After fitting my rebuilt damper and going for a test drive last Thursday, while being very satisfied with the improvement in running smoothness, I detected a slight, intermittent misfire in top gear at around 60 mph. So I pulled the plugs and cleaned them and checked the points gap yesterday. Today I was planning on another drive to check things, however, a backfire on engaging the starter indicated that the timing was off.
I pulled the dizzy checked and cleaned the points, but on refitting the dizzy with the engine at TDC on #6 (front) and the timing mark just coming into view I noticed that the dizzy wasn’t fully seated, in fact the only way it would seat fully was to rotate the drive through 180 degrees! Now, of course, the rotor is pointing backwards to #1! The bizarre thing is that when facing forward it will engage the drive it just will not fully seat flush with the block. I recall that when moving the dizzy to get the points to their open position I put a screwdriver in the top of the shaft engaging the screw below the rotor and was surprised that the drive did rotate. This, on reflection, is clearly what put the timing out! I have put over 30,000 miles on this rebuilt engine with, I suspect, the drive incorrectly engaged. Is that even possible!! I have removed the oil filler cap and confirmed that the engine is on the compression stroke.
I am at a loss as to how to resolve this situation? Any thoughts, suggestions appreciated.
I am sure the clever people will be along shortly… but in the mean time… If you have not loosend the clamp the dizzy it will only go back in the same timing as you took it out regardless of where the pistons are. To determine compression I would take out a spark plug , put your thumb in the hole and turn the engine with a spanner to feel compression. The drive in the engine could go back in many positions, but as long as the the dizzy doesn’t foul on the engine does it matter that much?
I stand to be corrected.
Jim
my mind is baffled with…what is the relationship of the flywheel TDC mark, with where #6 will be, relative to the number of flywheel full 360 deg rotatons (mark at window) to have #6 at TDC compression. Another way,…if #6 is at TDC on compression…how many times does the flywheel mark rotate and pass/come to window…to be back on TDC compression. Will the flywheel mark be at TDC again when #6 is TDC on exhaust, and…Another way…is it correct that each 120 deg of rotation brings one of the …
The driving dog has a very small offset, so one way is correct and it will drop in easily, but the other way will sort of fit if you really cram it in there. You can rotate the rotor as you go and feel that one way feels better than the other way, but the wrong way can be a fooler if you are not aware of it.
Next big question is, did you disturb the orientation of the 20 tooth drive gear in relation to the crankshaft? If so, your driving dog position may be wrong.
Last question, did you take the distributor apart? If so, the rotor position in relation to the driving dog may be wrong, 180 degrees off.
Having read through all the previous posts on this issue I have been enlightened by most, but still confused by others (not hard at my age).
Clearly, the only way my dizzy fits correctly is 180 degrees out from where it should be with the rotor pointing forward to approximately 3:00 o’clock (it in fact points to 9:00 o’clock). My dizzy (DVXH6A) has never been apart in my 56 odd years of ownership, but who knows about those first 10 years? From reading one of Rob’s later posts it would appear that the cam in the dizzy can be put in two ways, thus be out by 180 degrees. I have never touched the gear that drives the dizzy, so, unless my engine builder did, that can be eliminated. What confuses me is that looking at the dizzy drive I can neither visually nor measurement wise see any “offset”, it is certainly not stepped like later engines and, even though I cannot see it, it will only go in so that the dizzy body fits flush with the block one way. Turn it 180 degrees and, while it engages, it sits up proud of the block.
The only thing that amazes me is that this means I have logged all those miles with it fitted incorrectly. The two ‘teeth’ at the base of the dizzy do not look damaged at all, unbelievable! Nothing else has been changed set up wise so I think my solution, in order to maintain the correct flywheel timing mark and the rotor pointing forward to #6 segment of the cap as they now are, would be to dismantle the dizzy and move the drive through 180 degrees. This would align everything the way the manual states and, importantly, allow the dizzy to seat properly.
If it used to run then I think trying to dismantle the dog will just make it more confusing .Do the thumb test then when you feel compression see where the rotor is pointing and obviously it should be pointing to the lead on the cap that goes to that cylinder . If you are convinced it is 180 out it is easier to move the plug leads 180 on the cap , but I don’t think that is the answer
For the purposes of setting the static timing, it makes no difference whether the distributor rotor is pointing to cylinder 6 or cylinder 1. Just point it to number 1 so the drive dog fully engages and then set the timing. The engine should run fine. (I really think the drive dog must have been set in this orientation before you removed the distributor.)
