340 Gets Terrible Gas Mileage

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volaredon said:
The part about get rid of champion plugs
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Why would you? They foul out quick. Even my lawn equipment runs like junk with champion plugs. I've seen them miss fire at high rpm's with a engine that has a high compression ratio.
 
Dan the man said:
Why would you? They foul out quick. Even my lawn equipment runs like junk with champion plugs. I've seen them miss fire at high rpm's with a engine that has a high compression ratio.
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I still think that his mileage problem is that the cam is probably radical, the carburetor isn't to big at 650 cfm's, I would go over the entire ignition system, fuel system, vacuum lines, correct timing. As I said earlier, 340's are a performance engine they wasn't designed for mileage. But I would think that a well tuned stock 340 should get mid teens anyway. I think that the compression should be checked out and see if the cam was degreed. You simply can't have a performance engine and mileage too,won't happen except for the late model high tech cars.
 
6 mpg?!
Wow! Are you sure?
Something is way out of whack!
Your timing doesn't sound too far off,
Id try a different carb.
what do the plugs look like?
is the gas tank leaking?...lol
Mine was when I bought it.

My 340 was getting 17 mpg with a Holley 670 sa.
Just switched to a 650 Brawler dp.
Mileage dropped to 16 mpg.
But it runs So much better!!
My car actually gets better Mileage in town
Then highway with a 904 and 3.73's
I hope you get it figured out.
 
My experience with LA engines has been mostly great. I drove a mostly stock 340/4spd/3.23 combo back and forth to work for about 8 years. City mileage was 10-12. Now my '85 D350/360/727/4.1/dually usually gets 7-8 mpg in town and If you drive 55 on the highway you might see 10mpg. So yeah the OP needs to find someone locally to sort his combo out. I hope he finds somebody!
 
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The TQ suggestion is a great idea, best performance 4 bbl carb there is. NGK plugs would also be great.

But the fact is that with the info provided & parts being used, you should be getting better mileage.
Three things to check as a starting point, relatively easily:
- float level. Should be 7/16". Unlike brand H [ & clones ], you do not need new gaskets if you are careful.
- check that jets & met rods are correct for that carb
- carb reassembled. Loosen met rod covers & swivel slightly so that the pistons are in view but captured; tighten screws. Start engine, put in gear, & watch pistons. They should go down & not move; if not, piston springs are too strong & you get poor mileage.
 
Bewy said:
The TQ suggestion is a great idea, best performance 4 bbl carb there is. NGK plugs would also be great.

But the fact is that with the info provided & parts being used, you should be getting better mileage.
Three things to check as a starting point, relatively easily:
- float level. Should be 7/16". Unlike brand H [ & clones ], you do not need new gaskets if you are careful.
- check that jets & met rods are correct for that carb
- carb reassembled. Loosen met rod covers & swivel slightly so that the pistons are in view but captured; tighten screws. Start engine, put in gear, & watch pistons. They should go down & not move; if not, piston springs are too strong & you get poor mileage.
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This is good info. Spark plugs- champion plugs are not at all what they were 10 years ago.
I quit buying them after several failures. Causes a diagnostic nightmare.
A feller can buy a tuning kit, springs, needles and seats all in one box. Handy.
 
Sport340 said:
Ok would like to resurrect this post. To refresh, I am getting 6 mpg on 1973 Dart Sport 340 that has a Edelbrock 650 with some sort of performance cam in it (do not know what cam is in it). I finally got a digital timing light and was able to determine initial and total timing. Initial timing is 24 at 900 rpm. Total timing looks to be 42 at 2800 rpm. I plugged the vacuum advance line that runs to the distributor when I checked the timing but it doesn't seem like there is any vacuum on the hose (put my finger over it). Trying to figure out if the way the timing is set is responsible for the bad gas mileage. If anyone has any thoughts I would be interested. Thanks!
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Please post close up pic's of all 8 spark plugs.
That will give us a quick view of whats going on.
FBO Ignition - Home of the HRR688 - Spark Plug Reading
 
