I have a 78-f250 with a 460...problem being timing off ..what should I set it to...+...-...got it off of family member who highly modified it but backfires every time I get on it through the carb.

 

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Sounds more like a carb problem.

 

nine times out of ten when an engine backfires out of the carb its because the timing is too far advanced, this makes it fire before the intake valve is closed completely and POW theres the carb backfire....the time between the valve closing and the plug sparking is only milliseconds, 2 degrees too early and its firing before that valve can seat and hold the combustion.....other causes of backfiring out of the carb could be a stuck valve or bent pushrod, but it would be constant, not just when you "get on it", you say thats when it happens and that tells me its because the vacuum advance is advancing too far because the initial advance setting is too high already.....you need to find out what YOUR engine is timed at now and post that number, do this with a timing light on the #1 plug wire and the vacuum advance off and plugged.....if whoever did the engine work advanced the cam, then finding the right timing could be hit and miss unless you have the specs on the engine work performed.we need more info on your engine to give the proper timing setting, generally it should be 12*-14* BTDC at 650 rpms in neutral...

 

If it is a timing issue, it would backfire out of the carb because the timing is too far retarded, not advanced. Too far advanced will cause detonation, but not backfiring. I run my 460 @ 16 BTDC.

 

if the timing is too far retarded then it fires after the piston has gone down from TDC, causing poor performance and dieseling after shut off, it will also make your exhaust a little louder because its firing closer to the exhuast stroke, i can go out to my truck right now and put it at 8 degrees BTDC and the only thing it will do is stutter and diesel, if i put it on 16 or more BTDC it backfires through the carb and is hard to start because it works against itself, maybe a modified engine is different but i dont see how..i have been up and down with the timing on my truck and i say this from experience not heresay, it runs best at 12* BTDC just like the sticker says on the engine, anything OVER that and it backfires through the carb, detination is when the engine fires too much before the piston reaches TDC on the compression stroke, causing too much resistance in the engine and killing it quite quickly....go outside and try on your truck, id bet money im right, thats how my 460 is and im sure its no different than any other motor made....like i said, i know this from experience on the many motors ive timed in the past, and its been the same story everytime.....it only makes sense that too much andvancement will make it fire closer to the time the valve is still open...when its retarded, it fires when the piston is on the down stroke and both valves are closed......

BTW: this isnt the first time ive had this arguement, ive had to physically show people on my own engine that what i say is true by advancing my timing above 18* and shooting a flame out of my holley that left soot on the underside of my hood.......

 

Too far advanced will cause detonation but also backfire. The plug is firing too much before TDC and like he said the intake valves isnt all the way shut so BAM.

 

Thread moved to proper forum.

 

Thanks for the advice I'll get more info on the parts in this. The distributor that was in it stripped the gear at the bottom. He pulled it apart cleaned it and threw in the old one...But never really set the timing ..It has run well but when you really get on it she pops.

 

i would bet a dollar that when it was put in, it was a tooth off, its very easy to make this mistake and not see it until you notice the engine running oddly, what you need to do is bring the number one piston up to Top Dead Center of the compression stroke, there are a number of ways to do this, the easiest way is to remove the number one plug and turn the engine over until you feel the compression blow your finger off the plug hole, then set the timing mark to 0 on the harmonic balancer. open the distributer cap and see if the rotor is pointing at the number one plug wire on the cap. If its off more than a 1/2 inch either way its off a tooth or two. you will have to remove the dizzy and turn it until it points the correct way...do the check above but dont pull the dizzy yet, just see if its off first and post back.

 

This doesn't seem to make much sense to me. Somebody help me out here.

Not doubting what anybody says about what has happened, but on the compression stroke both valves should be closed already way before the piston reaches TDC and certainly before the spark ignites the fule mixture. If the intake valve is closed how can too much advanced timing cause a backfire through the carb? Could be a bad valve. If the intake valve is open on the compresson stroke causing the backfireing through the carb., then it's a valve timing problem, or I'm I missing something?

 

you gotta think about how fast an engine is doing every thing, if you have it advanced a little bit too much , then its firing when the piston is about 3/4 of the way up the compression stroke, this causes an engine to be hard to start when its warm, if your advanced WAY too much, its firing before the valve is totally seated, its doesnt sound right i know.........also, i just reread your first post about it being "highly modified", if the CAM was too far retarded at the timing gear, then you can get backfires from the carb because its closing the valve too late, this might be what lxman1 was thinkin about with his post..i would find out the specs on your modifications if possible, it will tell you alot.........