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Triumph TR6 - Too lean machine

Hi Guys,
I'm having trouble with my mixture control,I get all the symtoms of too lean even with the carb adjustment all the way to the rich side. I've just replaced the diaphragms,balanced the carbs,set the timing,adjusted the valves.My bypass valves were rebuilt and are adjusted about 10 turns from fully CW and my temp compensators are equal and appear to open at the same time (viewed on the car while hot)
I have 18lbs of rock steady vacuum and the car idles at 900 rpm smooth and steady but when I put it under load as I start driving it misses/pops and the only way I can smooth it out is to richen the mixture by pulling out the choke part way then it smooths right out and runs great...then at say 40-60 mph I push the choke back in and it's fine till I have to stop at the lights then I start all over !!
I also get the popping as I decelerate.
I removed the dampers and made sure the were in the same position for fully rich or lean but I still have the problem..Any thoughts appreciated..

Hey Charlie

What condition is your fuel pump in. New they only put out by book, a couple of pounds and some of the fellows have stated great improvements by replacing? If its just getting enough at idle.Then on acceleration when it has to give most gas at still low RPM it might not be up to it. Higher RPM more flow even for weak pump? One thing though loosen your gas filler cap I have had a blocked vent trick me but not on a TR. A split in suction side of line will do it too but you should smell gas.

Let me know,
B Brayford

Just one thought. Its bloody cold and damp lately. For others Charie is only 20 K away from me. What weight of oil are you using in your dampers? I realy don't know if that would cause it but it might if its single viscosity.

B Brayford

Hi Bill,
My pump appears fine,I have the original one (55k on it) and it has the glass bowl which is always full even with high revs...a mechanic friend is insists that it is not a fuel pump problem, though I may change it out anyway and then I'll have a spare.
I use reg engine oil for the dampers but I did try 3 in 1 oil and it made no difference.

Charlie did this just start since its been cold? Have you done a pressure and volume test on pump first? Don't go buying a new pump its simple to do.

Because of Ricks problem I have been testing all kinds of things. Mines apart so I can't do running tests though. When I get tired of grinding and welding on frame. Read lazy. I pull the carbs out and look at them. Those temp. compensators were interesting. They looked closed but internaly were not seated due to carbon build up and age wear on my 72. I have never changed them in 18 years. After cleaning I was testing putting them in beer fridge freezer and then on top of oil stove and back. I don't know about yours but I could not get a good seal at all. Some of the people I know say close the Temp.screws and leave them closed. Close the BPs CCW adjust carbs. your float and needle should be only fuel and air source and try. Then bring out BPVs relevant to engine braking. You are now dealing with fewer posibilities.

Apart from all else I've read on this bbs those BPVs were only designed to aleviate high vacuum coming off throttle at speed. Engine braking. I would close them up ccw not to tight during testing. I am a great believer in getting things to work as originally designed and I can't see them originally funcioning as a tuning device. My opinion only.

Let me know
B Brayford

Hi Bill,
Actually this all happened since I replaced the diaphragms. I ran the fuel line into a bucket and had good steady flow and while running and revving it up I pinched the line tightly and the car ran great, then I removed the TC's and taped the holes and the problem was still there ...I'll do the BPV's next.

Charlie--The decel popping is a giveaway to what you already know--a lean mixture. I'm sure you've checked for vacuum leaks, but some have said that a vacuum leak can develop in the brake booster can. Remove the booster hose from the intake manifold and plug the manifold port. Check for performance change (be careful, you'll have real hard brakes).

Since everything was fine before you replaced the dashpot rubbers, I wonder if something didn't get installed wrong. You didn't remove the carbs, right?

Rick O.
Rick Orthen

Hey Charlie

Started after diaphrams. Lets walk this through. Ok as RickO mentions all sypmptoms say running lean.

On and off throttle with your head upside down are the floats moving up evenly and smoothly. Mark the needles are they moving exactly with floats. Hard to see without marks. Magic marker and a light works.

