Hi all I am new to the forum and hopefully I can find some useful info. I bought an 84 Dodge D250 w/ a 318 in it. It fires up and seems to run pretty decent, except it is backfiring through the carb. I installed new head gaskets, intake gasket, and reseated all valves. I have good compression on all cylinders. The backfire at an idle is very intermittent, like pop then anywhere from 5 seconds to 20 seconds it will pop again. Under load it runs like crap and barely goes down the road and the popping is constant. When I unplug cyinder 3 spark plug wire the backfire goes away. Cap and rotor are new. Wires new. Swapped plugs and does the same thing. I am kinda lost at this point and I am in the process of doing a leakdown test because I know a compression test is not a tell-all. Anything else I should be looking for, any advice would be highly appreciated. Thanks all.

 

If you have good compression on all cylinders a leak down test will not be helpful. I would suggest you verify the firing order, check the air gap for the pick-up coil/s, and check the timing.

 

Seal, thanks I appreciate the info. do you know how to check the air gap? Or where I can find a procedure for doing this. And why qould it only be one cylinder making it backfire, it goes away if I unplug #3 wire. Thanks

 

The air gap must be set using a NON MAGNETIC feeler gauge. Single pick-up coil setting is 0.006 inch, and with dual pick-up coils the start coil is 0.006 inch and the run coil is 0.012 inch. Just line up one of the reluctor wheel teeth (the star wheel on the distributor shaft) with the pick-up coil tooth that you are setting. It is like setting points. The proper size BRASS feeler gauge should just fit in the gap with slight resistance being felt. This should be checked with every tune up even knowing that it doesn't normally change.

Just a guess here about your #3 cylinder question but my bet is that it is not the only offending cylinder. If #3 is the only problem the distributor cap, ignition wire, spark plug, or a valve on that cylinder is probably the problem.

A little history might help here. When you changed the plugs, what did the #3 plug look like? What problem were you having to make you change the intake and head gaskets? Exactly what do you mean when you say reseated the valves? Was there much carbon on the valves and in the cylinders?

 

If you have to go internal pull that valve cover first and pays close attention to the exhaust push rod and rocker. I had a exhaust push rod bend and come out of place and still had good compression. Had the popping back thru the intake also.

 

Thanks for that info on the air gap. When i first got the truck all plugs were a bit white in color, a sign of a lean fuel condition I believe. Plug number 3 was just a dry black, looked as if it was not firing at all. I lapped the valves using valve grinding compound and a valve lapping tool. There was carbon on the back side of the valve, but not bad. I just dont understand how the backfire goes away when i remove plug wire from # 3 and its the only one that will make it go away. I have good compression, i think anyway 125psi wet and dry. Also, on my dist cap, there are 2 spots that have "1" with the word Rotation in between them. Left of the first "1" there is an arrow pointing clockwise and to the right of the other "1" an arrow counterclockwise. Something like this < 1 Rotation 1 >

 

are you sure you got the firing order correct? 1-8-4-3-6-5-7-2

 

The cap will fit small & big blocks which have different rotation. and wire locations.
On the 318 # 1 wire will be at the front toward the carb. as the above diagram shows.

 

ok good to know yeah i had the firing order right.

 

have you checked or adjusted the timing?