The spark lugs in the photo were taken out of my 62 340 horse. They are AC R45 plugs. They were removed because the engine runs rough on start up and sometime hesitates when driving. I suspect the plugs are fouled because of the black appearing tips. I know it is not NCRS correct but I plan on replacing them with Autolite 85 or 285 plugs which I have been told are less problematic. Is the Autolite 85 plug too cold? If so what heat range or type of plug would be better? Also, this engine was rebuilt with flat top pistons so compression ratio is estimated to be 9.5 or 10 to 1. Otherwise heads, distributor, cam, intake & carb are stock. The car is driven about 1500 miles per year, usually conservatively on suburban streets and I am most interested in reliability over performance.
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check your plug wires with a ohm meter
to make sure you have no opens or extra high resistance ones. also make sure the plug wires are all the way down into the cap tower. -
Re: check your plug wires with a ohm meter
I just checked. All wires (dated repros) are between 13000 and 18000 Ohms. I have read that 3-5000 Ohms per foot is the standard. The longest wires are at the 18K end and the shorter wires at the 13K reading. Does this appear OK?Comment
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Re: check your plug wires with a ohm meter
they sound OK but some people have had trouble with misfire with using certain repro wires.Comment
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Re: Fouled Spark Plugs?
Greg, I am not sure but, it looks like plugs #1 & #2 are blackish in color. The rest look to be a light grayish. If my antique brain recalls correctly, you are looking for a light beige brown to a light grey coloring to indicate good burning not fouled. Are #1 & #2 "oily" and black or just black. If they are oily then I would suspect some oil blowby from either the rings or valve stem seals. Of course an uneven fuel/air mixture to cylinders 1 & 2 is also a possibility if black dry soot. On my 427, it is notorious for rich mixtures on the outboard cylinders and leaner in the center of the engine.
David Nims 19639Comment
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Re: Fouled Spark Plugs? - DRIVE IT!
What you have isn't a plug problem. Those plugs 3, 5, 7, and maybe 6 look like they are hot enough, just burning deposits. Look at those gray ashy deposits, that is hot oil/gas that is burned.
It looks like you have oil in there (common, what SBC doesn't puff smoke on startup?) and too rich part of the time (choke in too long).
Your problem is 1500 miles per year and conservative driving.
Odds are the choke is in a LARGE percentage of your driving time. Then you never clear it out with a good romp.
A 340 hp wasn't made for Mom to take the kids to school in or pick up groceries, it was made to burn down the highways and dust a T-Bird at the light.
Since you are suggesting a non-NCRS adjustment, here is a better one. Take that repro/original distributor cap and repro wires off. Set them on a shelf for the next NCRS meet.
Buy a good new Delco cap, buy a set of good new wires, and run them.
Take the car out into the country or out on the interstate and romp on it a bit. Run it up through the gears, 5500 at least two of them, and put about 50 miles on it. Then check the plugs.
It isn't going to break if you drive it like it was meant to be driven. If it breaks doing this, you need to have a real serious talk with your builder, restorer, or previous owner.Comment
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Re: Fouled Spark Plugs? - DRIVE IT!
Thanks for the response. You are right it needs to be let loose more often. The plugs are actually more black than it looks in the picture. I have heard more than once that the repro wires are a problem. If the internal resistance of the wires is in spec why would the wires not perform? I'm no electrical engineer. I have a dual point distributor cap which I bought several years ago a Bloomington. It was in a GM box if that means anything. I still use points vs electronic so I don't want to lose the dual window cap. I hate to change wires as it is a pain to do and the repro wires are like new.Comment
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Re: Fouled Spark Plugs? - DRIVE IT!
I'm not a big fan of Autolight plugs, but if you want a non-resistor type equivalent to AC heat range "5". The NGK B4 and Denso W-14U are the same as the non-resistor AC 45 and they both have a good corrosion resistant coating.
It's tough to see the plugs in the photo in sufficient detail, but off-white deposit build up on the insulator is usually oil ash, which can be caused by too low insulator temperature for the driving condtions (too cold a plug) or just high oil consumption. Black wet deposits are also indicative of oil consumption. Dark grey dry looking deposits are indicative of rich mixtures.
As stated, these SHP engines don't like putting around town and idling a lot. They need to be run once in a while - like a race horse. There's nothing wrong with 1500 miles per year, unless it'a all short trips at low speed.
What's the rate of oil consumption, and what's the typical trip profile?
DukeComment
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Re: Fouled Spark Plugs? - DRIVE IT!
Typical trip is a one hour drive on suburban streets with short interstate highway time. Oil consumption is minimal. I never have to add oil. I change oil once a year when putting the car away for the winter. I had some ignition problems that were point and coil related. I think those problems may have caused the plugs to foul. I would prefer to us AC plugs but I have read on this site that they are problematic. I guess my main question is should I use a hotter plug for reliability and resistance to fouling or just use 45s and run it harder? Spec is AC44, pictured plugs are ACR45.Comment
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Re: Fouled Spark Plugs? - DRIVE IT!
