at the moment. It is just open under the hood. I plan on getting a charcoal canister soon.
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First off I hope the hell you're not spring break fluid around on your motor! I hope it was brake cleaner! LOL
 
A fuel pressure gauge would tell you if you're running out of fuel at any certain time or getting too low on pressure.
 
If she was going lean in third gear, there would be a lot of bucking and farting and misfiring/backfiring going on. You didn't mention any of that.

In the absence of drama,Verify the choke is coming ALL THE WAY off:If the choke mech is not coming off, the secondaries will not open.
After that; verify your timing systems.
If that checks out, time for a compression test.
 
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Check your fuel filters....

The one in the tank can get old and disintegrate... The 'sock' on the end of the fuel pick-up tube is known to break down and clog the tube... Then make sure the in line fuel filter is not clogged either.... .
 
So its a thermoquad? Is this a stock setup? No 340 in 76 IIRC. It would be 360 I think, but I’m old and forgetful. Pictures always help.
 
krazykuda said:
Check your fuel filters....

The one in the tank can get old and disintegrate... The 'sock' on the end of the fuel pick-up tube is known to break down and clog the tube... Then make sure the in line fuel filter is not clogged either.... .
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They’re are both brand new, so I don’t think they would be clogged.
 
furrystump said:
So its a thermoquad? Is this a stock setup? No 340 in 76 IIRC. It would be 360 I think, but I’m old and forgetful. Pictures always help.
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The carb is a 4 barrel Edelbrock, part #1407. The original engine was a slant six, but it was swapped for a 340 before I got it.
 
krazykuda said:
Check your fuel filters....

The one in the tank can get old and disintegrate... The 'sock' on the end of the fuel pick-up tube is known to break down and clog the tube... Then make sure the in line fuel filter is not clogged either.... .
Click to expand...
had this happen to me in an aspen R/T, the sock would collapse and starve for fuel...shut the car off for a few minutes, restart then it would work fine for a bit till it happened again....took a while to track that down...good call kuda
 
So it falls on it’s face in third gear when you are at full throttle through first and second? Put a fuel pressure gauge on it. NOT a liquid filled gauge. If you still have 5lbs of fuel pressure in third it might be the needle and seat. You can empty those carbs it the motor draws enough. They make a larger needle and seat. I run them.
 
Same sized fuel line as was there for the /6? Probably too small.

And what fuel pump do you have? Can be bad out of the box or if a straight stock one, running out of capacity.

As said, start with fuel pressure test at the carb inlet and watch the fuel pressure gauge as you get to the 'weak' speed. Ought to stay in the 5-7 psi range all the time. If it does, then yo have eliminated the lines and pump and tank and can focus on the carb.
 
I had the same problem with my duster
Turned out the little "rubber" fuel line that connects the pick up to the hard line had too much of a bend in it and would collapse on itself
 
Are you sure the vent line actually vents? Checked the fuel cap?
 
furrystump said:
So it falls on it’s face in third gear when you are at full throttle through first and second? Put a fuel pressure gauge on it. NOT a liquid filled gauge. If you still have 5lbs of fuel pressure in third it might be the needle and seat. You can empty those carbs it the motor draws enough. They make a larger needle and seat. I run them.
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liquid filled fuel pressure gauge? whats wrong with them,curious?
 
They warm up and go crazy. The fluid expands and makes the reading fluctuate. I had a liquid filled gauge that while idleing it would go from zero to 10 lbs. back to zero. I had heard about not using the fluid guage. I drained the gauge rock steady at 6lbs. Still use the gauge.
 
furrystump said:
They warm up and go crazy. The fluid expands and makes the reading fluctuate. I had a liquid filled gauge that while idleing it would go from zero to 10 lbs. back to zero. I had heard about not using the fluid guage. I drained the gauge rock steady at 6lbs. Still use the gauge.
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Great tip!
If I ever get curious about my fuel-pressure, I'll try to remember your tip.
 
I’ve found very little parts for a 45+ yo car to be new. They are all refurbished and mostly poor at that. Even aircraft parts. Since there are so many variants and aftermarket, describe your fuel system tank-carb.

Electric or manual pump? Regulator? Pump cam could be flat or wrong pump, cam, cover configuration. How do the filters look? Some filters you must cut open. Any kinked hardlines? Check carb float level. Tank could be full of sealant chips, you never know.

Thats even if this is a fuel delivery problem.

Posting a video would help. AJ is right, it would be fuckin pissed off and violent.
 
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