Fast idle

[sandandlime]

Hi all,

 

Looks like I spoke to soon.

 

Did around 50 miles yesterday in reasonably hot weather (pushing 35°C) and was again plagued by a build-up of revs. Once back home I investigated things and concluded that the issue is that the carburettor return springs just aren't closing the throttle discs sufficiently well.

 

With the idle screws fully backed-out, I can pretty much stop the engine by pressing and holding (not excessive pressure) the throttle disc assemblies closed. Once released the engine ticks over fine. When the throttle is blipped, however, the tickover increase as the throttle discs have not fully closed. Whilst probably a bit tired, the return springs don't seem to be too bad and are still tensioned on idle. Just to be sure, I have ordered new replacements.

 

I checked that the fast idle screws are clear of the cams but my question now is could I have missed something in the linkage that could lead to a failure to properly close?

 

As always, all advise gratefully received.

 

Tony

[Ransomes256]

Hi,

 

Are the spindles and bore in the bodies in good condition?

The problem may be due to sideways movement not allowing the natural return.

 

Neil

[manray1]

a good cleaning with carb cleaner should fix the problem.

A bit of oil on spindles and let's go.

[Ashley James]

I agree with Neil. I think you need new throttle spindles and probably the body rebushing. Your symptoms are classic.

[manray1]

Very often there is no need to rebuilt the carbs.

I had the same problem in the past and a good cleaning fixed the problem.

[stuart]

Try re tensioning the return springs.

Stuart.

[peejay4A]

Also it might be worth checking that the throttle pedal return spring at the bulkhead is in place and that the long operating rod is not adjusted too short.

[Guest ntc]

Try re tensioning the return springs.

Stuart.

 

 

Yep worked for me

[sandandlime]

Thanks for all the input.

 

Having refurbished the carbs about a month ago I really don't think that the spindles/bushes are an issue.

 

Neil, you might be on to something. There is a bit of sideways movement which could be the problem. I'll have a good look at that one.

 

Pete, the throttle pedal return spring is correctly in place but, like the throttle return springs, it does seem a bit tired. A difficult one though as I'm not sure just how strong it shoud be. Presumably though it shouldn't take any part in closing the throttles, only returning the linkages? There is obviously quite a lot of adjustment on the linkages but I've left these well alone onthe basis of 'if it ain't broke...'. Is there any decent guidance to the correct set-up of these?

 

xTry re tensioning the return springs.

 

I like the sound of that one Stuart but how do I go about achieving it?

 

Thanks again,

 

Tony

[Guest ntc]

Come on Stuart you cannot keep it secret forever

[Badfrog]

Having refurbished the carbs about a month ago I really don't think that the spindles/bushes are an issue.

Thanks again,

Tony

 

 

They may if you did not screw the butterfly discs correctly. If they don't fit the ventury diameter real snug, then you can't idle. I would re-check carefully the setting and motion of the throttle mechanism.

 

Badfrog, ask me how I know .....

[PeteT]

Hi tony it's worth having a peek under the spindle return springs to make sure the bush hasn't worked its way out. It happened to me and you can't see it because it's obscured by the spring. Im still suffering from an unreliable tickover even though the carbs are new. If I brake hard it stalls! I'm still working on solutions. Regards Pete

[sandandlime]

Well I've had a good look over everything and can find nothing out of order. Spindle and discs all seem okay, as far as I can tell. I think that the issue is associated with lazy return springs, both on the carburettors and the throttle pedal linkage - and it is the latter that appears to be particularly sad.

 

It may also be exacerbated by non-optimised linkage lengths?

 

Could it be that the effective weight of the throttle pedal and the linkages are the culprit?

 

I'll have a look at the return spring on the bulkhead and see if I can beef it up somehow. (We are booked for a rally in Bordeaux at the weekend so ordering a new one from the UK is not going to arrive in time. Angoulême was our first choice but we were too late with our application; maybe next year.)

 

All the best,

 

Tony

 

[mandarawessels]

on the subject of accelerator pedal return spring: I can see the hole in the pedal part that sticks out into the bay, but where exactly does it attach to?

Any help appreciated.

 

John.

[Ashley James]

If you've overhauled the carbs it might be worth making sure the butterflies are centred properly and not sticky.

[stuart]

How to increase return spring pressure on the throttle levers. (apologies for the writing but its not easy with a mouse )

Stuart.

[peejay4A]

There is a small tab welded to the bulkhead below the lever.