As to Champion plugs;
My 11/1 alloy headed 360 has run the very same copper Champions for over 100,000 miles. They will still idle at 550rpm, pulling the ol' Barracuda around the parking lot at under 4 mph. And she will hit 7000 right after that. They were new in 1999.
I planned a track day in 2004, so pulled the plugs for a look, and screwed them right back in. She later that day, ran 93 in the Eighth with those already ancient plugs.
As to the TQ carb;
For fuel mileage, that spreadbore/metering rod design is really hard to beat. I used to install them on anything/everything.
Having said that, there's nothing so wrong about a 650 Holley that I would waste money to replace it; you will very likely Never save the purchase price, in fuel economy, for several years.

IMO, I'll bet 100points that your cylinder pressure is very low, and your cruise timing is just not there.

When the cruise timing is retarded, peak cylinder pressure is never achieved.
But it's worse; the A/F mixture may not finish burning in the chamber but rather continues burning/chasing after the falling piston. The still expanding gasses exit the cylinder at or near 100* ATDC, before transferring all their energy to the crank, and instead put it to use heating the ports and exhaust system.
At the other end, with headers, and a "big cam", overlap may be so large that at low rpm, a sizeable slug of just-inducted mixture may zip right across the top of the piston, and exit into the exhaust system, before the exhaust valve closes. That is literally sucking, your fuel mileage right out the tailpipe.
A street engine with a big cam, for fuel economy, has to be geared to cruise well over 2000, perhaps up to 2400 rpm to shut the door to this phenomenon ...... which itself is counter-productive to fuel economy.

Big cams in low-compression engines, kill cylinder pressure. This makes the engine lazy and sluggish at low rpm; so you are forever deep into the throttle to get some reasonable acceleration, and during cruise, the throttle will be open further than would be the case in a high-compression engine, both of these traits kill fuel economy.
 
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Sport340, I'm sure you'll let us know when you've tried everything suggested. I know this'll take a while so start with basics. You may eventually find that the cam is installed a tooth off. Hope it doesn't come to that!
 
The "Champion plugs suck" thing is just total bullshit and wrong information. The only other thing I'll address regarding that is there are only a small number of plug manufacturers in the world and chances are pretty high Champion plugs are made by the same plug manufacturer that also makes the plug you run, instead of Champions.

That said, I'll add this as well. Not everyone "knows" how to calculate mileage correctly. More often than not, I see people making huge mistakes trying to figure mileage. Remember it takes multiple times calculating and then you have to average those calculations together over time, mileage and gallons burned. A lot and I mean a LOT of people just slap get it wrong wrong WRONG. Most of the time those mistakes work out to give incorrect results going the "WRONG" way.

So, before I touched a wrench on anything, I'd double and triple check MYSELF to make DAMN SURE I was doin it right.

Cause you may not be.
 
From 1970 to 1985, I had several hi-compression 340s, at least one with hi-mileage, and not a one of them could be made to deliver under double-digit mileage. After that, I switched to 360s.
 
Sport340... Is your speedo reading correct. the rear gear may have been changed or the trans but now it doesnt have the right speedo drive gear. kinda throws the calculations off.
 
AJ/FormS said:
Just asking; was that 340 a transplant?
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Yes my 74 Challenger came with a 318 removed it and put in a 73 340 behind the stock 74 drivetrain which happened to be a 904 276 rear 15 inch rims 235 wide tires and achieved 22 MPG
 
AJ/FormS said:
As to Champion plugs;
My 11/1 alloy headed 360 has run the very same copper Champions for over 100,000 miles. They will still idle at 550rpm, pulling the ol' Barracuda around the parking lot at under 4 mph. And she will hit 7000 right after that. They were new in 1999.
I planned a track day in 2004, so pulled the plugs for a look, and screwed them right back in. She later that day, ran 93 in the Eighth with those already ancient plugs.
As to the TQ carb;
For fuel mileage, that spreadbore/metering rod design is really hard to beat. I used to install them on anything/everything.
Having said that, there's nothing so wrong about a 650 Holley that I would waste money to replace it; you will very likely Never save the purchase price, in fuel economy, for several years.

