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PatrickB

TR6 PI CP - high idle/tickover speed

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Sorry my first post is a request for advice - I will at least try and add what I have found so far about the problem, and tried to resolve it.

Really enjoying using my first TR6 now it's warmer and dryer - commute, kids' taxi, any excuse...

 

It's a well maintained 1971 CP TR6, with no recent mechanical changes. Two weeks ago I noticed the idle speed increased to around 1400-1500 RPM on the return leg of a 40 mile round trip. It did the same at the weekend. Last night and today it was up to 2-2500 RPM at idle. Apart from that it's fine, pulls well, revs well, plenty of power, no problem.

 

Consensus is from my own logic and various bits of internet wisdom, it may be mechanical, but it is probably an air leak on the intake system, as you can't get an engine revving like that without a fair amount of air getting in somehow.

 

So...

Mechanically looks fine to me - the choke(cold start) cables both return back to where they should be. So does the throttle cable, with just a little slack. The throttle linkages/bar that moves the butterflies settles back nicely on its adjusting screws. Visually the butterflies all look to be closing OK.

 

Air leaks - before starting the car I pulled off the hose (engine end) that goes to the fuel metering unit. A good suck on this then put my tongue on the end of the pipe - not the nicest taste but seems to hold the vacuum ok by sticking to my tongue for a good few seconds.

Then, with the engine warmed for 1 mile and running (2500 revs idle!) pulled off the brake servo hose and blocked the inlet - revs drop to about 1500. Screw in the air bleed valve at the other end and revs drop a little - then block off the air inlet for the bleed valve, which makes no difference. Oddly, now putting the brake servo hose back on instead of blocking the inlet with my thumb, we are back to 1500 rpm. So in summary, with closed throttle & choke, blocking both inlets completely (which should stall the car in theory), we are still at 1500 rpm.

 

What with kids and a day job, and being a TR6 novice I don't really have time to embark on a voyage of discovery to get to the bottom of it and I am worried that if I don't sort it very quickly I'll miss the Summer.

 

So any ideas please -

1/ what else could I easily check?

2/ any recommendations for a friendly local expert (Windsor, or Reading, Berks) who can help sort it out in time for Summer?

 

I can see a few people have had the problem over the years so I'll write it up here once it is sorted out.

Thank you!

Patrick.

 

 

 

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Hi Patrick,

Welcome to the forum. Though from the detail in your post I suspect you have been lurking and learning - and why not !!

 

Looks like you have covered most of the likely trouble spots. Did you check the butterflies with the plenum off?

- they should be shut with not room for more than very thin feeler gauge.

Its worth checking that the three manifolds are firmly clamped against the head. The underside of the inlets share a stud with the exhaust - one nut tightens a bridge piece to ears on both manifolds. Cold engine job ! I gather there are tricks for spraying WD40??? to check for air ingress there. Not sure, I think the idea is you see smoke appear in the exhaust... Anyone done this?

I think you'd have already spotted tired rubber balance hoses linking the manifolds.

 

Peter

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Hi Patrick,

Sorry to hear of your problems. It sounds as if you are far more knowledgeable than I am about such things, so I'm not able to offer any guidance in respect of fixing the issues.

However, when I was looking to buy a TR6 last year, I visited a garage in Berkshire (just south of Reading) that specialises in classics (mainly MGs :huh:). I've not used them for any work on my car, but the people there were very friendly and helpful when I was looking to buy and seemed knowledgeable.

Name: Beech Hill Garage - http://beechhillgarage.com. I can't recommend them as such, as I haven't had work done there (and I have no connection with them), but if they are near to you and you and you don't get things sorted out, they might be worth a shot. After all, you don't want to miss out on the summer months.

 

If you do use them, let us know how you get on.

 

Darren

Edited by TR5tar

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Patrick

 

Welcome, try as Peter suggested and tightenall the manifold nuts as I have recently had this problem

 

Then get some spray carby cleaner and spray it around the manifold, throttle bodies and vacuum hoses to see if it gets sucked in and increases revs - if so there is your leak, if not seek professional experienced help

 

Let us know how it goes

 

Graze

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Definitely there is something wrong with the brake servo

when connected the idle may rise a bit but than it should

be the same when the hose is locked.

 

Rest was perfectly explained by the other experts, nothing to add....

If revs are too high too much air must come into the manifold.

Rubber is most likely the cause, loose bolts the next and something

with the throttle plates also because you have six of them that need to play perfectly together

and none of the bearings and connectors is allowed to stick, that will affect all the others, too.

