driftpin

1940 Misfires Under Load

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Hello all!

 

Through good fortune and family, I have acquired a 1940 Buick 56S. It runs and idles fine, but under load it starts to misfire badly on what I believe to be multiple cylinders. The ignition has been converted to electronic, but is otherwise stock. Could someone help me figure out the right troubleshooting path?

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The first question is, did it misfire before it was converted?

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Welcome to the AACA Discussion Forum. It is difficult to diagnose with such limited information, but if the plug wires are old, I would suggest you replace the plug wires to make sure they are good. If it still misfires, I would suggest you remove the electronic ignition and return it to the original setup with good points, condensor, and coil. The original components are typically very reliable and easy to diagnose. An aftermarket electronic ignition system could cause problems that are difficult to diagnose and difficult to repair. I put 1502 miles on my stock 1937 Buick Century in one week in July, driving it from NC to Ohio for a tour and back home. These cars can be quite reliable as originally equipped. 

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Thanks for the tips all! 

 

After reviewing the shop manual, my money is on the vacuum advance. I think the ignition & centrifugal advance are probably fine given that the condition does not occur at high rpm in neutral, only while under load. Now I just have to figure out how to diagnose and fix it!

 

I received the car with electronic ignition installed, but I think I have the OEM in a box of spares. It looks like the plug wires are old, but no shorts, so I will probably just replace them and the plugs as a maintenance item if nothing else.

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Electronic ignitions produce a higher voltage which could cause marginal spark plug wires to break down causing misfires.  It would be especially noticeable under load. If the spark plug gaps have been increased as if often done with electronic ignition that would also increase the firing voltage. I would replace the spark plug wires and check the plug gaps.


Steve D  

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driftpin, welcome. My first Buick was a 1940 when it was just a used car back in 1953.

 

  Electronic ignition are GOOD. I run one on my 1950. Never had a problem that was not self induced.  They do not like solid core plug wire, though. In fact, Pertronix RECOMMENDS  carbon core, I believe.  Is the problem misfire or backfire?  

 

  Ben

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1 hour ago, Ben Bruce aka First Born said:

They do not like solid core plug wire, though. In fact, Pertronix RECOMMENDS  carbon core

 

Thanks Ben! That was one of the first things I checked, all stranded! 

 

I believe it is misfires, as it does not produce a loud note from the tailpipe. But this is my first car older than 98’, so I am not nessicarily smart enough to know the difference 

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  One only feels a misfire. One hears a backfire, either from the tailpipe or from under the hood.  If under the hood, this is usually from an extreme lean condition. As in not enough gas. 

 

  Ben

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Is the ground wire on the breaker plate in your distributor ok?

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Still have the original breaker plate with the 3 pivot balls? Could be sticking.

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When cars misfire under load it usually means it isn't getting enough gas, and if it's getting gas, not enough air. I hope you didn't change the air filter to one of those little 5 inch air filters I always see at car shows. I never could understand how a car could run with 5 inch air filters, but people use them. They are show filters, good for idle or going 10 MPH into the show, not good under load. These cars need to breathe.

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Also check the intake manifold to make sure it isn't leaking. Cars with leaky intake manifolds run fine at low speed but when you leadfoot it, the manifold leaks and they don't have enough vacuum to bring in the extra gas you need under load.

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11 hours ago, driftpin said:

It looks like the plug wires are old, but no shorts, so I will probably just replace them and the plugs as a maintenance item if nothing else.

 

How do you know that?  One of the best ways to check the wires for shorting is to go out with the car after dark, start it, and then with a spray bottle, spray the wires with water.  If you see blue arcing anywhere, the wires are bad.  Simple and cheap check.

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Are the wires inserted into the cap and/ or attached to the plugs into the correct order? Check to be sure distributor is / was inserted correctly into the block . Just one tooth off did this on my 39 so it's worth checking.

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Posted (edited)

Driftpin,

All good ideas above, but I had the same issue with my '40 LTD when I first got her.

1. replaced the plugs and wires. plugs were AC-46 green striped, wires were "replacement" resistant type stranded from NAPA.

2.pulled the dist and points plate and cleaned up the centrifugal weights and plate with scotchbrite and re-oiled them.

3.REPLACED the 3 balls on the advance plate with plastic sliders. A really good idea !!!

4. got new points, condenser, ground wire, and cap and vacuum advance unit, from NAPA

= no more miss under load.............

 

Mike in Colorado 

Edited by FLYER15015 (see edit history)
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On 8/18/2019 at 3:44 PM, FLYER15015 said:

Driftpin,

All good ideas above, but I had the same issue with my '40 LTD when I first got her.

1. replaced the plugs and wires. plugs were AC-46 green striped, wires were "replacement" resistant type stranded from NAPA.

2.pulled the dist and points plate and cleaned up the centrifugal weights and plate with scotchbrite and re-oiled them.

3.REPLACED the 3 balls on the advance plate with plastic sliders. A really good idea !!!

4. got new points, condenser, ground wire, and cap and vacuum advance unit, from NAPA

= no more miss under load.............

 

Mike in Colorado 

 

Mike, all very good items, and logical as well,

but did you do all the above and then test drive, 

or

execute each item in sequence, and test drive between each step to note the difference?

 

Ages ago (1970-71?) driving a Citroen DS-21 Pallas from Ft. Wayne to New Orleans we started to experience a miss under load.

I changed the plugs at a rest area with a spare set (always keep spare beelts, hoses, clamps, electrics from last service).

It was better, but not proper until I swapped out the plug wires at the next truck stop.

