QuestionQUESTION: the last few times I have attempted to take my car for a drive I get about a block away and when I am switching from second gear to third it starts to sputter and sometimes back fires and stalls. I have been able to restart an nursed it back home in first and second gear. I tuned it up and replaced the coil.(seemed to have intermittent power to the coil that is why I replaced it)
ANSWER: Hi William,
Any time you have a engine failure you must run some standard tests. As we did in the dealerships I worked in. The tests must be run in order and nothing skipped (no stones unturned) You do need a few tools but none are expensive and you may already have some or all needed. It is difficult to wipe out of your mind any perceived failed part but you must test systems first not parts. Then when you have clearly made an ID of the failed system, you can then test the parts in that system. Never toss money or parts at the problem until you have an ID on the failed part in the failed system.
There are only three systems on a gasoline engine that make it run. Compression, Fire and Fuel.
They MUST be tested in that order. The tools needed are a Compression gauge, fuel pressure gauge, a 12v test light and a timing light. (Plus a set of hand tools)
First --- Remove the (+) wires off of the coil and remove all four spark plugs. Then Run a compression test on all four cylinders with the throttle wide open, this is important.
You must see from 125 to 180 PSI on all cylinders with little difference between cylinders. Any reading down close to 100 PSI is a large problem. ONLY after the compression test can you proceed to Fire (Ignition) And only if it passes the compression test. Don't skip this.
Second --- Put the (+) wires back on to the coil and place each spark plug on it's plug wire and lay each plug on a metal part of the engine. The plugs are still out so place your thumb over # 1 plug hole and have someone spin the engine while you watch each spark plug fire. As # 1 piston comes up on compression it will blow your thumb off of the plug hole with a "Pop" sound. At the same time you should see # 1 plug that you connected to # 1 plug wire and laid it on a metal part of the engine, you will see it spark (with a "Click" sound)
The spark must be blue in color and as thick as a pensile lead. The "Pop" should seem to be causing the "Click". If it is "Pop-Click" or "Click-Pop" the timing is off enough to cause a power loss. Or if you see a hair thin spark colored Orange or Yellow. (not checked in bright sun light) that is a weak spark. Check each cylinder the same way. This confirms firing order and checks for many other problems in the cap and distributor. If and only if all is correct up to this point should you look at Fuel.
Third --- Put the plugs back in and wires on the plugs and now you will need to tell me if the British Columbia version of the Spitfire is the Euro version dual SU carburetors or the US version with one Stromberg carb. But either one must have the fuel pressure tested. Put a "T" in the fuel line at the carburetor/s so you can see the fuel pressure as the engine is running. Use a long hose from the "T" to the gauge just in case you need to see the fuel pressure while under load (on the road). Read the pressure at idle and then rev the engine several times and set the RPM up to about 3500 RPM (cruise speed)and watch the pressure. You should see from 1.5 PSI to about 2.0 PSI at all times. If you see questionable pressure readings you should do what I do and run the hose out from under the hood (bonnet) and place the gauge under a wiper blade and drive the car and watch the gauge.
Backfiring and loss of power in higher gears is a common symptom of fuel starvation. This test is for fuel delivery only not a test of the carburetor itself.
You will have to tell me which carburetor/s you have to test the carburetor/s themselves.
let me know,
Howard
---------- FOLLOW-UP ----------
QUESTION: The Carburetor was the same as the American a Stromberg , but I replaced it with an new single SU Carburetor.
AnswerBoth the Stromberg and SU are cinstant velocity carburetors but I too like the SU over the Stromberg.
If and only if you have run the other tests should you go into the carburetor.
First remove the top center cap and lift it up about an inch and push it back down and you should feel hydraulic resistance. don't screw the cap back don't yet. Now remove the air filter and look to see that the holes in the front of the carburetor are all open through the gasket and into the air filter. The holes just inside and up must be open and not blocked by a gasket. Now lift the piston all the way to the top and drop it. It should hit bottom with a "Clunk". If not, something is wrong. Now remove the screws holding the top pot on to the carburetor and lift the pot off of the carburetor and lift the piston out being very careful not to damage the metering needle. Set it aside with it's pot in a safe place. There is a long soft spring between the piston and the top pot be careful not to damage it either.
Spin the engine over for 30 seconds or so to be sure the fuel pump has filled the float chamber and remove the top off of the flaoat chamber and note the fuel level in the float chamber. The float level should look as though it is high enough to hold the float needle cloased. Shake the hose that goes from the float chamber vent to the charcoal canister to see that no droplets of fuel are in it and then confirm that is is not stopped up either. Take a shop rag or such to soak all of the fuel in the float chamber out and take a piece of hose and place it over the jet itself and with your mouth blow down into the jet to blow the remaining fuel in the jet up into the float chamber and note if there was any trash in it.
Note the top of the jet itself as to it's position in the bridge of the carburetor to see about how much it is recessed below the surface of the bridge. It should look like it is recessed about 1/8 inch or so below the surface of the bridge.
If everything looks good so far put it back together and look on the scale of the fuel pressure gauge to see if it has a section to measure vacuum. (fill the float chamber before putting the top back on.
Start the with the vacuum gauge on a straight vacuum source and note vacuum at idle and slowly open the throttle and watch vacuum. Run it up to at least a fast cruise (4500 RPM) and at no time should the vacuum drop below 5 in hg.
If all of these tests pass and it still runs badly you should connect the timing light to the coil wire and run the leads out from under the hood and place the timing light under a wiper blad and tape a piece of cardboard on the windshield over the timing light so you can see the flash of the light in the car while you drive and also pu the fuel pressure gauge so you can monitor it too and drive the car and watch the light and gauge to see if you can see a failure while driving.
Somewhere in all these tests you have to find the fault.
let me know what you find,
Howard
