If you've spent any time under the hood of a classic British or European car, you've probably had a few choice words for the 175cd stromberg at one point or another. It's one of those components that defines the vintage driving experience—equal parts brilliant engineering and absolute mechanical frustration. Most of us know these carbs from old Triumphs, Volvos, Saabs, and even some Jaguars. They were the "constant depression" alternative to the more famous SU carburetors, and while they look similar from a distance, they have their own specific set of quirks that can either make your engine sing or leave you idling roughly at a stoplight.
The Zenith-Stromberg 175 CD is a side-draft carburetor that relies on a vacuum-operated piston to regulate fuel and air flow. When it's dialed in, the throttle response is actually quite smooth. It doesn't have the sudden "gulp" of air that some fixed-jet carbs have. Instead, it feels linear. But getting it to that point? That's where the fun—and the greasy knuckles—really begin.
The Heart of the Matter: The Diaphragm
If your car starts running like it's had a heavy night out—stumbling under load, refusing to rev, or just feeling generally sluggish—the first thing you should check on a 175cd stromberg is the rubber diaphragm. Unlike the SU, which uses a precision-fit metal piston, the Stromberg uses a flexible rubber diaphragm to help lift the air slide.
These pieces of rubber live a hard life. They're constantly flexing and sitting in a bath of fuel vapors and heat. Eventually, they develop tiny pinholes or a full-on tear. When that happens, the vacuum can't lift the piston properly, the needle doesn't move where it should, and your air-fuel mixture goes completely sideways. Replacing it isn't a massive job, but you have to be careful. If you don't seat the tab correctly in the groove, you'll have a vacuum leak before you even finish tightening the screws. It's a cheap part, so if you're even slightly suspicious of it, just swap it out.
The Mystery of the Damper Oil
We've all been there: you unscrew the plastic cap on top of the carb and peer into the little well of oil. Maybe it's dry, or maybe it's filled with something that looks like old mud. This little reservoir is crucial for the "acceleration" phase of your driving.
The oil provides resistance to the piston. When you stomp on the gas pedal, you don't want that piston to shoot up instantly; if it did, the air would rush in too fast, the fuel wouldn't catch up, and the engine would lean out and stumble. The oil slows that movement down just enough to keep the mixture rich while you're picking up speed.
What oil should you use? Ask five different mechanics and you'll get six different answers. Some swear by straight 20-weight dashpot oil. Others tell you that standard 10W-30 motor oil is fine, while the "old timers" might suggest ATF (Automatic Transmission Fluid) because of its cleaning properties and consistent viscosity. Personally, I've found that lighter oil makes the car feel more "zippy" but can cause a bit of a flat spot if the weather is hot.
Dealing with the Temperature Compensator
One of the more annoying bits on the side of the 175cd stromberg is that little plastic-covered rectangular box known as the temperature compensator. Its job is to lean out the mixture when the engine gets hot to prevent a rough idle. Inside, there's a bimetallic strip that bends as it heats up, opening a small air bypass.
The problem is that these things are notorious for failing or getting stuck. When they go bad, you'll find yourself with a mysterious vacuum leak that only shows up once the car has warmed up. Many people choose to "block them off" with a blanking plate, but if you want the car to run exactly as the engineers intended (especially if you live in a climate with big temperature swings), it's worth the effort to pull it apart, clean the gunk out, and calibrate the strip. You can actually test it in a cup of hot water with a thermometer, which is a great way to spend a Saturday afternoon if you're a bit of a nerd about these things.
Tuning Without Losing Your Mind
Adjusting the 175cd stromberg is where most people get intimidated. Depending on which version you have, the adjustment might be a simple screw on the bottom, or it might require a special long-reach tool that goes down through the top of the dashpot.
The later "emissions" versions are the ones that usually cause the headaches because they weren't really designed to be easily tinkered with. You have to use that special tool to turn the jet carrier itself. A tiny turn goes a long way here. I usually tell people to start by getting the car fully up to operating temperature. If you try to tune a cold Stromberg, you're just chasing your tail.
Once it's warm, listen to the exhaust. Is it spitting? Probably too lean. Is it puffing black smoke or smelling like a refinery? Too rich. The "lift the piston" trick works here just like it does on SUs. If you lift the piston about a sixteenth of an inch and the revs rise slightly and then settle, you're in the sweet spot. If the engine dies, you're way too lean.
Vacuum Leaks: The Silent Killer
The 175cd stromberg is incredibly sensitive to vacuum leaks. Since the whole operation of the needle and piston depends on the pressure differential, any "extra" air getting into the system throws the whole thing out of whack.
Common spots for leaks include the throttle shaft bushings. Over decades of use, the metal shaft wears down the holes in the carb body. You can spray a little carb cleaner around the ends of the shaft while the engine is idling; if the RPMs jump, you've found your leak. Fixing this usually involves re-bushing the carb, which is a bit more involved than a simple driveway tune-up, but it's the only way to get a rock-solid idle if the wear is bad enough.
Also, don't forget the gaskets between the carb and the intake manifold. These often get brittle and crack. It's such a simple fix, yet so many people overlook it while they're busy messing with the jet settings.
Why Bother with the Stromberg?
With all these potential issues, you might wonder why people don't just swap them out for a modern EFI system or a Weber conversion. There's a certain charm to the 175cd stromberg, though. When they are set up right, they provide a very smooth, "gentlemanly" driving experience. They don't have the harshness of some aftermarket setups.
Plus, there is a real sense of satisfaction in taking a 40-year-old piece of alloy and rubber, cleaning it up, and hearing that engine purr perfectly. It's about the connection to the machine. You aren't just a driver; you're the one keeping that mechanical heartbeat steady.
If you're struggling with yours, don't give up. Clean the components, check that diaphragm for the hundredth time, make sure your oil level is consistent, and take your time with the adjustments. These carbs were built to be serviced, not thrown away. Once you understand the rhythm of how the air moves through that throat and how the needle responds to your right foot, you'll see why they were the standard for so many great cars for so many years. Just keep a spare diaphragm in the glovebox—it's cheap insurance for the road ahead.