Frustrated beyond belief
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Senior User
Joined: Oct 2002
Posts: 164
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From: Mount Vernon, Washington
Frustrated beyond belief
Any ideas on this would be great. Here's the sitcheeashin. 65 F-250 w/72 300/6. All stock engine. Idles perfectly and will drive if I give it just a tiny, tiny bit of throttle but starts to start "popping/backfiring" with anymore than a little throttle. Seems to be coming from the carb not the exhaust. Also the tailpipe is under the truck, so I can't see what color the exhaust is when this is happening. Did this randomly a few times before but now is constant.
Perhaps unrelated, for quite a time before this began there has been a whistle coming from the engine. It sounds like an vacuum leak but I couldn't seem to find one, even replaced the carb. The "whistle" starts after the truck is warm. Could the intake manifold be cracked and cause this? Oh and all the ignition parts are new. I broke down and bought somethin else to drive till I figure this out but I hate not bein in my Ford
Thanks-Mike
Perhaps unrelated, for quite a time before this began there has been a whistle coming from the engine. It sounds like an vacuum leak but I couldn't seem to find one, even replaced the carb. The "whistle" starts after the truck is warm. Could the intake manifold be cracked and cause this? Oh and all the ignition parts are new. I broke down and bought somethin else to drive till I figure this out but I hate not bein in my Ford
Thanks-Mike
Post Fiend
Joined: Jan 2000
Posts: 5,860
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From: Metro Detroit (Redford)
Frustrated beyond belief
Shot in the dark: Could a couple spark plug wires be on the wrong plugs so they fire at the wrong time?
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INLINE SIX POWER!
'95 F150 XL300 Cubic Inches of Low RPM Truck Torque! And twin-I-beams too!
"Drive a stick young man! There'll be time for automatics when you're old and unable."
New User
Joined: Feb 2002
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From: Cottonwood USA
Frustrated beyond belief
a couple of questions and we could probably solve this!! does the truck have a vacuum advance or a mechanical advace. if it has a vacuum advance it could have a hole in the diaphragm. also does the truck have an ecm? mine backfired horribly at speed when the ecm went bad. also the first ecm i bought was also bad!! brand new out of the box it didn't work!! so i checked everything else before i realized the new ecm was bad!! azronster
78' f-150 4x4 351 windsor
79' f-150 4x4 300 6-cylinder
78' f-150 4x4 351 windsor
79' f-150 4x4 300 6-cylinder
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Senior User
Joined: Oct 2002
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From: Mount Vernon, Washington
Frustrated beyond belief
Checked the plug wires and they are good. No, there is no ecm and yes it has a vacuum advance. I moved the advance by hand and it seems to have some resistance to movment(IE:it doesn't just slide in and out like the diaphram is bad)but maybe that type of testing is inconclusive, maybe I'll get a vacuum pump and try that. Someone suggested that the intake manifold on these engines have a tendancy to crack, can anybody back this up? Thanks for the help.-Mike
Hotshot
Joined: Oct 2002
Posts: 18,488
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From: The Big, Oregon
Frustrated beyond belief
I'd put money on the intake, but probably not a crack. Does it have an exhaust leak? If so, locate which cylinder, and you have found your intake leak as well. The only other things I can think of would be a cracked intake valve, possibly a cracked or burned exhaust valve, or a possible cracked head, between intake and exhaust.
Jared
Jared
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Frustrated beyond belief
[updated:LAST EDITED ON 20-Nov-02 AT 11:17 PM (EST)]Popping and backfiring leads to me to think it is an ignition / timing problem - especially when it's out of the carb. Pull a vacuum line of the intake and see how it runs....vacuum leaks usually have a lopey idle and sometimes won't allow the engine to run without giving it gas. The intake and exhaust manifolds are known to crack, but I don't think that side of the engine is your problem. You mentioned ignition is new, have you checked the points and condensor or did you upgrade to electronic ignition? The pre-73 ('68? - '72) forum is great when it comes to these old dizzies.
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Senior User
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From: Mount Vernon, Washington
Frustrated beyond belief
>Popping and backfiring leads to me to think it is an
>ignition / timing problem - especially when it's out of the
>carb. Pull a vacuum line of the intake and see how it
>runs....vacuum leaks usually have a lopey idle and sometimes
>won't allow the engine to run without giving it gas. You mentioned
>ignition is new, have you checked the points and condensor
>or did you upgrade to electronic ignition?
