Hi,
Doug's given you a good list of things to check.
Something that's largely beyond control is so-called "Winter" gasoline formulations that are mandatory at a lot of gas stations and tend to make the engine run a little rougher. The exact formulation varies from one part of the world to another and from state to state in the U.S., but generally have increased ethanol in them (up to 10%), making for a slightly less efficient fuel, i.e. slightly lower octane. About all you can do to compensate is retard the timing a degree or two (which in turn will make the engine slightly less peppy... and probably mean a little bit lower gas mileage) or start to use a higher grade of fuel (unless you are already using premium). You might ask at the station where you gas up, if they have changed the formula recently (i.e., at about the same time you started to notice the engine missing.
With some missing like you describe, first thing I'd do under the hood is check all the wiring connections are tight and clean. Next thing would be pull the plugs, to see if one or more is fouling, or if there is any other indication of possible problems, or if the gap has changed on them. Most spark plugs' gap tends to open up over time, increasing the possiblity of missing. If the plugs are gapped right, but are a bit old, a new set might help too. But the condition of the older used plugs might give some good clues about the problem, so don't toss them out yet!
Another thing to check is that the correct spark plugs are installed. Of course, first this means what type were specified originally. But also there are now at least a few different types of spark plugs: standard, platinum (very thin electrode) and multi-electrode are the main ones that come to mind.
I tried "platinum" plugs in my TR4 and ended up going back to standard type. On the other hand, platinum worked fine for me in a water-cooled VW motor. It seemed to cause some missing in the TR4 (used with stock points-type Lucas distributor and sport coil). Still, platinum might work fine the the 6-cyl motor, so I wouldn't rule them out without trying them.
I don't recall ever trying the multi-electrode type in the TR, but have been unimpressed with them in other cars. It just seemed an unnecessary extra expense. But, again, they might work in your car.
In terms of specific brands of plugs, I generally stick with NGK now. I've tried some others over the years, just ended up using NGK most of the time. Some folks swear by Champion or Bosch or whatever. I suppose most major brands should work fine. NGK work for me and I've use them in most cars and motorcycles I've owned.
Another thing that might be worth a try is a hotter or colder plug. You might already be aware, when talking about hotter and colder spark plugs, it's not that the plugs themselves make hotter or colder spark. It really has to do with the plug's effectiveness as a heat sink within the engine, reducing temperatures within the combustion chamber. So a plug that's called "cooler" is actually helping keep the engine run a little cooler by drawing off some of the heat produced by combustion. A good thing up to a point.
It's always a compromise, the right plug for an engine is one that manages to keep itself clean (i.e., doesn't foul) by producing a good strong spark and full combustion, at the lowest possible temp rating. A hotter plug can overcome a little oil fouling, for example, (i.e., worn rings or valve guides), but too hot a plug can lead to pre-ignition.
The plug needed might change as time goes by and an engine ages - it wears and loses some compression or gains some carbon build-up - or is required to run on different gasoline. So, on older engines sometimes it pays to try different ones and see how they do. If any of the old plugs in your engine appear to be fouling, and this happens consistently, it might be worth trying one step hotter.
So, basically it's a matter of running a plug that's as cool as possible without fouling, and set it with as large as gap as possible, short of missing.
Regarding plug wires in particular, I installed a set of Magnecor 8.5mm on my Land Rover many years ago when it had less than 20K miles on it. Replacing the 18 month old OEM wires made a surprisingly noticible difference: clearly eliminated some engine missing and measurably improved fuel mileage. 7 years later, it's probably about due for a new set!
Now, the LR has a high energy ignition: fully electronic and computer mapped with a separate coil for each cylinder. I think it's more likely to benefit from something like the Magnecor design. But, it's probably overkill for TRs which use a lower voltage system. There are less expensive, but still very high quality wires available from MSD, Mallory and others. Most will probably do fine with TR ignition systems, I'd just suggest an upgrade to an 8mm wire.
It might also be a weakening older coil. If you change that out, make sure the new on is a coil that's compatible with the electronic ignition you are using. For example, I suspect some early failures of Pertronix electronic conversions are related to using that particular ignition trigger in conjunction with a Lucas Sport coil or other higher output coil.
Check the condenser and any other coil/distributor related connections, too. Usually these will cause hard starting or failure to start at all. However, faulty wiring and connectors have caused more than a few hard-to-track-down ignition problems. I generally use a little dielectric grease on the spark plugs and all wire connections, too, to keep help any trace of moisture out and reduce the chance of any corrosion.
Sorry, but I don't agree with Adrio. Changing out the thermostat really shouldn't make a difference. If it does, there is something else going on. The engine should end up in just about the same warmed-up, normal running temp range no matter which thremostat is used. A t'stat merely effects how quickly or slowly the engine warms up and reaches it's operating temperature. This is certainly true of the 160F "Summer" and 180F "Normal" t'stats, and would usually be true of the 195F "Winter" as well. A minor exception is that the 195F "Winter" thermostat will in some cases cause the engine to run just little hotter if it's being driven gently and ambient air temperatures are quite cold. In other words, a 195F t'stat can act to prevent over-cooling in very cold weather, as well it's primary purpose which is to provide a quicker warm up so the heater & defrosters become effective sooner.
It works the other way around, too. If a car tends to overheat, changing to the "cooler" 160F thermostat shouldn't have any real effect either (unless the old t'stat it replaces was simply failing and never opening fully). So, in a sense, the 160F "Summer" t'stat might appear to serve little purpose. But, it does add some margin if and when it begins to fail, insuring it's wide open well below the normal operating range, which sort of assume the t'stat will degrade slowly and gradually, but will continue to open fully.
The 4-cyl. engine requires a t'stat be in place all the time, it shouldn't be run without some sort of restricition in there. I'm not sure if this applies the 6-cyl. engine too. You might be able to run it without any t'stat installed, and see if that makes any difference in the way it runs. It should make little no noticible change in the way the engine runs and marginal change in the operating temp range, once the engine is fully warmed up (due to the total lack of restriction, not due to the design rating of the t'stat).
Hope this helps! I suggest you try one thing at a time until something seems to help, so you'll know what to look for the next time it happens! Let us know what you find.
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