Backfire out exhaust ?
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Junior User
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Backfire out exhaust ?
Just saw this link and been posting in m block and not getting help. so, 351m all stock but timming set and dual exhaust. all vac lines on with valves, new exhaust. no leaks on either. carb has to be set at 4 whole turns out stock 2b. as soon as I put the vac line on the air bypass valve the after rev letoff will backfire out the exhaust. I put the timing set at 0 and not -4 as stock would be. any help please. 
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Joined: Jun 2003
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From: Houston/Hope BC
Excuse me, but several of us have responded to your post in the 335 forum. Posting the same question in more than one forum is not very efficient for you or any of us who take the time to try to help you. Please pick one and post a referral to it in the other and and we will do what we can to help you fiqure out the problem. Now would some one please explain to me what an "air bypass valve" does?
Last edited by Ecuri; May 13, 2004 at 10:18 PM.
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Well exxxcccuuussseee ME. The air bypass valve is the inline valve between the smog pump and the air check valve. The smog pump pump air into the engine for the smog equipped engines and the check valve is the about 5 or 6" long 15 degree bent with a bell like end ( the valve ) that screws into the front corner of the intake manifold. If you dont know what these parts are you most not worry about smog laws. Oh, some people cal the ABV a air management valve. I was tring to get the right link as to get someone to answer. I have seen posts that get blown off if there in the wrong link !!!!!!
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Well I would say its the valve then, we don't have emissions/ my 351w was bare (emissions wise) when I bought it. Backfire can mean there is a lean condition, vacuum leak, if the valves purpose is to open at a certain time with a certain vacuum signal then I would think your getting too much air too soon, hence the backfire/ lean condition. So my vote is faulty valve, does it backfire when you unhook the vacuum signal from the valve? If not then I would definitely say it the valve. later
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I have changed both valves, all lines, the EGR, new intake set, new exhaust, rebuilt carb and umpteen times for all for leaks. This is 1 point I jst have been kicking around , I put a new brake booster on, since it works with so much vac pressure could there be a leak in it ? The carb mix screws are out 4 full turns and should be 2 to 2 1/2 turns. This I dont get. Im getting to the point theres no point in opening up the hood because Ive been over so many things so many times, I dont know what else to do. Sorry for the above atitude, Im get so 
at this Bronco.
at this Bronco.
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Well a lot of people spray a little brake cleaner, while the engine idles, at possible points on the engine that could be causing a vacuum leak, if rpms increase you have found it. If you do that and nothing comes up then I would Double check your initial timing, get number 1 cylinder at top dead center on the compression stroke, then what I do is line the pointer up with the initial timing that I want, stock specs would be 6 BTDC, then I mark number 1 terminal on the distributor caps base remove cap, loosen distributor, line the rotor up so it points to the number 1 terminal mark on the lower cap. If its possible that the initial timing could be incorrect, and you find it is, I would reset the carb screw back to 2 1/2 turns out, then test drive it.
It gets frustrating when things don't work just so......I get that. Good luck let us know how its coming along
It gets frustrating when things don't work just so......I get that. Good luck let us know how its coming along
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From: Houston/Hope BC
Checking in my book on emmisions controls... the basic idea I get about what they are calling the air-bypass valve is that it is supposed to dump the fresh air from the pump to atmosphere during deceleration. If I read you right from the first post, this sounds like the problem right? Backfire on deceleration?
There are multiple schemes for controlling the valve and they can become very complex. The simplest has one vacuum hose running to manifold vacuum. (Normally open style, there are also normally closed styles also.) You may have some other component in the vacuum contol lines upstream that are malfunctioning. I would start by putting a vacuum guage in the control line and see if it is registering high vacuum on de-cel and/or that the ABV does indeed dump its air to atmosphere under high vacuum. You may need to located the vacuum diagram for your particular engine. But basically you want to see the valve start dumping air when you decelerate. GL
There are multiple schemes for controlling the valve and they can become very complex. The simplest has one vacuum hose running to manifold vacuum. (Normally open style, there are also normally closed styles also.) You may have some other component in the vacuum contol lines upstream that are malfunctioning. I would start by putting a vacuum guage in the control line and see if it is registering high vacuum on de-cel and/or that the ABV does indeed dump its air to atmosphere under high vacuum. You may need to located the vacuum diagram for your particular engine. But basically you want to see the valve start dumping air when you decelerate. GL
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351w, air, backfire, backfires, backfiring, bronco, deceleration, decelleration, exhaust, ford, management, model, timing, truck, v8, valve
