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Triumph TR6 - Confusing Spark Plug Readings

Here's the details:
It's a 1971 tr6 with triple Zenith Strombergs. I'm using a Crane Ignition box couple with a 40k Lucas coil.
Spark plug readings is 1,3,5 are light tan (weak mixture)and 2,4,and 6 are dark dark brown (Rich mixture).
The logical bloke (like myself) knows that each carb is allocated two spark plugs and that these spark plugs should be the same reading.
So I'm confused!! Readings from each carb has one spark plug that reads extreme weak and one spark plug that reads extreme rich. There's no happy medium.
Is the issue in:
1)My ignition system?
2)Is it timing?
3)Could it be my air/fuel mixture?
I've got a car show this weekend that's a few hundred miles away and I sure don't want to be stuck.
HELP!!!!!!!!
benji

Benji,

check your high tension leads, cap , and plugs....I think that the trouble is in the ignition HT side. Its a weird one to me too Benji.

Rob
Rob Gibbs

Benji-Another possibility might be a leaky intake manifold gasket at cylinders with the light tan plugs. Long shot, but possible. Maybe a colortune gadget would verify the plug color.
Berry Price
BTP Price

No leak at the intake. I'll replace the cap and high tension leads and see what happens.
Berry, are you going to be in McMinnville this weekend? I'll be there if I can get this issue resolved.
benji

Benji,
Since the "front" cylinders (1 , 3 , and 5) are showing the lighter colour and the "back" cylinders (2 , 4 and 6) are equally showing a darker colour I would be inclined to suggest it is mixture being separated slightly.

The how and the why is really a guess but it may be the design of the inlet manifold or simply due to rapid accelleration of the car causing the inertia of the fuel and air to become more obvious.

The heavier mixture is sent to the rearmost of the two cylinders showing a slightly darker plug colour while the cylinder at the front of the pair is running slightly lean and therefore a little lighter in colour.

If it was electrical or timing most likely all would be the same (ie: Poor spark leading to a dark / sooty build up on the plug ends)

Cheers , Pete.
Peter Thomas

It would be interesting to switch the plugs, lean for rich along the line. Just to see what happens, wouldn't take long. At the least, you'd burn the carbon off the rich ones. I wouldn't lean it out any more at this point.

Maybe the front runners for each carb are a little hotter, the rears are in the "shade" of the fronts, and hot air is rushing by. I think the acceleration theory is a little far fetched, but it made me think of this wacky one, too. The other side of the coin is maybe the front runners are cooler for the same reason, air rushing by. That would blow my theory out of the water.

Tom

Tom,
Yeah I have to admit it really is a long shot but as I said not having any pics to go on it is a remote possiblity.

I will be intersted to know what happens.

Cheers , Pete.
Peter Thomas

Benji - unless you have a non standard inlet manifold and carbs I think it is highly unlikely it is fuel related.

I would be looking carefully at the distributor.

Firing order is 1,5,3,6,2,4. You have a problem in one half of the firing sequence.

I suggest that it may be worn distributor bushings causing the shaft to rotate off centre. This would happen if you have points controlling the spark as the pressure of the cam exerts force in one direction, pushing the rotor towards three contacts and away from the other three in the dizzy cap. Reduced and increased contact distances will result in a different quality of spark to each set of three cylinders.

R

Roger H

I put my money on Roger !!
Clever thinking !!
Eric de Lange

Good go Roger. But Benji has a Crane ignitor box that I believe is point-less (uses optical trigger). Bet the box has to be tweaked.
Rick Orthen

Bent shaft in the distributor perhaps or maybe non concentric lobes or cutter disc?

I think Roger is on the right track given the firing order and the fact the problem appears to be in one portion of the rotation.

Good thinking there Roger.

Cheers , Pete.


Peter Thomas

Cheers guys but we ain't there yet...

Wassup Benji - any results from the cap and lead change? It could be as simple as the dizzy cap not being seated correctly so you are doing the right thing in sorting the easy and cheapest items first. Keep us informed as it is an interesting one.

R
Roger H

Went on that 4 hour trip last weekend and I found that all the plugs matched in color. Peter Thomas from Victoria, Australia was right on about the rapid accelleration of the car causing the inertia of the fuel/air mixture. This sent the heavier mixture to the rearmost sparkplugs. When I had the weak rich sparkplug readings initially, I was in some stop and go traffic.
Thanks to all for your help. I love this BBS!!
Benji
benji

I love it! And I called it a hare-brained theory or something! I'm glad your trip went well. I haven't driven the TR in days, it's been kinda rainy, and I'm too lazy to put the top up.
Tom

benji,
Glad to hear you now have it sorted.

Tom,
I really must admit it was something of a long shot. I took a guess based on something similar I had happen with a Mini of '66 vintage during my club racing ( alas not winning !) days.
The confounded thing always loaded up cylinders two and four on one particular circuit which was also had the majority of the turns in one direction.
Even then it took a long time to track it down.

Cheers , Pete.
Peter Thomas

Benji - great that you have it sorted, but with all respect to Pete, I am struggling with the concept of mixture inertia being the cause of the problem. Foremost is, that if this were the case, why wouldn't all engines suffer from the same problem? Why wouldn't twin carby engines have problems with cylinders 3 and 6?

