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850T Distibutor advance springs

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GUZZIPETE View Drop Down
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    Posted: 12 Aug 2018 at 21:37
On strobing the timing on the 850T, it seemed that the flywheel marks for full advance are reached very early, in fact just above a fast idle. I decided to change the distributor advance springs in case they had become weak. I noticed that the spring with the elongated eyes was quite loose, with play between the end of the eye and the spring post. Initially I thought that this could be the possible cause of the issue until I fitted the new spring and found that this 'play' was exactly the same.

My question is, is this normal? Is this designed to allow the bob weights to expand a little, before they are resisted by the springs at all, and then the first springs takes the second stage of load before the second spring adds further resistance at higher revs. 


1971 Moto Guzzi V7 Special
1974 Laverda 750 SF2
1979 Laverda 3CL

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jpc View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpc Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 05:39
The 850 T and Le Mans were fitted with Marelli S3 11 A, the T3 and Convert with S3 11 B, they give very different curves and total advances, 26° vs. 31° respectively (the number is stamped on the side of the bottom flange).

Your description fits the B version, where the "looser" spring comes into effect at 3000 RPM and flattens the curve up to 6000 RPM (max adv. 31°, initial adv 0°)

The A version gives a straight progression from 8° (initial to 1500 RPM) to 34° (4400 RPM & up).

Make sure you have the right "distributor" type and the right markings on the flywheel. The starter ring has 96 teeth, so that's 1 & 3/4° per tooth interval.



Edited by jpc - 13 Aug 2018 at 05:51
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Mike H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mike H Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 11:36
^ What he said, they're deliberately different to get an initial steep slope advance, then a shallower slope advance from 3,000 rpm.

Personally I've never had much joy with strobes, always preferred the static method, like the old fag paper trick, i.e. put a fag paper between the points, when they open it goes loose. If this is at TDC, then is pretty much about right for 4 degrees BTDC or whatever, at idle. Or can and have used a multimeter on Ohms range. Get both cylinders right, then can fine tune later by tweaking the angle of the timing body during a "try-it-and-see". Getting the idle and bottom end right is better for most practical everyday purposes, rather than fussing about what happens at max. rpm.

HTH

"Chicken nuggets don't dance on a Tuesday."
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gianni Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 12:28
Are there any other differences between the A and B type distributors other than the springs?
Le Mans 1, Spada, Cali 1, T3 Mongrel, Le Mans V, Quota x 2, Stelvio TT
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpc Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 14:16
Yes, one gives 26° total advance, the other 31°.

I'm not quite sure the springs are different, but some springs are advertised as specific to the T3 for instance: https://www.ebay.com/itm/MOTO-GUZZI-750S-CONVERT-T3-SP1000-V7-SPORT-ADVANCE-RETARD-SPRING-SET/182528697257?hash=item2a7f8ee3a9:g:00UAAOSw3utY71YM


Which is all pretty confusing because the Tonti 750s, LMs and Ts share the same A type distributor...
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GUZZIPETE View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GUZZIPETE Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 14:24
Thanks for the info guys. I will check it out and get back.

I know strobing can be a bit hit and miss, but I like it use it a a base line.  A friend Normski, also of this parish I believe, listens for the spark whilst watching the flywheel marks, which seems to work well. He also has struggled in the past with the strobe light method which seems to mislead at times.

How the bike runs and idles should always take priority over text book marks I guess. After all, the bikes are 40 years old and vary widely in wear through use and less precise factory tolerances back in the day.
1971 Moto Guzzi V7 Special
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpc Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 14:44
Anyone who can hear the sparks with the engine running would have me worried.
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GUZZIPETE View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GUZZIPETE Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 14:56
Yes, definitely! 

I probably should have pointed out that this is with the ignition on, plugs out, distributor cap off and and turning the crankshaft by hand.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpc Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 15:00
LOL
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gianni Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 15:42
Originally posted by jpc jpc wrote:

Yes, one gives 26° total advance, the other 31°.
 
 
Thanks.
 
How is that achieved?
 
My T3 Mongrel is twin-sparked and I think I added a washer under the auto advance unit to reduce the total advance by constraining the initial position. Interested to see if I could do it more intelligently.
Le Mans 1, Spada, Cali 1, T3 Mongrel, Le Mans V, Quota x 2, Stelvio TT
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Mike H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mike H Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 15:44
AH!! Big smile

Neat trick tho.

(EDIT: This is a reply to guzzipete's ^)



Edited by Mike H - 14 Aug 2018 at 11:00
"Chicken nuggets don't dance on a Tuesday."
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpc Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 16:31
Originally posted by Gianni Gianni wrote:

  
How is that achieved?
 

Just my luck to run into a persistent fellow  Smile.

I just took an A version I have on hand apart, but don't fancy dismantling the only B version I have on my T3 just to compare the two right now.

It looks to me that the most economical way to go from 26 to 31° (or whatever) is to modify the slots in the bobweight arms. That would do it.

Correction: you would modify the slots in the fiber plate UNDER the bobweight arms, that's what constraints the travel of the cam.




Edited by jpc - 13 Aug 2018 at 16:41
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gianni Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 19:44
Thanks jpc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpc Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 21:16
You're welcome. I in turn would be interested to know what ignition timing you find useful with the twin plugs, and what bore size, C.R. and inlet size are associated with it. Standard cam ?
(I have a spare pair of big valves heads for such a job, would you say it makes it a better engine ?)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gianni Quote  Post ReplyReply Direct Link To This Post Posted: 14 Aug 2018 at 00:13
Engine is otherwise standard T3 except for a billeted Le Mans flywheel, Lafranconis and 36mm carbs (upped from 30mm so I could get the accelerator pumps).
 
The twin-plug setup is 2 x 6v Lucas coils in series each side running off a T3 distributor. Can't remember the timing details except I had to reduce the amount of overall advance.
 
Bike is designed for London so lots of low end power and virtually unstallable.
 
When running right it is good but I have had lots of issues with the ignition which may be due to old condensers and coils, and confused plugs. It cost me knowt to do (for various reasons) but the likely £200-£400 commercial costs would (IMHO) be better spent elsewhere like on an SS2/P3 cam. YMMV.
Le Mans 1, Spada, Cali 1, T3 Mongrel, Le Mans V, Quota x 2, Stelvio TT
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