‘74 X19 miles per gallon (mpg) not very good. Likely solution?

Dan Sarandrea (Phila) said:
Let's start with two old school checks that (1) are well within the ability of most home mechanics and (2) are appropriate for an old school car.

Before you being, pop round to your local major auto parts store that loans tools (actually they don't loan, you buy them and then bring them back for a refund). Assuming you don't own these already, acquire a vacuum gauge and a 3/8"drive torque wrench. Then ask the salesperson for an old-school spark plug gap gauge, the kind that uses different wire thicknesses rather than a flat style or feeler gauges:

41KGZ7E74GL.__AC_SY300_SX300_QL70_FMwebp_.jpg


For both of these tasks, we'll need someone with good knowledge of the US-spec 1974 1300cc for details denoted by *** to help us out with some details.

First task: pull all four plugs, lay them out on a paper towel, take a pic and post it. While they're out, check the gaps***, adjust if necessary, and re-install to the correct torque spec.***

Plug condition and color can tell us a lot about rich or lean mixture.

Second task: connect the vacuum gauge to manifold vacuum (where?***), start the car, and record the gauge say 15-20 sec with your phone. If you have a helper, it might be interesting to a "cold start" version of this as well.

Vacuum gauge behavior can tell us a lot about valve timing and valve train condition.
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Why do you prefer the wire gapper to this kind:

 
tvmaster said:
Why do you prefer the wire gapper to this kind:

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I was taught that since the wire style uses a cylindrical shape to measure, it is less subject to measurement error as compared to using a flat shape. If a flat shape is used, care has to be taken to ensure the tool is held so the measurement surface is parallel with the gap being measured, a consideration not applicable to a round or cylindrical shape.

I suppose that would have been important in the infancy of internal combustion when ignition systems were just barely capable of pushing a feeble spark across a gap, and when spark plugs were expected to be cleaned and gapped on a daily basis ;)
 
Dan Sarandrea (Phila) said:
I was taught that since the wire style uses a cylindrical shape to measure, it is less subject to measurement error as compared to using a flat shape. If a flat shape is used, care has to be taken to ensure the tool is held so the measurement surface is parallel with the gap being measured, a consideration not applicable to a round or cylindrical shape.

I suppose that would have been important in the infancy of internal combustion when ignition systems were just barely capable of pushing a feeble spark across a gap, and when spark plugs were expected to be cleaned and gapped on a daily basis ;)
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So you’re still keen on the wire then? Thanks.
 
dllubin said:
The wire gauge will measure between two points in space versus two planes in space for the flat gauge.
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And? Remember I know zero about engine maintenance. Happy to learn though. I'm guessing it's YouTube time using those words. Thanks D
 
tvmaster said:
And? Remember I know zero about engine maintenance. Happy to learn though. I'm guessing it's YouTube time using those words. Thanks D
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I think Dan explained it pretty well up above. If what you are measuring are not two absolutely parallel surface (like a valve shim and cam lobe), you can run into issues with a flat gauge. On a spark plug, the surfaces are likely not flat or parallel, particularly if it has already been used. The wire gauge just measures the distance between the two exact points it is touching. For a spark plug, you use it to set the minimum gap between the center electrode and the ground electrode. The spark will tend to start where the minimum gap is because the electric field will be highest. There are other effects that can alter this slightly such as what is going on in the combustion chamber and the spark plug geometry in the electrode area. All that said, I've never had a car that the plug gap was super critical so I suspect that any type of gauge you buy will get you close enough. If you happen to install a high energy ignition (such as an MSD capacitive discharge unit) they will often tell you to set the gap significantly larger than stock. This is because the higher voltage can give you the same or larger electric field across a greater gap (electric field = voltage/distance) and the greater gap will ignite a greater amount of fuel mixture more quickly.

