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Triumph TR6 - Idle speed changes when hot

I'm running into a problem where the idle speed is fine when starting out on a drive, but after running the car fairly hard and then slowing down in traffic, stopping for lights etc. the idle speed is too low and engine nearly stalls. The engine is running great with plug reads showing good mixture, no stumbling or hesitation. Everything seems good.

What changes the idle speed. I'm running dual SU HD8 carbs.

Thanks in advance

John Parfitt
John Parfitt

Mine does the opposite. Just after it is warmed up, it runs at about 800 RPM. Later when it is really hot, it'll idle at 1200 - 1300 RPM and then I get this loud vibrational squeak in the hood latch spring. So I hook my toe under the gas pedal, pull up on it and it then idles about 400 to 700 RPM. There is no vibrating squeak at that range of RPM. The next time I come to a stop light, it idles fast and squeaks again.

I run the original SU's that came with the car in 1958 with a full carb re-build on both carbs 14 summers ago (over 78,000 miles).

I think it is grit in the TR3A carb linkages (ball and socket type). I suppose I should clean them out - or at least lube them. Maybe one or more of the balls have developed flats on them. They ofen do. I'll take a look some day. My return spring is OK. I just looked at it. Might be the white plastic balls that support the cross bar below the battery that got grit in them too.

So if you set your idle to be 1300 RPM hot and check for lube and clean linkage connections, it'll probably do exactly like mine.

Don Elliott, 1958 TR3A
Don Elliott

My '74 suffers from the exact same condition; waiting to see other responses.

Since you're not encumbered with temperature compensators or by-pass valves, it may just be the heat from the warm engine & manifold. Hotter air is less dense then cool air, and since the piston sucks in a constant volume, there is less "charge" by mass in the cylinder when warm which would lead to less power and a slower idle.

An alternative is that when things warm up the clearances between the butterfly and the carb close somewhat and choke the flow even further. I don't favor that explaination, though, because the carbs are insulated from the manifold, and the butterfly is cooled by fuel evaporation.

It might help to close off the engine coolant line that heats the manifold as that's not needed in warm weather. I installed a ball valve in the line that is closed during Summer & open in Winter. If you look closely you'll see it in the closed position here:
Brent B


Intersting thoughts. In my case I'm running a cold air induction with a K&N filter out in front of the radiator shroud so air temp won't be an issue for me.
(After a hard drive my carbs are cold to the touch with condensation on the outside of the carb bodies)

But you might be on to something in regards to the hot coolant running through the manifold. That would be one thing that changed during the drive. I'll try the valve there.

John Parfitt
73 5 speed.
John Parfitt

I have similar problems sometimes idle goes up to 1500 or more and sometimes it just wants to die unless I work the throttle to keep it running. Bizarre little cars but I guess that's why we love 'em. Some suggest to adjust temp compensators, bypass valve screws, add fuel pump pressure regulator, Doh! So which one or all must we do? Any body really know for sure? Would be glad to hear some more stories about the infamous ZS carbs and the mysterious hunting idle.
B.R. Horne

Nothing mysterious about them.

If you change one thing from original as in most Hot Rodding situations you have to re-engineer it all. Yep changing air filters is Hot Rodding!

Thats what makes it fun?

Keep a log book! Mark date milage and alteration. A pattern will develop. Gives you something to ponder in the library or reading room?

Bill Brayford

I keep a log of what I do when I adjust's just too hard otherwise to remember when working on the many parts of the car. I set my temp compensators to open about 135 degrees and I found that by adjusting the bypass valves the idle will start to climb. I started off by turning them both fully clockwise (CW) which causes the valve to float and gives a higher idle. I then turned them CCW about 9 turns each and was able (along with timing reset and idle screws adjusted) to idle about 800 - 900. It used to go up and down but stays close to this speed now. It's been about 3 weeks since this and she is running great.
Use a magic marker and mark a point on a screw driver and count the turns you make..try different settings.
I think 11 turns from fully CW is the max you can go before the end of the adjustment.
I bought one of those screw drivers with the long flexible neck and various works well for that second bypass valve.
Charlie Ballard

Hi Charlie and el. I have not played with the bypass valves before. I now have three. Today just doing the turns thing I encountered resistance after only about 2 turns in either direction. Am I being too gentile or is it 11 turns you say! The temp compensators I have screwed down tight. Do you or others think this has efect on the wandering idle. Did I mentionn todays run was great brown plugs good pull in all gears to 5000+ and then 1500 or 0 at a stop sign. Go Figure!!!
Regards All Brian's TR6
B.R. Horne

BR--With performance like that, I think I'd take the high road and simply blank-off the bypass valves (BPV's) with gasket paper. As far as the temp compensators, they shouldn't be allowing any bleed air in if nutted down fully UNLESS the 2 rubber gaskets are shot (particularly the small one that seals the cone valve face). Check them out.

Rick O.
Rick Orthen

Hi Bryan,
I'm sure there is something amiss with your bypass valves..try removing them and taping the holes with duct tape for a test run..if there is an improvement then you are on the right may be that only one is causing the problem and not all 3.
I agree with Rick, your temp compensators should seal when closed (cold) and they should be open when the car is hot, I think if you set them up it will improve things.
Those were the two things I worked on that gave me a noticable difference.
Charlie Ballard

Gentlemen and Ladies,
I like the function of the bypass to remain. Allowing closed throttle decelleration without pulling excess fuel into the manifold.
If you prefer to use this function instead of bypassing them, you can adjust them as follows.
with the valve removed from the carb, I used a Mity-Vac tool to apply vacuum to the appropriate port on the vavle (this varies with the year model). By applying vac in small increments you can observe the vac level at which the valve begins to open. if your valve opens at a level lower than 21" of Hg, your valve needs to be adjusted. The ZS Sromberg manuals including the Haynes reprint specify that the valve should begin to unseat (open) at about 21"Hg.
In my case I needed to replace the 30 year old tired spring inside the valve, by finding a suitable replacement at the local hardware store, as the original spring could not be adjusted to more than 10"Hg.
I can also report the both valves now work as intended, without the warm condition fast idle (no 2000rpm idle now).
This is of course my opinion, and "your mileage may vary".
Bypass the "bypass valve" if you like, but if you want it to work, this is how I did it.



This thread was discussed between 15/05/2004 and 20/05/2004

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