I have a query with my XK120, and I hope you can help. The car's performance seems a little lackluster to me. I installed a manifold pressure gauge on the windshield washer tube to see whether it would work. At 750rpm, the gauge would read just 10", instead of the correct 18". As a result, I went in search of leaks and came up empty. At 2,500 rpm, it could only reach a diameter of 18". If I twitched the throttle, it would go between a high of approximately 20" and a low of about 8". The engine has 9/1 compression, and I just examined it and found that each cylinder has between 180 and 190 pounds of pressure. The electrical system, made by 123 of Holland, keeps time at 10 BTDC. To compensate for the lightened flywheel causing the gearbox to clatter at lower than 750 rpm tickover, the engine is equipped with D-type cams, 2x2" HD8's with UO needles, a jet set approximately 1/8th inch down, and volume screws turned up about 6 turns. When I turn on the electronic choke manually from a cold start, the engine turns over, but only at 500 revolutions per minute until I apply throttle, at which point it will stay at 1,000 for roughly two minutes. No matter where I set the nut on the electronic choke, the engine never accelerates over 500 revolutions per minute (rpm) after being started.
Is there a lack of suction in the manifold system as a result of this?
Low pressure in the manifold!
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newjimmy54
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This is only conjectured on the basis of recent events.
The 2.4-liter engine in my MK2 was woefully inadequate. But I had no clue since I had never driven a 2.4L that had been tuned to perfection.
Even after having the servo booster rebuilt and replaced, I knew my power brakes still weren't up to par. The vacuum was just about 15 or lower.
When the clutch failed and I had to remove the engine, I tore it apart to patch the leaky head gasket (among other problems) from the outside.
Piston tops were marked with intake valves because the cams were advanced 30 degrees.
After I replaced the rings, bearings, lapped the valves, and adjusted the cams, the vacuum was back where it belonged.
Get a cam setting tool ASAP if you don't already have one.
Remove the cam cover, locate TDC, and double-check it.
Removing the exhaust cam cover is a lot simpler on side draught engines than it is on downdraught 2.4s, which need removing the stupid cabs first.
The 2.4-liter engine in my MK2 was woefully inadequate. But I had no clue since I had never driven a 2.4L that had been tuned to perfection.
Even after having the servo booster rebuilt and replaced, I knew my power brakes still weren't up to par. The vacuum was just about 15 or lower.
When the clutch failed and I had to remove the engine, I tore it apart to patch the leaky head gasket (among other problems) from the outside.
Piston tops were marked with intake valves because the cams were advanced 30 degrees.
After I replaced the rings, bearings, lapped the valves, and adjusted the cams, the vacuum was back where it belonged.
Get a cam setting tool ASAP if you don't already have one.
Remove the cam cover, locate TDC, and double-check it.
Removing the exhaust cam cover is a lot simpler on side draught engines than it is on downdraught 2.4s, which need removing the stupid cabs first.
In fact, I just tested the manifold gauge on a friend's XK140 and got a reading of 15 inches; this is indicative of a delayed ignition, but by how much I am unsure. I've been advised that a vernier gauge will work for timing the cams, so I won't need to rent a cam setting flat plate tool. Incorrect valve timing is shown by the 10" mine being visible.
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newjimmy54
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- Joined: Thu May 21, 2020 3:30 pm
Absolute TDC is the true goal. To find #1, you may have to dig a little. There were two sets of timing marks on my car; the one on the flywheel was so off that the timing was all incorrect. Perhaps the problem with the 30 advanced camshafts was due to this.
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newjimmy54
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- Joined: Thu May 21, 2020 3:30 pm
Remove the spark plugs, remove the distributor cap, then rock the crank to inspect the lower cam chain. If there is a slapping sound and a great deal of motion before the distributor rotor begins to turn, maintenance is required.
Adjust the top timing chain tension using the right tools.
Locate the TDC and make a note of it.
Verify the two camshaft settings.
Put back together, making sure there are no sloppy joints or missing parts.
Time it out.
Modify your carb intake.
Prove it out on the road.
Adjust the top timing chain tension using the right tools.
Locate the TDC and make a note of it.
Verify the two camshaft settings.
Put back together, making sure there are no sloppy joints or missing parts.
Time it out.
Modify your carb intake.
Prove it out on the road.
Yes, I will make sure to tighten the top chain and inspect the bottom chain as indicated. My D type camshaft, with a duration of 270 degrees and an overlap of 60 degrees, will offer a poor idle because there is not enough suction at idle, but all the power comes in further up, as I learned today from talking to camshaft suppliers. The secret lies in the cam's overlapping construction. If I am not using these cams for racing, according to the specifications for Rob Beere cams, Piper cams, and Newman cams, they are too aggressive. The D type cam has a basic overlap of 60 percent, or 30 times 30. I need a cam with either a 20:60 or 25:65 field of view (40:60 or 50:50 overlap, respectively).
The 200 horsepower 4-cylinder engine is quite powerful in comparison to other 4-cylinder engines, but it's not very road-friendly.
The 200 horsepower 4-cylinder engine is quite powerful in comparison to other 4-cylinder engines, but it's not very road-friendly.
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BentleyOwner
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- Joined: Sun Jan 01, 2023 8:20 pm
While this is not exclusive to Jaguars, I have found that many aftermarket Triumph cams only produce 10–15 in Hg at idling speed. That fits up with what you've been feeling and hearing about the situation, therefore it's probably true.
That's exactly what I've concluded, therefore I guess you're correct. The engine itself is OK; the problem is that the car's designer designed it too fast for the heavy traffic conditions typical in the United Kingdom. In order to make the car more tractable, I want to swap out the camshafts. The dilemma is how to keep from losing too much power with 9/1 compression, big 2" SU HD8s, and a beautiful free-flowing stainless steel six-branch exhaust setup.
By the way, GT6s are among my favorite cars. In 1973, at the age of 19, I bought my first 1967 wedgewood blue mk1, and in the 1970s and 1980s, I had two additional mk3s. My final vehicle, a dark red 1974 model with very low mileage, was purchased by the head of the Triumph sports six club in 1985.
By the way, GT6s are among my favorite cars. In 1973, at the age of 19, I bought my first 1967 wedgewood blue mk1, and in the 1970s and 1980s, I had two additional mk3s. My final vehicle, a dark red 1974 model with very low mileage, was purchased by the head of the Triumph sports six club in 1985.
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Lucas_Smith97
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- Joined: Mon Apr 13, 2020 7:30 pm
Using hot cameras always results in a reduction in vacuum. This is why many high-horsepower street engines rely on vacuum pumps to keep the engine's vacuum steady at low RPMs. Once upon a time, I worked for a company that had a V-12 XJ; the shop foreman had a brilliant idea to increase power, and the owner gave his OK. Carbureted cams do not function well in a fuel-injected V-12 engine. A lot of effort was required since first it was discovered that the vehicle was severely lacking in air, then the air issue was identified, and last the ECU had to be modified to accommodate the increase in air, which necessitated the use of a new injector, and so on and so forth. The shop has since closed, and I still have no idea whether the Jag was ever fixed to run properly without the factory cams.