Distributor advance spring information
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
Thanks to Tobin Peever for sending his thoughts to me via email. In Tobin's own words:
In rebuilding the distributor, I followed your excellent directions and photographs. Very helpful! Can't thank you enough for putting these valuable Guzzi resources in one place and maintaining the site. Very helpful!!
I installed 2 of the lighter, 8-winding springs from MG Cycle on the advance weights as you show and you say Mark Etheridge also endorses.
After static timing, the bike ran reasonably well but the ignition seemed retarded to me. So I put a strobe on it and noticed that the ignition really didn't start advancing until 3000 RPM or so and the curve was really delayed. I am not exactly sure of the rpms as my strobe does not have a tach. When I advanced the timing by turning the distributor, the bike ran much better but putting a strobe on it confirmed I was sitting at about 15-20 degrees advanced at idle rather than at the 10 degree mark. My hypothesis for what was happening was that since the two springs were of equal length, the advance weights had to move both springs and it took a lot more revs to generate enough centrifugal force to get the advance weights moving against both springs rather than just one with the stock setup. The consequence was that the advance curve was really delayed.
When I went back to the stock spring setup by replacing one of the shorter springs with the longer, heavier 6-winding spring as per stock, the advance curve looked to be perfect! The ignition started advancing much earlier and topped out at exactly 38 degrees at 4000-5000 RPM or so. Again, I need to get a strobe light with a tach on it to make sure I am getting the correct advance at 1200, 2000 and 3600 RPM as per manual but it looked pretty close and the overall range of advance from 10 degrees at idle to 38 degrees at 4000 RPM was spot on. When I was assembling the bike, I checked the marks on my generator pulley with a degree wheel so I know that they are dead on.
Just curious if anyone given you any feedback about the use of two shorter springs on the advance mechanism? I certainly have a lot less experience with these bikes (this is my first loop Guzzi) than either you or Mark Etheridge so maybe I am missing something. However, if my observations are correct, I would be very worried about using the two short-spring setup due to the risk of way too much advance at high RPMs.
Since the mechanism is theoretically going to give 28 degrees of total advance, starting at 15-20 at idle is going to give way too much once the revs get up there. I guess it's possible that the two springs don't allow the complete 28 degrees of advance but it would be hard to know that without revving the bike to 6500 RPM to check!!
Interested to hear your take on this!!
My reply to Tobin:
Your observations regarding the use of the shorter advance springs is correct...using shorter springs will definitely preventearlyadvance. I believe Mark prefers these because of today's gasoline, though I don't recall for sure.
I know on my V1000 I-Convert I tried two of the shorter Tonti springs, but it didn't work well at all unless I finally got high enough RPMs to max them out. So, I went back to the normal springs on it and that configuration has been working well for me.
I honestly don't recall what I'm using in my Ambassador just now. I'm sure I tried two short spring at some point in time, but I'm not sure what I have in there now. I may have liked them, or found them way too restrictive, I don't recall. I do know that I've fit some bikes with two shorter springs with no ill effects. I guess it all depends on how it works on each machine??? I know that isn't a great answer, but I know I don't have all of the variables.
I know that whenever I set the dynamic ignition timing, that I rev the engine to achieve full advance and then set the timing there. That way, full advance is correct and the curve falls downward from there. This eliminates any risk at full advance and lets the curve fall where it may. The advance weights are prevented from spinning out too far...so once you've hit the full advance with them, they won't go any further out with additional revs.
A reply from Tobin:
Thanks Greg! Do you know if all loop frame Guzzis were fitted with advance springs of the same weight? I ran across this article about Lucas automotive distributors and there was a bewildering array of different advance cams that were used in different cars. You might find it interesting as well.
My reply to Tobin:
As far as I know, all the weights are the same.
That is a very interesting article. Now if I only had a machine on which to test (and graph) advance curves!