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Triumph TR6 - Rising Idle speed as temp warms up

MY '75 is behaving badly this summer. I bought it in the spring and the idle was always fine ,with warm or cold engine. When we got into summer here in Texas, the car's idle begins to progressively rise as the car gets hotter. The linkages are all fine, the carbs were rebuilt several years ago and I just checked the diaphrams and adjusted the needles. All looks fine. It starts off around 850 but rises smoothly over 10 or 15 minutes until it idles around 1800. Its maddening. When I had the air filter off the slides appear to be resting at their bottom, and the big spring above both looks fine. They were running rich so I leaned them up just a bit. As an aside, I run Pertronix, slightly colder plugs, 8.5 compression, and stock ZS CD-2a, and I've been told therer are no servicable parts in those plastic bodies on the side of the late model carbs...
Any ideas on the cause are appreciated.
Kent
Kent Bracken

Kent,
I believe you need to check your bypass valves on the right hand side of both carbs. I had the same thing with an uncontrolled idle about 2000rpm and mine were floating under running conditions. When they float they allow fuel to "bypass" the throttle plate. Maybe you need new diaphrams for the bypass valves. There is also a spring inside the two halves which may be weak. Some of the years had adjustable bypass valves without having to take it appart. Mine did not as they had a brass cap on the end. If you have a small flat head screw then your's are adjustable. Look in the Bentley manual for adjusting instructions. I modified mine to where it would not allow much bypass at all by adding shim washers inside to push the brass dish more firmly up against the body of the carb. Others have totally blanked their off or removed them.

You can do a quick test to see if it is the bypass valves. I used some thick gasket material from autoparts store and cut a piece to fit both bypass bodies and then installed it. The totally blanked off the bypass plumbing and solved my issue. I did not want to totally blank it off so I did the modification above. With the dish firmly up against the carb body there is a small slot that still allows a minimal amount of bypass. I fealt that was better than totally blanking it off. The engineers put it there for a reason (over run conditions where the engine is running faster than the throttle plates indicate).

Once you get the bypass issue solved, go back and adjust your mixture again as you fooled with it to lean it out.

Good Luck,
MRankin
1971 TR6 CC61212L
MRankin

Hi Kent
I suspect your bypass valves as well...I think you'll have the type with the slot screw..since they may just be mis-adjusted adjust them both fully clockwise then back about 7 turns each ( the limit is approx 11 turns before the end of adjustment)..and see if that makes a difference. the rear is trickier but can be adjusted on the car with a flexible or small screwdriver, you may have to move the linkage down by locking it fully open.
If no difference remove them and tape all the holes then see it that makes a difference etc. The rubber diaphragm should be supple and not hard with no tears..they are easy to work on.
Also check your temp compensators ..I adjusted mine to open between 135 and 140 per my wifes oven thermometer.

Charlie
Charlie B.

As discussed on this BBS many moons ago, the by-pass valve can be made to be adjustable...if yoiu have the little round brass plate covering the adjuster screw. Simply remove the BP valve and pop out the brass cover plate. Now they are adjustable.
Rick C
Rick Crawford

Great!
I'll get into it tomorrow and see what I've got there. Many thanks.
K
Kent Bracken

I disassembled the front carb bypass valve and found it in reasonably good shape. A little crud, but otherwise the gaskets on both sides were good and the assembly seems OK. I reassembled and adjusted the small flathead screw all the way in, and then all the way out, no change.
So I disassembled the carb nearest the firewall and found it massively gunked up (this carb has always been rich). There were cakes of crud in all parts of it, and the gasket material around the little brass center was completely eroded. I cut out a new gasket, carefully reproducing all holes, and mounted it all up. No external leaks, it seemed OK. I adjusted the little flathead screw all the way in, and all the way out, no change.
What is the working component here? Is the fuel flowing through the little pinhole in the brass disk? How it the adjuster supposed to be working? (these are '75 Stromberg CD2s)
Thanks,
K
Kent Bracken

Kent
I feel for 'ya lad. I have the same problem. It seems worse on the front carb (same carbs as you). Talk about "racing" at the stop light:)!! I have turned then the Charlie amount and makes no difference at all other than I hear a load (kinda load) hissing sound from the front carb. It is kinda like a "vibrating" hissing.....the brass part of the valve is slapping around... floating?? I have said in past threads I hate these stupid valves, and I still do!
I installed new valves and I think it made them worse.

I have the front valve fully CW and this seems to be working other than I have severe "popping" on deceleration...I know this is not good. I think I will do a rebuild on the carbs this winter. There appears to be a leak at the throttle arm.