IMO, I'll bet 100points that your cylinder pressure is very low, and your cruise timing is just not there.

When the cruise timing is retarded, peak cylinder pressure is never achieved.
But it's worse; the A/F mixture may not finish burning in the chamber but rather continues burning/chasing after the falling piston. The still expanding gasses exit the cylinder at or near 100* ATDC, before transferring all their energy to the crank, and instead put it to use heating the ports and exhaust system.
At the other end, with headers, and a "big cam", overlap may be so large that at low rpm, a sizeable slug of just-inducted mixture may zip right across the top of the piston, and exit into the exhaust system, before the exhaust valve closes. That is literally sucking, your fuel mileage right out the tailpipe.
A street engine with a big cam, for fuel economy, has to be geared to cruise well over 2000, perhaps up to 2400 rpm to shut the door to this phenomenon ...... which itself is counter-productive to fuel economy.

Big cams in low-compression engines, kill cylinder pressure. This makes the engine lazy and sluggish at low rpm; so you are forever deep into the throttle to get some reasonable acceleration, and during cruise, the throttle will be open further than would be the case in a high-compression engine, both of these traits kill fuel economy.
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100,000 miles on a set of plugs? You must keep your engine in perfect condition and have a excellent fuel injection system. Idling is hard on plugs. I say that you have a rare set of plugs
 
RustyRatRod said:
The "Champion plugs suck" thing is just total bullshit and wrong information. The only other thing I'll address regarding that is there are only a small number of plug manufacturers in the world and chances are pretty high Champion plugs are made by the same plug manufacturer that also makes the plug you run, instead of Champions.

That said, I'll add this as well. Not everyone "knows" how to calculate mileage correctly. More often than not, I see people making huge mistakes trying to figure mileage. Remember it takes multiple times calculating and then you have to average those calculations together over time, mileage and gallons burned. A lot and I mean a LOT of people just slap get it wrong wrong WRONG. Most of the time those mistakes work out to give incorrect results going the "WRONG" way.

So, before I touched a wrench on anything, I'd double and triple check MYSELF to make DAMN SURE I was doin it right.

Cause you may not be.
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Champion plugs have been junk for years. I've done enough tune ups over the years, so I'm speaking from experience. I've had lawnmowers quit running because of them.
 
Dan the man said:
Champion plugs have been junk for years. I've done enough tune ups over the years, so I'm speaking from experience. I've had lawnmowers quit running because of them.
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Then you had something else wrong and not enough experience to know it.
 
Dan the man said:
100,000 miles on a set of plugs? You must keep your engine in perfect condition and have a excellent fuel injection system. Idling is hard on plugs. I say that you have a rare set of plugs
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Everything's perfect in his world, dontchya know?
 
RustyRatRod said:
The "Champion plugs suck" thing is just total bullshit and wrong information. The only other thing I'll address regarding that is there are only a small number of plug manufacturers in the world and chances are pretty high Champion plugs are made by the same plug manufacturer that also makes the plug you run, instead of Champions.

That said, I'll add this as well. Not everyone "knows" how to calculate mileage correctly. More often than not, I see people making huge mistakes trying to figure mileage. Remember it takes multiple times calculating and then you have to average those calculations together over time, mileage and gallons burned. A lot and I mean a LOT of people just slap get it wrong wrong WRONG. Most of the time those mistakes work out to give incorrect results going the "WRONG" way.

So, before I touched a wrench on anything, I'd double and triple check MYSELF to make DAMN SURE I was doin it right.

Cause you may not be.
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It's correct information, champion plugs are not any good
 
RustyRatRod said:
Neither are misinformed opinions. Yours.
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I've worked in the automotive industry for years, I've never known anyone to have good luck with them. My main job at garages was tune ups, so I'm speaking from experience not just a opinion
 
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