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Triumph Repair/Rough idle when warm


I have a 1975 Spitfire. The previous owner replaced the single carb with 2 HS4s. The car starts and idles fine when cold. It will idle fine initially without the choke but after its well warmed up and I pull up to a stop light, the idle is terrible and if I don't blip the throttle or add a little choke, the engine will stall. We had a hot summer and I thought it might have been related to that but even in the fall with outside temperatures in the 50s the problem continued. I thought I might have issues with heat affecting the fuel but I've insulated the fuel lines and carb bowls to no effect. Any thoughts on what I should have a look at first? Thanks for your advice, love this website.

Hi James,

I found while working in dealerships that diagnosing by symptoms was mostly a waste of time. The only sure way to find a problem is by testing systems first and there are only three systems that make an engine run. Compression, Fire and Fuel and they MUST be tested in that order.

You need to forget what might be the item causing a problem and look at each of the three systems then when you find which system is failing then narrow the search by testing only. This method will always lead you to the fault 100% of the times.

First check the compression and don't skip this by saying "I'm sure it is ok"
You want to run a compression test (throttle open)and see at least 125 to 170 PSI on all four cylinders and only a small difference between cylinders.

If it tests ok then and only then should you test ignition. Most people don't have access to a scope that can test an ignition system in seconds. If you don't have access to a scope there are tests that are easy to run with only hand tools. Remove all four spark plugs and connect each plug to it's wire and lay the plugs on a metal part of the engine. Now, have someone spin the starter and watch the plugs spark (not in bright sun light). You want to see a thick spark (about as thick as a pencil lead) The spark should be blue in color. A spark that is thin as a hair and yellow or orange in color is a weak spark and can cause many problems.

If the spark looks ok on all cylinders check the ignition timing and set it if necessary.
Only now if these are ok can you test fuel.

First test of fuel is the fuel pressure. On your car you need to get a "T" fitting and a short piece of fuel hose and extra clamps(from any auto parts store) and install it in the fuel line from the pump and then connect a low pressure gauge (some vacuum gauges have a low fuel pressure section) These gauges are not expensive. connect the gauge on the "T" and the fuel line back to the carburetors. You want to see from 1.5 to 3 PSI and no more with the engine reved up. If that is ok then proceed to the carburetors.

The two SU carbs will out perform the one Stromberg they replaced but they do require to be set up and adjusted correctly. This is a little too much to type in this program so you should look at my tech tip on this subject on my web site at

Follow the directions and let me know if you have any questions and how it turns out.  


Triumph Repair

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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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ASE Master Auto with L-1 certification up to 2000

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