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Triumph Repair/rough idlingl 78 spitfire


QUESTION: The car is only idling  at 200-300 rpm and very rough, fighting to stay running, unless warm will stall.  Changed the plugs. 3 were fouled 1 was relatively clean, also replace wires.  Removed plug wire from each plug, engine speed decreased, also heard the "sparking" sound when putting wire back onto the plug.  Changing the plugs and wires had no apparent effect.  The cap and rotor did look a little worn, waiting for a replacement.  Have not run a compression test yet.  What would be my next steps?
Thanks for the help!!!


I found it is a waste of time to try to find a running problem by the symptoms. Working on these cars every day for many years I found it was necessary to just run the tests.

The tests are the same on all the cars I worked on. There are only three items that make a gosoline engine run and they are "Compression, Fire and Fuel" and they MUST be tested in that order.

"Compression" must be from 145 PSI to 170 PSI on all four cylinders and be done with the throttle open. There should be very little difference between cylinders. It is a waste of time looking at Fire or Fuel until this is done.

"Fire" (Ignition) can be tested without expensive equipment. While you have all the plugs out from the compression test just connect all the plug wires to each plug and lay them on a metal part of the engine and spin the engine and watch the color of the spark and how thick it is (not in bright sun light) The spark must be blue in color and as thick as a pencil lead.
Now put your thum over the number one plug hole and spin the engine. The compression will blow your thum off of the plug hole with a "Pop" sound and the plug will spark with a "Click". It should seem as though the "Pop" caused the "Click". If it goes "Pop-Click" or "Click-Pop" then the timing is possibly so far off it may not run. This don't mean that the timing is correct but it is close eniugh for the engine to run. Check all four plug holes and plugs the same way and that will show that the firing order is correct.
If that checks out ok, set the timing accuratly.

"Fuel" Now and ONLY if the first two tests were ok do you look at fuel. The 78 Spitfire has a mechanical fuel pump so you first have to put a "T" in the fuel line at the carburetor and a fuel pressure gauge on one end of the "T" fitting. (use clamps on the hose connections).
You need to see from 1.5 PSI to 3 PSI on the gauge. It is difficult to test the float level on a Stromberg carburetor and also difficult to test the coolant operated choke on the Stromberg carb. So, take a spray can of starting fluid and spray into the intake a little and start the enigne. If you start up but it dies quickly then try to keep it running by spraying. If that keeps it running then you need to remove the carburetor and remove the float chamber and the choke and check the float level and the choke operation. The Stromberg choke systems are noted for failure. (note- most vacuum gauges have a section to test low pressure fuel pumps)

If it didn't start with the starting fluid then you didn't do the first two test (Compression or Fire)

You must do each test in order and not skip any of them. If you proceed as I have outlined you can not fail to fix the car.


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QUESTION: Howard did the tests in order compression on #3 was a little low but in acceptable limits, timing off manually set the timing,  set the idle car idling much better.   Thanks again for the help!!!  

I have looked around and cannot find a definitive answer on what type of coolant to use or what mixture.  What is the best coolant to use for the 78 Spitfire?

You are welcome. The coolants in 78 were just regular antifreeze made of Ethylene Glycol and the mix was usually 50/50 or 40/60 with water. I don't know if any of these new coolants will affect the Spitfire or not so unless you get info from a reliable source I would not use any of the new stuff. I will soon look into Propylene Glycol as an alterative coolant but have only looked at the properties so far. I am interested in it because it may be used pure with no water and no pressure cap and is non toxic. Small animals and even birds are attracted to Ethylene Glycol antifreeze and it is deadly.


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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.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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