[mandarawessels]

thanks peejay4A

I'll have a look and see if I can find it!

[mandarawessels]

found the tab, spring is very close to the big lead to the startermotor!!

But it all seems to work OK.

Cheers

[sandandlime]

Stuart,

 

Nothing wrong with the writing; mine's not much better these days normally! There was a time when I couldn't think without a pencil my hand. That changed to a trackball, then a mouse and now it's a touch screen. I've just about forgotten what a pencil is!

 

Coming back to topic, however, my arrangement is different to that shown in your photo. I don't have the concentric springs. Instead I have a pair (both sides) of approx. 2" long coil springs with hooks either end. These hook into a hole in a bracket fixed to the throttle spindle and fixed to a bracket beneath the carbs. I'll take a photo tomorrow if my description doesn't make sense.

 

Ash,

 

With respect to butterflies being centred, well I tried damn hard and I think I got it right but I suppose that it only has to be slightly off to cause the problem. Strange thing is that we did 170 miles on Monday without issue but the problem materialised as the weather became warmer. Coincidence?

 

Overall, I still suspect springs and linkages. But I could also be braking up he wrong tree. Time alone will tell.

 

Thanks to all for the advise,

 

Now back to last night of the proms!

 

Tony

 

 

[stuart]

Tony, sorry I thought you were referring to TR4 carbs. The 4a ones do indeed have springs going down to the central plate. But if you have any leaks around the spindles where they exit from the bodies then as they warm up this will allow more air in which will up the tickover. Try spraying a bit of carb cleaner on the exit points of each spindle while the engine is ticking over, if the tickover increases then you need them re-bushing (Keep the carb cleaner away from the exhaust ) You could try oiling the plastic bush where the throttle pedal shaft goes through the bulkhead and check that the bellcrank locating bolt is correctly assembled and well greased, that also goes for the main and secondary link ball ends especially if they are of the type where you can adjust the pressure of the spring pad (The ones that have a split pin across the end screw.)

Stuart.

[Rob Salisbury]

Hi, had this happen to a pair of Strombergs I had rebushed, .... ie OK when cold , sticky throttle when hot, took the spindles out and polished the bush contact areas in a lathe using 1200 wet and dry. No problems on reassembly. Worth a look.

Cheers rob

[dingle]

I have had an experience similar to Robs. I used to re-bush carbs with teflon or delrin. They worked fine when the car was cold or even if hot and running. But the spindles would stick when the car was brought up to running temp and then shut off for a few minutes. On re start, the spindles would stick. The problem was caused by the engine heat "soaking"into the carbs and the uneven expansion of carb body and bushing. The solution was to not make the bushing a tight fit in the carb body.

Anyway,back to the case at hand. If the problem of a high idle speed when hot isn't caused by inadequate tension of the return spring, the spindle sticking in the bushing may be the culprit, especially if the problem just occurred after the carbs were rebushed.

Berry

[Menno van Rij]

Yep worked for me

 

 

And me.

 

When the engine is idling fast, see what happens when you lift the accellator pedal with the tip of you shoe. If the revs go down, your foot is acting as the return springs.

 

Menno

 

(adding: must say that my car's engine tend to idle slower when the under bonnet temps are high like yesterday. Even with a heat shield, the carbs get hot.)

 

M.

Edited September 9, 2012 by Menno van Rij

[sandandlime]

Well I've been looking in close detail this afternoon at what's going on. I can't say that I've got to the bottom of it but I may be onto something with the linkages (or it may just be a red herring?).

 

I removed the throttle pedal return spring and was surprised to find that it was much stronger than I had previously thought. However, the whole linkage assembly seems just very 'dead', and stodgy. It just doesn't pull back. Whilst the spring is pretty tough, it just doesn't seem man enough to balance the whole linkage assembly.

 

I wonder if its just a question of throttle linkage lengths? I've looked back through the forum to see what has been said before about throttle linkage lengths and haven't found anything specific, either in the forum or the WSM. For good or bad, I have tried extending the long control rod to extend the bulkhead spring and introduce a bit more tension but with not a great deal of success. (it also had the effect of dropping the height of the pedal considerably so is probably not the best approach.

 

So, does anyone have any tips for the correct linkage setup?

 

Hi Menno,

 

Good idea to try lifting the pedal. I'll give it a go and see what happens. Given that it's often pretty hot this far south, it might also be a good idea for me to buy/make a heat shield for the carbs so I'll look into that.

 

Thanks to all again.

 

Tony