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454 mag backfiring wonít rev up.

Old 08-10-2021, 04:41 PM
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Default 454 mag backfiring wonít rev up.

I rebuilt a 454 mag mark IV engine and itís not revving past 2300 and popping out the intake. Iíve been changing things for the past month just wasting money with no results. Itís bored .030 over, 10.29:1, 185 on each cylinder, comp camshaft broken in properly and verified after no lobes were destroyed, thunderbolt ignition. Iíve changed cap, rotor, coil, plugs, thunderbolt ignition module, reset valve lash, swapped carb with one from a running 454 engine, drained all 75 gallons of gas and put fresh 93 in, new fuel pump. Vacuum gauge reads 13 inches at idle, sprayed starter fluid all around intake with no change in rpm. Timing is 8į at idle and advances smoothly while revving up. I think Iíve checked everything but Iím lost. Any help would be appreciated.
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Old 08-10-2021, 05:05 PM
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I would be checking fuel pressure while eng is under load. We had a member recently suffering very similar issues and it turned out to be a collapsed inner lining of fuel supply hose severely restricting fuel flow.
Ethanol laced fuel and old school hoses don't play well together.

Another possibility is excessive spark plug gap. 0.035" is the almost universally accepted gap for any configuration. This is especially important for forced induction or high compression engs.
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Old 08-10-2021, 05:10 PM
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What gen TB ignition is it
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Old 08-10-2021, 05:26 PM
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It is I believe second gen thunderbolt IV on the triangular plate. V8-24. How can I check pressure under load? I have the rubber fuel line from tank to filter than itís all metal lines from there to mechanical pump on the block to carb. Cut the metal line and put some kind of T between pump and carb?
Plugs were gapped correctly, have tried different manufacturers plugs as well.
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Old 08-10-2021, 05:47 PM
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What did compression test say ?

Fuel psi when backfiring ?

Any chance you can borrow that guys dist (maybe pickup issue ?)and tb module ?

carb you borrowed is from same hp 454 ?
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Old 08-10-2021, 05:58 PM
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Can you easily replace the fuel line from the tank? If your throwing parts at it the hose is cheap enough.
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SB (08-10-2021)
Old 08-10-2021, 06:44 PM
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Originally Posted by carnutsx2 View Post
Can you easily replace the fuel line from the tank? If your throwing parts at it the hose is cheap enough.
Yup ^^^^
ck anti siphon:valve/ pickup/hose/other eise psi
12v to coil
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Old 08-10-2021, 07:54 PM
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you check plug wires?is the cam a swap cam?what cam did you get?
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SB (08-11-2021)
Old 08-10-2021, 09:51 PM
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-Compression - 185 on all cylinders (~3or4)
-carburated so no connector to test fuel pressure
-carb I swapped with was from another 454 mag I built for a friend a few years back. Both motors are 454 mag
-donít have a distributor to swap with, donít know what tb module is but cleaned the ignition module in distributor
-I will change 3í rubber line to tank. Iíll post results when it stops raining (Thursday)
-checked plug wires 40x, has brand new marine wires. Under load timing light didnít pick up any cross firing.
- cam is comp cam, cam is bigger but not anything crazy
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Old 08-10-2021, 09:53 PM
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Iím a new member so I canít post a video but I uploaded a video on YouTube. You can see it pop out the primary as soon as the secondary opens.
YouTube search ď454 mag not revving and backfiringĒ
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