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Discussion Starter · #1 · (Edited)
I'm in the process of rebuilding the 6-pack on my 340 T/A engine and found something interesting in the center carb. The jets are .070 and there is only a plug where the power valve is supposed to be. I believe the stock jets should be .064 and aside from an aftermarket Mallory distributor, the engine is completely stock. I'm curious as to why someone would do that? I plan to return it to stock. The front carb lost seal on the fuel valve, but before that tit ran good, I thought it might be on the rich side, but was going to deal with that after the carbs were done. Any thoughts would be appreciated.
 

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I'm no expert, and you'll likely find more help on one of the E-Body forums. That said, I think if a cam was changed to one that did not create enough vacuum to keep the original power valve closed during idle, people blocked it off, and installed larger jets to compensate. Not a good setup as you'll be running maximum enrichment all the time

A Guy
 

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That discovery is evidence of a modded engine or the previous owner sourced a carb from a motor that had a a cam that was not stock. If you're certain that your internals are stock, getting the carb back to stock will make the car run properly and you won't smell the distinct odor of a rich running motor.
 

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Discussion Starter · #4 ·
Thanks for responses. I can see evidence of previous carb rebuilds and the original owner tried to do some DIY fixes on the fuel system, chasing some problem on the center carb. It has the correct Holley part numbers, but no Mopar numbers, so the changed carb is very plausible. The fuel lines were a horrible braided steel mess and I mentioned the distributor which has no vacuum advance, but I'm pretty sure the engine has never been apart. I suppose he could have done a cam swap at some point, it has a bit of a lope at idle, but sounds like other T/A's that I've heard over time. It's a one owner and he had a son, so anything is possible.
 

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I'm in the process of rebuilding the 6-pack on my 340 T/A engine and found something interesting in the center carb. The jets are .070 and there is only a plug where the power valve is supposed to be. I believe the stock jets should be .064 and aside from an aftermarket Mallory distributor, the engine is completely stock. I'm curious as to why someone would do that? I plan to return it to stock. The front carb lost seal on the fuel valve, but before that tit ran good, I thought it might be on the rich side, but was going to deal with that after the carbs were done. Any thoughts would be appreciated.
Here's a "thought"...... how about some pictures?

Very cool car and as a "survivor" it's got to be REALLY COOL.

In a forum devoted to a late model car that can at very best described as merely pretty good.......your car is simply awesome.

I'm just old enough to remember these cars in traffic as daily drivers. Fact is they are the reason I bought my own retro take on the old 340 T/A vs some other version of the latest Dodge Challenger.
 

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Based on my memory, the correct jets for a 340 Six Pack would be #63, with a 6.5 power valve. I have driven and worked on the 340 Six Pack powered cars, but I don't have as much experience with them as with the 440 Six Pack and Street Hemi. I am pretty sure about the power valve, reasonably sure on the jet sizes. I used to change out the springs in the vacuum pots to change opening speed of the secondary carbs, but I have seen mechanical linkage on them, and even a screw in the equalizing links to pull them open. By the way, you have a beautiful Challenger there, glad to see it being cared for.
 

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Discussion Starter · #10 ·
Based on my memory, the correct jets for a 340 Six Pack would be #63, with a 6.5 power valve. I have driven and worked on the 340 Six Pack powered cars, but I don't have as much experience with them as with the 440 Six Pack and Street Hemi. I am pretty sure about the power valve, reasonably sure on the jet sizes. I used to change out the springs in the vacuum pots to change opening speed of the secondary carbs, but I have seen mechanical linkage on them, and even a screw in the equalizing links to pull them open. By the way, you have a beautiful Challenger there, glad to see it being cared for.
Thank you for that. Since I first created this post, lots of work on the car. I think the carbs are right now, set-up as you have described, waiting for all the parts to arrive to restore the ignition. Hopefully the last issue under the hood to resolve. The previous owner painted everything hemi orange poorly, paint was flaking off everywhere so I spent a lot of time over the winter making it right.
 

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Discussion Starter · #11 ·
Thank you for that. Since I first created this post, lots of work on the car. I think the carbs are right now, set-up as you have described, waiting for all the parts to arrive to restore the ignition. Hopefully the last issue under the hood to resolve. The previous owner painted everything hemi orange poorly, paint was flaking off everywhere so I spent a lot of time over the winter making it right.
Forgot to mention the valve covers. OEM replacements are ordered, they will be painted to match and swapped out when they come off backorder.
 

