Horrible MPG/Fuel odor troubleshoot

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rtee007

Deplorable Texan
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Jun 10, 2014
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Location
Baytown, Texas
Strong gasoline odor while driving with all windows down.
No visible gas leaks anywhere.
Very poor MPG (130mi for almost a full tank of gas)
5.9L Magnum crate with a Summit double pumper carb.

No leaks anywhere.
 
Way to rich on the carb along with openings or leaks in and around the trunk and rear end area is what it sounds like.

Exhaust all the way out the back?
Trunk seal?
Holes in the metal or body plugs missing?

The vacuum at the rear of the car from the car moving through the air WILL suck that exhaust into the cabin, and running way rich compounds the problem.
It would explain the mileage also.
 
Way to rich on the carb along with openings or leaks in and around the trunk and rear end area is what it sounds like.

Exhaust all the way out the back?
Trunk seal?
Holes in the metal or body plugs missing?

The vacuum at the rear of the car from the car moving through the air WILL suck that exhaust into the cabin, and running way rich compounds the problem.
It would explain the mileage also.

X2... It's also possible that your ignition system is outta wack too. Check all your spark plugs and make sure they're all sparking.
 
I would check your vacuum at idle in gear and at cruise and then adjust or replace power valve accordingly. Probably need to set idle air bypass as well to get those transfer slots covered.
 
Run your finger around the inside of the tailpipes. If it comes out sooty black, you are pig-rich.

If you are running that terribly rich you probably should change the oil right after fixing the carb, and long before you work it hard. Gas-diluted oil is not a good lubricant. It does however make a pretty good fire-starter.

Holley makes a power valve plug. Sometimes I bolt one of those in.Then I defeat the secondaries, and lean out the primaries, til the engine goes lean around 10 inches of vacuum,when attempting to accelerate in 2nd gear.Then I tune the PV, for full load. Then I reconnect the secondaries, and start leaning those out. The timing has to be close to right before you start, or you may be doing some parts over again; which is kindof normal.
 
Do those carburetors have power valves?
 
Do those carburetors have power valves?

I see your point. I saw DP and jumped to conclusions.
I don't know anything about summit brand carbs, but they have to have some form of power enrichment, probably activated by falling manifold vacuum. I would imagine.
In that case, substitute power enrichment system for PV. It's only a little trickier, to tune.
 
Make sure you are not running too rich and also check for vacuum leaks. Sounds like something with the carburetor for sure.
 
I was thinking hard that it is a carb issue. But dont know squat about this summit carb.
Never had an issue before. Car runs like a bat outta hell. But the smell of fuel will make you sick.
I did change from an M1 single plane to a Eddie Air Gap(previous owner used that as it came with the crate engine). It is a 5.9 Magnum crate engine with some mild mods. Could this be the trouble?

Pipes run all the way out the back.
Open 3.23 8.75.
4 speed standard.
 
So what's the inside of the tail pipes look like? (black and sooty?)

And does the car have places where the exhaust can come up from the back, like a trunk seal or body plugs (or other holes?)
 
Since when are the jets on the idle circuit?

When modifying carbs for certain race applications you eliminate the power valve. They recommend upping the jet size 10 steps. They also recommend making little "V" shaped wire inserts to restrict the idle circuits which are affected if they do not have the factory restrictor orifice.
 
When modifying carbs for certain race applications you eliminate the power valve. They recommend upping the jet size 10 steps. They also recommend making little "V" shaped wire inserts to restrict the idle circuits which are affected if they do not have the factory restrictor orifice.

I don't know who "they" are, but IMO, they are wrong. I've read all about that too and known people who've done it. It just never works as well as tuning with a power valve correctly.

Also, what I was getting at is, if it has a power valve and it is ruptured, that could be the whole problem.
 
I don't know who "they" are, but IMO, they are wrong. I've read all about that too and known people who've done it. It just never works as well as tuning with a power valve correctly.

Also, what I was getting at is, if it has a power valve and it is ruptured, that could be the whole problem.

Agreed on the power valve statement. Before we knew about the check valve trick or Holley doing it themselves it could get expensive going through power valves if you had a backfire. The plugging of the power valve was mainly recommended for applications where idle vacuum was so low because of a radical cam that the vacuum at WOT was enough to close the weak valve and cause a lean out. Not a good thing. Intermediate fuel mixture was always over rich but for "on-off" drag applications it was functional. It also became a crutch for lazy tuners or handy for out board carbs on mechanical six pack setups.
 
do you have a mechanical fuel pump? if so, maybe it's leaking internally or spraying on the exhaust.
 
If it is an AFB knock-off, then it has metering rods. Which are controlled by manifold vacuum acting against calibrated springs. Perhaps the calibration is off, or perhaps the metering rod calibration is off, or perhaps the manifold vacuum is off, or perhaps the timing is insufficiently far enough advanced to bring the vacuum up to what the springs are calibrated for. If the system is borderline out-of-whack, and the vacuum advance is not working, the springs can elevate the rods to where cruise might be rich.
Notice all the perhaps and mights.
Start with; making sure the choke is disengaged, that the engine is getting up to temp (180 to 195), that the valves are properly lashed,getting the timing into it, and ensure that the vacuum advance is working, and ensuring that there are no vacuum leaks, and make sure the Vcan is hooked up to the spark-port.Once you have those bugged out, you can move on,to the carb.'Course by now the plugs are probably done.
 
I have the correct answer:

"I don't know."

ONE mile per GALLON?

Sounds to ME like you have someone with an "overactive" siphon hose.
 
My Duster once was getting 8 miles to the gallon (on the freeway). And the exhaust smelled terrible.
Come to find out my Carter fuel pump (M6270) was putting out too much pressure (about 8 psi) and one of the brass floats in the 650 AVS developed a leak which filled up with fuel. Therefore causing a very rich condition. I Replaced the brass floats and installed a regulator and set it to 5.5 psi. I was getting 13.5 on the freeway (woot!)... Wait... What's this, why do I have green/yellow deposits on three of my spark plug on the passenger side... Doh! Yep blown head gasket... Darn! Replaced that as well and new sparkplugs.
Now I'm getting around 17 mpg on the freeway on a warm day and about 15 -15.5 mpg when its about 35 degrees outside. (darn airgap).
Sorry for my little story, but my point is: it can be a combination of things. (hopefully not your case) It can be frustrating to "find" more issues when you're not meaning to.

By the way what kind of fuel pump do you have?
 
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