I put in a new set of points and condenser, cap, rotor,etc. No new coil though. The engine idles fine but coughs when I get on the gas with a load on the motor, its fine to rev in neutral. There's really only one vacuum line running from the carb to the vacuum advance and puting my hand over the intake doesn't speed or slow the motor any, so I don't think it's a vacuum leak. I did notice the advance springs on my dizzy aren't tight, but I don't think this would make such an overnight difference. I have a electronic dizzy but no ignition box, what year truck would I need the box from to switch over and see if that takes care of the problem? Thank a bunch-Mike
>ignition / timing problem - especially when it's out of the
>carb. Pull a vacuum line of the intake and see how it
>runs....vacuum leaks usually have a lopey idle and sometimes
>won't allow the engine to run without giving it gas. You mentioned
>ignition is new, have you checked the points and condensor
>or did you upgrade to electronic ignition?
I put in a new set of points and condenser, cap, rotor,etc. No new coil though. The engine idles fine but coughs when I get on the gas with a load on the motor, its fine to rev in neutral. There's really only one vacuum line running from the carb to the vacuum advance and puting my hand over the intake doesn't speed or slow the motor any, so I don't think it's a vacuum leak. I did notice the advance springs on my dizzy aren't tight, but I don't think this would make such an overnight difference. I have a electronic dizzy but no ignition box, what year truck would I need the box from to switch over and see if that takes care of the problem? Thank a bunch-Mike
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Joined: Nov 2002
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From: West Coast
Frustrated beyond belief
Sounds like it is leaning out.
Vacuum leak is a good suspect. How about your brake booster?
Carbs work off of the velocity of air passing through the venturi. The velocity of the air creates a vacuum which draws the fuel up from the fuel bowl and through the metering jets to mix with the air. As you open the throttle, more air flows to the engine, but the velocity will drop until the engine RPM begins to increase. To compensate for this, the carb has an enrichment pump that squirts a little shot of fuel as you mash the gas pedal down.
The enrichment pump is called different names on different carbs and they do fail pretty commonly. The symptom usually falls along the lines of the engine will run OK if you slowly push down the gas, but faster causes stumbling, to the point that in same cases, snapping the go pedal down will stall the motor. Holley carbs have a "power valve" that is notorious for blowing if there's even a slight backfire.
To diagnose: Engine off, look down the throat of the carb and operate the throttle. You should see a shot of fuel being squirted as the throttle opens. (on some carbs, this might be hard to see).
Vacuum leak is a good suspect. How about your brake booster?
Carbs work off of the velocity of air passing through the venturi. The velocity of the air creates a vacuum which draws the fuel up from the fuel bowl and through the metering jets to mix with the air. As you open the throttle, more air flows to the engine, but the velocity will drop until the engine RPM begins to increase. To compensate for this, the carb has an enrichment pump that squirts a little shot of fuel as you mash the gas pedal down.
The enrichment pump is called different names on different carbs and they do fail pretty commonly. The symptom usually falls along the lines of the engine will run OK if you slowly push down the gas, but faster causes stumbling, to the point that in same cases, snapping the go pedal down will stall the motor. Holley carbs have a "power valve" that is notorious for blowing if there's even a slight backfire.
To diagnose: Engine off, look down the throat of the carb and operate the throttle. You should see a shot of fuel being squirted as the throttle opens. (on some carbs, this might be hard to see).
Postmaster
Joined: May 2002
Posts: 3,241
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From: Broken Arrow, OK
Frustrated beyond belief
My 90 has done this exact thing twice in the last 2 months. Both times it was the same thing; a bad spark plug in #3. Don't know why the plugs are going bad, but the truck runs like *** when it happens. Does fine at light throttle, but anything above 1/3 makes it spit, sputter and generally make a mess of itself. I found the problem by going out in the driveway after dark and running the truck with the hood up. Every once in a while I could see spark coming out of the wire boot.
Freshman User
Joined: Nov 2002
Posts: 42
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From: West Coast
Frustrated beyond belief
Well, just because it's new doesn't mean it is good. 
But you are likely looking at a vacuum leak. Could be a cracked manifold or any number of things. It would have to be a pretty big one.
Could be something else. Good luck to you.
But you are likely looking at a vacuum leak. Could be a cracked manifold or any number of things. It would have to be a pretty big one.
Could be something else. Good luck to you.
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Senior User
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From: Mount Vernon, Washington
Frustrated beyond belief
Your definetly right about being new not always being better. I've had my fair share of battles with "new" carbs being crap, but this doesn't seem like a carbeuration problem. Do ya happen to know if these engines run a timing chain or are they gear drive?
Postmaster
Joined: May 2002
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From: Broken Arrow, OK
Frustrated beyond belief
They are gear drive, but there are some gears that are a plastic/metal composite that can slip. Typically if an engine has jumped time it won't run at all, or the idle is horrible.
Post Fiend
Joined: Jan 2000
Posts: 5,860
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From: Metro Detroit (Redford)
Frustrated beyond belief
Was there any engine work done before the popping/backfiring began? Perhaps the rebuilder got a rocker on backward or wrong in some way that an intake valve is not operating and is just held open. Or as happened to my '77 300 I6 the nuts backed off all the time and once the rocker fell of the stud while I was cruising.