View Full Version : Idle rpm creep?

02-15-2008, 07:56 PM
This Tinster stupid question No. 791 is not about a
problem, just a curiosity.

Forum blaggard Edward Teach himself, fine tuned my excellent
Jeff Palya restored carbs. Ergo, I know my fuel delivery
is just about perfect.


At temperature, sitting in my garage, the idle is ever so
smooth at 900 rpms.

Out in the street, after a few minutes of bumper to bumper
or a zoom down the expresso, the idle creeps up to a smooth
1,100 rpms. It stays at 1,100 rpms until I shut down the
engine and let it cool down. It then is back at 900 rpms.
Anyone else notice this idle creep effect?

Inquiring minds what to know!!


02-15-2008, 08:06 PM
The idle rpm on my TR3A wanders all around, depending on ambient temps, how long and how hard it's been driven.

My SWAG is that the temperature and pressure (hence density) of the air being sucked into the carbs changes, thereby changing mixture and idle rpm.

But of course it doesn't have the temperature compensators found on a TR6; which I understand are supposed to compensate at least partially for this problem. Be interesting to know what Ed/Jeff did to adjust them, since AFAIK the factory never published that information, and various rebuilders seem to disagree on how they should be set.

The Stags do have the compensators, but I don't have enough experience to really comment on idle rpm (except I haven't particularly noticed it as a problem).

If you still have the carbon canister linked to the fuel tank, it's possible the mixture is also changing due to fumes sucked from the tank and/or canister.

02-15-2008, 08:18 PM
Randal- No pollution control devices and no water pipes
for heater either. A fume pipe from the gas tank straight
down to the ground. A hose from valve cover to air filter
box. That's it.


02-15-2008, 08:56 PM

A very slight misalignment between the carb shafts can cause such a problem. As the manifold expands slightly when warming up, the misalignment is exaggerated and the problem sets in. After cooling down, all is well.

That is why I removed the beautifully machined solid couplers that Jeff made for me when I installed the triple carbs. There is even more movement in the three individual GoodParts manifolds and you must use the flexible couplers.

Try this little trick. After warm up, when the idle has crept up a little, loosen the nuts just a little on the flex coupler and see if it drops back down. If so, tighten it back up and I believe that you will have fixed your problem.

02-15-2008, 09:23 PM
If throttle by-pass valve is open when it should be closed, a little extra of the fuel/air mix will bypass the nearly closed throttle disc and end up in the intake manifold and eventually the combustion chamber. That little bit extra fuel will give an increase in idle speed by about 200 rpms.

If your TBV's have an external adjustment screw, you can increase the tension on the spring that is supposed to keep the diaphram valve closed except during engine overrun, such as downshifts and deceleration from speed.
Again, if you do have the external adjustment, you can test to see if they are seating properly or open by a light push of the screws when the idle is higher than normal.
You will only be getting a 16th of an inch or so of movement,but that's enough to manually seat the valve.
If your engine speed does drop back then you can increase the spring tension by CCW turns.
On my 74, I turned the adj. screw CW until the screw stops, which is minimum spring tension, then CCW 6 1/2 turns
The temperature compensator is really sort of an "auxillay choke". It is designed to be in a closed position until the bimetal strip starts to warm up enough to pull the plug (valve) and allow additional air to enter the mixing chamber of the carb. That air comes from the port under the right side air box attachment bolt's threaded hole.
I believe the intention was to allow for a quicker driveability while the engine was warming up because when it is cold (relativly speaking) the mixture is richer than when it gets warm and opens admitting more air.
That is one reason that carb adjusting is done when the engine is warm.
Therefore if your engine is warm, the temp. comps. should be open and letting in the right amount of air, which should not affect your idle speed, especially with a warmed up engine.
All that being said there are other things that could cause the idle to act the way it does. Linkage? Throttle shaft friction? Vacuum leaks?
And from living in the Deep South, I can tell you my ZS prefer cool weather.

02-15-2008, 09:37 PM
A hose from valve cover to air filter

You should have a little more vacuum on your valve cover, you might have more pressure than you need. Try dropping a hose straight down below the engine and let speed suck a vacuum. Keep it away from the exhaust.

02-15-2008, 10:57 PM
On my TR6 I had the same problem; mine turned out to be the 2 little horizontal plungers on the carbs that are actuated by the carb throttle linkage. On mine they were sticking and causing the throttle to fail to fully return to the throttle stops when the engine got warm resulting in a fast idle.
I took em off and all's well, they are there to close the fuel bowl vents at idle so when you turn off the engine the anti run-on device functions properly but none of that seems to make any difference.
As a test, try pushing the throttle linkage on each carb closed by hand with the engine running and see if the idle slows down.

02-16-2008, 12:19 AM
My Spitfire has the same idle problem with its single Zenith-Stromberg. Thanks for pointing me to the throttle bypass valve. I'm going to adjust that this weekend.


02-16-2008, 09:35 AM
OK. Now, if the diaphram that surrounds the actual brass valve is brittle or even cracked as old ones often are, then increasing the spring tension on the valve may not be the solution. The diaphram is available from the usual sources, though.
And then there are those carbs that have the "tamper-proof" TBV. In order to adjust them, they have to be removed from the carb , and except for the accessibility to the adj. screw, the internals are the same.

02-17-2008, 06:40 AM
On TR2-4A cars failure to return to normal idle can be down to dry swivel linkages between the accelerator bracket on the firewall and the carbs. The bell crank beneath the front carbie can also become dry due to exhaust manifold heat.

Check that adjustable swivel linkages are not set too tight.

Lube these and drop a spot of oil onto the spindles each side of the carb bodies for smooth mechanical return of the butterflies to their idle setting.

02-17-2008, 11:36 AM
qoute...no water pipes for heater either...just wondering out loud here....maybe the manifold is running to hot with no water cooling? Any body else running a dry intake and with what results?
I know from experience that the cooler i run the engine for smog testing the better the result at idle test.

02-17-2008, 02:48 PM
Don't forget the check the fastners on the intake manifold. They seem to loosen with a few heat 'n cool cycles.

02-17-2008, 11:00 PM
Methinks the Triumph sump contains many quarts of oil, (8 or so if I recall correctly) methinks oil thins out as it heats up, most of us run a thicker oil in the cars, I assumed from your post that the higher idle was a normal function of the car and oil warming up/

It can also go down again when the car gets really hot and the underhood air thins, tending to richen the mixture.

At least that has been my experience with the half dozen SU carbed cars I have owned.