Author Topic: Carb backfire after warm up. Please help  (Read 566 times)

Offline ric812

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Carb backfire after warm up. Please help
« on: August 30, 2018, 04:22:26 PM »
Join Date: Mar 2010
Location: Clarksville, TN
Posts: 12
Random backfire through carb.
I have a 1979 GMC Sierra 15 with a 250 co 6 cyl.with a Rochester varajet ii. I have driven it twice to work which is over and hour away one way with no problem, as well as driving it locally almost daily during the first three weeks I was driving it. Then one day I drove it to work, about an hour away, without any problems but halfway back it started backfiring through the carburetor so bad it blew off the vacuum lines. I attempted to drive it another 5 miles down the interstate while having to stop every 2 to 3 mies to replace the vacuum lines, in between it would run fine then backfire. I have replaced the spark plugs, wires, rotor, distributor cap, coil (factory HEI system)as well as replacing the carb with one that was tested and tuned on an engine before I got it (http://www.nationalcarburetors.com) . This is not a constant backfire , it only happens after driving a few minutes then I stop , shut it off , check everything , replace vacuum caps and lines and when it starts back up it runs fine for a few miles and backfires again. I have set the timing to 10 degrees BTDC ,with the vacuum advance disconnected, per the sticker on the truck. The distributor does not show any signs of arcing or crossfire, the firing order is correct, a vacuum guage does show any leaks. I pulled the plugs and this is what I found, #1,2 and 5,6 cylinders look like the first picture which is #6 and #3,4 cylinders looks like the second picture which is #3 cylinder. Please help I am lost.

Thank you for any assistance.


Offline bd

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Re: Carb backfire after warm up. Please help
« Reply #1 on: August 30, 2018, 04:44:23 PM »
Cylinders 3 and 4 are running mates, meaning that the pistons in those two cylinders arrive at TDC and BDC at the same time.  Run a compression check to verify engine condition.  Inspect the fuel filter and measure fuel pressure.  Use some aerosol carburetor cleaner and physically spray for vacuum leaks.  If the basic checks described don't reveal any problems, replace the distributor.  More than likely it has a bad pickup coil or module.
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Henry

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Re: Carb backfire after warm up. Please help
« Reply #2 on: August 31, 2018, 11:54:30 AM »
Hi Ric812:
The picture of spark plug 6 looks normal to me. The picture of plug 3 looks like it has a dry sooty coating which indicates the cylinders are not firing or have a weak fire. Does the backfire occur very soon after overnight cold soak?...or after it is warmed up after a while of driving? If it occurs only when  the engine is fully warm after a bit of driving, I would concur with BD that it may be an intermittent pick-up coil or module...both are hard to test if they become intermittent when hot...they may test fine when cold. When the module or pick-up becomes intermittent, cylinders 3 and 4 are getting a full charge of fuel/air and not burning it...then there is a time when the valves are rocking and the unburned mixture is detonated by the hot exhaust...hence the backfire through the intake. In any case, I would concur with BDs diagnosis procedure and sequence.
Regards,
Henry

Offline ric812

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Re: Carb backfire after warm up. Please help
« Reply #3 on: August 31, 2018, 05:44:17 PM »
Thank you so much for the information. I fell like I have a direction to go. The truck does not backfire untilnit is fully warmed up.

Offline ric812

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Re: Carb backfire after warm up. Please help
« Reply #4 on: September 02, 2018, 05:08:09 PM »
I have ran a compression check on #1, #2, and #3. Three has the black plug. It shows about 120 psi but leaked down slowly, within a minute. I had to stop but I will get the rest tommorow.

Offline Henry

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Re: Carb backfire after warm up. Please help
« Reply #5 on: September 03, 2018, 10:12:12 AM »
Hi Ric812:
My 76 service manual has 130 psi compression for the 250 engine. Maximum variation between cylinders is 20 psi. So as long as all your cylinders are at least 110 psi and the highest is 130 psi, all should be well with the valves, rings, head gasket. A slow leak down after cranking is not a problem, as long as the pressure comes up pretty quick on each crank when cranking.
Regards,
Henry

Offline bd

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Re: Carb backfire after warm up. Please help
« Reply #6 on: September 03, 2018, 11:07:37 AM »
I have ran a compression check on #1, #2, and #3. Three has the black plug. It shows about 120 psi but leaked down slowly, within a minute. I had to stop but I will get the rest tommorow.

The compression gauge leaking down slowly means the gauge itself has a leak - possibly a faulty or fouled Schrader valve.  Disregard the loss of pressure since it has no bearing on engine condition.  Allowing for variances between gauges and techniques, 120 PSI is acceptable.  Typically, each cylinder should build to maximum pressure within about five compression strokes with ~85% of maximum pressure occurring by the second stroke.
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline ric812

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Re: Carb backfire after warm up. Please help
« Reply #7 on: September 03, 2018, 02:19:25 PM »
thank you for the info. That makes me feel a little better, maybe I can figure this out.

Offline JohnnyPopper

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Re: Carb backfire after warm up. Please help
« Reply #8 on: September 03, 2018, 02:24:11 PM »
Don't forget to have the carb throttle wide open when doing the compression test... ;)

Offline ric812

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Re: Carb backfire after warm up. Please help
« Reply #9 on: September 04, 2018, 05:37:58 AM »
Finished the compression test. All the cylinders were between 120 and 130 psi. So per previous information I will order a new distributor and replace the fuel pump to see if that fixes the problem. I will also check for vacuum leaks with carb cleaner. If there is anything I need to check , please let me know. Thank you all for the help.