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Discussion Starter #1
Ok, here are all the details i can give and all I've done so far. I have a 1948 sedan delivery in my shop right now that backfires under load but not in park or neutral. It happens between the 1800-2500 rpm range.

Engine specifics are: 355 at aprox 10 to 1. comp 12-210-2 cam/ .454 lift 268 duration. 882 heads. RPM air gap intake with a 1405 edelbrock and stock type hei dist. Comp roller tip 1.55 rockers on press in studs. No known head or intake work. Build has aprox 50 miles on it other than break in time. EVERTHING is new (except heads and block).

What I've done: Ohmed out plug wires and they showed high resistance so i replaced with MSD 8.5 super conductor wires. 3 fouled plugs (r45ts) I replaced all with Bosch platinums because store didn't have AC's in platinum. Set timing at 12 deg. Re-adjusted rockers and no studs were pulling up. Re-tuned carb idle screws by wide band. No o2 sensor in system, put deep in tail pipe. I say this because I cant monitor air fuel when I am driving it before I'm asked if its rich or lean when under load. I also pulled cap and button to inspect wiring of dist and everything looked fine, there was metal shavings on the pickup. I figure that to be from being a new off the shelf part and not a higher end dist. Tomorrow I'm replacing cap, button and coil and getting a rebuild kit for the carb. I'm also going to check for broken valve springs because i forgot to when adjusting rockers in.

What has happened thus far: After plugs and wires it improved but not fixed by any means. After readjusting of the valves it ran better below and above the effected RPM range but no change to the popcorn. Also, if you bring the four barrel in before the popcorn it has less but it is still there.

What I'm left with, being as the plugs and wires noticeably helped I'm feeling it is ignition related and not fuel delivery or cam condition. However, when adjusting the rockers (with engine not running) I found 4 soft lifters. I'm attributing this to bleed off from spring pressure on those lifters. My question is after Cap, button and coil if need be am i overlooking anything else in the ignition side and could the springs be pulling to much timing to soon and cause this. If cap and such doesn't remedy the problem I'm going to rebuild the carb as a little bit of a desperation move as I really don't think its the cam. Any and all help is appreciated and i will post what the remedy is once I figure that out.:confused:
 

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I'd be pullin' the intake to eyeball the cam and lifters and particularly the intake gaskets to verify they are sealing all the way around each and every port. I hate chasin' spooky stuff like this.
 

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It could be a lack of ignition advance; usually if it has too much advance you'll hear it ping under a load.

Be sure the mechanical advance is not stiff or hanging up. It has to be free from friction as you move the advance plate to cause the weights to deploy. Also be sure the springs are providing enough tension to pull the weights back in at idle.

Do you have full battery voltage going to the HEI? No ballast resistors or resistor wire should be used. Keep the plug gap to around 0.035"-0.040" max, no need for more gap than that.

Be sure the clamp holding the distributor down is able to provide a ground pathway to the block. Also be sure there are good grounds between the engine and battery, chassis, firewall, etc.

Check to be sure the coil has the ground wire or strap (depending on the type) under one of the coil hold down screws (see drawing). The other end of the ground is the center terminal that the pigtail from the distributor plugs into.



Check the condition of the carbon electrode that is in the cap. It can wear or break, or even get installed upside down- the small spring goes against the coil. There should be dielectric grease on the rubber grommet that surrounds the center electrode, too.

Any cap cracks or carbon build up can cause a misfire under load. You're replacing the cap so that'll take care of that if it was the problem.
 

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Discussion Starter #5
Ok, here's an update. Replaced cap and button and found out that the mechanical advance isn't retracting when the rotor is tightened down. Just picked up an accel dizzy. Go
 

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Discussion Starter #6
I hate phones. Going to pull the old one and see if its repairable before I put in this unit. I'll update when that's done.
 

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Discussion Starter #7
So, after doing plugs and wires that it definitely needed and a new cap and button I put in the accel dizzy. Problem is now completely solved. I can now feel a flat spot in the carb however. That's an easy fix though. Ultimately the problems were the plugs being fouled and a few bad wires both of which I attribute to timing issues caused by the bad dizzy. Hope this helps anyone who has a similar problem and makes it less frustrating.
 

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Discussion Starter #8
I would like to thank everyone who has jumped in and offered suggestions and advice. i appreciate all of the knowledge and help. Thank you all.
 

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Discussion Starter #9
I know this is old but wanted to update something. After all this there was a spark plug laying in the lifter valley. Not left there by me. It dropped down between the cam gear and distributor gear and popped the distributor out the top breaking the strap and all. Thank god I was only a half mile from the shop. It also ended up having a couple bad cam lobes and one bad lifter as well. Hence, the perceived dead spot. New cam and lifters and all is well now.
 

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. Thanks for the update... when you mentioned "soft lifters" I figured you either had a lifter adjustment technique problem and/or valvetrain problem... is it running really strong now?
 
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