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SU CARB SET UP


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Hi all,

The carbs (SU's) on my 4a need to be set up as the plugs are black and sooty, the car has been laid up for some time and rather than take it to the local Classic car tuner and pay a fortune I would like to try adjusting them myself.

I have checked the w/m and u-tube but they all seem to have different ideas on what to do.

All I would like is someone to point me in the right direction to an article or video that explains the best method from scratch to use.

 

I have dumped the petrol and replaced with fresh, I noticed the old petrol about 2 years or so had an awful smell about it and looked very dark in colour.( I put it in a modern fiesta)

 

Thanks in anticipation.

 

Note the ignition is electronic so should be ok.

Sorry if this has been done before but I believe it would be very helpful to a lot of owners who would like to do this job themselves.

 

regards Lee

 

 

 

 

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This is a helpful article, although not TR specific it covers the HS type SU carb

http://www.mintylamb.co.uk/?page=sutune.htm

What about other things like tappet clearance and engine ignition timing?

Peter W

 

PS what state are the air cleaners in? Dirty clogged filters will affect the running and mixture as they restrict airflow.

PPS As the article writer says 'take it for a drive'

  1. Go for a hearty drive to get the car to operating temperature.

    Go on! Drive it like you stole it!

Edited by BlueTR3A-5EKT
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As Peter has said you need to take it out and Drive it hard for a while first. It needs what we used to call an"Italian service" i.e. drive it hard at high revs until you have used about half a tank of fresh fuel and then see what it runs like.

You will be surprised what a difference just doing that will make.

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Thanks guys, just fitted the new wheels and vibration now at a minimum for once, after giving it an 'Italian service' with fresh petrol.

 

Still a little vibration but think this may be propshaft.

 

Will start on the carbs tomorrow,

 

cheers Stuart and Peter,

 

Must say it was fun, even had my photo taken by a nice yellow camera

 

Regards Lee

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This is a helpful article, although not TR specific it covers the HS type SU carb

http://www.mintylamb.co.uk/?page=sutune.htm

 

Except for the error on first page:

"If you are unsure of what oil to use, just use engine oil until you know any different - or can judge what to use. A thinner THICKER oil will richen the mixture on acceleration".

Peter

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How?

Thicker oil reduces vacuum created piston lift therefore reduces effective nozzle size of Main jet? Therefore thicker oil will weaken mixture?

No?

 

Iain

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Because the throttle plate will be more open than at cruise, possibly even wot if you're treading it. That means a higher depression behind the slide and therefore more fuel pulled through the jet before the damper allows the slide to rise and reduce the depression back to cruise value.

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Hi Iain,

just what Pete said but to see it another way-

 

When you open the butterfly the area on the valve side of the piston gets more depression. This will suck more fuel out of the main jet.

And so richen the mixture.

The piston will rise with this depression and bring the mixture back to normal - however a thicker oil in the dash pot will slow the piston rise

thus richen it for longer.

 

The depression WILL richen it. The thickness of the dash pot oil varies the time constant for how long it is rich.

 

Roger

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How?

Thicker oil reduces vacuum created piston lift therefore reduces effective nozzle size of Main jet? Therefore thicker oil will weaken mixture?

No?

 

Iain

 

Iain,

Thanks for the question, it helps me in preparing the IWE talk. This is the sequence of events:

Upon suddenly opening the butterfly, for a brief moment the piston acts to block the air flow released past the now-open butterfly. The transfer port passes this unusually large depression to the suction chamber, and this tries to lift the piston. The damper resists this piston rise and the depression, which is greater than the 'constant depression', accelerates fuel flow out of the jet, enrichening the mixture. This is despite the needle not having moved out of the jet much at all. After ca 1-2 seconds the piston has reached its new equiliribum position and the normal 'cosntant depression' is reached and the damper has not longer any effect. So Thererefore a thicker oil will slow down the rise of the piston and give a bigger transient depression and hence a richer mixture, and for longer.

 

 

I'm hoping the explanations will be easier when I can wave my hands about at Malvern. There is a lot more to SUs than meets the eye.....I used to think they are simple.

Peter

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