QuestionI took my 74 MGB GT out for a "longer" run today. It was rebuilt, and a pair of HS6 were installed with a new intake manifold. No other upgrades. The timing is correct and there is fuel getting to both bowls. It runs fine at idle and even at lower RPM (<3000). But when I got out on the highway and got the car to about 3500 RPM, it started to sound like it was misfiring, like the balance between the carb's was lost. I did use a air synchronization tool, to make sure that both Carbs are balanced at idle. The car is very peppy accelerating, but when it gets to those higher rpm's, it seems the carbs get out of balance. Is it possible that the air/fuel mixture is too rich at this RPM, or is it possibly something as simple as dashpot oil not being even? could it possibly be the coil? I did have the distributor cap and rotor replaced. The vacuum advance seems to be in order as well. Any help diagnosing/correcting this problem would be greatly appreciated.
AnswerHi Rob,
If the butterflies are in sync at idle, they are in sync at all RPM.
If you adjusted the jets like you are suppose to by lifting the spring loaded pin under the edge of the top pot and got the correct response then the jets are set at all RPM unless you changed needles to the wrong needles. Ignition timing must be set to specs and test both advances to see that they are correct. Check valve clearance and check compression to see that the engine is ok.
As far as a misfire at high RPM, you must see if you still have the correct fuel pressure at the RPM that it starts missing. It must be from 1.5 to 3 PSI at the time of misfire. This can be tested by putting a "T" in the fuel line close to the carbs and run a long hose out from under the hood and put a fuel pressure gauge under a wiper arm so you can see the pressure at speed. (be sure to put clamps on all connections)
To see "Fire" at speed you just need to clip a timing light onto the COIL wire and run the timing light out from under the hood and put it under the other wiper arm and if you are running the tests in bright sun light you may have to tape a piece of cardboard over the end of the light so you can see the flash of the timing light at speed.
With these two tests you can narrow the problem down to a small area. As it stands now you may have one or more of a thousand possible faults and you will probably run out of money before you run out of possibles if you try guessing at what is wrong.
A timing light and a fuel gauge and connections are not expensive compared to what you can spend tossing parts at the problem.
I learned in shops not to even think about what it might be and just test to find all problems. I am often surprised what I find but always find the fault.
Howard