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Thanks for responses. I can see evidence of previous carb rebuilds and the original owner tried to do some DIY fixes on the fuel system, chasing some problem on the center carb. It has the correct Holley part numbers, but no Mopar numbers, so the changed carb is very plausible. The fuel lines were a horrible braided steel mess and I mentioned the distributor which has no vacuum advance, but I'm pretty sure the engine has never been apart. I suppose he could have done a cam swap at some point, it has a bit of a lope at idle, but sounds like other T/A's that I've heard over time. It's a one owner and he had a son, so anything is possible.
its possible that if it has an aftermarket cam, the timing advance needed to be re-curved.

running no vacuum advance does two things -
kills the mileage
a lot of wear and tear on the engine due to timing not being advanced a lot of the fuel mixture doesn't have adequate time to burn efficiently and lost power as well

probably with increased cylinder pressures (longer duration cam) and the stock curve - the former owners noticed spark knock (with pump gas) and disconnected the vacuum advance

often the trick that is done is reducing the mechanical advance, changing base timing and re-work vacuum advance to make everything function as it should.
-today's fuels are far different than what was around in the pre-unleaded fuel days
 
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Yeah, I didn't mention anything about the distributor, but I noticed that a Mallory distributor was mentioned. If it has a vacuum advance cannister, it would be best to have it hooked up and working. Most aftermarket distributors have provisions for easy advance curve adjustments, and the vacuum advance curve may be adjustable as well. If this is true of the Mallory, it would probably be better than tweaking a stock distributor. If not, the stock distributor can be recurved, and the vacuum advance adjusted, with a little more effort.
 

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Yes, the center carb acted as the primary carb, the outer carbs were controlled by vacuum based on engine demand. Worked just like a vacuum secondary Holley four barrel.
 

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Discussion Starter · #16 ·
Yeah, I didn't mention anything about the distributor, but I noticed that a Mallory distributor was mentioned. If it has a vacuum advance cannister, it would be best to have it hooked up and working. Most aftermarket distributors have provisions for easy advance curve adjustments, and the vacuum advance curve may be adjustable as well. If this is true of the Mallory, it would probably be better than tweaking a stock distributor. If not, the stock distributor can be recurved, and the vacuum advance adjusted, with a little more effort.
I'm pretty sure the Mallory is bad, but it does not have a vacuum advance canister and it only makes about 18 to 20 degrees total advance, which is probably why they dumped all the fuel in there in the first place. I have a new one (electronic) but a mopar style with vacuum advance, will be installed once the new harness arrives. That should improve my fuel economy, not that care all that much, but 7 to 8 mpg is really no fun.
 

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Yeah, 7-8 mpg is pretty rough. My '69 440 Six Pack got 11.7 mpg once I got the carbs and distributor dialed in, and that was with 4.30/1 gears. I did swap in an aftermarket ignition system, hoping to gain a little more power. It was a waste of money, it ran as well as the stock system, using points did, but NO better. No gain in mileage or power, but at least I didn't have to set points anymore.
 

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Discussion Starter · #18 ·
Yeah, 7-8 mpg is pretty rough. My '69 440 Six Pack got 11.7 mpg once I got the carbs and distributor dialed in, and that was with 4.30/1 gears. I did swap in an aftermarket ignition system, hoping to gain a little more power. It was a waste of money, it ran as well as the stock system, using points did, but NO better. No gain in mileage or power, but at least I didn't have to set points anymore.
I've not checked it since I changed over the center carb metering block, would be a bit better for sure, but I do want the vacuum advance back.
 

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I'm pretty sure the Mallory is bad, but it does not have a vacuum advance canister and it only makes about 18 to 20 degrees total advance, which is probably why they dumped all the fuel in there in the first place. I have a new one (electronic) but a mopar style with vacuum advance, will be installed once the new harness arrives. That should improve my fuel economy, not that care all that much, but 7 to 8 mpg is really no fun.
a 340 ought to deliver 12 -13 around town and ~ 16 - 17 on highway, depending on final ratio and what cams are in it with the proper tuning and timing advance

yours is losing a lot of efficiency and power w/o correct timing advance
 
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a 340 ought to deliver 12 -13 around town and ~ 16 - 17 on highway, depending on final ratio and what cams are in it with the proper tuning and timing advance

yours is losing a lot of efficiency and power w/o correct timing advance
That sounds about right from what I remember with my old 340 with a mild cam and 3:55 gears.
 
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