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I have a 305 that's been rebuilt in my full size... Before I took it in to have the new heads and all new seals put on, the engine.. maybe once a week, would sputter/backfire... btw its a TBI with computer/sensors.. anyways it would back fire like once a week... well then I paid $1100 and had brand new heads put on, new seals on everything, etc... and it started backfiring about a week later like once or twice a day and there nasty back fires... well i replaced the plugs and it still did it...

well then i replace the plug wires. it stopped back firing and ran like a dream for about a week and then i took it to the shop to have all this custom work done to the suspension/body... well its been there probably a little over 2 months and id say its been driven maybe 10 times for a max of 5 minutes and not once on the actual road but to the front and back of the shop in a little longer then 2 months... and its back firing again.. and the engine has gotten wet... So.. im wondering...

The plugs have rust on them, could they be giving bad spark? Could the new plug wires be giving bad spark since they've gotten a little wet/maybe they got rust on them? I know the gas in it has to be old so that isnt help, the oil has been sitting for 2.5 months im assuming it wouldnt be a bad idea to put better oil in it, i have a new distributor cap and plugs to put in and rotor button. Do you guys think that these may be the problem of why its back firing? It doesnt do it all the time... just like.. if i try giving it gas to power brake it, it'l back firing and a flame... not too big... will shoot out of the throttle body... but if im just reving it up or giving it gas to get around the shop, it wont back fire and it'll run fine...


Thanks for any input guys
 

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A couple of things come to mind. 1st check your dist cap for arcing inside, replacement would be cheap. Then I would run the valves, it almost sounds as if an intake valve may be a little tightly adjusted or hanging open for some reason. I would rather have valves a bit loose rather than tight.
 

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my truck was doing the same thing and it was the timing being to advanced. put a timing light on it and i believe that the timing should be 4 degrees retarded, or maybe 6. depending on whether it is a heavy duty 305 or not (i dont know how you would consider a 305 heavy duty but thats what my book said).

i hope that helps a little.
 

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Backfire

Doc here::pimp:

Install those new tune up parts and be sure the coil is dry when you transfer it over to the new cap.

Check for Vacuum leaks, Carb , ported thermo switches, vacuum advance (if so equipped)Power brake booster, transmission modulator valve/line

You didn't say what year the engine was.. but if it's AIR, check the vacuum lines to that.

Check your EGR Valve for leaks and proper operation.

Be sure your PCV isn't randomly sticking open on throttle up.

Check your Mechanical advance for Stuck or rusted weights.

Static timing for the 305 is 4 deg BTDC Vacuum advance disconnected and plugged at 650 RPM curb idle.

It might also be a slightly over-adjusted intake valve, have the place that did the job recheck them.

Hope it helps!
Doc :pimp:
 
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