Help with Backfire

Small Block Mopar Engine

  1. SGBARRACUDA

    SGBARRACUDA ROY FABO Gold Member

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    Are they saying the retainer bottomed against the valve guide?
     
  2. Alex.Sciortino

    Alex.Sciortino Active Member

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    He thought maybe there was coil bind. I just bolted a head back on however, and there was about .101" in between each coil. The lifters did leak down, so that would be less with it running, but I can't believe it would be enough to cause coil bind.
     
  3. SGBARRACUDA

    SGBARRACUDA ROY FABO Gold Member

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    I would check for coil bind and retainer to guide clearance with a solid lifter. That will take all speculation away. You may need and adjustable push rod to set clearance at zero. That will let you know exactly what you have for clearance.
     
  4. moparownr

    moparownr FABO Gold Member FABO Gold Member

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    I agree solid lifter and adjustable push rod, see what is there for clearance. I would also check the specs on what your kit came with, so many bent, it would seem that one part or another does not meet the machine tolerance as assembled. My personal old car saying......"Just because its new, don;t mean it works!"
     
  5. yellow rose

    yellow rose Doctor of Thinkology.

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    What's the status of the heads? I've seen it for decades now...guys starting fresh engines and they set the timing down to 20-25* total because the think they are saving the engine. But your not. You are killing the engine.

    With no load on the engine, slow timing makes combustion continue way too long. So long in fact that when the exhaust valve opens, you are just finishing the combustion cycle. All that does is load the exhaust valve with heat the valve can't dissipate and the valve grows and snags a guide. If the valve is stuck in the guide tight enough, it can hit the piston and knock the head off the valve.

    If they are quite so tight, you hang the valve open enough to keep it off the seat...which means that valve will be hotter on the next cycle because it can't dump it's heat load without being on the seat. Now, the valve is off the seat so you have some clearance. The lifter and pushrod come up against the rocker with say a mere .100 last. By that time (.100 lift) the lifter and the pushrod are coming at a pretty good speed and BANG the lifter shoves the pushrod right into the rocker arm cup. Since the valve is pretty well stuck in the guide somethings gotta give. It will poke that pushrod right through the rocker.

    As my theory stands, you need to inspect every valve and physically look at and measure each stem and look for signs of heat in the valve head itself and about 3/4 inch up the stem. These areas get extremely hot from slow timing and can't get rid of the heat. Then they stick in the guide and it puts more heat into them. Depending how tight they may be in the guide, you should probably be able to see at least some galling on the stems. You should see clear signs of over heat and the guide grabbing the valve.

    I can't say that the engine builder didn't finish the guides to the correct size, because I have had this happen to me. I tell the customer no LESS than 45* total timing on start up. Of course, they all ask their buddies who build engines for the Xbox racing team and compete against those clowns with the Wii system and they all say the sure way to kill this thing is a bunch of timing. Of course, that's dead wrong. I'd much rather have more timing than normal (35 is pretty normal for a sbm so 45-50 is great for break in. Absolutely nothing less than 40. Keep the heat in the exhaust pipe and not the valve

    So you have some things to look at for sure. Remember...slow timing is a parts killer.
     
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