I can understand the idea as you have three carbies with each one feeding a dual-branch inlet. However, inlet vacuum is created by suction of the down stroke of one piston at a time. When one piston is drawing in mixture, the other is closed - why then would the mix travel to a closed port in favour of the open one? The mix is not being forced into the manifold, it is being drawn in, one cylinder at a time.

I am always interested in discussing things that I don't understand so it would be great if we could go further with this....

Regards

Roger
Roger H

GREAT QUESTION ROGER! I DUNNO???(AMERICAN WAY OF SAYING I DON'T KNOW)
WHAT I DO KNOW IS THIS...
I MADE A FEW MODIFICATIONS TO MY ENGINE THAT MAY CHANGE THINGS. THEY ARE AS FOLLOWS:
1)HIGHER ROLLER ROCKER RATIO @ 1.65/1 WITH COMPETITION VALVE SPRING.(I ADDED THE SPRINGS TO MAKE SURE THE VALVES SNAPPED BACK BEING THAT ROCKER RATIOS ARE HIGHER.)
2)HIGH PERFORMANCE STREET CAM FROM RICHARD GOOD.(GP2)
3)TRIPLE ZENITH STROMBERG CARBS.
ROGER, I THINK ARE ANSWER LIES IN THESE UPGRADES. BUT, AGAIN I JUST DUNNO...
benji

Alas - that is the problem with reading plugs. One time it's screwy, the next it's not. My suggestin it to "read" them only after a long drive where you have not idled long. GOT THAT???
Brent B

Roger,
I agree totally.
I would like to gather as many helpful opions as well since each help all of us gain a better understanding of these things.

Anyone else had something like this happen and if so what was the outcome when you hopefully solved the problem?

Cheers , Pete.
Peter Thomas

Benji.....
A quick question, in your 1st. post you said thatyou are using a Crane ignition with a 40k Lucas coil.Would you mind letting us know which Crane system you are using? Is it the XR700 version by any chance?

Dale
Dale

Dale from Toronto,
I'm using the Crane XR700 box. It's not adjustable.
benji

Thanks Benji,
Would you confirm the voltage to the coil, the + side, is 12 volts? Also the voltage to the XR700 ignition module, from the same coil terminal is also 12 volts?

Thanks,
Dale
Dale

Benji
Rod sent me pics of your 6.
Very nice looking ride. You did a good job on her.
Hope your problem gets resolved.
I think Roger hit the nail on the head.
Make note guys this TR6 has TRIPPLE ZS carbs.
Rick C
Rick Crawford

To Roger H...
I have been following this thread for some time, and your comments on Aug.2 struck a bit of a chord, re firing order etc.In my case the car is a '73 engine in a '76TR-6, Allison [now Crane]ignition,Lucas high output coil[12v] with the ballast wire disconnected from the coil, and a direct feed,12v, from the fuse box to the + side of the coil.During a lengthy problem solving exercise regarding a low speed stumble/hesitation situation I replaced just about everything except the glove box lid....and was looking around for one of these. The problem turned out to be unrelated to the ignition system,and was a burned through intake/exhaust gasket allowing exhaust gas to mix with intake. As part of the problem solving re the above, I replaced the electronic ignition system with a new Crane xr700 unit, and came acrost a rather confusing situation.The installation instructions, mine dated 7/05,in figure 19, p10, show a ballast wire as needed, BUT only with the Crane xr3000 system, however the xr700 ballast resistance notes, p13,para. 1 states that "...or other system without ballast resistance, you will need to add a 1.2 to 1.9 ohm ballast resistor..." Following para 6,same page in bold face is stated "...for maximum performance, any external ballast resistance must be bypassed" ALL VERY CONFUSING, so a phone call to the Crane technical people in Florida came up with the following hard and fast statement." THE CRANE XR700 SYSTEM MUST BE USED ONLY WITH A BALLAST SYSTEM, THE IGNITION MODULE MUST OPERATE ON 6 V ONLY". So in my case a 6v coil was installed, the resistor wire was re-installed, [fortunately it had been disconnected and tied back only] and now on our merry way.

I wonder in all of this if Benji has an ignition problem, PERHAPS caused by excess volts to the module?

All rather long winded I'm afraid, but does anyone have any comments/suggestions? [Other than keep it shorter in the future}

Dale
Dale

The choke is on one side of the carb and the "leaning" devices (bypass valve, trim screw, and temp comp) on the other. I don't know how its possible, but the sides they are on correspond to the lean and rich cylinders. Coincidence? Air leak on one side of the throttle shafts due to accelerator set-up? Maybe the triple set-up brings out inherent leaks more than in twin-carb operations.

Jeff
JB Fetner

I was having problems with a spark plug loading up with carbon and took my plugs out many times to look at condition. I found the only good way to get a reading is to travel @ highway speed for several miles, turn off the ignition and pull over with the engine off. Any idling or stop and go will skew the readings. Every time I took the plugs out they were different. I set the mixture with the "lift the piston" method and forgot about plug colour. Depending on wind speed and direction, I get 25 to 29 miles per Canadian gallon (1.2 US gal or 4.56 L). Good enough.

Don
Don Ho

This thread was discussed between 31/07/2006 and 18/08/2006

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