Triumph Repair/Backfire through carb. on 1977 MG Midget
Howard, you answered a few questions for me before about my Tr-6 and were very helpful. I have a problem with backfiring through the carb. under load only, not at idle, on my 1977 MG Midget(I believe it's the same engine, 1500cc that's in a Spitfire?). Spark is good to all cylinders-still I replaced the rotor and cap, it has electronic ignition. Compression is 140 in all 4 cylinders. I adjusted the valves. Timing set to specs. Put vacuum gauge on vent behind throttle on carb.-read 13 PSI. Rebuilt carb(Stromberg). New fuel pump and fuel filter. Sprayed for leaks on intake manifold-can not see or hear any leak. While driving car in 2nd or third gear get back fire with loss of power- pulled chock out, it made no difference.One last strange thing. When you pull the spark plug wire off the plug at#1 or #4 the engine will die. But when you pull the plug from #2 or #3 the engine will lose RPMs but not die. What do you think Howard?
I don't have a clear answer for you but will try to lead you to an answer. First, The port on the base of the carburetor is "ported vacuum" meaning at idle you should see little to no vacuum on that port until the throttle is started to open. But under no circumstance will you ever see Pressure. The only way you can possibly see pressure there is if your engine is either turbo charged or supercharged.
Next is that number one plug wire can not possibly kill the other 3 cylinders. Nor can the number 4 plug wire kill the other 3 cylinders.
What can happen is that carbon tracs inside or outside the cap can rout the spark to other cylinders and if that other cylinder happens to be on an intake stroke the combustion will blow pressure into the intake manifold and destroy all vacuum and cause the engine to die. This is easy to test. Just short the plug wire to ground without removing it from the plug. Also put that gauge on the carburetor port and run the kill tests while watching the gauge.
If none of this nets you an answer run a pressure test on each cylinder with shop air pressure. You put a cylinder on TDC of the compression stroke and put the car in 4th gear with the brake on and pressureize the cylinder with an adapter for your air hose. With air applied (120 or more PSI) open the throttle and listen in the carburetor and listen at the tail pipe. then remove the radiator cap and watch for any raise in the coolant level at all. Do all 4 cylinders that way.
Let me know,