Do plug read. Dark plugs rich. White plugs lean. You know. Determine what carb is doing what. As I went through with RickC you have to know exactly what is going on especialy with 2 carbs. Your plug read will tell most. Thats your Datum Line.

Needle or diaphram problem. Thats were it started right?
Fuel intake. Needle lifts via atmospheric difference, venturi action sucks fuel through port. Simple. If one needle damaged and not seating one carb will run rich at idle. If port blocked one carb will run lean always. Needle not moving up carb will run lean at throttle. Thats the basics.

Damaged needle running rich not likley problem unless you have compensated by tuning for a rich problem by leaning out other carb? One carb will run the car at idle through vacuum balancer on intake. Thats were it gets crazy and you have to have a datum point. All the fancy balancers in the world won't beat a plug read.

I am not sure what relates to your situation. Why were you changing your diaphrams? If one was bad your carbs may have been out of whack already and compensated for. My rule is if the area I was working on was workable before I changed it. I screwed up in that area.

Take the plug read and get a starting point.

My youngest Daughter now at U.Vic International Law and myself do the logic thing back and forth. Years ago Einstein created a puzzle he felt less than 3% of mankind could solve. Various Mensa like groups have it posted. Shannon in Grade 10 worked it in 25 minutes and sent it in. Correct and welcome to the club. I can't decipher it so I don't belong.

If one more Tr guy comes up with a lean carb problem after a fairly minor replacement. Maybe the dark DR.Lucas. I'm going to create a whole new Mensa IQ rating and send it in. If George removes the left screw on his rear tail light why does his engine idle rough and run lean. These cars were in a sense hand built. Oh the horn doesn't work due to the fibre washer in the ground path. Don't bother the Guv.just slap a cable on that puppy.

In 18 years actually 15 running I never had major carb problems on my 72. Minor adjust here and there. My BPV adjusts were still covered.

Sorry for the long post. Thinking while doing.
Let me know

B Brayford

Hey Charlie
I phoned around to a couple of guys I know that do Brits for a living. And the answer to my question of why I never had trouble was "Your just Bloody Lucky".
B Brayford

Charlie, I had the same problem and still do, in a way. I just finished rebuilding my carbs a couple weeks ago and they still ran lean when tested. I also rebuilt the distributor at the same time and it hadn't run very well since. Because it was running lean, I assumed it was the problem; however, I took a look at the plugs and they looked pretty good so I did a static timing. It's running great now. Just a thought. bill
B Vogan

Hi Charlie

I have been looking at Buckeye and the TRF parts book etc. etc. There are a lot of Trs. coming up with lean symptoms after minor service by experienced TR guys not only on this sight but others and only seems to be in the last year or so that I can see. Makes me wonder about a parts quality problem?

What year is your car? I noticed there is a different part# for the diaphrams starting at 1973. Pre 1973 is ZEB17421 after 73 is ZEB19420. I don't know if they are interchangable or what the difference is. Maybe you know or someone out there can tell us.

If your pulling a stable 18 vac. your timing at idle should be OK. Don't know your car and distributer vac. set up as far as advance and retard goes. What does your vac drop to on throttle?

From what I can see the temp. compensators if having a problem sealing. Would happen off throttle lower vac. level to seat them.

The BPs should only cut in at high vac. deceleration somewhere in the 23 range I would imagine but thats a guess. I don't think it would be much less by the sensor design. And should cause more problem at idle if floating and letting in air.

Lean problem. You have both cranked rich. Leaves you with maybe an external vac. leak? Air valve or needle not functioning properly sticking? Plugged orfice crap in the bowel? Fuel starvation. Not enough gas getting to float bowel?

Just as a tip to everyone. A fellow down in Aukland NZ. had a lean problem and it turned out to be his fancy Chrome valve cover. The oil filler cap used was a vent type. Not air sealed as the TRs are or should be. His vacuum leak was there.

Still think we should send this in to Mensa.

