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AlfredHitchcock

Recommissioning fears

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Hi peeps

 

After more than 2 years slumber I woke the beast last weekend.

 

Fresh petrol, charged up battery and refilled coolant. Plugs out, spun up oil pressure, plugs back in and primed fuel bowls.

 

Turned the key and engine fired into life instantly.

 

Now the thing is that I guessed at the ignition timing and had the H6 carbs mixture mid position (12 flats down) after rebuilding both and it ran quite nicely. But it was a bit smokey when revved up.

 

So I thought I would read up on basic timing and made myself a test light to do the basic set up.

 

Following instructions I put my No 1 piston at tdc and the advance/retard adjuster on the side of my dizzy mid position and rotated dizzy anti-clockwise so the test light was just coming on at the leading edge of the cam on the dizzy. Then dialled in 4 degrees advance.

 

First thing was the amount the dizzy had to be turned to get it to correct position. It was MILES OUT!! ~I reckon 15 degrees advanced.

 

Restarted the engine and it fired up instantly again and I noticed 2 things. 1) It had stopped smoking and seemed to idle better and 2) It seemed to be getting hot a lot quicker (I have an electric fan so no engine fan) than before and the fan cut in after a very short time, maybe 5 minutes.

 

Unfortunately my temp gauge has stopped working so I couldnt see how hot it was but it didnt get overhot and the fan seemed to cope.

 

I then set up the mixture with a colourtune and found the front carb very lean but the back carb ok.

 

Now I'm a bit paranoid about this because I'm sure I holed a piston years ago running very lean or badly tuned so although it seems to be OK I welcome advice from more knowledgeable peeps on the above and or the next step.

 

Cheers

 

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With modern less volatile fuels you will need about 12 degrees advance at tickover and 32 degrees all in (10 degree cam in distributor gives a 20 degree span at the crank)

Edited by Drewmotty

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Simply means when it has advanced as far as it can with increasing revs. There is a limit stop inside the distributor which prevents the ignition from advancing (from the static or idle setting) more than a fixed amount.

So while watching the strobe, gradually increase the revs until the ignition does not advance any further, & do this with the vacuum advance pipe disconnected. It should end up at around 30 - 35 ° BTDC

 

Bob.

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I then set up the mixture with a colourtune and found the front carb very lean but the back carb ok.

 

 

Good idea to find out why the imbalance in the carbs. Ther's always a degree of "charge" robbing with two carbs and a balance pipe but if it was me I'd like them there or about each.

Strip down the carb covers and remove the piston and check out the height of the Jet down from the carb bridge, ( depth micrometer or even graduated slide rule( the bit on the end) they should be about the same. If they are then the mixture imbalance is being caused between the jet and the inlet manifold, remove and check both needles are the same profile and fitted correctly in the piston, refit the carb pistons and lift and let them drop to ensure each falls onto the carb bridge with a soft "click" as it touches down ensuring there is no interference between needle and jet or the piston and bell cover.

If all this is ok check that there isn't an air leak in the manifold, normally at the end where the weak mixture is and often caused by the manifold fouling upon a nut or head face preventing it sealing properly against the head, the over rich carb putting in enough fuel to compensate to a degree in static running.

 

Try using this carb compendium site it has lots of excellent original information on it including the SUs

https://sucarb.co.uk/technical-h-type-carburetter-tuning-multi

 

Mick Richards

Edited by Motorsport Mickey

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Thanks for your help.

 

Mick when I rebuilt the carbs I couldnt get the needles out and decided that they didnt look to have any wear so left them as they were. They were stuck fast and I had visions of snapping them off. They went back in as they were.

 

How are the held in?

 

I dont get a soft click after lifting them and they are quite stiff to lift up against the damper, but always fall without friction.

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Thanks for your help.

 

Mick when I rebuilt the carbs I couldnt get the needles out and decided that they didnt look to have any wear so left them as they were. They were stuck fast and I had visions of snapping them off. They went back in as they were.

 

How are the held in?

 

I dont get a soft click after lifting them and they are quite stiff to lift up against the damper, but always fall without friction.

 

Remove the damper first before dropping to prevent suction stopping the fall.

 

The needles are secured by a screw which is accessed through the side of the piston, ( check out the diagram on the SU publication) undo the screw and see if the needles are then loose, if not gently tap the needles onwards to loosen them and then draw them out.

Don't mix them up between the carbs, then check for any marking on them for comparison...but don't trust it. It's not uncommon for the needles to have been altered by rubbing with wet and dry paper to change the profile. Draw a little diagram and measure down the needle length about every 2 or 3 mm with a micrometer or vernier, make a note of the dimensions and compare the figures against each other, they should be about the same.

Are both Jets the same way down on both carbs ?

 

Try the piston drop without needles in, if they fall easily and you can hear the click as they hit the bridge that's your datum, then add the needles and again check the drop and the click. If the needles are bent against the jet they'll stop the drop onto the bridge, they should be able to be straightened if needed.

There's good information in this publications drawings and procedures, it should help you tune and rebuild the SUs.

 

Mick Richards

Edited by Motorsport Mickey

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Remove the damper first before dropping to prevent suction stopping the fall.

The needles are secured by a screw which is accessed through the side of the piston, ( check out the diagram on the SU publication) it's item 4.

 

https://sucarb.co.uk/mediafiles/su/tech_su_07b.jpg

 

undo the screw and see if the needles are then loose, if not gently tap the needles onwards to loosen them and then draw them out.

Don't mix them up between the carbs, then check for any marking on them for comparison...but don't trust it. It's not uncommon for the needles to have been altered by rubbing with wet and dry paper to change the profile. Draw a little diagram and measure down the needle length about every 2 or 3 mm with a micrometer or vernier, make a note of the dimensions and compare the figures against each other, they should be about the same.

Are both Jets the same way down on both carbs ?

Try the piston drop without needles in, if they fall easily and you can hear the click as they hit the bridge that's your datum, then add the needles and again check the drop and the click. If the needles are bent against the jet they'll stop the drop onto the bridge, they should be able to be straightened if needed.

There's good information in this publications drawings and procedures, it should help you tune and rebuild the SUs.

Mick Richards

Edited by Motorsport Mickey

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