Total advance on XK engine

One thing I don’t think I saw mentioned is that if you are directly measuring the angular displacement of the rotor, keep in mind that the distributor rotates only once for every two rotations of the crankshaft, so 5 degrees measured in the distributor is 10 degrees at the crankshaft.

Hi Ron. I had British Vacuum in New Hampshire rebuild and change the specs of my distributor when I converted my S1 XJ6 to triple SU’s (among other mods). He definitely sells vacuum capsules and other parts as well. No affiliation but he did a very nice job.

That looks like a good resource. I have used Advanced Distributors for a Lucas BMC 1275 distributor for my Morris truck, with good results.

Nobody beside Distributor Doc seems to sell springs, which may be because it’s too much of a compromise compared to a rebuild/recurve. When I get the time, I’ll think I’ll try to snug my springs up, then get a rebuild this winter.

So, progress report here.

I pulled the distributor and examined the initial advance spring, which to me looked stretched and weak.


notice how the last coil looks stretched. So, I snugged up the ends and tightened it.

Upon reinstalling, I noticed a prolific leak from my front carburetor, which resulted in 6 hours of work to fix, plus getting some nastiness out of the float bowls and changing the filters. I’m afraid my tank cleanout wasn’t fully successful, but the fuel is now clean at my see-through in-line filter. Anyone who has removed and reinstalled a front carburetor on a Mk10 knows why it took so long - b^& of a job.

So, I static timed at 10 degrees and started the car. Idle speed dropped about 75rpm from before the distributor work and crank timing decreased from 15 to 12 at 550 idle, which was smoother, not perfect. Went for a ride. New hard miss on acceleration, not improved by switching in the ASC, so probably not a lean miss. I verified vacuum assist working. Today, I gathered data for an advance curve, also scaled the ported vacuum.

Compared to the curves posted by Davidxk ,mine is flatter in the 1000-200 range, but I’m not sure it’s enough to cause a miss. Spring might be too tight. Might be something else going on. Also noted some sooty plugs, which has been an ongoing issue with this car - the air filter arrangement on the Mk10 is a real choke point, I’m learning. Tune the car with the air filter off, then have to lean it out when you put it on.

Also, the distributor cam was stamped “9”, which should give me a total advance of 28 degrees, but I obviously have a lot more, off the scale at 3000rpms. If I read the stamping correctly, this is a 1971 distributor in a 66 car. The vacuum assist bumps the advance up through the whole curve, but may not have as much effect if there was load.

All this is interesting, but doesn’t lead me to any strong conclusions other than getting the distributor recurved when I can. Plan is to replace the plugs with some NGKs, get my air/fuel meter working (new gauge ordered) and see about getting my hands on some high quality new points, condenser, rotor and cap. Maybe prayer.

Any thoughts?