sounds like you got two wires out of the proper firing order
Backfires only when when punched
Thread Starter
Instructor
Joined: Jun 2005
Posts: 248
Likes: 1
From: Apopka Florida
Backfires only when when punched
Just replaced the wires with MSD 18.5, new plugs and distibutor cap by MSD without an access to the dwell angle. Car sound good but when you go to hit the pedle it back fires alot. Timing is at 10 and idle is at 5. sounds great until acceleration. with the air cleaner off you can see the backfire coming out of it. Any help? 
Last edited by 73'454folendore; Jul 18, 2005 at 03:31 PM. Reason: add pic
Burning Brakes
Joined: Feb 2004
Posts: 958
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From: shawnee ks
Originally Posted by 73'454folendore
Just replaced the wires with MSD 18.5, new plugs and distibutor cap by MSD without an access to the dwell angle. Car sound good but when you go to hit the pedle it back fires alot. Timing is at 10 and idle is at 5. sounds great until acceleration. with the air cleaner off you can see the backfire coming out of it. Any help?
Safety Car
Joined: Aug 2001
Posts: 3,892
Likes: 13
From: Pettis Performance 565 with two stages of Nitrous Supply nitrous 1.082, 4.61 at 155, 7.17 at 192
Originally Posted by 73'454folendore
Just replaced the wires with MSD 18.5, new plugs and distibutor cap by MSD without an access to the dwell angle. Car sound good but when you go to hit the pedle it back fires alot. Timing is at 10 and idle is at 5. sounds great until acceleration. with the air cleaner off you can see the backfire coming out of it. Any help?
Thread Starter
Instructor
Joined: Jun 2005
Posts: 248
Likes: 1
From: Apopka Florida
Thats between 8-10 on the timing light at idle which is about 5. If I jump up the idle screw the timing jumps to 12+. I'm trying to figure out whats the best rpm to set the timing. Thanks
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Safety Car
Joined: Feb 1999
Posts: 3,631
Likes: 14
From: Fountain Hills AZ
Check the function of the accellerator pump. When you quickly open the throttle there's a sudden increase in airflow into the engine. If the accellerator pump isn't providing enough extra fuel soon enough the mixture goes lean and you get a backfire. With the engine off, watch the pump nozzles while opening the throttle with your hand. Fuel should squirt as soon as the throttle moves.
Intermediate
Joined: Jun 2004
Posts: 48
Likes: 0
From: Orlando FL
It's not common but something quick to check if all else fails. My 350 ran fine as long as it didn't get over half throttle. Then it would bog and backfire through the carb. The previous owner sold it to me after he spent thousands trying to diagnose the problem. He had no mechanical skills and too much money. Shop after shop did tune ups, set timing, replaced the carb and replaced the didtributor and coil. So with no results he lost interest and sold it to me for cheap. It turned out to be a wiped exhaust lobe on the cam. It wouldn't open the valve far enough to let all of the airflow out before the exhaust valve shut and the intake opened. Thus all the blowing out the carb action. Just check to see if your rockers are tight. If one is loose, turn the motor over to see if it is opening as far as the others.
Probably not what's wrong with yours but it's something to keep in mind if nothing else fixes it.
Probably not what's wrong with yours but it's something to keep in mind if nothing else fixes it.
Drifting
Joined: Oct 2004
Posts: 1,259
Likes: 7
From: Ontario
Originally Posted by 73'454folendore
Thats between 8-10 on the timing light at idle which is about 5. If I jump up the idle screw the timing jumps to 12+. I'm trying to figure out whats the best rpm to set the timing. Thanks
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
You need to get your total timing set at 36. This will usually get you about 16-18 initial. If it still pops through the carb upon sudden throttle opening, you have a lean condition. Richen it up a couple of jet sizes and/or make sure sure your accel pump is producing an instant pump shot.