ZS carb idle issue

[sujitroy]

I have an auto Stag with ZD CD 175 carbs. I rebuilt the carbs, but have noticed when the car is at running temp and I've been out for a drive the idle speed increases, slightly, from around 800 rpm to about 1000. I carry a screw driver with me and reduce the idle and the car runs OK. If I leave the car over night, I have to adjust the idle screw again. I've rebuilt the carb myself with new seals and O rings. I have balanced the carbs. I downloaded the TR 6 ROM and it basically says the same thing regarding tuning. However, when I left each air valve, the engine speed does not change. The car pulls without hesitation OK. The carb are more biased towards being lean from setup. Both idle trimming screws are closed off. The by-pass valves have been sealed off. I have adjusted the temp. compensators.
I have installed a secondary spring to make sure the throttles stop on the idle screws. I'm not suspecting an air leak as if I pull the oil filer cap off the car wants to stall. The car is a Fed Stag and has a carbon canister where a line from the cam cover goes into each carb and the canister. As an experiment, I may block this line off. The line to the canister has a reducer to it
Prior to swapping out the carbs, the car would randomly stall when coming to a stop. but I didn't encounter this type of idle issue.
I've rebuilt another set of carbs and they are running well on my manual Stag. I don't plan on touching this car and swapping carbs.
Any suggestions appreciated.

[MikeParker]

Sujit,

I had exactly the same thing for years (decades in fact!) It sounds like the temperature compensators. These bleed air directly into the inlet manifold when the carbs get up to full temperature. That is for emissions reasons. There is less condensation/wetting within the manifold, so more petrol stays as vapour when hot. Without the extra air, the mixture would be slightly too rich (for emissions standards). The answer is to screw up the set-nuts so that they never open! Job done.

[sujitroy]

Hello Mike,
Prior to rebuilding this Fed. carb set, some folks on the TR6 FB forum said to close the compensators off completely and close off the by-pass valves. The valves are closed. However, I have seen a couple of documents and YouTube videos on how to adjust the compensators. I feel that I've adjusted them the same on my other Stag which runs OK. I have a spare set of un-adjusted compensators, and will try your suggestion, of permanently closing them, and report back.
Sujit

[trunt]

Even if you manually close the temp compensators the are not necessarily closed.. The rubber gaskets are probably ok from your rebuild but have you checked that the plunger itself is nice and seated when it’s closed? neither of mine were sealing correctly, I used the suck test!
I unsuccessfully tried to fix them, in the end had to use a spare pair, it’s idling perfectly now. The other option is to plug the hole that feeds air from the flange at the air filter end.

I’m pretty sure that the cam cover always feeds the carbs so best leave that alone, maybe just close the line to the carbon canister, can’t remember exactly where but there are restrictors in those canister lines, maybe check if any are missing?

Terry

[MikeParker]

Even if you manually close the temp compensators the are not necessarily closed.. The rubber gaskets are probably ok from your rebuild but have you checked that the plunger itself is nice and seated when it’s closed? neither of mine were sealing correctly, I used the suck test!
I unsuccessfully tried to fix them, in the end had to use a spare pair, it’s idling perfectly now. The other option is to plug the hole that feeds air from the flange at the air filter end.

I’m pretty sure that the cam cover always feeds the carbs so best leave that alone, maybe just close the line to the carbon canister, can’t remember exactly where but there are restrictors in those canister lines, maybe check if any are missing?

Terry

I agree that the compensators tend to leak because the o-rings/ gaskets may have gone, but that will tend to be a constant between hot and cold. It is the compensators opening and effectively increasing the gas-flow through the engine that causes the revs to go up by about 200 RPM. That was my experience. I closed mine off a few years ago and that stopped the excessive rev rise on warming up. I did notice when I rebuilt the carbs recently that all the rubbers had gone and so they must have been leaking, but it did not seem to affect the AFR readings much.

