Cylinder Compression Checks

[Patrick67BJ8]

@Patrick67BJ8

I asked because I had a similar issue - although it felt more like a hesitation than a miss if that makes sense. Wound up replace the coil, HT leads, plugs, dizzy cap (Pertronix was "new"). These were swapped out over the course of 6 weeks or so - plugs were last to be changed. Regardless, the issue got better and appeared to have been fully resolved when using a gas treatment. We can only get 91 Octane here in CO (and corn-free is hard to find) but the combination of things seemed to work. Good luck sorting it out!

Thank you for your suggestion on possible cause.

 

[twas_brillig]

This has been a fun read! Someone with a real problem and a cheerful attitude; everyone else contributing real information - which is like most of the posts on the Forum, but still fun. I've used compression tester and leak-downs in the past, and learned some stuff here. Good fortune with the search.

 

[Patrick67BJ8]

I will be working on my car's miss issue next week. I have everything I need to begin working on it except dedicated “time” due to grandchildren graduation from colleges.

 

[Patrick67BJ8]

Progress Update!

I concentrated my engine miss to the ignition system. Carb were rebuilt, great fuel delivery, etc. I had lots of suggestions and a few personal emails from BCS folks too. I use a Pertronix and I didn’t think it to be bad. The dizzy cap was what appears in good condition. My plugs looked good and I gapped them to .30 with a special tool I purchased that really helped me to get a perfect gap.

I never had sparkplug wires go bad so as a last resort I decided to test them at the suggestion of a “fellow Healey in crime” friend. I needed to measure the plug wires ohms and that’s when things began to happen! I am a visual type so I found a YouTube video on testing the plug wires and proceeded to grab one of my many multimeters, but none had good batteries! I grabbed my free Harbor Freight multimeter and went to work. Bingo! Two plug wires had a very high resistance and wiggling the ends fluctuated meter scale. A couple of more wires were suspect too.

Watching the YouTube video was easy to follow the process of testing each wire and how to determine ohms per inch! After changing the wires my Healey started right up immediately which it had not done so for quite a while!

Cheers,

 

[Michael Oritt]

@Patrick67BJ8

I asked because I had a similar issue - although it felt more like a hesitation than a miss if that makes sense. Wound up replace the coil, HT leads, plugs, dizzy cap (Pertronix was "new"). These were swapped out over the course of 6 weeks or so - plugs were last to be changed. Regardless, the issue got better and appeared to have been fully resolved when using a gas treatment. We can only get 91 Octane here in CO (and corn-free is hard to find) but the combination of things seemed to work. Good luck sorting it out!

Patrick--I happened upon this string from May which I somehow must have missed reading and was really looking forward to learning the results of the leakdown test. It looks like you feel the issues were due to the plug wires and that the new wires entirely solved the problem. Though a leakdown is much more revelatory (and it certainly never hurts to do one if only for practice) the 10 psi disparity from the first test was certainly within normal limits. Glad things worked out as new wires are definitely cheaper than a valve job or worse.

The "ohms per inch" test makes me wonder if all wires should be of the same length or if the difference in resistance between wires cut to "visual" length is significant. I have seen cars where all wires were the same length and assume that must have been the reason. It also reminds me of something I saw a long time ago: I was hanging around the shop of an old-time mechanic and mentioned that I had ignition noise in the (AM) radio that varied directly with the engine speed. He told me to disconnect the coil wire, tie a loose overhand knot in it and reinstall. The noise went away. Can anyone explain this phenomenon?

 

[Bob_Spidell]

The difference in resistance due to secondary wire lengths is insignificant compared to the resistance of the plug gaps. Some resistance is desirable to limit current; voltage produces spark, but current is what causes plugs and points to erode (lightning is extremely high voltage, but very small current).

I'm skeptical of the 'put a loop in the coil primary to reduce RFI' but, theoretically, it could create some inductive impedance in the wire, somewhat 'smoothing' the pulse from the points. That's why the electronic tach in BJ8's requires a loop around their pickups; the loop causes a pulsed magnetic field that drives the tach's internals (it's essentially a pulse counter). Simple analog noise suppression circuits typically contain a looped coil and a capacitor, 'tuned' to damp inductive noise in certain frequency ranges.

 

[Michael Oritt]

I'm skeptical of the 'put a loop in the coil primary to reduce RFI'

Bob--I heard it with my own eyes!

 

[Patrick67BJ8]

Patrick--I happened upon this string from May which I somehow must have missed reading and was really looking forward to learning the results of the leakdown test. It looks like you feel the issues were due to the plug wires and that the new wires entirely solved the problem. Though a leakdown is much more revelatory (and it certainly never hurts to do one if only for practice) the 10 psi disparity from the first test was certainly within normal limits. Glad things worked out as new wires are definitely cheaper than a valve job or worse.

