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Engine issues carb leaking into engine

Benjamin Nelson

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Hey everybody, I’m having some issues with the 440 in my 72 challenger. When I shut the engine off, after about 30 seconds I hear a trickling noise and I took the air filter off and saw that one of the barrels was filling up with gas. I have a video that shows what I’m talking about. The second issue is when the engine is running and I get out of the car, the driver side exhaust sounds good but there is a chuffing noise coming from the passenger side exhaust pipe. I have a Holley 850 double pumper carburettor and a stock fuel pump, engine is a stock 67 440. I have cherry bomb glass packs and 3 inch exhaust all throughout. Any help is appreciated as I’m pretty new to this.

thanks, Ben

Carburettor not working

Engine exhaust issue
 

Chryco Psycho

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It could just be heat boiling the fuel in the carb pushing the fuel into the intake , blocking the the heat crossover by using a 1215 intake gasket to block the port .
I would do a compression test , you might have a bad valve making the weird noise in the exhaust .
 

Benjamin Nelson

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I’ll check the compression, I doubt it’s fuel boil because I can walk over to the engine and put my hand in the intake manifold, I’m not running a thermostat so it’s running under 100 degrees, I can hold my hand on the radiator.
 

moparleo

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850 double pumper on a stock street motor is overkill. Too much Carb. Replace it with the proper type and size. Your carb problems will all go away and it will run much better. Is the carb/manifold hot when it is doing this ? Was this setup on the car when you got it or are these your mods ? If so ,why?
Why is the choke plate missing ?? Trying to get more HP ? Also replace the plastic filter with a metal one. Fire hazard. What is the running fuel pressure since you went through the trouble of putting a gauge on ?
 
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Carolinachallen

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I agree that a 850 is too much carb for that engine, but that's not the reason for the fuel spill over. I would check the float level first.
 

moparlee

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I’ll check the compression, I doubt it’s fuel boil because I can walk over to the engine and put my hand in the intake manifold, I’m not running a thermostat so it’s running under 100 degrees, I can hold my hand on the radiator.
Benjamin, why aren't you running a thermostat? You need a least a 180 thermostat to get any kind of decent combustion.
 

Chryco Psycho

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Weird the factory used the 850 CFM Thermoquad on the 440 , 750 is too small for a 440 , running a 650 you might as well install a 2 bbl.
I agree you need to run a 180* thermostat to get proper combustion , running it too cold will not help anything .
I know he has plans to race this car & I assume will be upgrading the engine in which case the 850 will be too small . the 440 should run fine with an 850 , it is still under the 2x the CI I use as rule of thumb for carbs & it is Not the problem for it leaking or pushing fuel into the intake , it comes down to 3-4 things , too much fuel pressure , unlikely with a stock fuel pump , sunken float not shutting off the fuel pressure , bad needle & seat leaking fuel past , or heat boiling the fuel after shut down .
 
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Chryco Psycho

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I have told this story a few times -
I was running a 440 Duster with 516 with 1.81/2.14 valves closed chamber ported Iron heads , it was a nothing special build , forged 10-1 flat top pistons & stock rods with 2400 gram bobweight it had a cam intake & headers , first I ran it with a 850 DP Holley , it ran well , I changed to a 950 carb & the car ran faster in the 1/4 , then I swapped to a 4500 series team G intake & a 1050 dominator , using the dominator the car ran its best times ever mid 11s @ 118mph , it had wipers heater stock bucket seats & was completely street legal , the Duster was fastest & got the best fuel milage with the 1050 at around 14 MPG , it had awesome response & had the best street drivability with the Dominator as well as the Dominator is a 3 circuit carb .
I have had the same results with many customer cars , you need 2x the CI , I would Only use a 750 on a 340/360 build it is too small even for the 408/412 La stroker builds , not even mentioning what you need for a 512ci build , 950 would be the minimum for 512 .
 

sir_veza

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Hey Chryco,
I know you have much more knowledge than me about mechanical issues and also helped me a lot with my project issues etc. (However if my math is weird, please set me straight). The formula I’ve ever used for calculating how much CFM an engine requires is: CFM x Max Usual Operating RPM x Volumetric Efficiency ÷ 3456.

A stock motor will have a volumetric efficiency of about 80%. Most rebuilt street motors with average bolt-on’s have a volumetric efficiency of about 85%, while race motors can range from 95% up to 110%.I’m using the .95% factor for this 440 motor

440 CID engine x 6000 max operating rpm = 2640000

2640000 x .95 = 2508000

2508000 ÷ 3456 = 725.7 CFM

That’s why I feel the 700 CFM would work
 

moparleo

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You seem to know more about his plans than we do. An aftermarket Holley double pump was not the factory carb. Jetting ? Power valve size ? Double pump for the street ? Why ? Not to get anybody mad but it sounds like rookie, magazine article add ons.
So unlike the Thermo-quad which the engine was originally built and tuned for is not a fair comparison. He says "stock": 440 but from the short video it looks like has done many things that are more like Magazine mods than thought out performance enhancements.
No Thermostat ? No choke plate ? Plastic fuel filter ? What is the rest of the story ?
He needs personal assistance where someone can actually see what he is working with and what exactly he is trying to accomplish. Then he can get good recommendations based on known info not assumptions and guesses as to what we are working with and trying to fix ?
 

