QuestionHi Howard, you helped me out on my previous project a TR2 powered kit car, when I was having problems, so I’m hoping you can help me on this project.
I now have an MGB based kit car, an NG TF, which is a 1930s roadster style, all based on a 1969 uk spec MGB.
It had been running fine last year & I laid it up for the winter having in mind a couple of improvements to deal with whilst it was sat in the garage.
1st was to re-route the choke cable - the original cable serving the twin su carbs was feeding from the top and the cable was operating on the inner wire of the cable. This was stiff to use and sometimes didn’t drop the jets down at the extent of the travel. I renewed the cable & reconfigured it to feed from the bottom and pull on the outer cable.
2nd I needed to replace the cb points as they hadn’t been replaced for a while and they were black & a little pitted. While I was at it I would replace the condenser. When I was turning it over in the winter it was struggling to start, so at that point I thought I’d also replace the coil. So points, coil & condenser all replaced on a like for like basis.
So spring has arrived, lets get the car out. Started very easily, take it for a drive, spluttering and popping; more choke doesn’t help, more acceleration the worse it gets. If I keep it at even revs it just about stays running. It is back firing either in the inlet trumpets or the exhaust, not quite sure!
Back to the garage. Recheck the cable which was actually holding the choke open, so I thought that’s the problem, unignited fuel in the exhaust backfiring. Test drive, just the same.
Back to the garage, recheck the points gap, check all leads (changed HT lead coil to distributor, as it had a poor connection to the new coil), still the same.
Changed back to the original condenser in case the new one was defective, still the same.
Did a static timing test, (not that anything should have changed) all ok.
So that’s the problem and I can’t resolve it – any ideas?
Many thanks David
AnswerHi David,
When working in MG dealerships we often received cars with various symptoms like yours and I learned to listen to all the symptoms. And a few would lead me to a section but rarely do they pinpoint the problem so I learned to smile and nod my head and when the customer left I just went about the procedure needed on all cars with "derivability problems".
We had the advantage of a scope to short cut most testing but most small shops didn't have a scope. But the procedure is the same.
You first need to isolate the problem into one of only 3 categories, Compression, Fire or Fuel.
they MUST be tested in the correct order too. Compression first and it does not matter if the compression was good a short time ago. Leave no stone unturned.
You must confirm that the combustion chambers are good with no question. Run the compression test with all plugs out and throttle open. You should see from 135 PSI to 170 PSI an all four cylinders and very little difference between cylinders.
If they have that and only if you have that can you proceed to "Fire" (Ignition).
You have done some of the work here already but there are a couple more things to look at. You have checked the timing so that is good. You didn't say if you replaced the coil wire with a new one or just another used one so check that and the plug wires with an ohm meter. I use 4K ohms per foot as a bench mark. Anything close is probably ok.
It is a good trick to also put a timing light on the coil wire (not a plug wire) and place the timing light (with the trigger taped down) under a wiper arm too. I found that you can spot even the smallest misfire from the primary of the ignition system by watching the flash of the light. It looks almost a steady light and even a small misfire will show up as a blink in the steady light. This only checks the primary of the ignition system.
Next test the coil, not just with an ohm meter for the winding resistance but a rough test can be done by holding the coil wire about 1/8 inch from the head and have someone spin the engine and watch the spark jump. It should be thick (about as thick as the lead in a pencil and colored blue (not in bright sun light) If it is, then while it is jumping slowly pull further away from the head and you should be able to get at least 3/8 inch away from the head and still have a spark. A hair thin spark colored yellow or orange is a weak spark. Don't do this a lot as it may make the spark jump elsewhere.
If all this is ok clean or replace the spark plugs
Now remove the distributor cap and hold the end of the coil wire about 1/8 inch from the center of the rotor and have someone spin the engine over and see if you can jump a spark to the rotor. If you can jump any spark to the rotor, it is bad internally even when you can't see the tell tail carbon trail on the outside. Wash the distributor cap and examine the cap under a strong light to look for carbon tracking inside and out side.
Now run a cylinder "Kill" test by running the engine at what ever speed it runs the worst. and kill each cylinder one at a time and note the RPM drop. This tests each cylinder to see if each is pulling it's own load.
Only now can you proceed to "Fuel".
Test the fuel pressure to the carburetors. It should be from 1.5 PSI to 3 PSI. Your symptoms don't indicate a lack of pressure but don't leave stones unturned.
While in the shop hold the RPM at which ever makes it run the worst and spray starting fluid into each air horn to see if it smoothes out any. Also connect a vacuum hose to the intake manifold and run a long hose out from under the hood and place the vacuum gauge under a wiper arm so you can see the vacuum while driving and run the car and note the vacuum under different driving conditions. At NO time even under full throttle should it drop below 5 in hg.
We had a scope in most of the dealerships I worked in so much of this could be tested in a matter of minutes but not every shop has a scope so I had to use these other methods.
Don't skip any test and do them in the order as listed above.
let me know the results.
Howard
