87 k20 350 tbi backfiring

Discussion in 'Chevy C/K Truck Forum' started by 87_k20, Apr 1, 2014.

  1. 87_k20

    87_k20 New Member

    I recently purchased an 87 k20 350 tbi. thing was a DOG driving it home I knew it needed more ass for my liking. so on went some longtube headers and away with the ****ty manifolds. next a friend of mine talked me into putting in a bigger cam. cam specs are comp cam cs x4 250x-11 dur @ .050 206 int 214 exh. valve lift .432 int .453 exh. g ot it all timed and valve lash adjust started it up and all was running smooth. nice gain in power. had no tags and with a snowy winter it sat for a while started it up and blowing flames like crazy only from the right side of the exhaust. figured I never done plugs cap and rotor or nothing so I got a set of ac delco plugs gapped to .45 and new accel cap and rotor. wires are new ac delco already on the truck. still back firing from right side only. at this point im lost. set timing about 8 degrees advanced. still no change. compression test got about 120 on the right bank and about 140 on the left. only thing I haven't done is get a chip made for the tbi computer. nor will I. plan on changing to carb once I get some morecash rolling in. any ideas helpful at this point. gotta put cats on to pass inspection but with the backfiring already blowing apart one muffler I don't wanna put the cats on yet.
     
  2. Crispyt

    Crispyt Rockstar 3 Years 100 Posts

    Could possibly be a vacuum leak at the intake manifold. If you took it off when you did the cam and didn't torque it properly or use a new gasket, one of the cylinders could be running too lean. Or its possible that maybe the intake was already warped a little bit and taking it off and putting it on again caused it to start leaking. Does it seem to idle higher or start higher? Also make sure you have good fuel pressure as well. Because your only getting it on one side I would lean towards a leak on the manifold, or a valve that isn't properly adjusted. When it was stored was the coolant mixed properly so it wouldn't freeze?

    You could try taking some starting fluid and spraying some where your intake manifold meets the cylinder head when it's idling and see if it changes the idle. If it does it's sucking air and has a leak.

    Anyways hopefully you figure it out, it doesn't seem like your compression is below minimum spec. on either side.
     
  3. 87_k20

    87_k20 New Member

    went thru the other day and readjusted all the valves again. none where to tight or to lose pretty dead on. hooked up a tach and it was idling low around 600 so I bumped the idle up to 800 and the backfire stopped for the most part let it sit and cool down. started it up again and before it warmed up it popped like once or twice. my buddy pointed out that I have a tbi spacer I didn't even think about it. gonna take that off soon as I get a nice day and no rain. im gonna check the intake gasket to just to be sure.
     

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