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Distributor discrepency.

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ReggieV View Drop Down
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    Posted: 05 Feb 2019 at 21:18
I have today got my re-build started, and have fitted the correct distributor for a LM1 which is the bike in question. The issue that I am having is that the timing is supposed to advance from 8* to 34* giving a total advance of 26* but my estimate of the amount of advance given by said distributor is about 18*to 20* in total and not 26*.
I have written about this issue on the forum a few years ago when this distributor was on my other LM1, and what I was advised to do was what I have done today, which is to set the ignition at 5,000 rpm to the correct markings and let the low speed running be advanced.
 
The flywheel is a correct one for this model and new. The base plate in the distributor is free to move on its pins. I fitted new springs from Stein Dinse a few years ago and the base has a bit of forward and backwards play when in at rest. Could this be the problem?
 
I would prefer to get the distributor to be working properly. Does anybody have any ideas? Thinking about it, maybe a spring that removed this play when at rest and pulled the base plate back further may address the problem? I could try new springs again possibly.
 
Slightly puzzled.
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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: 06 Feb 2019 at 14:00
The MarelliS 311 A provides 26° of advance, was fitted to all Tonti 750s and 850s except the T3 and Convert (S 311 B, 31°).
At rest, the stronger of the two springs is slack, but the weaker one should have no slack. If it's not the case you'll be losing a portion of the advance range, and the idle speed will not be stable because of the "wandering" initial advance caused by the slack.

This is distinct from the camshaft worm drive to distributor pinion slack, which is unavoidable.
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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: 06 Feb 2019 at 20:25
Yes springs should be different, the 'weak' one has round eyelets, the  'strong' one has an elongated eyelet at one end, so that it doesn't start pulling until shaft has rotated to take up the slack. 18 - 20 does kind of sound like it's only doing the top few degrees where the strong spring operates. As though you've only got two strong springs?





"Chicken nuggets don't dance on a Tuesday."
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ReggieV View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ReggieV Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 2019 at 15:39
Thanks for the replies.
I did have a look in a few weeks ago and one of the springs was "slack" which I wasn't sure if that was correct, so thanks for confirming that for me, but I can't remember enough about how "tight" the other spring was or whether it took the base plate back to the start of its travel?
 
I will be having a look inside in the next couple of weeks and will report back, or maybe sooner.
 
Thanks.
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ReggieV View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ReggieV Quote  Post ReplyReply Direct Link To This Post Posted: 18 Feb 2019 at 22:26
I've had a look in the distributor today and there is one spring that is "engaged and the second spring has an elongated loop and is not engaged. The advance plate will not retard any further than where it is being held with the shorter spring.
I did fit these springs new from Stein Dinse about 5 years ago. Therefore I can only assume that one or both are too strong to allow full advancement of the plate. I put the distributor back on the bike and there was still a restricted amount of advancement.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpc Quote  Post ReplyReply Direct Link To This Post Posted: 19 Feb 2019 at 07:15
Your primary spring does look a little "butch" compared to one of mine. Hard to tell from a pic, though.


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ReggieV View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ReggieV Quote  Post ReplyReply Direct Link To This Post Posted: 19 Feb 2019 at 10:46
jpc wrote; "Your primary spring does look a little "butch" compared to one of mine. Hard to tell from a pic, though."
Thanks for the picture jpc.
 
It certainly does. There are the same number of coils in the spring but it does look heavier duty.
I'm loathe to order a new set of springs from S. Dinse in case I just get the same springs. I'll possibly give Gutsibits a ring and see if they have any used springs available. A long shot, but I won't know until I ask.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpc Quote  Post ReplyReply Direct Link To This Post Posted: 19 Feb 2019 at 11:25
The wire diameters for the pictured distributor are 0.80 (primary) and  0.70 (secondary).
This is from an "A" type dizzy fitted to a (crashed) LM1 that showed a correct advance curve.
The primary is "gun black" colored, the other shiny.
This is finicky stuff to get right, good luck.
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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: 19 Feb 2019 at 20:15
Originally posted by ReggieV ReggieV wrote:

I've had a look in the distributor today and there is one spring that is "engaged and the second spring has an elongated loop and is not engaged. The advance plate will not retard any further than where it is being held with the shorter spring.


Possible the 'short' spring isn't short enough. You could take them both off and then twiddle the cam to see exactly what its full range of movement is. Question

The position of the 'long' spring does seem to suggest it's got slack to go back some more.

"Chicken nuggets don't dance on a Tuesday."
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