bad backfire
Thread Starter
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Senior User
Joined: May 2004
Posts: 398
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From: london, ontario
bad backfire
hi all, my 69 f100 has been acting up for months, it sound fine when idling, but if you touch the gas it backfires out of both ends, then stalls. i have done the points, cap, rebuilt the carb. then the mechanic said the carb housing was gone(it's only 3 years old) so i blew money on that. still has the same problem. the engine is fairly new with only 35000 miles, and has run great for a couple of years, i luv the 300's power. if only it would run.lol
any other ideas? the mechanic has given up, parked it on the road facing the wrong way, with the windows open and the keys in it. nice guy
any other ideas? the mechanic has given up, parked it on the road facing the wrong way, with the windows open and the keys in it. nice guy
Posting Guru
Joined: Jul 2003
Posts: 2,222
Likes: 1
From: East Central Florida
300 CID I6 doesn't use a timing chain. Cam is driven by 2 gears directly, Can't slip.
Look for air leak in the intake side/system, maybe a cracked hose, leaky gasket, something small. Also check Dizzy shaft bushings for excess side play, or drive gear play, Check Vacuum Advance to make sure it works correctly. . .
Make sure your Ignition wires aren't leaking or cross firing, and the cap is not cracked or getting condensation inside it.
What carb are you using on this engine?
FBp
Look for air leak in the intake side/system, maybe a cracked hose, leaky gasket, something small. Also check Dizzy shaft bushings for excess side play, or drive gear play, Check Vacuum Advance to make sure it works correctly. . .
Make sure your Ignition wires aren't leaking or cross firing, and the cap is not cracked or getting condensation inside it.
What carb are you using on this engine?
FBp
Thread Starter
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Senior User
Joined: May 2004
Posts: 398
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From: london, ontario
i've changed the intake/exaust gasket, the cap is new and dry, the vac advance works when i suck on it(nice taste.lol). the wires are only1 year old, and weren't the cheap ones, but i'll check them. the carb is the stock carter 1 barrel, with the automatic choke. no vacuum leak that i can find, nothing being used so just rubber caps to check. the dist didn't seem to have much play when i pulled it, so i stuck it back in. the plugs were clean before this, now their probably black, but i'll clean them tomorrow.
thanks all
mark
thanks all
mark
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Joined: Jun 2006
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just my 2C worth, but i would double check points again. they may have slipped a little(dont take much). had a 71 360 that would run the same way. in fact, backfired so bad that split muffler open like a ripe watermelon. would idle perfect, but the second you touched accellerator, would stumble and start backfiring. turned out points were off just a little.
Posting Guru
Joined: Jul 2003
Posts: 2,222
Likes: 1
From: East Central Florida
If no leaks are present, and obvious things appear to work correctly on inspection,
Imtermittent backfiring from the intake & exhaust can only be a result of wandering Ign timing or the valves in the engine randomly staying open intermittently. Since the valve problem is niether common, nor obvious in this case, that leaves ign timing.
Just because Ign wires are pricey doesn't male them "good". Accel & Moroso make high buck plug wires that last for 500 miles in competition & won't go 250 miles in traffic.
Are the counter weights & springs in your Dizzy working right?
Also like Dennis says is your dwell within spec?
FBp
Imtermittent backfiring from the intake & exhaust can only be a result of wandering Ign timing or the valves in the engine randomly staying open intermittently. Since the valve problem is niether common, nor obvious in this case, that leaves ign timing.
Just because Ign wires are pricey doesn't male them "good". Accel & Moroso make high buck plug wires that last for 500 miles in competition & won't go 250 miles in traffic.
Are the counter weights & springs in your Dizzy working right?
Also like Dennis says is your dwell within spec?
FBp
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Joined: Sep 2003
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From: Bridgeville,CA
This sounds way too familiar. I had the same problem with by 69 when I replace the 240 six with a 360. I hooked up the vacume system the way that the used motor arrived. I was attaching the vacuume hose to the fitting at the rear of manifold. Wrong. I finally realized that reguardeless of how it arrived the hose was supposed to go to the carb base. The way it was plumbled the right rear cylindar was running lean and backfiring. After putting the hose on the carb base no more back firing.