Trip profile sounds okay, but don't baby it.
What "problems" have you read on this site with AC plugs? That's a new one to me.
If you want to stick with AC plugs clean and reinstall what you have, but since spark plug cleaners are pretty rare nowadays, the best course would be to buy a new set.
The fouling could have occured when you had the ignition problems, and once fouled the plugs must either be cleaned or replaced. It's tough to burn off.
Check them again at the end of the driving season. It would also be adviseable to go throught the idle speed, mixture adjustment procedure and set it up on the ragged edge of leaness.
DukeComment
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Re: Fouled Spark Plugs? - DRIVE IT!
I think I will try AC R45 plugs again as I am going to have car judged this summer. As I remember, the idle adjustment calls for closing AFB idle adjustment screws starting at 1.5 turns open to achieve highest possible vacuum. I have done this in the past but honestly didn't see much change in idle vacuum. Is there a better procedure or am I doing this wrong? Also the book calls for 800 RPM idle which I do get with a slight lope with Duntov cam. What idle speed do you recommend? Also, I am using a 270 coil that came on the car. Do you have an opinion on the functionality of the new repro 091 coil that is correct for this car? If they have the same reputation as repro wires I don't want to waste the money to purchase one.Comment
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Re: Fouled Spark Plugs? - DRIVE IT!
I asume you have ST-12 I would refer you to that as I expect it has an idle speed-mixture setting procedure, but the initial setting for my '63 340 (which has vacuum advance) is 1.5 turns from the seat. I think 800 is a little low and recommend you increase the idle speed to about 900. (IMO Chevrolet's recommended idle speeds for SHP/FI engines are unrealistically low.)
Before you set the idle speed mixture, I would recommend checking the timing and the function of the centrifugal advance. The advance might start by about 700, so you should lower the revs to check and set initial. If you have a dial back timing light check to see where the centrifugal starts and verify that it's reasonably within spec. The OE curve is fairly slow, so you should be able to run a little more than the specified initial timing, which is probably ten. If you can run 14 without detonation, that's what I recommend.
If you have a CA model with PCV, check that the valve and plumbing are clean and in good working order.
Once the timing is verified, set the idle mixture screws at 1.5 turns out from the seat and set the idle to about 900. The engine should be fully "normalized", so do this after a good drive.
Turn the screws in about a quarter turn. Any change in vacuum or idle speed?
If not, turn another quarter turn in. At this point you will probably see a reduction in idle speed and vacuum, which indicates too lean a mixture, so turn the screws back out a quarter turn. With a little experience you develop a feel for the ragged edge of leaness, which is how I've accomplished the procedure for years and rarely even bother with a vacuum gage or test tach.
I don't have any experience or opinion on the quality of the rebuilt O91 coils. By "270 coil" do you mean the last three digits of the part number? The 091 was used on all engines except FI from in the '57 to '62 era. You can buy a generic 12 coil that will fit from NAPA et. al, and save whatever OE coil you have for judging. When the shielding is on a replacement coil will not be obvious. Coils can slowly degrade with electical load and heat cycles and cause increasing misfires. You probably can't tell if a plug misfires once out of 25-50 cycles, but this can affect smoothness and contribute to plug fouling.
DukeComment
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Re: Fouled Spark Plugs? - DRIVE IT!
Thanks for the detailed procedure. I have the ST-12 which I think is where I got my procedure. I wish the 62 340 had vacuum advance also. The 270 I refered to is the number on the coil, same as 091 which is the specified correct coil for my engine. I don't know how old my 270 coil is or what engine it is intended for. I think I will buy one of the repro correct 091 coils and try it out since I am going to have the car judged.Comment
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Re: Fouled Spark Plugs? - DRIVE IT!
I forgot to mention that before you check timing, you should check the dwell and make sure it's in the ballpark. As dwell changes, so does timing. Higher dwell retards timing and lower dwell advances it especially on the trailing set of points.
I don't see the 270 coil listed for Corvette, but there were many of them. For years the 202 coil was the replacement for all previous Corvette coils. All have similar resistance, but inductance, which is not specified for any, may be different.
At least for driving, I recommend vacuum advance distributors for all early engines that didn't have them. It should not be too hard to integrate one with your engine as long as the vacuum can will fit under the shielding, but I'm not sure if it will.
Duke
DukeComment
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Re: Fouled Spark Plugs? - DRIVE IT!
The shielding is the same for the 300 HP as the 340 SHP so I assume the vacuum can would fit under the shielding of a 340. The 300 HP has a vacuum advance. I would love to add vacuum advance but correctness would suffer. I know the 1962 340 SHP and FI engines are considered high performance but most were street driven. Seems like GM could have added vacuum advance before 1963. The normal base timing for the 340 SHP is 10-12 degrees. If I add more the engine doesn't knock but the starter works harder during a hot restart.Comment
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