why the differance?

[GARTHXS]

Looking at an old thread it says that a UK model e and f the timing is 5° btdc and the us models are 10° btdc. Why is it different

[crazy steve]

Well, just a guess but the E was touted as a performance bike in the US and the additional initial timing will certainly improve performance, but make the bike more sensitive to fuel quality and a bit harder to start (also a bit harder on the starter). It's interesting to note that the US-spec E was the only model that had the 10 degree initial advance (Clymer incorrectly shows the E/F as being the same; they're not). This was all about drag strip times...

There's also some discrepancies in the parts fiche; while they show the mechanical advance unit as being the same for the E/F/SF (as well the 2H9), if you look in the FSM for the E model it shows 26 degrees total advance, while showing 31 degrees for all the others (there's your 5 degrees difference...). You'll also not find the vacuum advance unit listed as a separately available part for the E; you had to buy the entire timing plate assembly, so there may be some differences there too. But at this late stage, due to aging in the parts, you would probably be hard-pressed to find the actual difference.

[3Phase]

... at this late stage, due to aging in the parts, you would probably be hard-pressed to find the actual difference.

Steve, the total advance wouldn't change much but the advance rate would change as the diaphragm and the spring in the vacuum pot aged. I think Ivan got that pretty well sussed when he discovered that the vacuum pots are adjustable if you remove the resin seal from the end of the diaphragm vacuum chamber. I haven't actually messed with any of the ones on my bikes but they could be reset/recurved on the bench with a vacuum pump and a slide caliper, then fine-tuned on the bike with a timing light.

I have seen several of the vacuum advance arms with a wallowed out, rounded hole where it connects to the pickup coil advance plate. It's probably from years of pulsing/rattling caused by a misadjusted #2 carburetor throttle plate or, worse, connecting the vacuum line to the manifold instead of the #2 carburetor nipple.

The advance arm itself is hardened steel so you can't just tap it with a small hammer or squeeze it back into shape because it will shatter ... go ahead, ask me how I know ....

To fix a damaged advancer arm without destroying it you'd have to drill out the hole and press in a small bushing, or weld the hole shut and then try to index and drill a new hole in the weld without breaking it. So far it's a lot easier to just find another vacuum advance pot from the correct year/model XS but that will eventually change as good used parts start to disappear.

[crazy steve]

....So far it's a lot easier to just find another vacuum advance pot from the correct year/model XS but that will eventually change as good used parts start to disappear.

Yep, that's a true story.... I've learned not to throw away anything unless it's just totally trashed 'cause you never know what might prove useful down the road. I think if I was trying to fix an elongated hole in one of these, I'd try brazing or silver solder rather than welding though...

I am a bit suspicious of the FSM specs for some of these parts, as they show the same curves for different years with different part numbers, but not knowing the reason for the different numbers (changed design for improvement/cost, or an actual difference in the curve?), it's not something I'm going to lose sleep over...

[3Phase]

... it's not something I'm going to lose sleep over...

I know what you mean. Look at the pages and pages of carburetor, distributor/vacuum advance specs in one of the old Motors repair manuals and then get all paranoid about tuning or buying parts. When you went to the parts store the guy would walk over to a shelf, pick out a part from a shelf with a relatively small selection compared to the assortment you would think would be required and say, "This one here is what you need!" as he plopped it on the counter. And by golly it would work!