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Senior User
Joined: May 2004
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From: london, ontario
well went over today, played with the points, adjusted timing by turning slightly, then trying to start. the only way i could get it to start was with full throttle, then it just seemed to try to run up to full slowely, if i let off it died. now i'm down to mabye the fuel pump, or changing the dist? it's getting fuel, but mabye the pressures off?
any other ideas guys?
thanks
mark
any other ideas guys?
thanks
mark
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Post Fiend
Joined: Dec 2003
Posts: 6,658
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From: "Islander"
Sounds like the idle circuit is plugged up as well maybe the transfer circuit. Air bleeds clear? For a cheap fast check remove idle mix adjusting screws and blast them as well the air bleeds near the choke horn with Berryman's B-12 carb cleaner then check running again. Carb needing rebuild and cleaning?
You stated vacuum functions, how about centrifigual and how smooth including the vacuum travel?
Backfire will cause carb damage, afterfire will destroy mufflers, which one do you have?
You stated vacuum functions, how about centrifigual and how smooth including the vacuum travel?
Backfire will cause carb damage, afterfire will destroy mufflers, which one do you have?
Thread Starter
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Senior User
Joined: May 2004
Posts: 398
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From: london, ontario
it was shooting out of both ends. the carb is new, it's only been turned over about 20 times. the old carb was rebuilt, but the mechanic said it was the housing. tomorrow i'll take out the screws and try the carb cleaner. i'll also clean the inside of the dist with carb cleaner, but when i sucked on the advance it moved.
thanks
mark
thanks
mark
Post Fiend
Joined: Dec 2003
Posts: 6,658
Likes: 4
From: "Islander"
Do yourself a favor and remove the points plate, cam and advance weights, clean and polish out all contact and wear areas, add a little thin viscosity teflon grease and reassemble.
Same with the vacuum advance unit where it rotates on the points plate as well all pivot points.
Cap & rotor up to the job with no carbon tracking? Wires checked with a ohm meter and the resistance readings?
Plugs known good like tested under pressure with a plug tester, yes a old tool that finds defective plugs you thought good but fail under compression.
Fuel pressure checked to eliminate fuel pump and filter?
Running a inline filter before the carb or do you already have crap plugging up your new clean carb passages hence crappy idle?
Coil voltage test besides a thermal breakdown or failure?
Bad gas like water in it?
Just a few items to check on first to get ya started.
Same with the vacuum advance unit where it rotates on the points plate as well all pivot points.
Cap & rotor up to the job with no carbon tracking? Wires checked with a ohm meter and the resistance readings?
Plugs known good like tested under pressure with a plug tester, yes a old tool that finds defective plugs you thought good but fail under compression.
Fuel pressure checked to eliminate fuel pump and filter?
Running a inline filter before the carb or do you already have crap plugging up your new clean carb passages hence crappy idle?
Coil voltage test besides a thermal breakdown or failure?
Bad gas like water in it?
Just a few items to check on first to get ya started.
Thread Starter
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Senior User
Joined: May 2004
Posts: 398
Likes: 0
From: london, ontario
well today i got it home, sorta.lol
changed the plugs, added some new gas, new fuel filter plus added an inline filter. sounds great till i touch the gas. still backfires bad with no power. it runs fine when cold, but when it heats up the problem starts. when it stops snowing i'll take the dist off and bring it inside to clean out. is carb cleaner ok for the inside the dist?
also would too much carb cleaner in the gas likely to cause this? there's a can in the back of the truck, and i don't know how it got there. there's only 1/4 of gas in the tank, and am not sure if the mechanic added it to my tank.
thanks all
mark
changed the plugs, added some new gas, new fuel filter plus added an inline filter. sounds great till i touch the gas. still backfires bad with no power. it runs fine when cold, but when it heats up the problem starts. when it stops snowing i'll take the dist off and bring it inside to clean out. is carb cleaner ok for the inside the dist?
also would too much carb cleaner in the gas likely to cause this? there's a can in the back of the truck, and i don't know how it got there. there's only 1/4 of gas in the tank, and am not sure if the mechanic added it to my tank.
thanks all
mark