The "ohms per inch" test makes me wonder if all wires should be of the same length or if the difference in resistance between wires cut to "visual" length is significant. I have seen cars where all wires were the same length and assume that must have been the reason. It also reminds me of something I saw a long time ago: I was hanging around the shop of an old-time mechanic and mentioned that I had ignition noise in the (AM) radio that varied directly with the engine speed. He told me to disconnect the coil wire, tie a loose overhand knot in it and reinstall. The noise went away. Can anyone explain this phenomenon?

I was told that the ohms per inch should be about 48 to 52. The wires to not have to be the same length, but I do not know the theory behind that.
attached is a YouTube video I used to “visually“ see how to do it.

I may still do a leak down test at a later date. I am going to be doing vacuum checks next to see my Vacuum at my rear carb vacuum advance point, Intake manifold and also the Brake Booster.

BTW: There is a lot of Austin Healey YouTube videos!

 

[Bob_Spidell]

Suppressor wires are supposed to be about 1KOhm/inch, stranded copper has very little resistance (duh). The length is inconsequential given the speed of the 'signal' (about 300,000km/sec):

Speed of electricity - Wikipedia

en.wikipedia.org

Side note/question: I wonder why 4-cyl ignition systems used the more modern wire terminals, at least at the distributor end, when the 6-cyl cars used the 'spear' type. I just redid the secondaries on my BN2, and they had the stranded copper stripped back, and the bare ends* soldered to the terminal. This would have been pretty labor-intensive, but reliable.

* Similar to:

7mm 8mm 180 Degree Brass Distributor Coil Terminals Straight Set of 50 Bulk Pack | eBay

Fits 7mm or 8mm wires. USA brass material.

www.ebay.com

 

[WHT]

"Suppressor wires are supposed to be about 1KOhm/inch, stranded copper has very little resistance (duh). The length is inconsequential given the speed of the 'signal' (about 300,000km/sec):"

Hi Bob, I think the latest quality performance suppression wire is better than it was a few years ago. And, I also cheated on the test. As with Burt Gummer in Tremors, you were denied critical information. Pat's wire resistance is shown in the attachment below (and, you can easily see two wires are out of spec).

IMO, the best spark plug wire for an old British car with a modern electronic ignition is one that has (i) low EMI transmission and RFI interference to protect the electronic modules in the distributor with (ii) NO MORE resistance per foot than is absolutely necessary for interference protection. Lower resistance spark plug wire usually allows the engine to start more easily and have a smoother idle. You want good EMI/RFI suppression, not higher wire resistance.

I assume most of us are using good quality spiral wound spark plug wire if we have an electronic ignition and are seeking better performance. However, everything in life is a compromise and this is no exception.

Low resistance spiral wire that effectively suppress EMI transmissions and RFI interference is more expensive and OEM automotive manufacturers often use higher resistance fiberglass/carbon core suppression wire instead. Saving several dollars is important to them. I doubt many of us worry about saving a few dollars on spark plug wire.

Today, good quality suppression spark plug wire usually has about 40 to 55 ohms per inch. Small variations in individual wire resistance can be caused by differences in terminal crimping.

Solid Metal Core Wire

We all know solid core wires have no effective EMI/RFI suppression and they should not be used with electronic ignition systems; and they have very low resistance per foot. They are great for older cars with conventional ignitions. Although, solid metal core wire can also work well with Pertronix, 1-2-3 and CSI distributors when used with suppression spark plugs.

Fiberglass/Carbon Core OEM Wire

The latest factory wires tend to be 5000 ohms to 500 ohms per foot (400 to 40 ohms per inch). Most modern cars today are basically computers on wheels and are very sensitive to EMI/RFI transmissions, and manufacturers are conservative as well as cheap. Higher end wire with lower resistance cost them more money. Regardless, the best OEM suppression spark plug wire has about 40 ohms per inch.

Low Resistance Spiral Wire
The most popular plug wire for high performance cars is spiral wound spark plug wire. It has excellent EMI/RFI suppression for electronic ignitions and also has a low wire resistance (less than 40 ohms per inch).

Typical Spiral Wound Spark Plug Wire Resistance:

Heli-Core Wire has 150 - 1,200 ohms per foot (13 to 100 ohms per inch).

MSD 8.5mm Super Conductor has 3 - 4 ohms per inch.

Regards, Bill

 

[Bob_Spidell]

Ugh ... brain fart. Can be up to 1.2KOhms per FOOT:

"The Society of Automotive Engineers suggests the maximum resistance should be 12,000 ohms-per-foot."

Defective Spark Plug Wires

Checking spark plug wires is an important part of changing spark plugs. Learn how to spot defective spark plug wires.

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