Let me know

B Brayford

Charlie I was just thinking on BillVs post. Is it possible that your new diaphrams might not be performing not quite up to par or maybe just new little stiff? I don't know exactly.

If you can hook up your vac. gauge and adjust timing to get your highest vacuum reading possible. And then give it a try. You may have already tried this but thought I would mention it.

B Brayford

Has a previous owner fitted the wrong needles? Use a micrometer and measure needle diameter every eight inch along its lenght and compare with a good needle. A needle that is too thick along its lenght would give a lean mixture. You could try emery papering it to get neare the correct profile.
Thick/thin dashpot oil will only affect the mixture transiently upon opening the throttle.
The wrong spring over the diaphragm (too stiff) would also lead to lean mixture. Measure its downforce on kitchen scales at measured compression lengths, and compare with good spring.

Peter Cobbold

GEEESSS Charlie, your thread is only 3 days old and you already have 13 posts. Are you trying to beat my record?
I wonder how many other guys out there are having the same problem as us and are hoping at some point we come up with an answer/fix. I am positive my problem is with the front BPV. I am running so lean at the front carb that I could take the 3 plugs out, put them back in their original box and take them back to the store.
I think tomorrow I will go for a ride and put her away for the winter and not worry about it till next spring.
Sorry to hear you are having same problem.
Ha Bill, what would/could cause an increase (higher than normal) HighVac at deceleration. You think around 23 but what would happen if it was significantly higher. I can actually hear my BPV "slap closed".
You wrote:
"The BPs should only cut in at high vac. deceleration somewhere in the 23 range I would imagine but thats a guess. I don't think it would be much less by the sensor design. And should cause more problem at idle if floating and letting in air." Man this sounds like my problem.
Charlie, you know than the help from Bill is going to cost you a wind blocker!
Rick C
Rick Crawford

I replaced the diaphragms because the "old" ones ( less than 5k miles since installed) were tearing,They are made by ROYZE and are made of a very thin rubber. The replacements are made by WALKER and are a lot heavier rubber and appear more durable.
All my plugs are more white than anything so that points to the carbs being equal. I'll richen the mixture tomorrow and see what happens though I'm not convinced that's the fix.
Thanks for all the input..Hello to Rick and Bill !!!
Someday soon Bill maybe I'll drive your way to see what your TR is up to !


Hi Charlie

Great your equal over all. Try to richen and see if you can get normal brownish plug read. Any one on each carb should do.

I agree. I am not convinced thats the fix. Non of the manufacturers or retailers want to hear 5K as the time to failure either on there diaphram. But I think after your input the problem is that every manufacturers product is designed to work with only there own matching diaphrams, valves and seals as in total rebuild. Only a theory so far but it makes sense to me. JohnD from U.K. seems to have an intimate knowledge of triumph parts. John can you comment?

The diaphram is in a closed area subjected only to differences in atmospheric pressure. Unless extremely abrupt changes in atmosphere or abnormal resistance from the air valve toast it they should not tear? If the BPs are not coming on line at the right high vac Hg that might cause it? Different parts different specs.? Lots of nightmares.

Hey Rick its 2 am the extreme vacuum is my brain but I am thinking on that.

Charlie I would really apreciate a visit at any time. Just give me a E or a call to set up.

Let me know

B Brayford

Well I took it out for a 20 mile run this afternoon and apart from constantly pulling on the choke to get the best/smoothest power it ran great but I still get that miss and backfiring "pops" out of the carbs..I richened up the mixture and still no difference.
I guess this winter I'll overhaul the carbs...again !!
Oh well it'll help pass those long dark Canadian nights.

Hi Charlie and Rick

Don't get on your carbs yet my advice. You've richen upped both carbs. Both running lean and still have same lean problem. Thats crazy. And I don't have an answer.

RickC mentioned in one of his posts his BP replacements were way more flexable than original. Maybe good maybe not. You mention Royze Diaphram much lighter than Walker? Did you contact your supplier about the 5k failure and if so what was the response?