Mike

[sujitroy]

Hello Mike,
Prior to rebuilding this Fed. carb set, some folks on the TR6 FB forum said to close the compensators off completely and close off the by-pass valves. The valves are closed. However, I have seen a couple of documents and YouTube videos on how to adjust the compensators. I feel that I've adjusted them the same on my other Stag which runs OK. I have a spare set of un-adjusted compensators, and will try your suggestion, of permanently closing them, and report back.
Sujit

I have a few compensators and took the best ones. I did make sure the plungers closed, opened and sealed a few times while adjusting them with hot water. Some spare plungers didn't close. I have some compensators where PO's had bent the bi-metal strips. I may seal up the holes with some epoxy glue as I have a few bent plastic covers that don't seal too well.

Sujit

[trunt]

I wouldn’t overly worry about the covers leaking, any leaks there are pulling air from the atmosphere just the same as it does from the proper drilling, (except its not filtered air) probably why they never bothered with a gasket. Same goes for the larger seal . The important part is weather the plunger truly seals when it’s closed and the small seal that is down inside the hole at the tip of the compensator which if not 100% can provide a bypass leak, so if you epoxy a couple up for testing make sure that smaller seal is good and still there!

[jbuckl]

Throttle linkage must not interfere or override the disc settings. (Idle)
sounds like it might be.
that’s why there should be lost motion at the cable quadrant & the pushrods must not be imparting any effect at idle.
what is the hot temperature at the compensators …. Aware you’re in a hot climate.
throttle discs must be centred in the bore before spindle screws are tightened.
spindle seals must be supple / sealing…..check with a volatile spray.

[sujitroy]

I have just read the Dealer Training on the subject of Carburetion. I see how the compensators work work and make the fuel mixture leaner when at operating temperature. The doc says when they open, it weakens the depression which allows the piston to fall slightly thereby cutting the available fuel and creating a leaner mixture. This I get, but with the throttle plate in the idle position, and the volume of air passing the throttle plate can't change, then why does the engine speed increase?



Is it possible that my carbs fuel mixture is too rich and the loss of fuel, from the lower piston, is making the fuel / air mixture just right for combustion?

My carbs are currently set to lean. It drives well and is lively.

Julian
I have an extra spring holding the carbs on the idle stops. I have lost motion within the quadrant. I read somewhere that someone does stronger replacement springs to hold the throttle plate closed.
throttle discs must be centred in the bore before spindle screws are tightened. I did this
spindle seals must be supple / sealing…..check with a volatile spray. New seals here.

Sujit

[Phil S]

A mate of mine was a triumph dealer when the stag was released. The unofficial fix they were told to do, but was never put on paper was to stop the temp sensors from working as they were a known problem

[MikeParker]

I...., then why does the engine speed increase?


Sujit

The temp compensators bypass the throttle plate. So they add air directly into the manifold. That will lean the mixture, but still increase the overall charge of air+ fuel into the engine, so increasing revs. I guess the only way it would not increase the revs would be if the engine was so lean that the extra air caused the engine to falter. Generally the engine is quite rich at tick-over, so this does not happen.

[trunt]

The temp compensators bypass the throttle plate. So they add air directly into the manifold. That will lean the mixture, but still increase the overall charge of air+ fuel into the engine, so increasing revs. I guess the only way it would not increase the revs would be if the engine was so lean that the extra air caused the engine to falter. Generally the engine is quite rich at tick-over, so this does not happen.

The temp compensators bypass the bridge, not the throttle plate as Sujit indicated and the description in the 73 emissions manual describes it, a quick look at my spare carb confirms this, the hole enters beween the bridge and throttle plate. . Its the bypass valve that byasses the throttle plate.

[MikeParker]

My apologies to both if that is true. I guess the same arguments still holds though in that it will still increase airflow through the engine, but to a lesser extent. The airfow circuit would consist of the throttle plate in series with the temp compensator and the normal path over the bridge, in parallel.

[Wheelz]

I think the theory is that as it bypasses the venturi, so it reduces the depression allowing the valve/metering needle to drop a little, and this reduces the fuel hence weakens the mixture. I read this in the Technical Section info.

If the idle speed increases when hot, then perhaps it's because the mixture is set too rich? You will find out if you close the TCs.

[trunt]

Sujit,

You say they are set to lean, do you mean the lean side of optumum or fully lean position of the needle?