Car is running, I need to get it up to operating temperature, road test and adjust the timing, however, thunder storms are pending, but it is running! I am amazed that I cannot discern any difference in the drive at the base of the distributor, I even measured the length of each ‘tooth’ from the centre and both are 8/32" and they are the same width. Still the unit only went in one way, go figure. Thanks for the help guys.
seems like you have it running…and most questions answered: …yes there is an original way the dizzy rotor cam was set from factory new…and same for the gears in the distib drive ear to crank…but for those, even if 180 out… or not to original, …still and never-the less the timing can be set up: once one sorts what the setup is: and re arranges the .wires to plug in firing order. The distributor dog fits in the slot only one way,…per the very slight and hard to see offset. Some say that with inappropriate force it can be in 180 out…I don’t think it will fully seat that way. Each tooth on the flywheel is 2.73 degrees 132 teeth total. TDC on whichever…1 or 6 (or any other) can be on compression, or on exhaust…put a small rubber stopper in the spark plug hole…and rotate the engine in the proper direction…by hand…or just roll the car forward in 4th ear…(do not peer in over the stopper…it blows out quite forcefully !!!) on compression…once you know it is on compression…then a bit of back and forth…to find exact TDC…there is a slight amount of dwell at the top of the stroke.: Now…look at the flywheel for the mark…and or on the damper…and enhance it with a dab of red or orange paint…now remember that this is TDC…and your static timing will be set BEFORE TDC… that means the tooth you use for timing and make your static timing mark on will show in the window BEFORE the TDC does…mark your timing tooth with WHITE paint…this is the one you want to see…perhaps with a timing lite. 2.73 degrees per tooth. (only rotate the engine…one proper way…as in operation: which by the way is counter clock.: .as from the position of a driver looking forward. .(.this can be confusing…as oftentimes when you look at damper or flywheel this is often seen from front…looking back…so from that view is clockwise…) It is OK to turn very slightly back and forth to find exact TDC.but .be in the habit of the final engine move being in the proper direction…this would be important in cam timing and timing chain adjustments. Nick
Maybe this discussion would be more beneficial with some pictures.
Start with the orientation of the distributor drive shaft in the block.
Page B32 in the XK120 Service Manual has this paragraph.
This means that when the #6 front cylinder is ready to fire, the distributor drive should have the fat part close to the block, slot away from the block, and look about like this:
There are 20 teeth on the brass drive gear, so if you are off, it will be 18 degrees or a multiple of 18 degrees off.
The offset driving dog is offset about 2/32" or 1.5mm
It should only go in one way. If you put it in 180 degrees off, it may sort of catch in the slot, but will probably not sit down in all the way.
Now the rotor should point forward.
If it points backward, your points cam shaft is in 180 degrees off.
I, along with many others on this forum, are beholden to you for both your knowledge and expertise on all things Jaguar and specifically XK Jaguars. It was by reading through the previous threads and comments on this issue (brought to my attention by Michael Moore) yesterday that I came upon your piece about changing the points camshaft through 180 degrees (which I ultimately did). Thank you!
I see from your posted photo of the driving dog offset how subtle it is. While I had my distributor out I simply could not see it, even after measuring (but I confess, I am not an engineer!). Making the change has allowed me to continue setting up the initial static timing on my XK as per the manual.
Good news is that today a road test proved everything is fine and the car performed well, that is until some character cut me off and upon applying the horn I was a little embarrassed to have it stick on. A quick nip down a quiet side street allowed me to disconnect them and, after returning home, that issue has also now been resolved!
Chris, I was once at the same place you are right now with my 4.2 saloon, I was trying to install the distributor in the same exact orientation it had it when I removed it. My problem was that I had ignored that the engine rotates 2 360 degree turns for each 360 at the distributor.
So #6 piston will be at TDC two times for each engine revolution; once at TDC compression and once at TDC at intake. Needless to say, other timing marks will do the same, You have to make sure when it is at TDC that it is at TDC on the compression stroke. Good luck,
Mike
ps-The other headache provoking problem I have created was to replace all the cables with the correct firing order buy in the wrong direction, The problem was not at all obvious to me for some time.
TDC…will be either compression.both int and ex valves closed…(that is after…the intake occurs which happens on a downstroke), or EXHAUST…as the piston rise pushes exhaust out of the open exhaust valve. Nick
