Frustrating misfire, PLEASE help...

mabigredtruck · #1 (**permalink**) 06-07-2008, 03:30 PM

i posted this on the 73-79 forum and was referred to this group in hopes of finding an expert. i have 75 f250 with the 390 and c6 trans. 2wd. it has a holley 4brl. carb (not sure how big, but has #65 primary jets in it if that helps). i bought the truck a few months back and it has run beautifully every day since, until, i cranked it the other day and it sounded like i had a dead cylinder. i tried revving to clear it up but it didnt help. i have been unable to isolate the missing cylinder(s). the truck will idle in park or neutral fairly well but when in gear with foot on brake it misses and chugs so bad the truck shakes and it almost dies. when excelerating from a stop it shakes a bit off the line but smooths out around 1300rpm. so far...i have replaced all plugs, wires, coil, cap and rotor. i have turned the motor over with valve covers off to see if anything wasn't moving that should be, all was fine, i have done a compression check, all good, i have listened with a steel tube as close to all of the intake and exhaust ports as i could and can hear no knocks or ticks. i tried pulling plug wires one by one while idleing to check drops and nothing stands out. all theo ld plugs even looked identical, nothing to indicate a dead cylinder. what do i check next??? someone suggested vacuum leak, but wouldn't i have a high idle or be able to hear it? please give me some advice, i'm at my wits end and may be forced to take it somewhere!

Tedybear · #2 (**permalink**) 06-07-2008, 05:15 PM

Vacuum leaks are not always something you can hear with your own ears. If you can hear one? It'll be probably pretty severe.

Hook a vacuum gauge directly to the manifold and see what you have. About 18-20 inches is fair and in a good range. From the sounds of it? In my own opinion this makes me lean towards carb problem. Blocked port/needle valve in the idle circuit. All it takes is for some tiny chunk of crud to get caught into the seat of the idle valve--and that will lean out the mixture resulting in low vacuum and rough idle. Once you get off idle and start using the other circuits? The problem goes away like magic.

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Freightrain · #3 (**permalink**) 06-07-2008, 05:56 PM

Yes, I'd start with going thru the carb. Have you reset the idle mixture screws(on the sides). I'd pull them out fully and check for crud, then screw them in fully(just til they touch don't crank on them). Turn them out 1 1/2 turns and then set them with truck running.

Carbs have three circuits, idle, transistion and main metering. Sounds like could be the transistion circuit is not feeding fuel.

Look at the airhorn of the carb, it will have a "listing" number on it that will tell you what size the carb is. Common is 1850, which is a 600cfm.

Tedybear · #4 (**permalink**) 06-07-2008, 06:31 PM

When you have the idle screws out? Pick up a can of B12 Carb/choke cleaner spray. Hose the crap out of the openings the screws go into. Any built up varnish or crud will come flying back out at ya...so watch your eyes!!. Also, HOLD the red pointy hose thing with one hand, and spray the crap out of the throat of the carb. (do this with the idle screws removed) Aim towards the butter fly plates at the bottom end. USE CARE not to allow the red hose thing to pop out of the can of carb spray. (don't ask how I know this--but let's just say it's a bearcat to try and remove that red hosey tube thing out of an intake manifold with the carb still in place). That should back-flush out the idle 'holes' and blow anything left out.

Yeah it'll be messy, so have plenty of rags handy for clean up.

(It's also highly let's say.....Fire?....this stuff ignites pretty quick)

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