Chryco Psycho

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I agree from a math perspective it doesn't work .
Holley has a 500 CFM 2 bbl & a 400 cfm 4 bbl , how is this possible well 2 bbls are tested at a different vacuum 3" vs 1.5 .
My belief is the carb ratings are not 100% accurate either , so maybe a 750 really flows 600 I am not sure .
All I can tell you from 45 years tuning & racing with carbs is 2x the CI is far more accurate than the math , making the 850 barely sufficient to feed a 440 which is why there were so many multi carb systems in the 50s & 60s , the Dominator was created around 68 as they could not get more air flow through a 4150 throttle body to get well past 1000 cfm , they even used a 4150 type 3 bbl carb , 2 primaries & a single oval secondary to get to 950 cfm ! In 71 Carter designed the TQ for Chrysler the small version was 800 cfm used on small blocks & the largest was 1000 cfm .
Lets look at the 6 pack , do the math 1350 cfm using 2 500 cfm & a single center 350 cfm carb , but wait there are rated as 2 bbls , rated as 4 bbls they flow approx 900-950 cfm Far too big for a 440 right so the 6 bbl was a dog right ? Nope the opposite it makes more power with more air flow . I have had bets with people lent them carbs & tuned 100s of carbs over the decades more air flow works , I never did try a 1150 dominator on my 440 but the 1050 was the best carb I ever ran in that engine .
I built a slant 6 milled the head .100 & ported the head & swapped on a alum 2bbl intake with a Holley 500 cfm 2bbl the power increase was rediculous even with the stock exhuast manifold in place , a 400 cfm 4bbl possibly would have been a better choice yet , which I did do on a friends slant !
I have no dog in this fight you don't have to believe me or trust me , but it works .......
 
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fasjac

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Hello Ben,
Making progress at your age and experience is great. I’d start with getting the engine to operating temp. Put a tstat in at least a 160-180. Definitely may have a carb issue but, get it to warm up first. Things will probably change. 100 degrees is just too cool.
 
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Benjamin Nelson

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Ok thanks for the help everyone.
Moparleo, if I’m being honest I don’t really know what I’m doing lol. The carburettor has no choke plate when I got it because I could t spend upwards of 600 dollars on a new carb, a plastic fuel filter was the cheapest option and it allows me to see if fuel is actually running and if it’s full of rust. The carb was used on a 502 ci big block Chevy and I havnt changed the jetting so I’m sure it’s running very rich, I pulled one of the plugs and it was more black than I would like it to be. I have a thermostat sitting somewhere that I will install in the car. When I was re sealing the oil pan the other day I noticed that I have a lot of slop in the timing chain. Probabaly close to half an inch Either way. Today when I was fiddling around with the carb I gave it a few taps with a large screwdriver on the secondary float bowl and that seemed to fix my flooding issue. I fiddled with the fuel air screws in the side of the carb and finally ended up around 18 inches of vacuum at 800 rpm or so. Was I was doing that it started popping and backfiring a little bit. I havnt checked the timing yet so that’s probabaly my next move.
 

Benjamin Nelson

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Also I’m 15 and havnt been able to work lately due to an injury so I can’t really just run down to summit racing and buy a new carburettor, we don’t have stores like that up here in Canada anyway.
 

moparleo

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15.. Congrats. Good for you. A lot to learn but lots of time to do it. My point is that the more info available, the more complete the story, The less assumptions and guessing get done and more things can be learned. Total cfm flow really doesn't matter in most cases as most people are not racing their cars and have little tuning experience.
With any project, learn to walk before you worry about running..
I find out that for some reason people always make things more technical and complicated than they usually are. And make a lot of assumptions based on THEIR personal experience.
I ran several repair shops and dealt with 1000's of customers over the years. I learned not to assume anything. Get as much detailed info as possible from the customer. To try to explain things in the simplest terms so that even the most non mechanical person can have some understanding about their car.
Mistakes are another word for experience.
Everybody makes them. Cars are like any other skill...Like learning to read or do math. You must have a firm grip on the basics and concepts before you start to worry about Literature or advanced mathematics.
Master the basics and the good stuff with come.
The Factory Service Manual is your best friend. It has the factory specifications, diagrams, information and instructions on all areas of your cars mechanical systems. Maintenance, suspensions, brakes, engines, transmissions, tune-ups and more.
Download as much as you want and study it. It s hard to try to diagnose and fix something when you are really not sure exactly what is happening and why.
Resource Library – MyMopar
Service Manuals, Parts Catalogs & How-To Guides
When you have questions just remember that we only know what you tell and show us, we are not there.
More information equals better answers for you.
Doesn't matter what your age is we are all Mopar.
 
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