Pelican Parts Forums - Engine backfiring through intake

I want to share this info with the rest of the Pelican members even though I posted it on the Atlanta Pelican Get Together 7/26. The posts are over a couple of days so it will run together.

Thursday - 7/23 Post

This evening I took the beast out for a drive up the interstate towards the GA Mall. Well, seeing that traffic was backed up in the SB lane, I decided to run over to 29 and go south. On the way over, the P-car started to backfire - not very loud, just a pop, pop every now and then on deceleration.

Well, I pulled into a vacant lot to check it out. The sound is coming from the right front side of the engine (#6) cylinder. Well, the car would idle ok but there was a slight miss and when you revved it up, it would start to pop but not consistently.

Couldn't hear anything rattling around, so I decided to take a slow drive to the barn. The strange thing is when you accelerate to about 3K, the poping stops.

When I got it home, I pulled the plug wires on the right side (looking forward from the rear) and checked the plugs. All were tight. I then checked the studs with a mirror and flashlight and they all look like they havn't moved or are broken (just the top end so far).

Haven't checked the compression or timing yet - distributor is still locked down and the cap is on tight. Will let it rest for tonight and have at it Friday night. Hopefully it's just something minor. Will keep you posted.

7/24 Post

Well, had some time to check the P-car. Found out two things. First the distributor cap center post underneath (the little carbon) fell out when I took of the cap to check it out. I did check the timing and it was right on. Did replace the cap and rotor.

Second and most important - I am getting backfire through the box from the #1 cylinder on the intake side. Did a valve adjustment several months ago and it may be possible that the adjusting screw is loose thus causing the problem (I hope) - otherwise it may be a bad valve (NOT).

7/26 Post

Well, I found the problem. Apparently when I changed the valve adjusting screw on the rocker arm, I did not manage to tighten the end caps tight enough on the rocker arm shaft. The allen head bolts loosened up and the shaft managed to slide out of the carrier and the rocker arm was just flopping around.

This was checked after I removed the rear driver's side chain tensioner cover to do an inspection. My next step was to remove the heads on the driver side to get at the valves. Anyway, I ordered a new rocker arm/shaft/end caps from Pelican. This should solve my problem.

Apparently, the blow back was due to exhaust valve not opening when the piston was decending from it's compression stroke to it's exhaust stroke. There was still pressure on the cylinder and when the intake valve opened, and it would blow gas back through thus making it seem like a bad intake valve.

Steve

1978 SC Targa 3.0

"A Porsche does more then just go fast in a striaght line