How to Fix Your Carburetor

by Matthew McGuire, Member of the Capital Triumph Register

Back in January, while I was replacing a leaking water heater valve, I noticed a strong smell of gasoline. I soon discovered the fuel line going into the carburetor had a nasty crack, just behind the jubilee clip, and was slowly dripping. After replacing the fuel line, I took a closer look at the overall condition of the carburetor.

It was filthy and covered in oily grime. I didn’t remember it looking so bad. But the Spitfire’s MPGs seemed to be getting worse lately so, considering the fuel leak and the overall condition and age of the carb, I thought it was a good idea to replace the diaphragm, seals, etc., and give it a good cleaning. Those who are not skilled enough to clean or repair your vehicle’s carburetor should just bring your car to an auto repair shop and let an automotive repair technician do the necessary work.

I knew it was a Zenith-Stromberg but wasn’t sure of the model. After much online research and questions to various forums, I discovered I have a Z-S CD150 and that this was the standard carburetor for the Spitfire. The Z-S CD175 was used on the TR6, Jaguars, MGs, and others, usually in pairs, which explained why the vast majority of information online is about the CD175.

At the beginning of March, I was ready to tackle the carb project and ordered a rebuild kit. By now, I was thoroughly educated in the fine art of Carb Refurbishment thanks to YouTube, so I “confidently” set about my task at hand.

The carb came off easy enough and, armed with three cans of Carb Cleaner, I methodically disassembled everything, kept the various bits grouped separately, and took my time cleaning each and every part.

Once the carb was removed, I discovered it still had the factory original brass ID tag. It had gotten bent at some point and was hidden from plain view while in place. The zenithcarb.co.uk website was a source of good information, and I learned mine (#3612) was made sometime in 1973–74. My Spitfire 1500 rolled off the line in February 1976, but I understand it wasn’t unusual for older parts to still be on the production line.

Everything went back together pretty easily and even the damper had the appropriate “thunk” when gently lifted through the intake. It was now time to put it back on the car and see if there was any improvement. The car started up on the second turn of the key and it didn’t need assistance from the choke. However, it was idling at 1100 RPM and would not accelerate when I pressed down on the pedal.

I expected some adjustments would be necessary so, I opened the dash pot, inserted the new special tool, purchased just for the occasion, and began fine tuning the mixture. As I was making the adjustments, something caught my eye. Smoke. White Lucas smoke.


A nasty little wisp of smoke was seeping up through the demister vents on the crashpad. I shut off the car and as I removed the key from the ignition, the IGN & OIL trouble lights remained illuminated in the speedometer. I quickly disconnected the battery and thought to myself, “What the expletive?!”

It took some time and gymnastic maneuvers, but I finally found the source. In the section of the wiring loom that passes through the firewall into the engine bay, there was a small abrasion that had sawed through the loom tape and cut into four wires, causing the exposed metal threads to come into contact with each other. Apparently, the top of the accelerator pedal shaft, where the accelerator cable inserts into the tip, had been rubbing against the wiring loom.

With the carburetor tuning now on hold, I had to first sort out this new problem. To make it easier for myself to repair the wounded wires, I decided to disconnect everything inside the engine compartment and pull the wiring harness back through the firewall. This gave me more slack to work with and room to splice in some new wire. Three wires were nearly severed completely through while the fourth was only lightly scratched. I merely taped up that one. After heat-shrinking the repairs, I tapped it all up, rerouted the loom to the other side of the steering column, pulled it back thorough the firewall, and reconnected everything.

Here is where I am supposed to say, “I then got back to adjusting the carb and she’s now purring like a kitten.” But you know where this is going. As long as I’m this far in…

Along with pulling the wiring harness to make more room, I decided to remove the driver’s side glove box and instrument panels to provide more space as well. Besides, the windshield spritzers were loose, and I couldn’t get to those otherwise. Also, I had a refurbished crashpad I’ve been eager to install.

After disconnecting all the gauges, I started to remove the dashboard and I saw the metal speedometer cable was still attached to the gauge and sliding out of its plastic sheath. Great, now I needed to order a new speedo cable. That arrived a few days later and, with the transmission cover now removed, I disconnected the hollowed-out speedo cable and had a brainstorm. Now would be a good time to swap out those old rubber transmission mounts! Good thing I already had some.

Eventually, I replaced the transmission mounts, speedometer cable, put the trans cover back in place, reglued loose vinyl on all the cockpit trim (as long as it was out), installed the “new” crashpad, tightened the spritzers, repaired the wiring harness, and put the dashboard and trim back in place. Oh, I forgot to mention, while the carb was off the motor and waiting for a replacement part, I replaced the old motor mounts as well.

Now, I can finally get back to adjusting that carburetor. Is it just me or is it always something?



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