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Triumph Repair/1975 TR6 fuel mixture


I have removed all the emission controls from the engine and capped the vacuum retard at both the carb and distributor. Car runs fine but at idle there is a lot of unburned gas smell in the exhaust fumes. I pulled the plugs and the color on the tips is perfect (light tan). I get poor gas mileage (18 mpg) but can't find any leaks. I am using premium gas and have tried it with or without ethanol. Any ideas? Thanks

ANSWER: Hi Gary,

Some of the emission control stuff can be removed and gain performance but some can not. For example if you remove all of the vacuum retard controls you have upset the advance curve of the ignition system and the engine can not perform well at all. (low power and poor fuel milage)

A vacuum retard unit on a distributor has direct manifold vacuum on the retard unit so it holds the timing in a retarded position while high vacuum is applied (low RPM)and as soon as you open the throttle the manifold vacuum drops low and allowes the retard unit to advance the ignition timing which boosts performance. Even though Triumph TR-6's had emissions related pieces on some of the Triumphs like delay valves etc, It still needed to advance the timing and you eliminated that.

ALL gasoline engines produce the most power and efficiency at an advanced setting just before detonation. This advanced setting varies with conditions and RPM and many other factors like compression ratio, temperature, grade of fuel and design of the carburetor system and exhaust system. That is why there is no exact setting of everything.

Here are some general things to work toward.
Use the grade fuel that matches your compression ratio. High octane fuel for high compression ratio, Low octane fuel for low compression engines. keep the engine temperature in a mid range. keep the fuel mixture a slight bit richer then Stoichiometric (normal for that engine) Set the ignition timing so that you get a slight "Pinging" on a quick throttle opening from idle while the engine is at operating temperature and then retard it by about 5 degrees. Then if you don't have a hill to test the car on accelerate in 4th gear with the clutch out from a low RPM (about 2000 RPM) if it "Pings" retard the timing a few degrees more. If it don't leave it there and check it to see what you new timing should be.

"Pinging" is detonation and will destroy an engine if left that way. To get an advance curve correct is a very difficult job without equipment. If you know a shop in your area that builds race cars, they will usually have the equipment and know how to change the advance curve of your distributor. In the late 60's and on into the 70's BLM was very poor at getting that correct.


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QUESTION: Thanks for your quick response. I'll look for into a shop that can test my advance curve. In the meantime,of all the ports on either stromberg carberator, which would you recommend reconnecting to the vacuum retard? Thanks again.

ANSWER: Look at the distributor first as many vacuum retard distributors didn't survive the years and the owners opted for the better performing older Vacuum Advance distributors. If the vacuum port on the vacuum unit on the distributor points toward the distributor, then it is a vacuum retard unit and must not be connected to any port on the carburetor, but MUST be connected directly to the intake manifold. (called straight vacuum)

If the vacuum port on the vacuum unit is pointing away from the distributor then it is a vacuum advance unit and MUST be connected to the "Ported Vacuum" on the mounting flange of the carburetor. or on top just next to the flange. To test a port for "Ported Vacuum" use a vacuum gauge on the port and you should see little to no vacuum at idle and just as soon as you slowly open the throttle you will see vacuum.

You can not install a vacuum advance distributor unless you have a carburetor with a "Ported Vacuum" port on the carb flange.

Many British car owners over the years found out that the pre-1968 cars performed much better so they often converted their cars to 1967 and earlier car distributors and sometimes carburetors also. So today you have to look carefully to see if any of those modifycations have been done. Either way just pay close attention to Ignition timing.


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TR6 distributor
TR6 distributor  
QUESTION: I am enclosing a photo. The port is pointing away from the distributor. So vacuum advance? I thought this was a vacuum retard.

The case of that distributor use to have both an advance unit and a retard unit and someone has removed the retard unit. Hopefully you still have the "Ported Vacuum" port on the carburetor. Usually on the front carb. Look for a vacuum port on the mounting flange of the carburetor or a port very close to the flange and then check the port by running at Idle and there should be little to no vacuum on the port but as soon as you slowly start to open the throttles, you will have vacuum on that port. That would be "Ported Vacuum" and the one you need.

The car is old so check the vacuum unit on the distributor to see that it does not leak and does move the advance plate inside the distributor. This is very important for performance. Then check ignition timing. 10 deg BTDC set static (not running)on 75 and 76 cars. Then check the timing with it running and see that it advances as the RPM increases. Do this with the vacuum advance disconnected. If the timing advances as you increase the RPM it shows that the mechanical advance is working in the distributor. (it doesn't mean it is right, it jsut proves it is not frozen up) Next set the RPM at a fast idle and steady at about 1500 RPM and check the timing and then plug and unplug the vacuum hose on the distributor and watch the timing change. This tells you the vacuum unit it changing the timing (it does not tell you it is correct but does tell you it is operating)

Let me know,


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Howard M. Fitzcharles III


Triumph TR-4 up & Spitfire, and Engine theory


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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