Fortify Your 1500: Improving 1500 Spitfire and Midget Engines

[Editor's Note: This article originally ran in the March 2013 issue of Classic Motorsports. Some information and prices may be different today.]

Story by Tim Suddard • Photography as Credited

While often lamented, Triumph’s 1500cc, four-cylinder engine is still owned and loved by thousands of enthusiasts. Almost 100,000 of these engines were used in the 1973-’80 Spitfires, and nearly as many were used in MG Midgets sold from 1975 and later.

Right off the bat, this larger engine delivered more torque than its predecessor. More torque, as we know, helps produce better acceleration numbers. 

But there were some problems. Detractors complained of durability and drivability issues.

These engines were known to be delicate, especially if over-revved. The three-main-bearing design did not lend itself well to hard use. Thrust washers needed to be maintained, or crank walk would destroy them as well.

The biggest problem with these engines, however, had to be the single, emissions-era Zenith Stromberg CVcarburetor. To save money and meet the ever-stricter U.S. emissions standards, American-market versions of this 1500 were fed by just one carb, not the dual SU setup found on earlier Spitfires. 

While adding main bearings and redesigning thrust washers is not an easy feat, at least one of the engine’s problems can be addressed: without much headache better carburetion. 

Some enthusiasts opt for a Weber DGV down-draft or single side-draft 40 DCOE setup. These are both good solutions, but we went with an even easier fix on our 1973 Spitfire. 

In England, Triumph Spitfire 1500s were sold with dual SU carbs. A nifty-looking, dual-inlet air cleaner wrapped in a black crackle finish was part of the package. The factory paired these preferred carburetors with a free-flowing, cast-iron exhaust manifold that emptied into twin pipes. These SU carburetors may be older technology, but they have served many owners very well through the decades. They’ve even won countless SCCA championships. 

For about $400, we sourced a set of these carbs, the proper intake manifold, a factory European exhaust manifold and a downpipe from Quantum Mechanics. This firm finds these parts overseas and imports them for American enthusiasts. Because these parts are designed by Triumph to fit the cars, the swap is a snap and the results are nothing short of remarkable.

Starting Simple


The first step is buying the right parts. The key here is getting the entire setup, including the linkages. Ideally, you’d also get carburetors that aren’t worn out. Remember, these parts may be nearing their 40th birthday, so go with a reputable dealer.

In our case, Quantum Mechanics sold us a brand-new downpipe to match the European exhaust manifold. Because our carburetors didn’t exhibit any throttle shaft wear, we didn’t need to have them rebuilt. To truly rebuild SU carbs, you must send them to an expert; replacing those throttle shaft bushings requires specialized equipment. 

Our carbs did need to be freshened, though, and we did that ourselves. We also redid the air cleaner with the original black crackle finish.

As always, once we test fitted the manifold, we sent it to Swain Tech Coatings to be coated. Every time we use this miraculous product, we’re impressed with how it knocks down exhaust temperature and holds up for years, looking like new for a long, long time.

Not-So-Complex Carbs


Freshening a carburetor is not as scary as it sounds, and companies like Quantum Mechanics supply the necessary kits. The process is mostly a function of carefully cleaning, disassembling and reassembling each carburetor while installing new gaskets. The idea is to remove all the accumulated junk, especially from critical areas like the float bowl and jets.

A shop manual is a good idea, but we have a little cheat: Just disassemble one unit at a time, and use the other as a 3-D shop manual of sorts. 

Make sure to clean and lubricate your throttle linkage, too. You don’t want anything to be bent or stuck, as this will affect your car’s operation.

Going Continental


After refreshing the carburetors, the next step is simply to replace the intake and exhaust manifolds with the European-spec pieces. The exhaust manifold goes on first, underneath the intake manifold. Use new gaskets and properly clean all mating surfaces. Bolting on the new carburetors is easy, and since this is a factory setup, the throttle cable will perfectly join the existing system. 

The American exhaust system from the downpipe back will need to be replaced, however, as it won’t mate to the European-spec downpipe. For about $200, we had a local muffler shop fabricate a new system. 

Don’t let this step scare you. The Spitfire’s exhaust system is epically simple, and any competent shop can build you a nice system. To avoid offsetting the soon-to-be-realized power gains, make sure the new system is not too restrictive.

