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gas exaust smell

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Category: The Garage
Forum Name: AMC V8 Engine Repair and Modifications
Forum Description: AMC-made V8 engine mechanical, ignition and fuel from basic repair to high-perf modifications
URL: https://theamcforum.com/forum/forum_posts.asp?TID=93650
Printed Date: Mar/28/2024 at 4:51am
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Topic: gas exaust smell
Posted By: ditdit
Subject: gas exaust smell
Date Posted: Apr/15/2018 at 11:38pm
I need some help if you all please , I have a 401 1968 AMX with 727 trany ,runs very well but after I shut it down after a trip and park it in the garage it has a bad smell of gas ,I am running a 600 cfm Edelbrock carb ,,I have tried running a Holly 750 vac secondary and the same thing ,on a open trip the pipes are brown ,I have tried retard, Advancing and dead on for timing ,I have 5  lbs of fuel pressure at the carb , the jets are stock , could the floats be off or too much jet size  ?????? 

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1968 AMX 401/727
1984 Eagle 4.0 org owner
1988 Eagle (time machine)
1980 Jeep J10 (under restoration)                 Carolina AMC (Senior Member)



Replies:
Posted By: mixed up
Date Posted: Apr/15/2018 at 11:53pm
with a holly carbs you might want to play with the jetting on it there always fat and check the flout level I set them so if you bump the car side to side it just trickles out

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69 amx 290 auto
65 220 290 4spd
80 ford fairmont


Posted By: PHAT69AMX
Date Posted: Apr/15/2018 at 11:57pm
What Fuel Pump?  Ya'know... a YouTube Guy does carbs with videos...
He shows / claims with modern gas gotta do like 3.5 ? or so psi !...
Yah... I know... but... vapor pressure... volatility... ethanol blend...
Ya know... the lower PSI Fuel Pressure... maybe somethin' to it...
Then the OBVIOUS question... what Intake Manifold...
And... is the Choke Stove Heat Crossover Blocked !?....
Any Carb Spacer?  Phenolic or otherwise?  Boiling Carb?


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Link to a http://www.youtube.com/watch?v=MySiKQsmWxU" rel="nofollow - Short YouTube Burnout Video



Posted By: Boris Badanov
Date Posted: Apr/16/2018 at 5:31am
As mentioned above, a boiling carb after shut down.

Are you using an aluminum intake manifold?

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Gremlin Dreams


Posted By: mmaher94087
Date Posted: Apr/16/2018 at 10:53am
"..1968 AMX ..". Original gas tank? They sometimes rust out on the top.

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Mike


Posted By: ditdit
Date Posted: Apr/16/2018 at 5:36pm
I am running a stock case iron intake , the exaust cross over is not blocked , the gas tank was treated about 10 years ago , I am running unleaded gas  and have tried regular ,middle and super blends , the vent on the gas tank is not blocked ,I am not running a spacer on the carb  , but I don't know if the fuel is boiling ,I will have to check it out    thanks guys

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1968 AMX 401/727
1984 Eagle 4.0 org owner
1988 Eagle (time machine)
1980 Jeep J10 (under restoration)                 Carolina AMC (Senior Member)


Posted By: PHAT69AMX
Date Posted: Apr/16/2018 at 6:25pm
Like the other thread about a 287 carby, the fumes you're smelling may be due to modern gasoline.
Fumes can also come from the correctly working vented gas tank.
Would imagine you've already checked it all over for no leaks.
If a stock type mechanical fuel pump, the diaphragm can age, allow fuel into the crankcase & oil,
or maybe even vent fumes out a small fuel pump casting "vent hole" above the diaphragm cavity.
Way Edelbrock Carbs are made, usually don't "drain themselves" after shutdown into the intake manifold.
Oh, is the Eddy Carb a Manual Chock Model or an Electric Choke Model ?
If the Needle Valves are "iffy" they could overfill the bowls after shutdown causing fumes.
If / ounce Bowls overfill causing fuel to flow out venturis, guess it could drain bowls by siphon action.
Or just today's "modern gas" is just boiling off more easily, maybe causing floats to drop.
Exhaust Heat Riser Valve still in place on passenger side exhaust manifold outlet?
If the fuel odor is a new occurrence, would image either a leak somewhere, or a boiling carb...

Your original question(s):
(revised) Jetting, if too rich, could cause gas smell once off and parked, unburned fuel in exhaust...
Incorrect Float Setting in either type of Carb could, yes.
Unsure about brown pipes, mine were always a nice pale gray when running the best.
Guess if it is too rich and/or has weak spark, unburned fuel could "collect" in the exhaust...
It won't burn there because it has no oxygen to use.
Pull the coil wire off the cap, place it near metal, and crank it over, it should SNAP, be white-blue.
If the spark makes no noise and/or is yellow or orange, it is a weak spark imho.
Also once sparkplugs get Fuel Fowled from any reason they are no good anymore they say....

