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Triumph TR6 - Mixture setting?

This week I've noticed that my car is running a little rough at speed. Almost sounds like a mis-fire, except, when I pull the choke out to the first notch, the motor smooths out and runs beautifully. Slow and hot starting is fine and idle is fine (maybe a little rough). I checked the dashpot oil which was ok according to the Bentley Manual.

I'm guessing the carbs are running lean and pulling out the choke corrects it. Any carb experts out there? Should I be adjusting the mixture setting or should I be looking somewhere else for the fix? I read on another site that newbie's should never never never try to adjust their carbs. Good advice? tks

Hi Mati

Check your plugs if there all an even browny/grey your mix is OK. If there all real clean and white your lean.

Timing will cause your symptoms as well. Different mixtures burn better at diffferent timings. Don't know what your running for ignition but points and plugs do go faster on older cars.

Might just need a tuneup?

Bill Brayford

Check the carb diaphragms first. If it was running good and suddenly went bad, there is probably a hole in one of them. That would prevent the carb air valve from rising like it should so not enough fuel gets pulled in. You can pull a spark plug from the front 3 and the back 3 to compare & maybe find the "lean" carb - the plug center electrode will be white if too lean.

•Unscrew & remove the damper from the carb top.

•Carefully unscrew the aluminum carb top ( the screws look to be, but aren't phillips, so be careful)

•lift out the diaphragm/air valve. Hold the rubber up to the light and stretch it out - hopefully you'll find a hole.

• If you've gotten this far you can figure out how to change the dia. and put it all back together.

One thing to keep around is a spare set of diaphragms - replace both when one goes bad. It's easy to do, and there are tabs to keep you from making a mistake.

As far as adjusting carbs - don't do that yet. Your problem sounds like a bad diaphragm, so adjustment is not needed. BUT - don't let that stop you from reading up on how things work. Once you do that and figure out what's going on, you'll be ready to get into it up to your elbows! And never, never, never listen to someone that tells you not to tinker with a TR - just ask here if you have questions as there's lots of experience.


Brent B

I have to interject here something that my Father ran by me years ago.

To wit:

"A rich mixture is easier to light than a lean one".

That having been said, I wonder if something changed. Perhaps a vacuum leak developed?

Just a thought.

JB Deatsch

Hey, Jim. Glad to see some more new blood here!
Brent B

You might have a vacuum leak on the carburator throttle shafts. To check for leaks, spray each end of the throttle shafts with a small amount of carb. cleaner. Depending upon the amount of the leak, the idle will either speed-up or the engine will die.

My guess the engine will speed-up if you have just noticed this problem and the throttle shafts have little wear.

The cheap fix is to simply enrichen the mixture or the expensive fix is to rebuild the carbs with reaming the throttle shaft bushings and installing new shafts.

I had this same problem on a '75 TR6 and enrichening the mixture was good enough.
L. Kiser

I checked the plugs before doing anything, and I found that all six plugs are identical in appearance. They all look like they have a brown/tan dust surface. I checked a few web sites for photo's of lean plugs, and I don't think that's the case here despite the symptoms. I forgot to mention that I do get a some exhaust popping on deceleration and when I checked the Buckeye site on carb-rebuilding - this was identified as another symptom of lean running carbs??

I guess I'll start with the less invasive tune-up and if that fails to correct the problem, I'll check for a vacuum leak.


Hi Guys,

I also have some popping on deceleration, on my 74.5 Tr6.
My mixture seems to be normal- perhaps a on the lean side looking at the plugs. The end of my exhaust pipe are blacky/grey. I did connect a exhaust gas analayzer & found I had a mixture of about 14.35 to 1.

I have however, disconnected the bypass valves on my strombergs (damn the pollution junk!) & feel the lean mixture occurs when I get off the gas and results in the popping noise.

Here is a post I found relating to the above.

Does the popping really hurt anything anyways?

Aivars Berzins

Hi Aivars

Depends on where the popping is coming from. Out back is OK may shorten muffler life by 5 miles or a month?

I don't know those carbs well enough to be an expert but I have some problems with some of the experts on the net?

I always thought the bypass valves let air in to decrease manifold vac. and prevent suction at the exhaust pipes under deceleration/engine braking?

Way that type of device works in most cars. Not what I read from that post I think? May be wrong.

The gas that doesn't get burnt. And there always is a bit goes into the exhaust system. You come off the go pedal shift down release the clutch and get popping? If your timing and general engine conditions are off you will get more.

Unburnt fuel mix hits nice clean air sucked up hot tailpipe by manifold vac and goes pop. Unless using short straight pipes. I don't think valve damage would be a problem?

