340 idles at 1500 won't stay running in gear

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SR77

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72 dodge demon 340 with a 408 stroker and a 727. Here's some back story. i was coming from a cruise and immediately noticed it running rough. as i came to a light i noticed the headlights dimmed, it started idling rough and eventually died. It started right back up, put it in neutral and played with the gas to keep it running. I had to keep doing this all the way home, slap it in neutral give it some gas.
Got home checked the battery is was 12.87v, checked the alternator it was putting out 14v at the battery Picked up a vacuum tester, in idle I'm pulling 10 inches and in gear it drops to 5. I do have a pretty aggressive cam but don't have the specifics, i believe it has a solid lifter since it was an older bracket racer running nitrous .
I've cleaned the carb, pulled the front fuel bowl and checked the jets. Switched the plugs and they where fouled from not running it alot, dry carbon couple where a little wet.
Right now it will start up and idle but i have to feather it up to about 1500 and it will stay at that with my foot off pedal running. If i put it in gear it dies almost immediately.
I was able to get it to idle then in gear and drive a little down my street but then i could feel it start dying. This was a couple days after it initially went down and it has progressively gotten worse. I've also noticed that i have like a black soot built up on the outside of the manifold on the drivers side down by the valve cover.

the motor was built in 08 by koffels place, didn't get any build specs just a few things i was told when i was talking to the previous owner over the phone while driving. It does have a holley street avenger 870 with electric choke, and a edelbrock rpm intake manifold edelbrock heads.

I think it's a bad intake manifold gasket, tried the spray test but didn't notice any thing. I've also checked the vac line to the pcv and at one point blocked off all of the vac ports on the carb .
I noticed the current intake manifold gasket is like a blue paper 2 piece gasket.
I'm thinking intake manifold gasket, if it sounds like it's bad what are some tips/ tricks for replacing it, I've never done one before
 
have you checked to see if the intake bolts are loose?
 
Did the engine backfire at any time during the cruise? Holleys have power valves that can blow out and cause this.
 
sure the needle and seat are clean? you say ya checked the jets but said nothing about the needle and seat..

vac leak, timing??
 
electric choke is opening

didnt check the bolts yet

havent checked the needle and seat. first time looking at the needle and seat, what would i be looking for? dirt build up, something cracked?
 
Gently close the choke while it is running. If it continues to run, it has a vacuum leak.
 
SR77 said:
electric choke is opening

didnt check the bolts yet

havent checked the needle and seat. first time looking at the needle and seat, what would i be looking for? dirt build up, something cracked?
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Pull them out and blow them out. Could be dirt that you can't see with the naked eye allowing gas past.
 
Based on the way the OP described the engine, I was thinking valve lash too. But then again, valves being out of spec wouldn't happen all of a sudden they way he described. Still, poorly adjusted valves could make it more difficult to find the real issue.
 
my hemi rr did that and found the air screw wound down by itself? couldnt wind it out all the way ,casting tight? so i poured alittle locktight down and prob. solved!
 
thanks for the replies, i at least have some other things to look at
 
This sounds exactly like what happened to me in my Duster. After taking carb off and sending back to the shop that rebuilt it, I took it out and noticed a lot valve noise. Tried to adjust and got to number 5 and I couldn't adjust it. The ball was gone off the push rod. Got it out and, decided to do a cam swap. While removing the intake I found the two center bolt on the passenger side were finger loose. I'm pretty sure that was the cause for it running like yours is now.
 
i checked the intake manifold bolts they where tight, pulled the needle seat and found whitish crust around the o ring, after cleaning it seen the o ring was turning white and was like dry rotted, it was still one piece but you could see very small cracks going around it. also the o ring itself was able to slide around, i dont know if it applies but i seen a video on youtube where someone was saying it shouldnt do that

going to get a new one and see what happens
 
toolmanmike said:
Did the engine backfire at any time during the cruise? Holleys have power valves that can blow out and cause this.
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Modern Holleys have a check ball to prevent this from happening. They've had them for 20 years.
 
Did you do the choke test? This costs nothing. No money, no labor. You simply ease the choke shut. No tearing anything down, no spending money. A five second check. .....but you tore something apart instead.
 
any O-ring you see that is white, dry rotted and cracking should be replaced.....thats a given. Still sounds like a vacuum leak to me. Mighty-vac every dashpot you have and see if they all hold pressure. Carb base gasket...?
 
RustyRatRod said:
Did you do the choke test? This costs nothing. No money, no labor. You simply ease the choke shut. No tearing anything down, no spending money. A five second check. .....but you tore something apart instead.
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checked the choke early on I'll post pics of the soot I'm getting coming from the front of the intake manifold on the driver side. it starts up by the front corner and works its way back
 
Where's your initial timing at? Should be between 14 and 20. Maybe your distributor hold down loosened up and your dist moved...
 
Get yourself a can of Ether and spray around the bottom of your intake manifold and at the base of the carb. If your engine idle changes then that is where your problem is.
 
Wow. The amount of recommended diagnosis tricks given in this thread in a short amount of time, and most likely previously proven tricks as well. This has to be one of the best gearhead forums on the net.
 
And the white crud is usually water. Are you running gas with alcohol in it?The alcohol attracts water like flies to...
And soot speaks to fatness, as in running very rich.

BTW ether is extremely flammable. Do not get it on yourself. It works really well, but I don't use it anymore except in extreme cases.
 
EL5DEMON340 said:
Where's your initial timing at? Should be between 14 and 20. Maybe your distributor hold down loosened up and your dist moved...
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That actually happened to me once when I stopped for gas. For some strange reason I thought to check that, and the distributor was loose.
 
replaced the needle and seat last night, no big difference. i did however get some carb cleaner and made another run at the intake manifold. this time i sprayed enough to make a puddle in one spot and heard it level out. so i have a vacuum leak one the drivers side i/m right in the middle. checked the bolts again and they are nice and tight.

which leads me to my next question, is there one recommend gasket or way of doing it, I've never had to do one before.

appreciate all the help guys, I'm still learning as I go with this car and this site has helped a lot.
 
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