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What causes back firing through the carb and compression test questions...

Discussion in 'The Garage' started by Zeus33rd, Sep 5, 2005.

  1. Zeus33rd

    Zeus33rd Smarter than you GMOTM Winner

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    Trying to diagnose the new 'Burbs issues and get in running right. If I mash the throttle it backfires through the carb. What causes this? The carb is newly rebuilt, all the vacumn hoses are new. New plug wires. Timing is 8* per the emmissions sticker. Could it be the advance springs/weights in the distributor are worn and not advancing spark the way they should?

    I also ran a compression test on the thing today. Every cylinder was between 85-95psi. Is that good? Bad? So so? I kinda thought it was too low, but I know nothing about this kinda stuff. I'm pretty sure the mtoor is just way tired and needs a rebuild, still like to knwo this stuff just to know. :D
     
  2. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    Those readings are way too low. Just as an example, my blazer has 226,000 miles on it and when i did a compression test i have anywhere between 180-195 cranking compression. Even those numbers surprised me but i'm not complaining. I would typically expect to find compression readings around 140-160 on average.

    As for the backfiring through the carb, make sure you don't have a flat cam, this was a typical GM problem in those years of vehicles. Easiest way to check is to pull the valve covers and crank the engine over while paying attention to what the valves are doing, if you see any that don't open as much as the others you have a flat cam.
     
  3. davesanrn

    davesanrn Registered Member

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    The old rule of thumb is backfiring through the carb is ignition and backfiring out the tailpipe is carburetion. Good luck-
     
  4. Zeus33rd

    Zeus33rd Smarter than you GMOTM Winner

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    Hmm...

    I guess the oppurtunity for you to get one of my diff covers has arisen...If ya wanna trade me some machine work on this motor. :D After hearing what you replied, I'm for sure gonna to pull the Goodwrench motor from the truggy and stick it into the 'Burb. I know the Goodwrench motor will pass smog. Then build the 'Burb motor for the Truggy. Maybe somthin like Summit engine kit part# SUM-CSUM355KIT or SUM-CSUM383KIT. What ya think?
     
  5. Mad-Dog

    Mad-Dog 1/2 ton status

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    Timing too far retarded...... valve timing retarded....burnt, bent or stuck valve.........broken valve spring etc etc

    Those numbers are very low and indicate a very tired motor :bow:
     

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