• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

TR6 Black sooty plugs continue - What now?

Share this page

T

tv44engr

Freshman Member
Offline
UPDATE: 03 AUG '15
I believe problem is solved.
Another TR enthusiast found :
1) front carb needle adjust was out of its threads - therefore not adjusting and stuck full-rich.
2) front carb gasket for new air filters was covering the "float vent" opening.

Also, he plugged the "temperature compensator" vent hole. ("not necessary in our climate" and could cause unwanted mixture changes)
Then set both needles to 1.5 turns cw from stop.

Then drove about 5 miles --stopped to check plugs and adjust needle 1/4 turn according to plug color and then repeated the stop/check plugs/adjust needle procedure, until all plugs had equal brownish/beige color AND the "stumbling miss" was minimized or gone!.
He suggests replacing needles and jets with new because they have "wear" issues over time.........

I trailered it back home so I still need to do more road testing to be sure -- but that day it was clearly "FIXED" . Thanks to ALL on this forum for their thoughts and advice - it was appreciated!!

tv44engr


1970 TR 6 Overdrive.
Stock motor, stock Z-S 175 carbs & stock manifolds.
It has: Pertronix, custom exhaust, foam air filters & light flywheel.

This car ran perfect for about a year, then when I took it off the trailer at my first ever car show after about a 300 mile trip, it came down with the symptoms below and has not improved!
After driving 5 to 15 minutes it begins running very rough as if flooding & then won't idle & dies at stop
signs ....... resulting in front 3 spark plugs fouled solid sooty black & dry .... rear 3 plugs appear only
slightly black .


So far we have replaced: fuel, in line fuel filter, front carb float, spark plugs (NGK BP5ES, Champ N12YC & Autolite 65), plug wires, coil wire, coil, distributor, distributor cap, rotor & air filters.

Swapped Pertronix with points/condenser -- currently back to Pertonix.
Both carbs were rebuilt and fuel pump replaced about 10 months prior.
Checked for vacuum leaks, checked for spurious spark under bonnet at night.
Compression checked a few weeks prior: approx 160 with one about 140.
Timing has been set & rechecked.
Valve clearances were set a few weeks prior to the problem.

The front carb: has been disassembled, cleaned & inspected 3 more times; the last time we installed a Grose jet; checked & rechecked float level, adjusted idles & synched carbs several times.


Where to look now?????

Tom
tv44engr@att.net
 
Last edited:
Sounds to me like a sunken float. I'd be either changing the float in the front carb (maybe swap front to rear); or finding a way to check the actual fuel level.

Also, how do you have the float bowl vents hooked up?
 
How are your carb diaphragms?

Cheers
Tush
 
Our GT6 had similar symptoms which turned out to be a sunken float as Randall suggested. However, I would check the diaphragms also.

You said you are running at least one Grose Jet. Did you clean it out internally with carb cleaner before use? The steel balls are coated to prevent corrosion while in storage. There are many reports of them leaking or sticking until the coating has been cleaned off.

I removed Grose Jets from my cars and only run the Viton tipped traditional style needle.
 
Thanks to all for ideas - so far.
1) Both diaphragms have been checked this week.
2) Front float changed out this week -even tho it seemed fine.
3) Rear float checked fine today.
4) Rear carb removed today, disassembled, inspected & cleaned - no issues found.
Still intermittent fouling/stalling then running fine for a few minutes!

NEW DATA: Compression check today: 150, 140, 120, 150, 140, 170.

 
You might also check that the carb pistons move freely and contact the bridge when lifted&dropped. If the dome screws are tightened evenly, it possible for the needle to bind in the jet. This will produce a rich mixture, similar to a torn diaphragm.
Berry
 
Only 2 things come to my mind..
Bad float in the flooding carb
Or, a faulty needle valve was replaced with typically faulty Grose jet.
 
Now both carbs have been disassembled , cleaned & inspected -- diaphragms looked fine & are near new; swapped out front float just because we had one; & both jets have been swapped; fuel pump removed & checked; new fuel filter again........ Starts & idles fine -- but plugs are fouled by the end of my driveway!

NEW DATA: Compression check: 150, 140, 120, 150, 140, 170.

Now I have scheduled 1) fuel line inspection, reaming & blow out; 2) new compression check; 3)leak-down test.......
Tom
tv44engr@att.net
 
I don't think compression has anything to do with your carbs flooding.
Some 70 versions of ZS carbs have the ability to change the mixture by adjusting the needles (not to be confused with needle valves).
Do you have adjustable needles or has that already been answered ?
 
I would measure the feul pump pressure before your next test. Could it be a strange pump that puts out too much pressure? I assume its not a electric pump.
 
Tom,
I fought a similar problem in a 73 TR6 7 or 8 years ago, but never tied it to one carb effecting it's three primary cylinders. Car would sporadically run rough and quit, then run OK. I don't know if the liner in the gas tank was stock or added, but a thin film lining the gas tank had started to dissolve and would randomly block the fuel outlet. Of course, it started all of this while I was in Atlanta for the Mitty vintage races 170 miles from home. i chased this problem for about 5 months before i figured out what was causing it.
God luck!
 
My thinking here is this : essentially , it runs fine until it warms up ,then it won't idle and runs rough , which is a symptom of the valve lash closing when hot ( as well as other causes of course)and you dropping one or more cylinders, isn't it?
You say you adjusted the valves just before it became symptomatic the first time yet didn't mention that you re checked them. You assume they are ok because you've done a compression test. Perhaps you did not adjust in the proper sequence or give the proper lash.
You do have deviations in your compression readings but don't mention whether you checked it cold or hot. Did you check it when it was symptomatic ?

You didn't mention what color the plugs were when it was running great , they could've been black then and you had no reason to check them or be suspect ! Black plugs are not " fouled" , they will still fire. "fouled" plugs are wet with gas or oil and can't fire.

It sounds just as plausible to me like you limp back home , ponder the problem while the engine cools down just enough,( maybe change the plugs or take a carb apart) then re start it and go through it all again.
 
You have a fuel pressure regulator? Perhaps you have too much pressure in the line flooding out the carburetor. My TR3 had a similar problem after being rebuilt "professionally". Front carb ran too rich no matter what I did. Long story short, the jet on the front carb was about 1/8" shorter than the rear, which caused the rich conditions.

For your situation though, perhaps you filled the gas tank with gas that had a high concentration of ethanol that is distorting some rubber components causing your rich conditions?
 
Thanks for the input.
The valves were set stone cold - Have not been checked when symptomatic.
Plugs had normal clean, beige color before.
What are the causes of "valve lash closing when hot" ??
 
thanks for the reply-
No fuel pressure regulator--I would like to try one ...........
I have never used ethanol gas.
 
Check tank for something blocking pickup intermittingly. You said ran fine but misses and idles OK. Check choke tolerances for not moving cable. Let us know where you are from, someone might live in your area and can help.
 
Back
Top