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Engine idle surge


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Hello,

I am drawing to the end of taking my gearbox out, replacing seals, and refitting.  As part of this project I have the car on axle stands and was going to run the car to allow the new oil to circulate the gearbox and overdrive.  However, upon starting the car the idle speed just keeps going up and up and something is clearly wrong.

I have backed off the fast idle screws completely, ensured that the choke screws are not engaging, and checked the oil in the dash-pots to no avail.  As I have done nothing to interfere with the engine or HS6 carburettors recently I am at a loss as what could have changed and need some help please.

Many thanks.

 

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21 minutes ago, john.r.davies said:

Almost always due to an air leak, in the inlet ducts.     A fine spray of something inflammable, WD40, brake cleaner, at suspect points can find it by an idle increase.

J.

+ 1 with John. You'll normally find the exhaust and inlet manifold fit underneath the inlet runners are holding off the inlet manifold from the head and creating a major air leak.

Mick Richards

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2 hours ago, Peter Cobbold said:

Throttle calbe outer jumped out of its recess ?

Not on my car Peter as it has the solid rod type linkage.  I have checked from pedal to carbs and everything seems free to move and return.

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2 minutes ago, MKTR said:

Not on my car Peter as it has the solid rod type linkage.  I have checked from pedal to carbs and everything seems free to move and return.

try checking for sunken float.

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Thank you all - I have taken a look and can find nothing obvious, and all is tight as can be.  So I am going to bite the bullet and take the whole carb set and manifold out tonight, and start again.  Still remains a mystery whey this happened but this is the joy of a TR I guess.

Mark 

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Update - spent the weekend stripping out the carbs and inlet manifold to see if I could identify a problem, but there was nothing particularly obvious except for a stripped brass manifold nut.  So put it all back together with some Locktite on the manifold side and sort-of got it running again - sort of.  Lots of popping, banging, and stalling until I managed to get a decent balance of idle screw and mixture to let the car warm up and idle 'normally'.  Now it is the simple task of balancing and mixture setting.

Crossing fingers all is well again.  Thank you for your pointers.

Mark 

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Well the saga continues as the car is still not running correctly.

Fresh fuel added just in case.  Carbs balanced & oil in pots, and throttle links move OK. Colour tune shows orange at idle and then blue with some revs, which previous threads suggest is about right for a TR.  There is some hesitation when I press the throttle pedal and holding revs at about 2000 rpm is a little lumpy, but nowhere as bad as it was.  I took a short drive down the road and she seems to struggle to pull until there is some momentum.  Is the mixture so rich that it is affecting idle and pick up or too weak that the engine has no power?

I know there seems to be a selection of things that could affect SUs though I am trying to stick to what could have suddenly changed - after all I only took the gearbox out.  Going to check floats and dirt again just in case, and double check air leaks at manifolds.  Any other tips?  If not I think I may have to try and get the car to a tuning specialist in Sussex if I can find one.

 

Mark 

 

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Hi Mark,

I must have missed when you tried spraying brake cleaner or other inflammable mix around (under) the carbs, did it show up with an increase the revs as suggested ? There doesn't need to be much of a gap for the engine to suck air through, it's a mighty pump, often it isn't obvious when you strip the manifolds off where ...or if there is a gap between the manifold face and head.

Mick Richards

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As I mentioned in other post I spent 2weeks with a similar problem.Rebuilt carbs twice changed disy , cap , leads.     It turned out to be faulty plugs 3new sets before I changed to different make then fine

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Thank you all for your thoughts.  I am at the end of my tether now and am stuck between a rock and a hard place - cannot resolve it myself and cannot get it to a TR or tuning specialist.

I am content to strip back and build up again but it is knowing how far to go.  This problem started after me putting the gearbox back in the car, having played with nothing else to do with fuel or ignition.  So it seems plausible that an air leak was to blame.  But now I have had the inlet manifold off, changed all the gaskets, checked floats, confirmed piston movement and dashpot oil, and tried new fuel.  I have even gone back to 12 flats down on the needle jets.

Since my last post, I even tried no flats on a carb and I was still getting orange with colour tune - surely it should be whitish blue as too lean?  Another indicator was using a gunson carb balancer, which went off the float scale - so had to revert to hose pipe sound check.

Based on comments I now plan to:

1.  check all manifold nuts and spray carb cleaner during a run to see if there are any gaps.

2.  confirm points and plug gaps

3.  Ignore colour-tune and go by road usability and plug colour (when I get it running)

4.  consider taking everything engine side off, including exhaust manifold and then build back up.

5. lock the garage door and forget it for a while?

Thanks again.

Mark

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Mark - did you take the exhaust manifold off? If you did and it's a 4A, are you sure you didn't put the manifold gasket on 90 degrees out because the stud holes will allow you to do that but the holes in the gasket wont match the outlet hoes in the manifpold and end up being significantly obstructed which will cause all manner of issues

Cheers  Rich

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20 minutes ago, rcreweread said:

Mark - did you take the exhaust manifold off? If you did and it's a 4A, are you sure you didn't put the manifold gasket on 90 degrees out because the stud holes will allow you to do that but the holes in the gasket wont match the outlet hoes in the manifold and end up being significantly obstructed which will cause all manner of issues

Cheers  Rich

Hi Rich - No not yet, only the inlet manifold on my TR4.  Looks like I will have to take everything off and start gain.  Should have put up with leaky gearbox and nor had these carb issues hey?

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