My Xj6 suffers with miss-fire and very bad starter. New plugs, new luminition electronic ignition fitted and discarded points and condenser. With their removal I had to discard the resistor as well, as per instructions. Bout a higher output ignition coil and had to fit ballast resistor, as per instruction. Calibrated both carbs with a help of vacuum gauge and set jets with help of depth gauge and managed to get a correct exhaust emission. Have checked compression which are almost perfect. With the help of Crypton engine tuning equipment managed to get the timing as perfect as I could with crankshaft pulley that has no marking. Yet the missing at idle is still there and when accelerating but once at speed miss-firing is not felt which is due to speed of the engine and as soon as the revs drop it can clearly be felt that one cylinder is out. Yet on the oscilloscope the height of coil discharge on the screen are almost the same on all plugs. Can any member please help this frustrated new member. I may have forgotten the very basic thing to be checked, but what? Can't think of any. Meanwhile I wish each and every one of you enthusiasts a very Healthy, happy and peaceful 2016.

 

Amir:


And a very happy new year to You.


One of the convenient things about this engine is the accessibility of the spark plugs. I have the following ideas for you:


1. Remove the spark plugs and conduct a compression test. If the readings are not within 10 or 15 percent of each other, the engine will tend to idle rough. If one is very low, it may not fire at all or only at higher RPM's' all the tuning in the world will not fix this issue..


2. The cylinder closest to the fire wall is considered # 1 on this engine. Finding the fire TDC will enable you to mark the crank pulley and verify timing. Again, convenient here. Turn the engine by hand till the piston is rising and you feel compression by applying a thumb to the spark plug hole. when it gets close to the top insert a wooden dowell. when the dowell is at the apex, that is TDC. Mark the pulley.
Write Out works well or a white Sharpie.


Enjoy !!


Carl

 

Agree with Carl, but you say the comps are fine?.

1) I am leaning to a tight tappet. Very common as the engine heats up. HOWEVER not consistent with the descriprtions you give above.

2) Lean mixture, and/or unbalanced carbies, meaning one is coming "on" before the other by a millisecond.

Failing that:

Is the misfire a "regular beat", or "erratic"?

Pull the plugs and note CAREFULLY whence they came, and eyeball the colour of the electrodes. An off colour plug will tell you which cylinder is hissy.

My years with the S2's is that to set them "as per the specs" is a tad tooooooo lean, and a simple SMALL turn of the mixture screws to the rich side takes that annoying misfire away.

If you still need emission testing up there, that may be a problem.

Balancing those 2 SU's is a learnt art, and it takes serious time, Machines etc simply will not balance then, it is a "feel and listen" procedure. I learnt mine with my MK7 waaaaaay back, and it took me days to perfect. Once done , that thing was a flier and smooth as.

Spark plug gap is too wide.

Final thought, the cam timing is out of synch, again a common thing and easily sorted.

 

Thank you Carl and Grant for all your suggestion. Will go back to the drawing board, as they say, and try all those that I have not check, namely Valve timing and clearance and report back.
Thanks again
Amir Manzoori
London