I've got a gauge similar to the one you posted the URL for above. They were giving them away at an auto parts store so I took one. I find that style useful for getting a quick reading for a gap since you can just stick it in and turn it until it touches both sides and read the number where it touches. Unlike the flat or wire gauges which measure discrete distances, this type has a continuous range so it can be pretty quick to find a gap size without the need to switch from one size to the next. Not exactly a replacement for a caliper or mic, but OK if you don't need precision.
 
dllubin said:
I think Dan explained it pretty well up above. If what you are measuring are not two absolutely parallel surface (like a valve shim and cam lobe), you can run into issues with a flat gauge. On a spark plug, the surfaces are likely not flat or parallel, particularly if it has already been used. The wire gauge just measures the distance between the two exact points it is touching. For a spark plug, you use it to set the minimum gap between the center electrode and the ground electrode. The spark will tend to start where the minimum gap is because the electric field will be highest. There are other effects that can alter this slightly such as what is going on in the combustion chamber and the spark plug geometry in the electrode area. All that said, I've never had a car that the plug gap was super critical so I suspect that any type of gauge you buy will get you close enough. If you happen to install a high energy ignition (such as an MSD capacitive discharge unit) they will often tell you to set the gap significantly larger than stock. This is because the higher voltage can give you the same or larger electric field across a greater gap (electric field = voltage/distance) and the greater gap will ignite a greater amount of fuel mixture more quickly.

I've got a gauge similar to the one you posted the URL for above. They were giving them away at an auto parts store so I took one. I find that style useful for getting a quick reading for a gap since you can just stick it in and turn it until it touches both sides and read the number where it touches. Unlike the flat or wire gauges which measure discrete distances, this type has a continuous range so it can be pretty quick to find a gap size without the need to switch from one size to the next. Not exactly a replacement for a caliper or mic, but OK if you don't need precision.
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How many mechanics, whether at dealerships or private shops, check this when changing spark plugs on ANY car - 50%?
 
Any competent mechanic would do it. It only takes a moment and allows you to adjust the gap if it needs it. There are some specialty plugs that are not adjustable.
 
tvmaster said:
How many mechanics, whether at dealerships or private shops, check this when changing spark plugs on ANY car - 50%?
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I think the older the vehicle the more a competent mechanic or home mechanic will pay attention to spark plug maintenance. On the earliest cars and motorcycles spare spark plugs were carried as consumable items. When I got started with cars in the late '60s the typical publications and "Chilton's manuals" always had sections on cleaning and gapping plugs, usually as part of the 3 month 3,000 mile "tune-up" checklist LOL. You were expected to have a plug gapping tool because that spark plug could have been used in 27 different models and no one "trusted" the gap as you were taking the brand new ones out of the box. There were tabletop spark plug sand-blaster cleaners you could buy "to save money." And many local repair shops had an Autolite chart on the wall that showed you how to "read" a spark plug.

Contrast that with today, plugs that last 100,000+ miles and when you get new ones they are pregapped, and they have a protective plastic sleeve around the electrode end. And now it costs more for one than 16 did back in the day. :cool:
 
dllubin said:
Don't assume the timing is correct at higher speeds just because it is OK at idle, especially if the vacuum retard has been messed with. The Ducelier distributor on that car has a very large centrifugal advance which is offset by the vacuum retard. For a good desmog, best thing to do is get rid of the vacuum retard, set static timing to 10 degrees, and recurve the centrifugal advance to get you ~32-35 degrees by about 3,000 rpm.
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Any advice on removing the distributer on these old cars? Simple?
 
tvmaster said:
Any advice on removing the distributer on these old cars? Simple?
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I would ensure the engine is at TDC, take a pic and mark where the dist is on the body of distributor (a punch on the base and an associated punch on the block) and the block as well as the rotor pointing. Pictures are always good things to have to assist you later.
 
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I am going to replace my spark plugs and I thought it is a good idea to reuse this thread. I am now going for NGK instead of Bosch but I do not know how to measure and adjust the gaps on these:
IMG_20230517_195036.jpg
 
Bjorn Nilson said:
I am going to replace my spark plugs and I thought it is a good idea to reuse this thread. I am now going for NGK instead of Bosch but I do not know how to measure and adjust the gaps on these:
View attachment 73459
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According to the specs at NGK's website, these should be pre-gapped to 0.035in / 0.9mm.

I think this is one of those situations where a wire-gauge style of plug gapping tool would yield a more accurate reading than a feeler gauge or flat-ramp style of tool.

Examples:

 
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