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CDS Rear Carb setting


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Hi.

I have an early 1971 Triumph GT6 Rotoflex Model Mk2 Engine fitted originally with CDSE carbs.

I have now fitted a pair of CDS carbs mounted on a Vitess 2 ltr manifold. I have done this in order to be able to tune the carbs a lot easier with the adjusters being below the carbs. I thought this would be okay however, I am having problems with trying to tune the rear carb with a colortune. I can just about turn the adjuster all the way up (anticlockwise) before it shows any sign of turning blue. In fact it has problems even showing a faint blue colour. This is from a starting point of 3 turns down as in the WM. I have swapped the pistons rear to front but same problem occurs. The carbs have been completely overhauled with everything i.e. new gaskets, jets, needles (6AC), etc. Pistons drop with a clunk. I have noticed that if i raise the piston pin then the colortune does turn blue. The timing is set to 13 deg BTDC (using 102 octane fuel) and the tappets to 10 thou. Plugs are BP6ES gapped at 35 thou as running accuspark electronic ignition. Floats are set to 17-18mm height.

Any thoughts advice please?

Regards, Ken

CDS.jpg

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Ken,

Are the carb to manifold gaskets on right? One way is correct, three ways to get it wrong, easily done (Don't ask :wub:) Did you change the head to manifold gasket? Any leak there will be troublesome.

102 Octane fuel?!! These engines were designed for 100, but these days only lower stuff is available and 13 degrees is too much, mine's set to around 8, I think. But I set it by ear turn the dizzy for max revs then back a gnats.

Doug 

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Hi Doug,

The carb to manifold gaskets on these CDS's are different than the CDSE's. They don't have the loop in them for the hole on the carb face. These can be fitted either way round. The hole on the carb face is covered by the carb to manifold block as it seems the hole does not do anything as it does on the CDSE carbs. If you get my drift.

The manifold to head gasket is new along with new studs and nuts etc.

We can get ARAL Ultimate 102 octane over the border here into Germany. We also have Euro 102 at certain pumps as well as Competition 102. 

i will try timing the engine as per your recommendation. Thanks.

Regards, Ken

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Is the rear carb happen to have the float upside down,  most cds have a twin armed float ,it will fit both ways up ends up with wrongnfuel heights

Is the alloy sealing washer under the jet top hat ?

Are both springs the same ??  And the diaphragm fitting rings the  same weight/thickness  .

Air box /gaskets is not blocking the air vents on the   front face

Any corrosion on the choke disc allows fuel bleed via the hose to the rear carb??

Pete

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Hi Pete,

Thanks for your advice.

I have checked the floats and they are correct, the alloy sealing washer is under the jet top hat and the air box gaskets etc are not blocking the air vents. I have had the choke disc apart and no corrosion present.

As far as the springs go, i have no idea whether they are the same as there are no markings or colour on them. How can i check this?

The diaphragm fitting rings would seem the same weight etc as when I swapped the pistons from the rear to front carb the problem remained on the rear carb however,  I did not swap the springs. What I don't understand is when I lift the piston pin on the side of the rear carb the mixture turns to blue, is this normal or would it indicate a problem?

I appreciate your help very much.

Ken

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Hi   do you use fixed needle air pistons or re use the ones rom the cdse  ??

Eg are the needles set to the same height in the piston ??

Simplest check on springs is put them side by side ...are they both similar ??

When you lift the pin how far ro get it blue ??

If you look down the throat with air box off at idle are both air pistons running a similar height from the jet bridge ???

Just thinking out the box for a clue 

Pete

 

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Hi Pete,

I am using fixed needle air pistons that came with the cds's (needle 6AC).

The needles are set to the same height in the pistons.

Will check the springs side by side.

If I lift the pin all the way then it turns blue.

I will have to check the air pistons running height at idle. The temp outside is -7deg C at the moment so may be a while before I can check this.

Will get back to you on this, Thanks.

Ken

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A quick check is remove the top cover

A look down the air piston, are the two  holes in its bottom   either side of the needle facing the , manifold side

You can get real odd running if the diaphragm location lugs are off position in the piston and its location in the body.

Pete

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Hi,

Eventually managed to get into the garage today after being ill for a week. I have checked down the air pistons and found that the two holes are either side of the needle facing the manifold side. I have removed the springs and put them on the bench to check whether they are equal or not. (please see attached photo's)(not sure whether equal or not)?

I have started the engine and let it idle for a while whilst checking the piston movement up and down. They look as though they are just a little bit above the bridge. When using the throttle it looks as though the rear one moves first and moves easier than the front. Is this an indication of a spring problem. I have not yet taken the springs from the cdse's and put them into the cds's to see whether that makes a difference. Does a weak or stronger spring cause a rich mixture or does this not have any influence on this problem at all?

Cheers, Ken

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DSCF0047.JPG

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Just a point when the carbs are re-assembled have you centred the jets as per the workshop manual. If not you may have a sticking needle stopping the piston from reaching the bridge. i.e.  Holding the piston up.

Looking at the springs I doubt they are the cause of the problem.  You could try swapping them around and see if the problem follows them.

