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Old 11-01-2021, 03:16 AM
Johng49 Johng49 is offline
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Default Need help with big block vacuum leak

Hello! IÂ’m new here, IÂ’m 22 and picked up a 73 challenger back in April and itÂ’s my first time working with a carbureted engine. IÂ’ve done lots of research, thinking, and tinkering. IÂ’ve had to fix/replace a bit on the car since I got it and IÂ’ve done pretty good all by myself. Everything is great but I canÂ’t fix one problem that I believe is some sort of vacuum leak. IÂ’ll start by describing my engine, thereÂ’s a whole story behind it but IÂ’m not 100% whatÂ’s all been done to it.

ItÂ’s a 361 big block, with 906 heads. When I had the valve covers off and the valley pan removed I noticed that the valve springs and cam seemed aftermarket but I donÂ’t know much about determining what they are. It came with an edelbrock tm6 intake and IÂ’ve put a Holley 750 on it. ItÂ’s an auto 727 with slapstick and an 8 3/4 rear with 3.55 gears im pretty sure.

When I first got it, if I would quickly jab the pedal she would fall flat on her face and occasionally backfire through the carb. The idle wasnÂ’t rough or at least nothing noticeable, so I figured I would try bigger accelerator pump nozzles and that seemed to help a bit with the fumbling but would still backfire. One day I started it up and I couldnÂ’t even get it to idle and using carb cleaner I found the one side of the intake manifold was leaking. I got new gaskets and a valley pan and it seemed to fix it.

The problem is it still backfired through the carb, and if I check the vacuum on the ported side itÂ’s not 0, itÂ’s like 3. I assume itÂ’s a vacuum leak and I was spraying around with carb cleaner again and it wasnÂ’t leaking in any normal spots that I noticed, but if I sprayed it into the air gap around the heat crossover in the heads, it would bog down. It does it on both sides of the engine. So when I got the gasket and valley pan the gaskets had the heat crossover open but the valley pan did not so I decided to cut a little hole the same size as the gaskets on the pan.

IÂ’m not sure why spraying into the gap would cause it to bog down but maybe thatÂ’s the source of my vacuum leak? My thought is that maybe itÂ’s getting in through where the crossover meets the intake somehow and then maybe thereÂ’s a pin hole in the intake? Any help is appreciated this problem is really stumping me.
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Old 11-04-2021, 01:45 PM
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creative1 creative1 is offline
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the purpose of the "heat crossover' is to move warm air from the cylinder heads to the carburetor to speed up warming of the carb. this helps prevent icing in cool weather. there should be NO vacuum associated with it. a properly built engine shouldnt need paper gaskets with the valley pan, the pan should be all the sealing needed. if it is being reused, you MIGHT need the paper gaskets to make up for the 'crush' that occurred during the original installation. if you blew the paper gaskets out (on the valley pan), that is a pretty violent backfire. a backfire on quick acceleration is almost ALWAYS a lean condition or a timing problem. yes, a lean condition CAN be caused by a vacuum leak, but that usually only involves 1 or 2 cylinders.

what you MIGHT be dealing with is improperly machined heads. if mopar heads are milled, they MUST be milled on both the deck and intake surfaces, and by different amounts, or the intake will not mate properly. 3" of vacuum seems awfully low, but you said you were measuring the ported side, measure your vacuum directly from the intake manifold. the ported vacuum is for vacuum advance for street driven, mostly stock engines. that is about all of the help i can give you, except that on my 440, if i try to use the paper gaskets with the valley pan, the intake bolts wont start. the fact that you appear to have blown them out indicates that something is wrong where your intake mates to the heads. also, the carb cleaner causing the engine to bog is an indication of an over-rich condition. the thing is that there should not be any connection between the heat riser and the vacuum portion of the fuel system, including the carb. again, the ONLY purpose of that heat riser stuff is to help warm the carburetor and intake manifold quickly, and by doing that, help prevent carburetor icing.

all this being said, i am sure that there will be others that have dealt with similar issues here, and will have information that you can use. i hope there is something simple. without taking the engine apart, i don't know of any way to identify things like camshafts and valve springs. even then things can be difficult. oh... if your manifold vacuum is less than 10", you will have some issues, and may have to install a system to address that issue. good luck.
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