Numbers Game


Theory is great, but how much does this swap really help performance? Before the swap—and with the original single-carb unit properly tuned—our Spitfire produced 49 horsepower and 66 lb.-ft. of torque at the rear wheels, as measured on a chassis dyno.

Remember, these cars were optimistically rated at about 70 horsepower at the crank. By the time you factor in a driveline loss of 15 to 20 percent, our low horsepower reading looks pretty accurate—maybe even a bit impressive. 

Our initial zero-to-60 testing was just as depressing. This measured sprint took us an alarming 14.7 seconds to complete—not very sporting for a sports car.

Perhaps our biggest issue was simply the original setup’s drivability. Like so many other emissions-era carburetors, ours caused the car to surge and run lean—especially at highway speeds. 

After this simple swap, our Spitfire was transformed. The leaning and surging had vanished. Measured engine output had improved nicely: We posted 65 horsepower and 78 lb.-ft. of torque during our next visit to the dyno. an improvement of nearly 25 percent. And real-world performance? Our zero-to-60 times dropped by more than 2 seconds. 

If there’s any drawback, it’s the legal ramifications. As always, check your local emissions laws. In some states, this kind of modification is a no-no. In others, these cars are too old to require routine inspections. If you’ve got the okay from Johnny Law, a straightforward swap to dual SU carburetors can net you a more fun and agile driving machine.

What's The Damage?

The cost for this setup is only about $400 to $500, depending on the source and condition of the parts. Budget a day to do the work, plus the cost of modifying your exhaust system, the carb-freshening kits and the paint and cleaning supplies. All in, you could easily complete this job for about $700 if you do the work yourself.

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View comments on the CMS forums
TR8owner HalfDork
7/1/18 4:24 p.m.

I was always much more partial to the 1296cc Spitfire engine than the 1500. Maybe because its the one I raced back in the day, but it always just seemed better balanced than the 1500 for some reason. As for the MG Midget, I preferred the 1275. Had one on the street will all the competition Cooper S parts. It went like stink. Always seemed like a sacrilege putting a Spitfire engine into a Midget.

clshore New Reader
7/13/18 8:59 a.m.

In reply to TR8owner :

The 1300 Spitfire and MG motors shared quite a lot; iron block, 3 main bearing crankshaft, similar bore & stroke, similar cylinder heads, similar carbs, similar power.

But the US emissions regs sucked away both torque and power. A larger motor was needed.

The MG motor was incapable of being enlarged, just getting it to 1275 had required engineering handsprings (offset conrods? Really?).

But the Spitfire could be easily stroked to 1493 cc to regain some performance.

Given that MG and Triumph were both owned and built by BLMC, choosing the Spitfire motor for both was a no brainer. At least the MG stayed 'all British'.

3/25/20 1:49 p.m.

I did the SU conversion source from Quantum Mechanics along with a 5 speed transmission also sourced from Quantum.  It has transformed the Midget into a nice little cruiser.  I tried the Weber DGV for a while but the SU's are more responsive plus I get better gas mileage ( the 5 speed helps).   I replaced the original exhaust header with a Pacesetter which increased the pipe diameter to about 2" but have learned there are better ones available.  I was not aware of the header from the UK.  I also experienced the thrust washer failure which ruined the crank so my advice is to keep your foot off the clutch as much as possible.  All in all, even though there are more desirable sports cars out there the 1500 Midget has been a fun car to have owned for the past 18 years.

murphmi New Reader
9/1/21 9:28 p.m.

I remember reading a brief article in Road & Track back in the 70s that with the two SU setup, there's a problem that at higher RPM the mechanical fuel pump doesn't deliver enough gas, causing fuel starvation. The solution is an electric fuel pump in the trunk, and bypass the mechanical pump. But not just any pump--SU carb float valves will allow gas by if it's under too much pressure, so you need a low-pressure pump or pressure restrictor in the system. 

The problem with the thrust bearing is serious, and every Spitfire (or GT-6, or TR-6) owner should know how to check crank end play. I know I was able to replace the thrust bearing in my TR-6 by dropping the pan and removing the rear main bearing cap, but can't remember if it the same on the Spitfire. A fair amount of work, but nothing too technical, and it saves the engine. And oversized washers are available if there's wear on the crankshaft. And it's easier/cheaper than replacing the crank and rebuilding the engine!

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