1984 c20 backfiring

[mikegpage]

Hi, I bougjht a 1984 c20 with 4.1 l6 engine.4 speed manual It starts easy , a little rough while warming up. When driving it backfires through the exhaust almost every time i shift, never backfires other times, new plugs, wires rotor and cap, timing set correctly, valves are all good. previous owner plugged some of the emissions and some hoses just missing. engine runs well other than backfiring, need to know where to turn next.

 

[Dennis Benson]

I had to connect up a lot of the emissions lines to get a 1975 GMC C3500 with a 350 to run right. Check to see if the vacuum hoses are cracked. Also, I found the distributor shaft was worn and replaced the distributor. Check to see if the shaft is loose in the distributor by removing the distributor cap and seeing if the shaft moves sideways. There is only a few thousands of an inch clearance between the shaft and the housing. Any more and the timing can be changing.

 

[sd pete]

I had an 83 sierra 5.0 and the distributor was worn out and was causing problems like yours.

 

[Kruse]

Does this have a smog pump? If it does, drive your vehicle a short distance with the belt off and see if it backfires. Sometimes there is a check valve on the big hose of these pumps that fail that will cause the backfire.

 

[mikegpage]

Wow, I took off the belt from the air pump and "no more backfire" Very Nice Thank you, Should I put in new check valve or take pump off??

 

[ericlb]

where do you live? unless you have anual vehicle inspections, loose the pump and all the associated junk, most of it did little if anything anyway , all that crap fell off my 83 somewhere and its run just fine for another hundred thousand miles without it,

 

[mikegpage]

i bought a new check valve for 11 bucks installed it and engine still backfires, i disconnected the hose and no backfire. i think i will loose the pump and plug off holes. no inspection in my state

 

[Kruse]

That"s probably your best choice. Most of the factory parts did okay, but the aftermarket stuff is marginal when new and will fail, corrode or rust out in under a year. The non-computerized smog equipment did very little to clean the air anyway.