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Old 12-13-2010, 10:11 AM   #1
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454 Backfire through carb

My 1990 250 MOHO will backfire through the carb when under power - accellration - going up hill - -
Any ideas - timing gear/chain?
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Old 12-13-2010, 10:25 AM   #2
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Sounds like electrical. Most likely cross fire under load.
Start with the distributor cap. Check for moisture both inside and out.
How old are the spark plug wires. If they are not properly routed with separation, they can cross fire.

Electricity, especially high voltage, will find the shortest path to ground. It can cross from one wire to another or across the inside or outside of a dirty or cracked cap and fire the wrong spark plug, which can be in a cylinder that happens to have an intake valve open.

Check the easy stuff first.
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Old 12-13-2010, 10:26 AM   #3
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how many miles is on it?
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Old 12-13-2010, 10:31 AM   #4
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Sounds like electrical. Most likely cross fire under load.
Start with the distributor cap. Check for moisture both inside and out.
How old are the spark plug wires. If they are not properly routed with separation, they can cross fire.

Electricity, especially high voltage, will find the shortest path to ground. It can cross from one wire to another or across the inside or outside of a dirty or cracked cap and fire the wrong spark plug, which can be in a cylinder that happens to have an intake valve open.

Check the easy stuff first.
thats is what i was thinking.
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Old 12-13-2010, 10:33 AM   #5
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Well; In addition to the above the 454 will do that with a bad gas filter and that is an easy replacement.

You sure it's not a throttle body? If so one filter on the frame rail passenger side.
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Old 12-13-2010, 10:40 AM   #6
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how many miles is on it?
Around 80K
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Old 12-13-2010, 10:43 AM   #7
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Well; In addition to the above the 454 will do that with a bad gas filter and that is an easy replacement.

You sure it's not a throttle body? If so one filter on the frame rail passenger side.
Good old 4 barrel - I'm having a little trouble understanding the relationship between my problem and the gas filters??
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Old 12-13-2010, 10:44 AM   #8
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I have a 1987 TBI/454 powered Suburban that I had prob with backfiring after a rebuild... turned out to be I had set my valve lash up wrong... it was holding the intake valves open during compression and power stroke.... So assuming you have not touched it recently, it could be a burned intake valve.
Also, as others have said, what miles has it done... be very wary of Timing chain wear.. see my post on this in mechanical section...
To recap, the cam timing chain sprocket is a Aluminum/Nylon piece wears and can jump, on mine it jumped a few teeth, and bent the intake valves, and pushrods. Your motor is from the same era too.
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Old 12-13-2010, 10:45 AM   #9
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Common problem on the 454 with the throttle body happened to me and that's why I carry a spare filter.
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Old 12-13-2010, 10:50 AM   #10
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Agree with all of the above. If that doesn't correct the problem then a compression test would be in order to determine if you have a burned intake valve (usually doesn't happen to intake valves because they are cooled by intake air/fuel).

Another possibility could be cylinder head carbon deposits acting like glow plugs causing pre-detonation (knocking) before the intake valve has closed; look at spark plugs and if carbonized that would be an indicator (caused by oil blow-by of rings or carburator to rich).
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Old 12-13-2010, 10:52 AM   #11
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Good old 4 barrel - I'm having a little trouble understanding the relationship between my problem and the gas filters??
Insufficient fuel flow results in an excessively lean mixture. This leads to increased combustion chamber temperatures that can cause the fuel/air mixture to ignite before the correct time, while the intake valve is still open.

By the way, congratulations on the correct use of backfire, through the carb or intake system. Out the exhaust is after-fire.
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Old 12-13-2010, 10:54 AM   #12
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Agree with all of the above. If that doesn't correct the problem then a compression test would be in order to determine if you have a burned intake valve (usually doesn't happen to intake valves because they are cooled by intake air/fuel).

Another possibility could be cylinder head carbon deposits acting like glow plugs causing pre-detonation (knocking) before the intake valve has closed; look at spark plugs and if carbonized that would be an indicator (caused by oil blow-by of rings or carburator to rich).
Definitely the carbon and secondary electrical issues described above and easy to check and correct. I'd start with the electrical first, move to decarbonizing, then look for a valve train issue.

Big blocks were pretty dirty combustion wise and carbon build up over time is common.
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Old 12-13-2010, 11:45 AM   #13
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I agree that it's due to a lack of fuel. Excessively lean mixtures result in backfires and the reason for this lean mixture under a heavy load (such as accelerating which calls for high fuel flow rates) can be a partially plugged fuel filter.
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Old 12-13-2010, 03:01 PM   #14
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We have a 454 in a 93 LY that did the same thing. Turned out I didnt put enough Sta-Bil in the last fill up. This started with a 1/2 a tank. I let it sit and idle till it ran out of gas. Fresh gas took care of it.
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Old 12-13-2010, 03:25 PM   #15
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80,000 miles, i put my bet on timeing gear. i go with keyair, seen it alot of times when i worked at chev dealer years ago.
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Old 12-13-2010, 04:44 PM   #16
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also look under the distributor rotor. they are know to burn the rotor bottom and even burn through the rotor.

check the plugs. a fouled plug can cause a backfire under load.

look for any broken or disconnected vacuum hoses. i like to check the basics first.

is the idle rough or smooth? is it worse when cold or wet?
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Old 12-13-2010, 06:23 PM   #17
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My money is on a bad rotor, very common to burn through on the HEI distributers. I'm thinking new cap & rotor. Adios, John
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Old 12-13-2010, 06:32 PM   #18
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Originally Posted by Pop Rivet View Post
My 1990 250 MOHO will backfire through the carb when under power - accellration - going up hill - -
Any ideas - timing gear/chain?
So when was the last service and complete tune-up and what was done?
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Old 12-13-2010, 07:00 PM   #19
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I hope it is electrical or fuel related, but these weak sprockets are an issue..

Mine started to misfire a few days before it went bang..
And here is what I found..
Every Intake pushrod is bent..



Every intake valve is bent, and you can see the mark on the larger Intake Valve head where it contacted the piston..





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Old 12-13-2010, 07:47 PM   #20
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Ours backfired when the coil was going bad.
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