I just picked up an XJS with a non-running V12. The previous owner stated that when turning it over, the engine would pop and backfire through the intake and sure enough, when I tried it, that is exactly what it would do. He felt that it may have slipped the internal timing that drives the distributor. Since I couldn’t get any history on what was done previously. My first thought was that I may be lucky and someone might have just put the spark plug wires on incorrectly. So I position the crank at top dead center on the compression stroke of cylinder #1A. I pulled the distributor cap off and the pointer was about two wires off the #1 plug wire in the advanced position. Once again going with the hope that someone previously making a possible mistake, I ran on the assumption that, as unlikely as it may sound, possibly someone put the distributor in a few teeth off. I rewired the plug wires to accommodate the alternate rotor position and gave it a try. Still the same problem. I would think if the timing had slipped, the rotor position would be in somewhat the retarded position if the rotor turns counter clockwise.
I am really scratching my head at this point. Since my manual for this won’t arrive for about a week or so, I don’t know if the distributor timing is picked up off the cams or another source. Do these engines have any history of internal timing issues? Any input would be greatly appreciated

Chris

 

I'm not personally aware of a history of internal timing problems but that doesn't mean it never happens. I've never had a V12 apart so I really can't comment with authority.

The dist is turned by a shaft running off the timing chain (or one of the timing chain gears).

Maybe pull the dist and look for gear damage? And/or have a helper crank the engine and make sure the dist rotor is turning properly without skipping?

If one of the timing chains/gears has slipped/stripped all the valves would be out of phase and you probably wouldn't have any compression.

I'd go back to square one: 1A at TDC compression, plug wires back where they whould be, rotor pointing to 1A on the cap, crank engine. Bring 1A back to TDC compression and see if the rotor is still pointing to 1A. If not something is definitely slipping. You might have to do some heavy exploratory work to find out what.

If the rotor returns to 1A as it should, and you have compression, then I'd start looking for a fueling problem as a cause of the popping and backfiring.

Others will chime in hopefully

Cheers
DD

 

Never turn the engine over backwards, by the way

Cheers
DD

 

Hi Chris-
I’m new to the V-12 world myself, but feel pretty comfortable around most engines. I have heard that the distributor drive gears can fail, but more problematic is the timing chain tentioner. It is absolutely critical not to rotate the engine in the opposite direction of how it normally runs. Apparently the timing chain tentioner will undo itself and cause major problems. My suggestion would be to remove the cam covers, disconnect the primary coil wires or ground the high tension lead to the ignition coil and bump the motor over with the key until the camshaft timing mark lines up with the marks on the front cam bearing cap. Now you are on TDC #1 Cylinder. Then remove the distributor cap and confirm the rotor is pointing to #1 Cylinder. The distributor cap should have the spark plug wire cylinder number on it so you can trace the plug wires and make sure they are not crossed. But before going thru this drill I would recommend a simple compression test first to be sure you have a healthy engine to work with. The shop manual is very useful but assumes one knows quite a lot. Hope this is of some help.
Bernie

 

Just curious Doug, why can't you spit it backwards? Does it mess up the tensioner?

 

My understanding is that turning the engine backwards can destroy an elderly tensioner (or some part of the tensioning system) that might otherwise live indefinitely.

Again, I'm not an authority but those who I've come to trust as V12 gurus offer the same (but even more strongly worded) warning.

Cheers
DD

 

Everyone, thanks for the great feedback. Initially I considered the cam timing with the thoughts that the first thing that would show up is low compression. I only checked one cylinder and it was close to 160 lbs. Even though one is not conclusive by any means, and eleven more good readings would be an absolute, it may tell me if the timing chain has slipped, I would think. I wonder if the timing was off just a little, could the compression still show up as good?
S3Jag, it is funny you mentioned pulling the cam covers and checking the cam timing marks. I was just looking at how involved it would be to gain access to those covers. That would eliminate that as a possible culprit. I may just take a look at that distributor gear first.
In regards to not turning the engine backwards to get to TDC, at this point, we are just using the starter to position the crank. Quite honestly, my wife now has a good understanding what “Just bump the starter” really means.