Monday, 27 May 2013

Tuning The Carbs

I know a few Triumph owners who shy away from tuning the carbs on their cars but really it's not rocket science if you have the right tools. Thought I'd put a blog article up explain how to tune the Stromberg carbs as fitted to the GT6 and many other Triumphs as it may help a few folks take the plunge an tune their own carbs.

Firstly, (or should I say lastly), always leave tuning the carbs until last. Set up the tappet clearances, check the plugs gap, if you have points set up the points gap (not necessary with electronic ignition), finally set the ignition timing with a strobe.

Next here are my recommended tuning tools, a Gunson's colourtune for setting the mixture (available for about £25 on amazon or ebay) , a webber syncrometer for balancing the carbs (about £36 on ebay) and if you have the later type CDSEV Strombergs the essential mixture adjusting tool (about £10 from Triumph Specialists).

Get the car up to normal operating temperature by taking it out for a run or leaving it idling for a while.

Then it's time to set the mixture. The Colourtune is a sparkplug with a see through section that allows you to see the explosions in the combustion chamber. When the mixture is set right the explosion burns blue in colour, if it's too rich then it burns Orange. The trick is to set the mixture up so it's just lean enough to burn blue. Screw in the colourtune plug in place of no 6 spark plug. Fit the shroud assembly with the little mirror and the HT extension and plug in the HT lead. If you have CDSEV carbs take the damper out of the carb top of the rear carb.
Colourtune installed (this is in No 6 cylinder)

Next start the engine and put the adjuster tool down the damper hole, make sure the outer part is engaged in the slot of the damper tube. Look into the colourtune reflection in the mirror and see the colour of the explosion, if it's orange then you are ruining rich which is where we need to start. If not turn the adjuster allen key clockwise until the mixture richens and the explosions are orange. Then turn the adjuster anti clockwise carefully until the explosions are just in the blue mixture range. If you have the earlier type of carbs the adjuster is a hex adjuster under the float chamber. Anti clockwise for richer mixture, clockwise to weaken.

Orange Explosion visible in the reflection of the colourtune in the little mirror.

Blue Explosion visible, mixture set correctly.

Using the adjuster tool on CDSEV carbs.

Next repeat the above with the colourtune in number 1 plug position and adjustments on the front carb.

Afterwards ensure that the damper pots are topped up with oil, I use 20-50 engine oil, some people prefer thinner oil such as 3 in one. Replace the damper pistons.

Topping up the carb damper oil

Now it's time to balance the carbs and set the idle. Undo the clamp on the linkage on one of the carbs so that the carbs operate independently.

Slacken one of the linkage clamps

 Remove the air filters. Start the engine and measure the airflow using the Synchrometer.

Measuring the airflow using the Syncrometer

Adjust the throttle stop screws of each carb until the same reading is obtained.

Adjusting a throttle stop screw to set the balance/idle speed.

Then increase or decrease the idle speed using the throttle stop screws a bit at a time on each carb rechecking the balance as you go until you get the minimum tickover speed  where the engine still ticks over smoothly. The manual says 600 - 650 rpm but I normally set it between 800 and 1000. Rev both carbs independently and recheck adjusting as necessary. Then when happy with the adjustment re-tighten the linkage screw.

Replace the air filters and that's it you are done. Take the car for a test drive. If you've done the job properly the car will run like a dream! However, if it has a flat spot or hesitation when accelerating then the mixture may be too weak so repeat the mixture adjustment stage with the colourtune and recheck the balance/idle mixture.

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