Ah, and yes, I agree with what Greyhound is talking about in the next post as a possibility also...


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Link to a http://www.youtube.com/watch?v=MySiKQsmWxU" rel="nofollow - Short YouTube Burnout Video



Posted By: Greyhounds_AMX
Date Posted: Apr/16/2018 at 6:47pm
How close are your tailpipes to your gas tank?

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1968 AMX 390 w/T5


Posted By: ditdit
Date Posted: Apr/16/2018 at 6:53pm
I did notice a few months ago after I let it sit for 4 or 5 days and tried to start it there was little fuel in the carb so I had to crank the engine longer till the fuel come up to the carb, there is no heat riser ,I do have a old mechanical fuel pump it is not leaking on the outside I have checked the oil for gas smell and it is ok. the smell is comming from the exaust pipes ,both of them

-------------
1968 AMX 401/727
1984 Eagle 4.0 org owner
1988 Eagle (time machine)
1980 Jeep J10 (under restoration)                 Carolina AMC (Senior Member)


Posted By: ditdit
Date Posted: Apr/16/2018 at 7:00pm
i forgot to answer one question ,the exaust pipes are all stock the way it was mfg

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1968 AMX 401/727
1984 Eagle 4.0 org owner
1988 Eagle (time machine)
1980 Jeep J10 (under restoration)                 Carolina AMC (Senior Member)


Posted By: PHAT69AMX
Date Posted: Apr/16/2018 at 9:13pm
I'll contradict myself, Jetting could cause the smell, unburned gas in exhaust pipes..
Revised previous post...
Exhaust pipes and gas tank, proximity can "boil the tank", another "new fuel" possibility...
Once when "in-between" getting an exhaust finished, drove mine awhile with half-pipes,
no tailpipes, exhaust ended at rear end, and yep, it would boil the gas tank, lol ! 


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Link to a http://www.youtube.com/watch?v=MySiKQsmWxU" rel="nofollow - Short YouTube Burnout Video



Posted By: mixed up
Date Posted: Apr/17/2018 at 1:16am
you say the carb drains down while setting after a long drive park the car t hen look in the carb see if its dripping into the intake could have 1 a sunken float or 2 high float level causing the carb to siphon the fuel out of the bowls or bad needles & seats

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69 amx 290 auto
65 220 290 4spd
80 ford fairmont


Posted By: Red Devil
Date Posted: Apr/17/2018 at 6:43am
If your '68 has no charcoal canister to store fumes from the fuel tank and carburetor bowl vents, and both just vent to atmosphere, it will be pretty tough to control fuel smell from normal evaporation in an enclosed garage.

Suggest add a charcoal canister with appropriate connections to tank and carb like later models, if possible. 

Hope this helps, RD 


Posted By: Ken_Parkman
Date Posted: Apr/17/2018 at 10:35am
Yep - old cars stink. We are spoiled by newer stuff.


Posted By: akimmet
Date Posted: Apr/17/2018 at 11:28am
A little bit of gas smell is normal, but a lot of gas smell indicates a leak, pressure build up, or running extremely rich.

You seem to have checked most of the obvious stuff already.
Gas smell from the exhaust is almost always from the idle mix being set extremely rich.

Try tuning your idle mix with a vacuum gauge, it is far more accurate than tuning by ear.

It isn't uncommon for some carburetors to have ruined idle mix screws from people over-torquing them. Take one out and check for flat spots on the taper of the needle.

Float mis-adjustment, can cause an extremely rich condition as well.

The carburetor bowls being empty after sitting a week is somewhat normal for an aftermarket carburetor. Modern gas simply evaporates far more easily.


Posted By: ditdit
Date Posted: Apr/18/2018 at 1:37am
thank you all for the sugestions 

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1968 AMX 401/727
1984 Eagle 4.0 org owner
1988 Eagle (time machine)
1980 Jeep J10 (under restoration)                 Carolina AMC (Senior Member)


Posted By: RUMBLR
Date Posted: Apr/18/2018 at 5:02am
Id say more to do with fuel tank venting than a carb issue.

Old rubber fuel lines could also be an issue.

PTFE fuel lines may assist, and a filter of some sort or carbon canister on the vent line.


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343 904 M20 - 11.83@115


Posted By: purple72Gremlin
Date Posted: Apr/18/2018 at 9:33am
Originally posted by Ken_Parkman Ken_Parkman wrote:

Yep - old cars stink. We are spoiled by newer stuff.
true



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