Limited Advance

I've got a vac advance distributor and the advance is limited. From reading before, it should be 7.5 up to 28 degrees at 3,000 rpm but it's not dvancing that far - maybe 22 degrees. The idle is set a bit more advanced now, 9.5 idle to 24 at 3,000 and it runs much better. Is this likely to be something that can be fixed / cleaned in the distributor advance, or TADTS?

 

It's likely that the advance mechanism is sticking. It's probably not returning rather than not advancing.
Warm it up, then take the distributor cap off and twist the rotor arm clkwise then let it spring back. See if it'll go back further anti-clkwise, then start her up without revving and recheck the timing. If it's now retarded, set at 7.5 and rev to check full advance. Now let revs drop off and see where tickover timing is now. did it go right back?

The other possibility of course is your tickover is too fast.

 

Tickover is fine, reading 740 rpm on the tacho. I've removed, stripped, cleaned and rebuilt the distributor, the only problem I noticed was that the vacuum advance unit has failed. Checking the advance now, I'm still only getting a bit more than 20 degrees at 3,000 rpm. Will set with a bit more advance now and then see the results with the replacement vacuum unit.

 

Just to clarify, do you mean you set the advance at tickover to 7.5 degs with the hose OFF an it then advances TO 20 degs maximum?

 

Exactly, although when I looked at the vac unit, hose on or off makes no difference as you can just suck straight through it.

 

I have a used one you can have for £15 posted.

 

*****, I ordered one from VWH already as per other thread.

 

The distributor is a VW part number 113 905 205 AN, it has 'remanufactured' stamped on it. According to Bentley, this part number has no mechanical advance (although the AN suffix isn't listed in Bentley), so I'm a bit confused.

Edit: Doing a search on the part number, JK are showing this distributor with the AN suffix.

 

For some reason your distributor does not advance anywhere near enough mechanically. Maybe you put it back together wrong?
I'd have said get an accupark electronic SVDA distributor from ebay for £59.95, but I guess you're too far into fixing this one now...

 

I didn't strip the advance mechanism, just cleaned it up and made sure it was all moving freely. It'll be out again to fit the vacuum advance unit, so I will dismantle it again and have another look later in the week.

 

Fitted the vacuum advance unit. The mechanical advance is still the same - 8 degrees at 800 rpm and 20 ish degrees at 3,000+ rpm. The vacuum unit brings it up to around 28 degrees at 3,000 rpm.

 

You're still lacking 10 degs mech advance...

 

1 more strip down and that's it.

 

Have you stripped it down this far?



Could be part of the mech advance system is gummed up and needs a good clean and an oil... The above was completely seized so goodness knows how it was even managing to drive at decent road speeds

Just to show you the rust build up due to a lack of oiling of the felt pad under the rotor arm!!

 

No, I just took the points plate off and cleaned the advance mechanism with brake cleaner and re-lubricated. It was all very clean and greased (has remanufactured stamped on the side as well) and moved freely, even before I cleaned it. It's worth another look anyway.

 

OK, final look at the advance mechanism. At least I can remove, strip, rebuild, install and time a distributor in well under an hour now. The advance mechanism has stops for the minimum and maximum positions and moves freely between those stops. So, I measured the rotor arm displacement between the stops and marked up the outer edge of the distributor body. A quick calculation revealed that the possible mechanical advance on the distributor is just under 13 degrees. Given that, in practice, I am achieving around 12 degrees, it seems that the theoretical and actual performance of the mechanical advance is the same, i.e. 12 - 13 degrees. The distributor part number fitted is one that JK advertise, 113 905 205 AN.

 

I think I remember someone having an issue like this a while back with a new dizzy... you have 2 options...

1. Massage the fully advance stops so you get more advance - Be warned tho... too much off and you'll advance too far...

2. Buy a new dizzy!!

 

Suppose it depends how soft the steel is, but it's going to be a pain to mod the stops right.

 

Coming round to this idea now, since it appears the existing is just wrong, for some reason. Are these any good for the money - seem quite cheap!

 

Worryingly cheap, but many TLB'ers have them and nobody has reported any problems.