Plug gap does make the difference

[Marvin Gruber]

The TR250 has had a stumble to it for the last year. I sat out to figure what it was. Car cranked well, little rough idle until it warmed up some. It would flutter when under power. I pulled the plugs, nice gray/brown color like the book says is good. Running a crane fireball ignition so I had the plugs gaped a little wider to .035. WRONG, set them to .028 and now runs great. First time I have really seen where the gap effected the performance that much.

Marv

 

[DNK]

Stock iggy?

 

[Brosky]

If you use the higher output coil with an electronic ignition you can go wider. I saw no performance difference and put mine back to stock.

 

[poolboy]

I think Dan Masters has an interesting take on Spark Plug gaps.
He writes:
"Bypassing the resistor is only one of the changes necessary to utilize the full advantage of the Sport coil, or any high output coil for that matter. Unless you also increase the spark plug gap, the plugs will continue to fire at the same coil secondary voltage as before, give or take a little, and will not give the hotter spark that you paid for. The voltage rise at the output of the coil secondary, although rapid, is not instantaneous. As the voltage rises from zero, as soon as it reaches the value high enough to jump the plug gap, it will. Since the plugs were firing at 20,000 volts with the old coil, that will fire at that voltage level regardless of the maximum voltage potential of the new coil. As soon as they fire, and the arc begins, the voltage drops to zero. By widening the gap, the voltage must rise to a higher value to jump the gap. Ideally, the plugs should be gapped to the widest setting that will still allow them to fire under all engine operating conditions. Usually, the coil manufacturer will give recommended gap setting, but these should be used as the starting point, and adjusted from there as experience dictates"

I know an old gearhead that adjust his plugs that way. He never uses a measuring tool, just his knife blade. He prys them open to the point where they fail to arc, then with a tap on the fender, closes them up a bit.

 

[Marvin Gruber]

Stock distr with crane fireball ign and matching coil. When I put that in last year I just cranked the car and rev'd the engine. It sounded good. It was weeks later before I got to go for a drive. Thats when I got the fluttering. Yesterday was first time I got to study on it. I have set the gap wider on other cars with no problems. I think the TR6 is set at .035 and it has pertronix. The 250 is the only one with crane ign. Maybe?

 

[SkinnedKnuckles]

Hey, Marv. I've got ask - did you do the "phasing" with the Crane ignition? That too can cause a miss if not set properly.

 

[cheseroo]

If closing the gap makes it run better, I'm wondering if the Crane is working properly or being all it can be.

 

[TR3driver]

The Crane XR700 is just an electronic substitute for points. In spite of the advertising copy, the voltage (and power) available to fire the plugs is still mostly set by the coil.

And the voltage required to fire the plugs also varies considerably with conditions inside the cylinder. Fuel/air mixture under compression is a much better insulator than 'open' air. Especially with a 6 cylinder, you may simply not notice that you are getting 1% misfires at high rpm (where the spark is weakest) and full throttle (where it takes the most voltage to fire the plugs).

 

[Marvin Gruber]

Don't think I was misfiring, the wide gap must have been giving me too weak of fire so engine just wouldn't throttle properly. It would catch up and run good after I held the gas steady. Brent, no phasing. The crane unit was from a parts car. I do still get a small burp which I think is a misfire occasionally. I reset the timing by ear, I may check it again after I run it some more. It is slow to wind up under power to 4500 rpm. The car has been stored for some time.

Marv

 

[TomMull]

Here, or what it's worth, is an observation made during my very unsophisticated racing days some forty years ago. The idea was to open the plug gaps as wide as possible to get maximum fire size for combustion. Don't know if this was valid but we did it. Run a lap or two at .020, then .022 and so forth until we heard a misfire or got slower. Then we'd go back a few thousandths and run at that setting.
What we found was that the increased resistance from a wider gap would make any defect the cap, rotor or wires very sensitive to arcing and scatter, causing misfire. Remedy was to replace the offending piece, if we had one, but if not, closing the plug gap would usually fix it.
Tom

 

[SkinnedKnuckles]

Marv - If the "phasing" as Crane calls it is off the optical sensor on the breaker plate can move during advance so that the rotor is too far from the cap electrode when the spark fires. This will cause a "miss" during acceleration that feels just like loss of fuel (ask me how I know, and whether I need to wear my glasses when doing the "phasing"). I bet you have an old distributor cap around that needs a hole drilled in the top. The phasing part is on page 14 of the instructions.

https://www.cranecams.com/uploads/instructions/90000700.pdf

 

[poolboy]

I think most of us know of Advanced Distributors and Jeff Schlemmer.
He said and once wrote about phasing being a particular problem with the TR6 distributor using either points or electronic such as Pertronix.
He said phasing and yet getting the correct timing range was aided by the addition of the timing adjustment wheel.
This :
https://advanceddistributors.mybisi.com/product/timing-adjuster-for-22d-25d-distributors

 

[Gliderman8]

Ken-
Last season my car seemed to stumble at higher revs, but it was inconsistent. I vow to get to the bottom of it this season. I have adjusted the timing to no avail. I do by the way have the pertronix.
Do you think this device would help? Have you used it?

 

[poolboy]

I do have it, Elliot.
Jeff installed it on my 74 when I sent it to him to get the dizzy recurved.
The 73 that I have now also has it and I assume TR6BILL had Jeff do it.
"Shinnedknuckles" gave an accurate description of what an out of phase distributor does.
Jeff told me a quick way to check is to look at the rotor and take note of the location of the carbon deposit.
He said he likes to see the carbon patch fron 1/4, to 1/3 of the way (clockwise) from the leading edge of the rotor.
Less than that and as the centrifugal weights advance the timing, the spark may have to jump a larger gap between rotor and cap, loosing potency in the process.

Basically you rotate the dizzy as you ordinarily would to set the timing, but instead of concentration on actual timing you line up the rotor with the contact in the cap, then concentrate fine tuning the timing with the adjuster.
Jeff will do all that if you send the dizzy to him for recurve and installation of the adjuster. Then all you have to do is fine tune your timing with the adjuster...or not. Me, I like to play with it from time to time and see how it affects my Vacuum readings.

 

[Gliderman8]

Thanks for the info Ken. Jeff did do my distributor. Is that fine tuning part new? I don't remember seeing it when I sent it to Jeff.

 

[SkinnedKnuckles]

Yeah - the "phasing" & timing may take an iteration, and the adjustment wheel would be nice in that regard. I don't have one, but that doesn't really make things difficult.

 

[poolboy]

I can go back about 4 or 5 years knowing about it, Elliot, but I don't remember seeing all the attention he has devoted to it on his website like it is now.
I'm not even sure he offered DIY parts back then like he does now.

 

[Marvin Gruber]

This is the first Crane system I've installed. Got no paperwork with it but I'll be checking this out shortly. Thanks guys, I knew I needed to post this to get to the bottom of it.

Marv

 

[Marvin Gruber]

BTW - this is a test, had there been an actual emergency ..........

 

[Andrew Mace]

...you would have been instructed....