SF0 750 - DMC2 Ignition - Top out at 40? BTDC?

Haggis

Haggis

Hero member
Location
UK
Hi All

It's well documented that 750s tend to hole pistons when ignition advance creeps towards being too far advanced (because of points mech wear, bad setting up, or whatever).

I want to aviod that fate, so, with a DMC2 - I'm deciding which ignition curve 'group' to use. There are 3 choices, topping out at 40, 37 and 34? BTDC.

My basic question is...
All things being equal (i.e. sensible mixture, cam timing, compression, etc, etc) are 750s happy with 40? tops? Or, does decades of empirical in-use evidence show us that this factory spec was a bit too far advanced?

SF0 in this case, but more of a general 750 query.

Cheers.

Paul
 
Whatcha talkin about Shaun? Are you telling me the cam joint ISN'T meant to look like this?
35013296064_90f3d66314_c.jpg
 
With everything else being as it should, it should run happily till the end of its days on 40? BTDC.

If you cannot attain a steady and reliable idle, please drop me a PM and I'll walk you through it. :D

piet
 
Never had an issue running full 40 deg advance, even when I went to SFC cams, valves, 2/1 and higher comp (10:1). But on my current 880 motor build with an ignitech sys I'll be dropping max to 32 deg.
 
I have to say Paul points are not a common problem for holed pistons, looking at the comments on my Facebook  group for 750s (Breganze 750 twins) from respected owners, it"s more like poor quality leading to  cracked heat manifold spacers or poor fuel or carb air leaks
.Not saying it isn t a good idea to have a DMC2  but points are fine if serviced, for many a mile too.
 
one thing often forgotten: it is not the maximum advance that is the problem but when it actually sets in... The points do not kill the piston on high revs, they kill them because the get stuck open at 40* in low-end, too.

So - full advance at 7.000RPM could be even higher than 40* potentially, depending on the whole setup, but it doesn't change anything or offers any benefit really (or if, just a marginal one). As long as the curve goes up constantly and the max is reached somewhere beyond 6.x00 or so, you should be good (talking about standard cams and pistons).

lowering the advance can make the engine running smoother, but also not with the maximum torque. But also that is just marginal... We had the 1000 SFC running on 4 different curves on the dyno. all showed almost identical curves. The range went from as low as 30* over 32* to 2 different ones with 36* - no significant change in peak power and minimal changes in torque, as at the point of maximum torque, all 4 ignition curves were very close...

just my 2 cents...
 
LJ said:
I have to say Paul points are not a common problem for holed pistons, looking at the comments on my Facebook  group for 750s (Breganze 750 twins) from respected owners, it"s more like poor quality leading to  cracked heat manifold spacers or poor fuel or carb air leaks
.Not saying it isn t a good idea to have a DMC2  but points are fine if serviced, for many a mile too.
Click to expand...

Maybe not the points themselves, but the condition of the entire points set-up.  Generations of bodgers have usually had their hand in that, resulting in bent and buckled base plates, pulled threads, etc, etc.  Add to that, normal wear and tear coupled with decades of neglected maintenance, you have a sure formula for trouble. 

I only recently had a bike in still fitted with points, one cylinder could be timed spot-on, the other wavered around with more than 15? discrepancy at full advance with the strobe flash jumping to and fro... just too much play in the pivot of the moving contact.    DPO had already replaced many of the fixing screws with larger ones, the fit of the secondary points on the base plate was sloppy, everything required fettling to fit back into the primary case half-decently.  Needless to say, the new owner will be up-grading to an electronic system very soon.

piet
 
Can t argue with that Piet,if that"s poor maintenance,and aging bikes will need a replacement, but that"s not what my post was on about holed pistons..
Many owners with high mileage twins on points.

Perhaps that"s called missing the point  :)
 
I found on my xt500 that if I set the ignition per the book, it linked a lot..it took trial and error to get it right..no linking and still being able to start it.It is sweet now I must say..I have a 12v upgrade and bigger light g coil but will keep the points.As Pete says the DPO had been there and the central weight advance was misassembled.Right now am in  Costa while my SP1 gets a set of tyres! Jeez rode it on the mismatched pair..original front so 17 years old...very wooden!! 6.5k miles one owner...it's a 'k in rocket. Unbelievable..and I had a Firestorm new in 98.  Jota on cover of Classic Bike guide! A surprise when in newsagent..must have used a pic from when they tested it a few years ago now.nice when you look at what else is in the guide..MV..rc30...Vincent.. just everythi g.And yes mine wears it's mudflap!😁
 
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