Cookie's Blog: GT6 Carb mixture issue solved

Over the last few years I've not been able to set the mixture properly on the rear carburettor on my GT6. Even on it's leanest mixture setting the mixture has been too rich on the rear although the front carb was fine to adjust. It's all a bit strange as I replaced the carbs with new ones back in the early 90s and had no issues adjusting the mixture properly on the rear cab until a few years back.

I changed the spark plugs recently and the rear 3 cylinders had pretty sooty carbs soI decided it was time to resolve the issue. Especially as I am doing the Club Triumph Round Britain Run in 3 weeks time and I want the car to use as low a fuel as possible over the 2,000 mile run as the petrol cost will be several hundred £!

The GT6 is fitted with Zenith Stromberg CD150CDSE carbs. Unlike the early GT6s which have adjustable jets to set the mixture by raising and lowering the jet from under the float chamber, the CDSE carbs have a fixed jet and the mixture is adjusted by raising and lowering the metering needle. This is done by inserting a tool with a long Allen type key into the top of the carb after removing the damper, the hex key fits into a hex socket screw to lower or raise the jet, the tool also has a spigot with a pin to engage in the carb piston to prevent it from rotating which could damage the diaphragm. The jet is raised to richen the mixture by turning the Allen screw clockwise or lowered to weaken the mixture by turning the screw anti clockwise. My rear carb was still too rich with the key rotated fully anti clockwise and the needle in it's lowest setting.

Stromberg CDSE mixture adjustment tool

The fixed jets are an interference fit pressed into the carburettor bodies, the top of the jet sits a little way below the bridge in the carb tract. I decided to check the height of the fixed jets in both the front and rear carbs by removing the carburettor pistons and using the depth gauge on my digital Vernier calliper. To my surprise the rear carb jet was 116.5 thou below the bridge whereas the front carb jet was only 78 thou.

Checking the jet depth below the bridge with a digital vernier calliper

Rear Carb Jet depth was 116.5 thou

Front Carb Jet depth was 78 thou

I can't actually find anywhere that states the correct setting, even the factory manual doesn't have that information and although I found information on some other marques of cars with Strombergs the setting differed between cars. I therefore decided that as it was possible to adjust the front carb to a correct mixture that the jet in the rear carb was obviously too low.

I removed the float chamber and float from under the rear carb and used a nylon faced Thor hammer to tap the jet upwards. It took a fair bit of gentle tapping but I ended up with the rear jet 80 thou below the bridge which was close enough to the front carb setting.

Tapping the rear jet up using a Nylon faced Thor hammer

After reassembling the carbs, starting the car and getting the engine up to temperature I adjusted the mixture on both carbs using my trusty Gunsons Colourtune and it was possible to get the correct mixture on both carbs. I then balanced the carbs using a Webber Synchrometer to set a balanced tickover. I had already adjusted the tappets and checked the timing in the last few days before tuning the carbs.