78 460 backfiring through intake
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
If as you stated the timing is correct, the v/a is working and you've tried two seperate carbs and you still have a sporadic backfire through the carb then as others have mentioned you need to start looking for a vacuum leak, it may not be enough to cause an erratic idle but under acceleration it may lean out the air/fuel mixture enough to cause a random backfire.
Senior User
Joined: Dec 2007
Posts: 118
Likes: 1
From: Colorado
When I initially started up my new 460 a few months ago, I had the same issue. I thought it was my timing, but it ended up as a vacuum leak around the carb spacer gasket. Definately check for vacuum leaks.
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Freshman User
Joined: Apr 2009
Posts: 49
Likes: 0
intake and gasket are all new. had mechanic friend do it dont really think there is vaccum leaks. havent checked with carb and choke cleaner but i put one hand over carb and it snuffs out very easy. strange thing about the two cylinders firing at once. was talking to another mechanic friend and he mentioned that too today. so i changed out the whole set of wires with another set and problem significantly reduced but still does it. now i dont know if that coincidence or not i will have to let you know tomorrow.
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Freshman User
Joined: Apr 2009
Posts: 49
Likes: 0
tried that already a couple times. advanced a bit, retard a bit. about 3 times each way same spaz thing. but thanks any help is great. never know i might have overlooked the obvious thats why i ask on here fresh mind helps. 10 fresh minds is even better. lol
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Freshman User
Joined: Apr 2009
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engine is used from another 78 f350 camper special with 75,000kms on it by the looks of things. dont think the odometer has been rapped around because the inside under valve covers and inside oil pan is ridiculously clean. old plug wires where stamped 1978 and i think are original wires. the ones i put on where from the 400 i pulled out but they have been replaced a while ago so not new by any means. but more recent than the ones that where on it. about 20,000km and ten years old. im thinking im going to grab a can of cleaner if no vaccum leaks then set of nice new wires. once i put the other set on i really had to try to get it to do it.
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MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Doesn't matter what anyone thinks, stuff happens. You won't know until you find out. Let us know what you find.
10 year old wires? It's time for new wires...
10 year old wires? It's time for new wires...
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Freshman User
Joined: Apr 2009
Posts: 49
Likes: 0
yup but from my experience when trying to suffocate it your rpms will go up a bit before stalling if vaccum leaks exist. im still gonna run around everything with cleaner when i go grab some. i dont trust anyones work even my own at times. and not to sure how much plug wires weather when truck doesnt move for 5 years. and only problem with checking it at night for stay spark is that its hard to get it to do it without load. so i need to borrow a midget to hide under the hood while i drive.
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Freshman User
Joined: Apr 2009
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well i found a vaccum leak but apearetly it supposed to be there according to edelbrock. its right near bottom right of electric choke pull off cap. dont make sense to me. im going to plug it anyways when i find a male plug with a head on it so i can get it out if i ever need to. anyone know the real purpose of this hole? nearest i can tell is it pulls ambient air over choke too pull off sooner if warm out. but its not metered and not filtered. so for what im doing and plan on having water flying around under there its getting plugged
Zombie Survivalist
Joined: Feb 2009
Posts: 14,809
Likes: 3
From: Rock Springs, WY, USA
Sounds like you may have found it. and as for wires, it is recommended to change them every 50,000 miles, or 3 to 5 years. wires are kinda like tires, let em sit long enough, and they are going to get bad. had the same problem on my 302 when i got it running. ran on 7 cylinders all the time and 6 most of the time. the wires looked new, so i didnt mess with them. then i looked at it at night and saw pretty blue arcs all over my motor. kinda cool. then i bought new wires and WOW. what a difference.
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