This is in regards to Ricks question.
High idle over 20 Hg. should only happen when ignition timing too far advanced? Very unlikely on these engines? At deceleration I have no way of knowing. Aukland a city of 300,000 has five shops with dynos don't know of 1 around here to do running tests. Perfect vacuum is 30Hg. only obtainable by my 17 year old son at supper table.

When you have nice idle and throttle response and brownish plugs front and back. Back off Clockwise BPs the same a little at a time to get rid of slap and pop hopefully? Thats the way they should work in theory.

When you changed the diaphrams did you make other adjustments right then that you can remember. If your vacs 18 your idle is good your needles are coming up no fuel starvation you should not have this problem. Yah I know you already figured that out.

I was wondering about shelac build up in the needle orifice? But highly unlikely over 2 carbs the same?

Still don't know the year of your car.
B Brayford

Hi guys
I've been luking on this BBS for several months now and I've gained a lot of valuable info. I own a 1974 TVR 2500M which has the same drivetrain as a TR6.
I had this same problem (running lean) after I rebuilt my carbs and it turned out to be the float level. Looking at the Haynes manual, Bentley manual and the instructions that came with the rebuild kit the figures for the float level are any where from .625" to .750". I initially set mine on the high side of .688" and couldn't adjust them rich enough but after reading the carb rebuild section on the Buckeye Triumph site I reset them to .625 and I seem to have adequate adjustment of the needles. Also I read some where that the idle trim screw should be turned all the way in as it was only used for fine adjustment on a fresh engine. I made the above changes and my car is running fine now.

Chris Taylor
C Taylor

Hi Bill,
The car is a 75.
As I previously mentioned my one year old diaphragms are a lighter weight of rubber and my problem appeared after I installed the thicker new ones I decided to try an experiment. I patched the tears with rubber cement and bits of the same rubber (from the original kit that covers two types of carbs) and went for a run and the problem was gone ! Can't say how long the repairs will last but two weeks should be plenty till I take it off the road for the winter.
I'll go for a longer test run tomorrow and keep you posted.

This all sounds alot like the running lean thread I started last August and I never did solve my problem. My question now is where do I find diaphragms that are made from a lighter rubber, or should I rebuild the used SU's I bought. By the way anyone out there know a good supplier for SU rebuild kits?
Curt Anderson

Hi All and Charlie

Boy I think you have hit on something by the # of posts!!!

OK the diaphrams that work are Royze. Only for a short time but work>? They are a major distributor 49 years in Bus. California. Seem to deal in everything so should stand behind and know there product. No tech. line though. Checked out they offer 1 only carb kit 67-76 Part# ZE-15K. TRF shows a different Diaphram # post 73 why?

Your 75 Carbs have a number of modifications over my 72 dealing with emissions? Victoria shows one only diaphram as well. I can't get at individual parts on Royze. Search Royze on Google.

I can't find any reference to Walker other than lawn mower carbs and mufflers? So I don't know about them.

Before I forget. I said there were no dynos around here DUH. Every drive clean Ontario shop now has a Dyno for stationary testing at speed. Don't know if they work ok for performance testing? Since some are on a 3 day waiting list for a 5 min. test $35. Might not want to dyno a Tr without lots of money. But things will improve. That would be the ideal way to set up the carbs of course.

Anyone with any info on this rubber business please post. Let us know product name and Part# if you have. Good and bad. It seems to cover a few different cars.


B Brayford

Charlie, I doubt diaphragm rubber stiffness would make any difference- ther are around 15 pounds of force ( dont recall precise figure) acting to pull up the needle. Colin's advice re. fuel level in the jet sounds good to me. Also if needle has been removed from the piston it has to be refitted precisely to keep the needle/jet mouth/fuel level relationship bang on. I fuellevel is too low and needle not fully recessed into its hole in the piston things could get horribly lean.
P Cobbold

This thread was discussed between 23/10/2002 and 30/10/2002

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