If you want a real show of that. Go out and watch some Super Modified outlaw stock cars on fuel, dirt tracking on a nice night. Flames and thunder are an exagurated 700 HP version of a 120 HP pop?


Bill Brayford

Thanks Bret.

I'm reading this thread with great interest since I'm experiencing the very issues that Mati is describing. Popping on deceleration (most annoying) and, as we know, the 'run with the choke out' syndrome.

I'll take a looksee at the diafragamums (I never could spell diaphragm) carefully and will see if I can detect... well, you know.


Jim Deatsch

Lordy, it looks like I've created a monster!!

I'm sucking back a Smirnoff Cooler right now enjoying the new threads and celebrating a nice afternoon under the hood. I put in new plugs, points, rotor, wires, adjusted my valves, timing and sync'd the carbs (the the best of my abilty) and even tinkered with the mixture setting (I love new tools!!)

My road test was bliss. I had new top end power, smoother running, and less pop on decleration, although a little was still present, it was smooth and much quieter. What a sweet ride it was (until it started to drizzle).

I think there's more work to be done, some refinements, but I need to thank everyone for giving me the necessary nudge to try this for the first time. I've done a little with MC engines, but this was in a league of it's own. Very satisfying.


Have closed off my bypass valves - still pops on deccelaration. Stopped this on my MG by advancing timing. Currently awaiting return of my (74) TR distributor after being converted back to vacuum advance, which I feel will correct a couple of problems ( what imbecile ever came up with vacuum RETARD, he was one !) Will let the board know results. Peter
Peter Gooch

Well DONE Mati.

Ain't it grand?

Jim Deatsch

Hi folks!

Thought I should share a quick update with everyone who assisted with my mixture enquiry.

I had completed the tune up and needed to go back and make some final adjustments. Well...I couldn't quite get it right..and over several attempts to set the mixture, idle and sync, I discovered that my damper's were not holding the oil for more than a day or so.

What a huge difference that makes!!! I pulled my air valves and knocked out the adjusting screws to replace the o-ring(s). I also discovered that the top o-ring/gasked on the damper piston was missing on both carbs. I'm getting parts today and I expect a whole new experience. When I topped up the oil, the motor ran smooth, less popping, etc. I very highly recommend the carb rebuild article on the Buckeye website to everyone. Worth reading even if you're not rebuilding. The explanation on how the carb/parts work is fantastic.

One last note, I also discovered that when tuning the carbs, the best approach is to adjust the sync, idle and mixture and then repeat and repeat. The manuals suggest starting with sync and then moving on. I found that you need to keep tracing your steps to get the best result. The knee bone is connect the leg bone theory applies. If you adjust one, the other two are affected, so when you've completed the third step, the first two have changed slightly.

And so it goes.
You should open a consulting service Mati. <G>
I've just won a set of Zenith Strombergs with manifold on Ebay.
Once I get the proper carburetion on my car I will pursue the issue in earnest. I've gotten the thing running fairly well with the SUs (no reason not to I suppose) but want the Strombergs if for no other reason than to be able to pick all your brains.
Now if I can just find out what LBC uses 175 SU carbs so I can peddle them on Ebay when the deed is done.

Jim Deatsch

Jim - You'll be pleased to know that many consider the SU's an UPGRADE over the Strombergs on a TR6, so you'll have no problem selling them on Ebay. I personally think one's as good as the other.
Brent B

After a night of bumbing around in the darkness, I made my way over to my part supplier..British Auto Sport.

On a day declared as a "state of emergency", BAS delivered two very special o-ring for my air valve adjusting screw. I rushed home and installed them to see if there would be any impact.

WOW. My car is running 300% better than before. No popping, no rough idling, no hestiation, no backfire. Nada!!!! Once again, many many thanks to the folks who encouraged me to tune my car. This is a beautiful car when it's running right.

fyi - the DPO (tks Jim)had installed two different o-rings and I don't think either of them were correct. Hence my dashpot oil was leaking. Problem solved, car runs beautifully. If only I could find a gas station with electricity to pump fuel. Line-ups are over an hour long to get some gas. Very few stations are open. Hmmm....maybe a BBQ would be nicer.

End of Saga.

I'm smarter than I was before Mati, thanks to you. Orings, indeed. Darn DPOs anyway.

I'm still waiting on the ZSs and will go from there.

Might have some SUs for sale, might have some ZSs for sale.

I DID acquire (from a buddy of mine) a VERY nice luggage rack.

Jim Deatsch

This thread was discussed between 31/07/2003 and 16/08/2003

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