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Old 04-29-2013, 10:07 PM
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426 Hemi dual 4's

Have a 1967 GTX 426 HEMI dual 4's. Idle's and starts good. Runs up to about 1800 to 2000 rpm and when trying to get any higher rpm the engine boogs and backfires out the intake. Reset both points to 27 degrees and total 38 degrees according to the book. Reset timing to 8 degrees and tried to checked total timing advance but could not get the engine to run good enough to check. The timing would advance when rpm was raised though. Rebuilt both carbs and checked for air leaks. Has good vaccumn and good compression. This engine was rebuilt and had been run for 500 miles then stored for 25 years. I replaced all seals and checked bearings and used a bore scope too check the heads for rust. I did not pull the heads because they looked good. The linkage to the carbs are set so the back or rear carb opens the throttle plate first so I assume the engine runs from idle to about 2000 rpm on the the rear carb, then opens the front carb with higher rpm. This is when the engine boogs and starts cutting out or poping out the carb. Where should I look next?

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Old 04-30-2013, 12:39 AM
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You replaced all the seals, inspected the bearings, looked in the cylinder with a bore scope for rust but didn't pull the heads or intake? You had to have pulled the motor, put it on a stand to pull the pan, rod and main caps. What seals did you replace?
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Old 04-30-2013, 04:16 AM
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Just a guess...weak valve springs from sitting 25 years
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Old 04-30-2013, 04:46 AM
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Originally Posted by poncho62 View Post
just a guess...weak valve springs from sitting 25 years
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Old 04-30-2013, 05:45 AM
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My guess is it's fuel related. What you're saying is that as soon as you begin to open the front carb it pops. ie a lean pop.

One thing to try is to disconnect the linkage to the front carb so it doesn't open. Then see if it will rev higher on just the rear carb.

Check the float levels, accelerator pumps, etc
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Old 04-30-2013, 09:21 AM
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Check the vacuum at idle and as u accelerate. I have seen cases where the exhaust has closed up, usually with a double wall pipe. The inner pipe collapses and you can't see it. Even found a couple of big rat nest in a pipe onetime.
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