1956 - 312 engine missing and backfiring.

Stuff101

Stuff101

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Jul 1, 2024
Thunderbird Year
1956
I just bought this car last week and brought it down from Denver to Southern Oklahoma. While driving it to and from town it started backfiring and missing, it was running great and
just started do this all of a sudden. I thought if was only while under a load but yesterday morning it was doing it while it was in park and the brakes on. Does anyone have
any thoughts on what I should check?

Stuff101
 

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I heard that old cars tend to act up with ex stream elevation changes. It's possible it being tuned for Denver it might need to be adjusted for where you are at in Oklahoma. Just a thought. There is people on this site much smarter than me. They might have an better explanation.
 
The mixture being off can result in poor running, however backfiring is not normally associated with it. If you still have a point ignition I would look at changing the condenser. It's cheap and quick.
You don't specify whether the backfiring is through the exhaust or carburetor,(or both) but a good look over at your wires and plugs is probably in order as well.
 
If both then my first stop would be the ignition system. A good start would be points, condenser, spark plugs. Also check the inside of the distributor cap for any carbon lines or Arc tracks. A tiny amount of moisture will also cause this. If you suspect moisture than hose down the cap and inside of the distributor with WD-40. Put the cap back on and fire it up.
 
I went to my local O Reilly’s to purchase some vacuum hose, they did not have the proper size to I purchased a vacuum pump with gauge and tested my
hose’s they passed but the Dual Vacuum Advance unit would not hold a vacuum. Need help can anyone identify this Dual Vacuum Advance unit and do I really
need a Dual unit for this engine? I am attaching photo of electronic ignition and Dual Vacuum Advance unit.
 

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If both then my first stop would be the ignition system. A good start would be points, condenser, spark plugs. Also check the inside of the distributor cap for any carbon lines or Arc tracks. A tiny amount of moisture will also cause this. If you suspect moisture than hose down the cap and inside of the distributor with WD-40. Put the cap back on and fire it up.
Thank you I will do that. I have also found that the Dual Vacuum Advance will not hold vacuum. Looking for a replacement.
 
While that does require attention,it shouldn't be the cause of what you are experiencing. Distributors don't get vacuum at idle, they run off ported vacuum. While I can't rule out a vacuum leak I feel it's unlikely that the vacuum advance system is the big issue.
That's probably a Pertronix electronic ignition, you could contact them for the ohm specs and make sure the module isn't going bad.
 
While that does require attention,it shouldn't be the cause of what you are experiencing. Distributors don't get vacuum at idle, they run off ported vacuum. While I can't rule out a vacuum leak I feel it's unlikely that the vacuum advance system is the big issue.
That's probably a Pertronix electronic ignition, you could contact them for the ohm specs and make sure the module isn't going bad.
I will contact Pertronix and I did order a new Pertronix and spark plug wiring new Coil Distrtibutor cap and rotor bug. Thank you so much for the advice. I am trying to get a Refurb Vacuum Advance unit. It looks like it has a new Holly 4000 and a Electric fuel pump. Have to wait on the parts to get here then hopeful I'll find out. No moisture or cracks in the Distributor. Waiting for a Timming light to check the timing. In the mean time I am going to try to locate TDC and verify that no slipped timing belt. Ugh I bought this to have fun, I never thought I would have to go back to my Teenage years LOL!!
 
Fortunately you have a timing chain. Not a belt. Now if you want to check its condition, follow along!
Make sure the distributor cap is off and the rotor is on. Using a breaker bar roll the engine over to tdc.
Now watch the rotor and roll the engine the OPPOSITE direction until the moves. Stop as soon as you see movement! Read where the harmonic balancer is on the timing tab.
This is the amount of slack in your timing chain. It should be replaced if over 8 degrees.
 
Fortunately you have a timing chain. Not a belt. Now if you want to check its condition, follow along!
Make sure the distributor cap is off and the rotor is on. Using a breaker bar roll the engine over to tdc.
Now watch the rotor and roll the engine the OPPOSITE direction until the moves. Stop as soon as you see movement! Read where the harmonic balancer is on the timing tab.
This is the amount of slack in your timing chain. It should be replaced if over 8 degrees.
Ok Thank you so much I was wondering how to check the Timing Chain to see if it needed to be changed without opening it up. You are a wealth of knowledge.
 
I had the same sort of problem with my '56 312, walked out of a store and it started running like crap. Luckily close to home and it limped into the driveway.
Turned out to be the cheap Autolite spark plugs, after changing to NGK's the problem is gone. ALOT of my mechanic friends tell me to never run the cheap Autolites or Champions. Opinions I value from a good source

The Pertronix units seem to be pretty reliable but some internet searching when my problem occurred showed people talking about frying some units by just having a plug wire coming off while running, other things mentioned are to NOT crank the engine thru with the ignition on as the high voltage from the coil could be fed back to the ignition amplifier (Pertronix unit) thus cooking it. I'm not a sparky but info worth looking at if I was running a compression check
I read that some problems occurred with the early Pertronix units from heat, newer systems apparently don't suffer from this
Others have complained about the old points system, mostly from cheap Chinese condensers being the problem

Opening the hood on a dark night while running can show shorts in the insulation of spark plug wires, changed mine and added extra insulator rubber tubes wherever they crossed each other and turned the block corner from behind around the head under the exhaust
Spark plug wire kits from thunderbird parts suppliers (Larry's, Casco, Tbird HQ) come with rubber blocks and brackets and nice instructions, remember to get extra set of insulator tubes and cut as needed you'll see why.

Hope it helps
All be well
 
A quick test of plugs is, with the engine running and watching for MOVING engine parts, is to use an insulated plug tool to pull the plug wire off each plug; the engine RPM should drop about 200 RPM. In the case of a plug where the drop does NOT occur pinpoints the issue.

Backfiring through the exhaust occurs during fuel starvation or a timing issue.....if you don't know this car, it might be worthwhile to conduct a fuel pressure AND flow check (they are NOT the same thing). An $18 vacuum gauge can be used for the test - just follow the instructions. You could have picked up rust or crud from the fuel tank, the fuel pump could be failing, or, even bad fuel.

A bad vacuum advance will result in a low idle speed and hotter running at idle but normally doesn't cause a backfire. In fact the vac advance is usually plugged when setting initial timing..
 
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I used NGK plugs exclusively in my 61 and 63 Corvettes unless they were being formally judged, they are inexpensive and ultra-reliable. I've installed a couple dozen Petronix Ignitor components in classic cars, the earliest units were failure prone and didn't have the enhanced protection of the newer versions. The Ignitor III is fairly bulletproof and I've never had one fail.

If you go back to points try to find some STANDARD (company name) versions (maybe NAPA ?)...do NOT use those Chiwanese "unipoints" with the condenser and points combined. - they're crap.
 
Do you purchase the standard NGK or the Hi dollar Irdum plugs.
 
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