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MG Car Repair/Stall when slowing from 65+MPH

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Question
1975 Mg midget ZS carb. Gas tank, fuel filter  replaced 500 miles ago.
After driving for more tha 20 minutes above 60 MPH and slowing down the temp gauge starts to rise. The engine starts to miss and then stops. After sitting for 30 minutes it will start up again and everything seems fine.
Hmmm?

Answer
Hi Mike,

In the dealerships we often would get cars with a large virity of symptoms. We were never able to put together a system of correcting a fault by symptoms.

We did put together a system of testing that was a 100% fix rate. Here is how it went.

All piston engines operate the same with only three systems to run. Compression, Fire and Fuel.
Every time an engine falters one or more of the 3 systems is at fault. So it is important that each of the 3 systems must be tested but only in it's failed mode (in your case the engine dies)
You can eliminate Compression because if Compression goes away, it will not return.

Fire and fuel can do that. So all tests need to be on only one system at a time. Fire (Ignition) should always be tested before Fuel.

We always avoid trying to think what the problem might be and it is always a suprise what it is but we always find it in our test procedure.

You will need a timing light for the first test or you can cary a plug wrench with you and road test the car and get it to quit. If you have a timing light connect it to a plug wire and the battery and put the timing light with the trigger taped down and put the light under a wiper blade so you can watch the light flash at the time of failure.

Or cary an extra spark plug and just pull a plug wire off of a plug and put it on your extra plug and lay it on any metal part of the engine so you can see the spark and have someone spin the starter (the car must be in it's failed mode to run this test)

If you have no spark the problem is for sure ignition. If you have good spark then it is fuel.

To test for fuel get a spray can of starting fluid and spray it into the intake while someone tried to start the engine. If it fires off but will only run as long as you spray then it is a fuel supply problem. Then you need a fuel pressure gauge and a "T" fitting to watch fuel pressure at the time of failure. You must see from 1.5 PSI to 3 PSI but never more then that or less then that.  

Let me know and I will give you the next test.

Howard

MG Car Repair

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Howard M. Fitzcharles III

Expertise

MG from 1956 (USA versions only) up and Engine theory.

Experience

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.

Organizations
Associate member SAE EAA member

Publications
Import Car magazine

Education/Credentials
ASE Master Auto with L-1 certification up to 2000

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