Twin carbs and a gas analyser

[100+]

So I have borrowed a gas analyser to tune twin strombergs on a Vitesse.

How do you know, given the exhaust pipe blends the output from each carb, which  carb is too rich or too lean?

Say you want a CO reading of 2.5%, could the rear carb be set rich (say 3%), and the front carb run lean (at 1.5%) and give you a 2% reading at the tail pipe, or does it not work like that?

Or do you set a datum the same on both carbs, then adjust them both by the same amount in order to arrive at the correct CO figure?

Also, does anyone know what sort of figure I should aim for, is 2.5 % ok?

Finally, I think I have hybrid carbs.  Engine is from a late GT6 mk3, with carbs with the yellowy plastic bit on the forward sides (CDSE are they?), but the jets are adjustable with a coin on the underside - not the stromberg adjuster tool which goes through the dashpot, so has someone retro-fitted CD150 jets at some point??

All help really appreciated, thanks! ;)

[Clive]

The gas analyser may not be the best way to tune them. Try this guide, OK its meant for SU's but the carbs are very similar and the strommies need you to lift the piston manually
http://www.mintylamb.co.uk/?page=sutune.htm

Otherwise I have used a colortune (2 are better, can do both carbs at once then) to get the mistures even. They sometimes need to be adjusted after, but then turn both jets the same amount.

And remember mixture is only one part of it, balance is just as important, as is getting the linkages right.
Then check that the gas ananlyser is happy!
Cheers
Clive

[Pete Lewis]

you can fit coin operated jets to the CDSE but make sure that the needle heights are the same on each damper which means you need either the slotted tool or a long allen key
depending on the needle holder vintage thats fitted and set the them flush with the damper bottom before you centralise and pre set the basic 2 turns down as per cd150  Pete

[G.in_Lux]

2.5%?
Good luck!

I reckon you'll be closer to 4%...

[100+]

Well I had a go today, set the points gap to 16 thou, timing to 6 BTDC and everything was going well. Then I let the analyser calibrate itself in still air for 10 mins and then whacked it in the exhaust pipe, whereupon the reading went off the scale! Way to rich, then.  Leaned it off a bit, and a bit more, until the jets were fully wound in.  Hmmm, still wouldn't budge off >10%co.  Paused, thought to myself I wonder if I've had a brain fart, so wound the jets to the full extreme in the other direction, and no difference.

So for me, the game was then to go back to piston lifting to get it about right.  Which went fine and she runs reasonably well throughout the range now.

I'll be getting a syncronometer to balance the carbs properly and 2 colortunes I think, which should sort me out.

By the way, the only problem now is that she runs on when you turn the engine off.  Other than carbon build-up, does anyone know what can cause this (thinking mixture here...)

Cheers, Russ

[Deleted User]

0r small adjustment in timing

[valencia1]

re run on, ign. timing, correct plug type, weak mixture?  run it for a while on current set up then see what combustion  colour your plugs are, also any pinking under load?

[Bob Bowling]

Running on like that can be caused by too weak a mixture. Had a Ford Fiesta which was set to run on a weak mixture and it needed an anti run on device - a valve which was held closed by the ignition circuit, as soon as you turned off it opened the valve and allowed air into the manifold to stop the engine.

[Paudman]

My Mk1 can be irritating like that - enrich the mixture, the plugs go sooty black but no run on; weaken the mixture as per Colortune, the plugs stay light brown but the car runs on for maybe 30 seconds after stopping.

[mikeyb]

490 wrote:

My Mk1 can be irritating like that - enrich the mixture, the plugs go sooty black but no run on; weaken the mixture as per Colortune, the plugs stay light brown but the car runs on for maybe 30 seconds after stopping.

strangely enough my mk3 is also similar - before and after rebuilt engine, before and after new carbs, before and after richer needles.  Needs a "richer than it should be" mixture to run nicely and not be sluggish/hesitant.  So much so that most of year round cold starts need no choke :-(

[GT6 M]

Is the engine speed no too high,

as a carb will let fuel thru, when engine has been turned off, but it is still causing vacuum,as it is ..slowing doon, = sucking fuel in,

higher the engine speed, the worse it is

also , if there are hot spots, or the comp is too high , it will diesel away too,

PI,s ..never run on, cos no fuel flow when ign is off

are your air filters ok, is the breather ok,  



Marcus