Hello all. I've got a bit of a mystery on my hands. My '67 F100 (with a 360 of unknown vintage)
runs fine until the throttle is opened up more than halfway, then it'll backfire upon letting up
on the gas. This does not happen if I leave the choke on. Also, once it's warmed up, and the choke is off, she'll hesitate off idle, sometimes
to the point of dying if I don't flutter the pedal. Again, this won't happen if the choke is
left on. Any Ideas?

 

scotty, did you read Mockys "Backfires through the pipes" thread for answers??


John

66 F100s
In the still cool hours of the night, you can hear chevys rusting away.

 

hello, i'm mocky, WELCOME TO HELL,(lol) sounds to me like you might have more of a carb. problem than what i went through, go to my post, backfires out the pipes, and click on the little box, next to the head at the top of the post, and e-mail me. i own a 71 F-100 302, i'll give you all the help i can... later, mocky

 

Read mocky's thread its a good one especially for back fires out the exhaust, he wears earplugs when he drives I think.
My question would be are you backfiring out the intake instead of the exhaust? If so you should check your accellerator pump look down the carb (engine not runnning)make sure you have a strong squirt that starts when the throttle is first moved then on through the whole stroke or at least the most part of it.
Also check for vacuum leaks and check your point gap and timing and vacuum advance.( Timing and points set wrong can cause exhaust back fires too) As for the off idle stall vacuum leaks and poorly adjusted points and timing can cause these problems too.
Also but if every thing turns out to be ok then try running your idle mixture screws out a couple turns ( If you have the exhaust back fire this may make it worse) these carbs are well knows for their off idle stumble every thing has to be just right mine is almost 100% except I don't have a heat riser tube so it tends to have a stumble until it gets really warm and the stumble never really goes completely away if its really cold. also changed my carb once and the rebuild came with a smaller jet I took the one out of the one I was replacing and it ran fine unfortunatly I can not tell you what size it should be.
Also it could have a partly plugged jet or your power valve could be stuck or plugged.
Do you have a consistant off idle miss? That should keep you busy for a while and thats not all I'm sure.

 

Thanks for the suggestions, I have a lot of stuff to check. I want to get rid of my points and put
in electronic ignition. Might that help?
Thanks, Scotty.

 

I'll be blunt. All the times i heard of this problem it turned out to be a worn camshaft. This explains the reason it runs with the choke on.

 

 

A worn camshaft? I certainly wouldn't be surprised
if it is worn, could that really cause the problem? I'm a bit rusty on my internal combustion theory. Did I mention that I get about
7mpg (or less)? Has anyone else heard of this?

 

my truck does the same thing but I have a burnt intake valve I beleive. I have a 400 with 165-175 of compression on all the cyl. except 1 which is about 10. I removed the valve covers and the valves are lifting fine, and I don't use oil and it dosent smoke. so I am assuming that it is an intake valve because I get a back fire through the carb when I get on it hard with out the choke pulled out. but, I haven't had time to pull the heads yet.
Scott
mickelsons(No Email Addresses In Posts!)

'98 Contour
'78 F-250 4X4 SC
'69 F-250 4X2
'52 International L-110

I would rather push a Ford than drive a chevy. (like you would have to push a ford. HA)