Rick C

Sorry to hear you will not be at Bronte this year Charlie.
Rick Crawford

Kent,
To explain the function of the bypass valve, it is merely intended to allow air (no fuel here) to flow from the carb side of the throttle plate to the manifold side of the plate. The idea is that under closed throttle deccel. that the large vacuum in the manifold does not suck fuel out of the venturi and cause backfire.
What happens as these devices age is the spring gets weak, and bypasses air at lower mainifold pressures thus raising your idle speed to around 2K.
There is a diaphragm with a brass disk that is held against the port by the spring. if the diaphragm is shot, the valve will not function correctly.
In the Haynes Stromberg Carb manual is the only reference to correct adjustment of this valve. It states that the valve should be adjusted to open at approx. 21 inches of vac (Hg).
This can be a challenge on some of the valve models as the vac port is through the mounting face of the valve itself. Some of them have a tube vac port that makes applying vacuum easier.
I found that the old springs in my 72 had to be replaced in order to get enough compression on the diaphragm to achieve the 21" of vac.
I also used a Mity-Vac to apply the vac and check my adjustment, this is with the valve off and looking at the port to see when the disk unseats.
It can be done, and when it is done, my carbs idled perfectly, and no backfires on closed throttle deccel.

Hope this is helpful.

Mark Riddle
Mark

I had the same problem...constant popping sound when gearing down etc and when I rebuilt my bypass valves it solved it.
Charlie
Charlie B.

Sorry..forgot to add...timing can play a part in popping exhaust..it has to be correct
Charlie
Charlie B.

OK. Many thanks for the info.
My diaphrams are bad, the springs are weak and the idle is 2K, so I'm guilty on all charges. I will order the bits necessary to rebuilt both valves. The guy before me said the carbs were rebuilt, but these two bypass valves look original inside.

While I'm waiting on the parts, I will continue to research the TBI solution, which I am very seriously considering. My carbs are problematic, and in places, pretty worn. The dashpot on the rear carb is not holding oil.

My first research shows TBI parts will run about $1500, and the biggest concern is getting a fuel return line to the tank. Also, does anyone know if the bimetallic temperature compensators are completely disabled on the TBI modification to the carb bodies? I want all that stuff out of the picture... I believe the TBI kit blanks the bypass valves but not sure what happens to the compensators.

On the TBI overall, if the DCOEs are $2200, the downdrafts are $1200, and new SUs are $900, one should consider the SUs and the TBI if you would consider downdrafts. What is the consensus on replacing old Strombergs with new SUs?

K
Kent Bracken

Update on the climbing idle progblem:

I Replaced both bypass valve assemblys, and related seals. adjusted as instructed, no change, idle still moves up steadily to around 2K. Then adjusted them all the way in and all the way out, no change.

Since the problem was the direct result of the warming up of the car, I pulled off both temperature compensators as well and checked the little plunger valves in them. Both were completely stuffed up, lots of black goop. We soaked them in B12 and worked them in and out about a 100 times, and the idle problem is now completely gone.

By the way, one of the diaphrams in the Bypass valves had a giant hole all the way around the disk. So its just as well they were replaced.
Just an FYI for all,
K
Kent Bracken

Kent,
Good to hear you found clogged Temp Compensators. Since this device allows additional air into the mixing chamber to lean out the mixture as the temperature of the carbs increase, sounds like you also were running on the rich side if this was causing the idle to run to 2000. There are many inter connecting devices on the carb affecting each other. May want to pull the plugs after a short run and see if they are black and sooty if too rich. Should be sandy tan if correct.

It sounds like you DID NOT dismantle the temp compensators and only soaked them in B12? It took me about 2hrs on the kitchen stove with a thermometer and pot of water to get both of mine to be syncronized. I had to take mine apart as the plastic piston was binding no matter what I did. I did not want to oil it as it would just collect dust as the air was pulled through the orfice. I polished the piston and also the bore before reassembly. As a note I found it easier to adjust the TCs as they were slowly cooling down as opposed to adjusting them while heating the water up. There is a good article on the VTR about adjusting these temp compensators.

Ain't this fun! Cheers! MRankin
MRankin

Well about the only thing I have not done to the D..BPVs is replace the spring. I have replaced both diaphrams with new..the new ones by the way, are 2 pieces as compared to the OE one piece.
Mark, very good explanation as to operation. I have heard ONE backfire and the normal to me:) popping on deceleration. The new name for these critters...DBPVs:)

Well winter is soon here so looks like I have a small project...GEESS, what else is new.Glad you got yours resolved Kent. I will also have a closer look at the TCs.
Rick C
Rick Crawford

This thread was discussed between 09/09/2005 and 21/09/2005

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