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Wed May 02, 2012 3:27 pm

Do these valves make a vibrating sound when failing? Something on my 79 fi bus has been for the last year while engine braking. I noticed that vibrating sound after installing a 72-74 style muffler a year ago. I thought it was just the sound of the new muffler at high rpms while decelerating. It stopped making that sound last week and started making a popping sound after descending a steep twisty mountain road. I use the gears to keep the brakes from overheating and slow the bus down. Today I had major bucking from my engine while climbing up the mountains. It comes and goes but seems to get worse as I agressively speed up and slow down at 60 to 70mph's. Could this bucking issue be a a result of a bad decel valve?

Wed May 02, 2012 4:59 pm

the decel valve only works with FI. If you have FI then IMHO maybe move this to the FI sticky cause it could get long and others later could benefit from it.

When the engine decels the mixture turns too rich to burn correctly. Vacuum to teh decel valve is what tells it when you are in decel. It lets a metered amount of air past the throttle which leans the mixture and keeps it from backfiring when more air is introduced in the exhaust - otherwise you'd have a mixture of fuel and air in the exhaust and when you opened the throttle the hot gasses coming out of the cylinder would ignite it, sometimes if the gasses are hot enough they will flame or backfire as they leave the exhaust. You see that on race cars in corners often when the headers throw out flames as the unburned fuel finds oxygen on exiting the headers and ignites. Remember - some of the carbon deposited in the headers and exhaust can be glowing from the heat so the can act as a glow plug.

So if you suspect the decel is bad then buy a spare and swap it. See if that helps. If that doesn't get it then IMHO have this thread moved to the FI sticky and we can chase it down there.

Wed May 02, 2012 8:25 pm

2bayday wrote: Do these valves make a vibrating sound when failing? Something on my 79 fi bus has been for the last year while engine braking. I noticed that vibrating sound after installing a 72-74 style muffler a year ago. I thought it was just the sound of the new muffler at high rpms while decelerating. It stopped making that sound last week and started making a popping sound after descending a steep twisty mountain road. I use the gears to keep the brakes from overheating and slow the bus down. Today I had major bucking from my engine while climbing up the mountains. It comes and goes but seems to get worse as I agressively speed up and slow down at 60 to 70mph's. Could this bucking issue be a a result of a bad decel valve?

Experiment. Pull the vacuum line from the decel valve and plug it. Leave the others alone. Drive.
Your options are worse, no-change, or better.
Colin