Those were the two primary culprits, but the other maintenance was likely needed as well at approximately 40-45,xxx miles. WE had bout it used a few months earlier following the wreck of our prior '67 DS-21.

Did a full service (like what you described except for plastic sliders - great idea!) when we got back home in Indiana.

 

 

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Although this may not be the problem, to test the vac advance pick up one of the MityVac kits and do the following:

1. Disconnect the vac advance line at the distributor and remove the dist cap

2. The kit should have several tapered adapters. Find the one that fits tightly into the advance unit and hook it up to the pump, with the pump set to VAC

3. Holding the adapter into the advance unit with one hand, pump with the other and watch the vac gauge on the pump

4. The breaker plate should start to move when the vac reaches about 6 or 7 inches Hg. Maximum movement occurs at about 14 inches.

5. Check that the advance can hold vacuum without any or minimal leakdown.

6. If no plate movement occurs but vacuum can be obtained and held, unhook advance arm from plate and check for free breaker plate movement. If plate does not move freely solve the issue and retest.

 

If vacuum cannot be obtained and/or held, unit is bad. If vacuum is good but arm does not move properly according to the above or sticks or is sluggish, unit is bad. There are still enough of these units floating around and vendors willing to rebuild that it's not worth it to put up with a bad one.

 

Cheers, Dave

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Marty,

I did it all at once, after driving the baby 120 miles from Denver to Buena Vista when we first picked her up from the "broker", of the estate.

Ran like crap all the way home, especially over an 11K ft pass on US 285.

Pulled the pan at the same time and found the oil pick up screen detached and laying in about an inch and a half of grey sludge.

That led to an oil pump rebuild and an ethanol proof diaphragm for the fuel pump, plus an electric pump and BIG clear filter back at the tank.

 

PS: Marty, Sara loves your white Explorer !!!

 

Dave,

I checked my new vacuum advance by hooking a brake bleeder hand suction pump up to the line at the base of the carb, and used a mirror to see if the plate moved, after I put the dist back in.

That way I know the tubing and connections are all good.

 

Mike in Colorado

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6 hours ago, FLYER15015 said:

 

Ran like crap all the way home, especially over an 11K ft pass on US 285.

 

My car did the same thing at 11K once. So I went into a pub and had a beer. Then when I got to 5000 feet it ran great again. 

 

This just proves that sometimes, all you need is a beer.

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Morgan.

If that is the fix, I should stay about half loaded, as we live at 8500 ft.

DW thinks I'm about half gone anyway........

 

Mike in Colorado

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20 hours ago, FLYER15015 said:

Marty,

I did it all at once, after driving the baby 120 miles from Denver to Buena Vista when we first picked her up from the "broker", of the estate.

Ran like crap all the way home, especially over an 11K ft pass on US 285.

Pulled the pan at the same time and found the oil pick up screen detached and laying in about an inch and a half of grey sludge.

That led to an oil pump rebuild and an ethanol proof diaphragm for the fuel pump, plus an electric pump and BIG clear filter back at the tank.

 

PS: Marty, Sara loves your white Explorer !!!

 

Dave,

I checked my new vacuum advance by hooking a brake bleeder hand suction pump up to the line at the base of the carb, and used a mirror to see if the plate moved, after I put the dist back in.

That way I know the tubing and connections are all good.

 

Mike in Colorado

 

Mike,

Great to know that the Explorer is still serving Sara well. When Nathan was away at college with it I wasn't able to give regular ongoing maintainence. I know you had several items to handle to make the 4WD front and rear wheel drive as quiet as new, as well as freshening up the interior. As I recall, Sara crosses the Great Divide twice each day, good weather and bad, we're delighted to know she has a solid and dependable ride - as well as a dependable family to care for her - best regards from Dale and me. Sorry we missed you earlier this year at Ocala, but if Dale's healh holds we hope to judge at Miami in February.

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I'm sure everybody in Colorado knows cars don't run well at such high altitudes, but us coastal folks have no idea. First time I drove across the country I was 19 and didn't know anything. The mechanics around 10,000 feet were telling me I had to buy new high altitude jets for the carb. They almost got me, until I said to myself, doesn't the altitude get low again after here? 

 

So I got a beer. It was 3.2 of course. LOL.

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Posted (edited)
1 hour ago, Morgan Wright said:

I'm sure everybody in Colorado knows cars don't run well at such high altitudes, but us coastal folks have no idea. First time I drove across the country I was 19 and didn't know anything. The mechanics around 10,000 feet were telling me I had to buy new high altitude jets for the carb. They almost got me, until I said to myself, doesn't the altitude get low again after here? 

 

So I got a beer. It was 3.2 of course. LOL.

 

Driving my (former) '27 Chevy Roadster up Pikes Peak on the 1990 Glidden Tour, I adjusted the carb at the base of the climb so that I could use the choke to modify the mixture as we made the ascent. She and I were still slightly short of breath at the 14,210 foot level, bot both still going.

 

Life is a bit different here where home is 3-1/2 feet BELOW sea level

Edited by Marty Roth (see edit history)

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1 hour ago, Marty Roth said:

 

Driving my (former) '27 Chevy Roadster up Pikes Peak on the 1990 Glidden Tour, I adjusted the carb at the base of the climb so that I could use the choke to modify the mixture as we made the ascent. She and I were still slightly short of breath at the 14,210 foot level, bot both still going.

 

Life is a bit different here where home is 3-1/2 feet BELOW sea level

 

Couldn't find a turbocharger and oxygen tank, huh?

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