 

You may look if ignition timing returns to normal when revs drop

but that is not the most likely reason for this problem.

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Do you have stop screws on the throttle butterfly shafts? If yes you could try and unscrew each just a 1/4 of a turn and see if this lowers the idle speed, this will upset the balance of each throttle body , and will need to be reset with a balance meter, but at least it may it a little more drivable. Make sure also that the little bolt on the choke control cam has some clearance with the throttle shaft.

If its a CP do you have just one air pipe between each throttle body, make sure these have clips on and the hose is in good condition.

If you sprayed some wd at these joints the revs shouldn't increase, if it increases there is your leak point.

John

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Thanks everyone for this great and rapid response!

I shall persevere and try all of the above at the weekend, hopefully it won't turn into a saga.

That will please the neighbours revving away on a Sunday morning!

I'll report back on progress

Thanks again

Patrick.

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just thought of something else. If not alreaady done check the wire going to the metering unit is not touching the bulkhead.

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Just checked and rechecked all of the above, everything seems fine.

Except - took off the vacuum pipe to the MU and blanked the inlet. Revs drop to normal. What I don't know is whether this is due a vacuum leak in the MU or because it now meters less fuel! Any thoughts?

Also - has anyone changed the diaphragm- is it a job I can do leaving the fuel metering unit in situ or does the whole thing have to come off?

Thanks again.

Patrick

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Patrick, Result!!

I think the calibrating spring unit lifts off after you have undone the four screws:

http://myweb.tiscali.co.uk/tlweb/tr6/injection/system/mu/cu_exposed.html

then there's just the central small nut that connects the diaphragm to the metering linkage.

http://lucasinjection.com/Lucas%20page%2011%20fig%2010.jpg

 

Cant see any reason for not doing it in situ.

Dont disturb the springs, in fact you can leave the black plastic cap on.

 

It should run a lot leaner with an intact diaphragm !!

 

Peter

Edited by Peter Cobbold

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Thanks Peter, much better than the drawings in the Triumph manual!

 

My question was:

 

What I don't know is whether this is due a vacuum leak in the MU or because it now meters less fuel! Any thoughts?

 

So I took the top of to have a look and the diaphragm looks just fine.

 

And I now realise I could have saved myself the trouble and my question above makes no sense - logically it had to be OK. Because if the diaphragm is split then it won't make any difference if I pull off the vacuum hose, as the diaphragm is doing nothing anyway!

 

So I think it's the hose. By next weekend I will have all new hoses and hopefully the correct idle again. I'll update here then.

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I read it as " took off the vacuum pipe at the MU ". Apologies. Still, all part of the learning process.

 

Whenever the space above the diaphragm is at atmospheric pressure the MU delivers the max fuel 'squirt'.

At part-load the depression in the manifold pulls the diaphragm upwards against the springs and the fuel pulses get less to match the smaller air volumes, and also (perhaps) to lean the mixture.

 

Peter

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Hi Patrick,

 

How are you getting on with this. My car is doing exactly the same thing. I will be working through the steps that you have taken, but it would be great to know what was causing your issue.

 

 

Cheers,

Stephen.

Edited by sfirm

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One trick I found was to balance the throttles using the linkages when the engine was up to temp. This allows the linkages etc to expand so that you get the proper synchronisation.

Tim

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Worth checking the throttle return springs are not broken - both the main one and also the small ones on each manifold.

Is the throttle cable frayed?

 

To check that the throttles are balanced - put the palm of your hand over each one - if it sucks it means the throttle is open (at tick over all the throttles should be closed and the air inlet should just be via the air bleed screw/valve - the exception being the very early 5s which are meant to be fractionally open as they don't have the screw) If a pair are open then you need to rebalance - if one of a pair is open then the butterly isn't sealing in the manifold which means that either the spindle is twisted or the disc isn't seated correctly - to seat it loosen the screws slightly and open the throttle and let it snap shut a few times before re-tightening.

 

To check the servo isn't leaking air into the manifold simply remove the pipe at the manifold end and stick your finger over the entrance- if tick over slows you have a leak on the servo or its pipe.

 

Check the air bleed valve isn't stuck open by sticking your finger over the pipe - it should cause the engine to stall.

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Hi, my issue was the result of a slightly pitted fast idle cam.

The springs were fine, it was the head of the adjustment bolt that was catching on the cam thus leaving the throttle bodies slightly open and getting progressively worse as the day went on.

 

I can't believe I didn't see it straight away!

 

S

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