Jaguar XK120, XK140 and XK150 | What to know before you buy

Photograph Courtesy Jaguar

[Editor's Note: This article originally appeared in the March 2015 issue of Classic Motorsports.]

Jaguar stepped back into the sports car market in 1949 with its XK120. The XK140 replaced the XK120 in 1954, and the XK150 arrived for 1957. All three cars are representative of Jaguar’s rebirth in the sports car world.

Expert

Gary Boffo
Welsh Enterprises
welshent.com

If it’s the driving experience that you want to enjoy from your Jaguar XK, then you might consider some modern upgrades. Though the vintage feel is what we like from these gorgeous classics, mid-20th-century technology can be disenchanting or even scary to some–especially in the case of the braking system. 

Modern disc brake conversions for both drum-brake and early disc-brake cars are a satisfying upgrade. Matching the performance of the Jaguar’s big six-cylinder with the stopping power of a modern brake system is comforting and adds fun to back-roads cruising.

The reliability of the old-school, non-synchro gearboxes in the Jaguar XKs has never been questioned. This vintage gearbox proved to be the workhorse of many British classics. Today, however, repair parts for these transmissions have become rare and costly. 

Using a contemporary five-speed gearbox conversion for your XK can be a cost-effective repair to stock Moss gearbox problems. Plus, owners enjoy smooth gear changes and the advantage of a fifth gear to keep the engine from working too hard on modern open highways.

No, we don’t have the frustration of a hand crank to start an XK Jaguar, but there are times when the procedure to fire the big-six can be trying. Replacing the ignition points with a Pertronix ignition, hidden neatly under the distributor cap, will provide the hot spark needed to compel this venerable powerplant to life. Also, the availability of gear reduction starters to spin the engine at faster speeds lends itself to easier starting in all conditions. 

Okay, all the Lucas jokes aside, the charging system provided by generators of any brand cannot come close to what an alternator can supply. Today, modern alternator electronics hidden in a generator case are readily available. These provide modern charging with a vintage appearance. Open the bonnet and everything looks undisturbed. Idle at night while the lights stay bright.

One common problem we see with these cars is a stumble under acceleration. There can be a myriad of causes, but there’s one easy item to check before you take your Jaguar XK to your favorite magician mechanic: the dashpot oil level in the S.U. carburetors. 

The oil damps the carb’s piston travel and keeps it from jumping up too rapidly, causing a fuel “gulp” and a temporary, quick over-enrichment. Low or no oil in the dashpot can cause this delay in pedal response.

Say you have an XK Jaguar that will not start. The problem always boils down to the holy trinity of the internal combustion engine: air, fuel and spark. If there is fuel at the carburetors and there is spark at the plugs, then it could be a result of the XK Jaguar’s electronic choke design. The two components that commonly create a problem in this system are the otter switch and the choke solenoid.

The otter switch is located in the intake manifold and is triggered by the temperature of the engine coolant. This switch closes the circuit for the choke solenoid, which is located between the carburetors. When actuated, this solenoid richens the fuel mixture to aid in starting the cold-hearted XK engine. While the engine is running, the coolant warms to a specified temperature, causing the otter switch to open the circuit and shut off the choke solenoid. This system works well as long as these two main components are working properly.

Expert

Paul Tsikuris
Tsikuris Classics
(863) 858-7981

Generally speaking, the Jaguar XKs have no more weaknesses than most other British cars. Rust problems, overheating and front bodywork damage from the weak stock brakes are the most common. The frames are massive, but the sills and rear frame extensions do rust. Check for poor door gaps as evidence of this. 

Rust problems can also be less apparent in these cars. Lead was used as body filler, and the sheet metal can rust under the lead. Another common spot to find rust problems is in the headlight and turn lamp housings. They sometimes trap water and rust out the fender underneath. 

When it comes to maintaining a Jaguar XK, there really isn’t too much “special maintenance” required. Preventive maintenance is the key to trouble-free ownership. The drivetrain found in these cars is almost bulletproof, but changing fluids regularly is cheaper than a rebuild. Cleaning the contacts inside the distributor cap will help the plugs burn the fuel more efficiently.

For modern driving, upgrades to the cooling system are a must. Alloy radiators, coolant recovery systems and electric fans–in hot climates–are beneficial. 

I have found that advancing the ignition timing to 15 degrees before top dead center helps burn ethanol fuel more efficiently and lowers engine temperatures. Modern electronic ignitions can help with reliability and consistent timing. 

I run dual fuel pumps on my cars, using the second as a backup should the primary pump fail. Five-speed transmissions transform the car, combining lower highway rpm, better gear ratios, and a synchronized first gear. Disc brake upgrades are also popular for safety and performance.

When considering a purchase, always buy the best unwrecked, rust-free original car you can. In the long run you will have less invested in a fine example than a basket case, entry-level car. One thing to keep in mind is that these cars were hand-built and bodywork is expensive. However, mechanicals are straightforward and are not a concern.

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Comments
VClassics
VClassics Reader
3/30/15 6:43 p.m.
The oil damps the carb’s piston travel and keeps it from jumping up too rapidly, causing a fuel “gulp” and a temporary, quick over-enrichment.

I think that's backwards. Delaying the pistons' rise subjects the jets to greater relative vacuum, momentarily drawing more fuel and preventing a lean stumble, not a rich "gulp." This is how SUs get by without accelerator pumps.

SSpro
SSpro New Reader
1/14/24 11:38 a.m.

These cars really brought the modern sports car era into existence. The XK120 was the fastest production car in the world and dominated club racing. Twin cams, bucket seats, four speed floor shift, real power and high speed was the formula for the next 25 years. Only about 10,000 of each of the XK120, XK140 and XK150 were made, so in my mind these cars are completely undervalued when compared to contemporaies of the day.

wspohn
wspohn UltraDork
1/14/24 12:32 p.m.

The 120 looks great but the 150 is the one to own - 4 wheel disc brakes is a definite advantage.,

CJ
CJ Dork
1/14/24 1:06 p.m.

Damn.  Where is Frenchy when we need him?

merkmia
merkmia New Reader
1/16/24 12:01 p.m.

This year it will be 60 years that I’ve owned my 1956 Jaguar XK140 Drop Head Coupe which I am currently having partially restored. Over the years I have used it as a long-distance road car, commuter car, second car and third car and have had it partially restored several times.

I was going to sell it a few years ago because the clutch was so heavy but instead I got a knee replacement so I’m okay again although I’m still thinking about selling it when the restoration is finished.  

 I also have a 2007 XK convertible but I can tell you that it is much easier finding parts for the ‘56 Jaguar than the ‘07 Jaguar.

Stewart in Miami

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