Dave

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Yes the springs look even enough  its  not them

Some diaphragms are too thick , they should be thin  gossamer type of flimsy rubber,, thick ones jam up 

Daves idea about centralise the jet is worth a check , we can spell,the method out if needed

An improvement in fuel atomisation from the jet would be to use the biased needles and pistons from the orig cdse

Set  the small delrin needle  washer flush with piston base , use underside jet adjustment to tune . It works well.

Were there any strange problems with the cdse before removing ???

Pete

 

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Hi,

Thanks for inputs, I have centred the jets many times to make sure the pistons drop onto the bridge and this seems to be happening. I am not sure whether the diaphragms that I have are too thick as they came with the kit I ordered to refurbish the carbs from Burlen. Is there anywhere that is known that produces the correct thickness of diaphragm?

I will try with the pistons from the cdse's. I will set the needle flush with the piston bottom. Do I need to use the same procedure for centralizing the jets or is that not necessary with biased needles? The cdse's were okay although did tend to suffer from flooding now and again. I was looking for something simpler therefore I went for these cds's.

Cheers, Ken

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Biased needles rub on the side of the jets, that's their advantage, no more fiddling about centralizing. I'd have stuck with the cdses and sorted out the flooding problem, what could it be? Rubber slivers caught in the valve, floats damaged or incorrectly set? Something simples?

Doug

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Hi,

I have today put in the cdse air pistons and springs making sure that the delrin washer is as flush as possible with the bottom of the pistons. The pistons drop onto the bridge with a light clunk. I have wound down the jets 3 complete turns.

Everything else checks out as diaphragms are practically new as is everything else with these carbs.

The car started after 3 short pulls and is running well. I have warmed the engine up and adjusted the throttle screws down to a nice idle of about 850-950 rpm. Both pistons are showing that they are off the bridge and when revved the pistons move equally.

Have not had time to check with the colourtune but that is a job for Monday although it looks as though I am getting somewhere now.

Once I get the fuelling sorted then I would like to get the timing as spot on as possible. Would someone be able to outline the procedure necessary in order to time via the dizzy. I have never fully understood this procedure and would be very grateful for any info in order that I can achieve this.

Thanks very much, Rgds, Ken

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Without any test lamp or strobe then its use your ears to start with

Set the points gap to 0.015" lucas  or 0.018" for delco

Run the engine up to temperature

Undo the dizzy base clamp bolt so you can turn the whole dizzy

Raise the ide to a round 1200 rpm  ie up a bit from idle

Turn the whole dizzy , clock wise advances  anti clock retards

Turn it slowly to get the fastest running position , then back this off to reduce the best by around 100 rpm 

Nip the clamp 

Take for a run , pick a slight inclined road and accelerate fromm25 to 40 mph in top gear

There should be no pinking  ( noise like marbles in tin can) 

If it pinks back of a little more until till acceleration is firm but no pinks

If the first drive does not produce pinking you can rotate clockwise untill it does , then back it off a bit to reduce the pinks

This takes longer to describe than doing the job

Best to always use 97+ron fuel , .

If you have a small bulb on a test lead we can explain  how to use this to check static timing let us know

Pete

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  • 1 month later...

Hi,

I'm interested to know whether a solution was found to problematic rear carb.

I have a similar situation which, in brief, is:

GT6 mk3, 2.5l engine, CDSE150 carbs from a triumph 2000. Timing set at 4 degree ATDC on strobe. Point at 40 degrees dwell on meter. Tappet clearances set when engine rebuilt but not adjusted subsequently. AIr flow on carbs balanced on meter at idle.

Current issue: Engine starts ok on choke. But with no enrichment (choke 'off') very unstable and rough idle. Idles at 1000rpm (ish) but then will do a burst of a few seconds at 2000rpm then drop down.

To investigate I put a ColourTune in cylinders no.1 and no.6. No.1 shows a good blue/yellow combustion. No.6 six  shows a very faint and very blue burn but will periodically 'flare up' to give a strong very yellow burn with fast, 2000rpm idle, for those few seconds.

My impression is that the rear carb is drawing air ok but is fuel starved but provides a burst of fuel intermittently.

I'd very much appreciate any wisdom or experience in how to proceed further in investigation and remedy.

Cheers

Chris

Beds/Herts. Spitfire 1500. GT6 Mk3.

 

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Chris,

The timing you quote is for an emission controlled car. I believe that for UK cars you should be aiming for a idle strobe setting of 10 to 13 degree BTDC. In my experience most carb problems lie in the ignition system 🙂, so altering your timing may well sweeten things up.

Wayne

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sounds more like the supply is obstructed , might be the dreaded rubber slivers in the float needle

speed increases as the pump supply pressures the feed.

and 4deg ATDC is way off the spec ,   Waynes spot on .

do check the air filter /gaskets are not covering the ports in the carb front face 

Pete

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Thank you for the pointers. Advice about the timing duly noted. I'll swing it back to, say, 12 degrees BTDC (static, not strobe) and leave it there for the time being.

My plan of investigation for tomorrow is examine the gaskets, float needle and float. Let's see if that leads anywhere.

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