timing, positive stop

[a-teamlambretta]

just doing timing on a motor using the positive stop method to locate tdc, as the flywheel and crank are brand new and also there are is no arrow or mark on the flywheel.
i have made a stop from an old plug with a bolt through the middle to use as the stop. i rotated the flywheel anticlockwise and made a mark on the mag housing and one on the flywheel at the same point. then rotated the flywheel clockwise untill hitting the stop , then put another mark on the mag housing . its aproxamatly 21deg between the two . i halved this and now have a tdc mark . my question is do i roate the flywheel aticlockwise back to the first marks i made on the mag housing and flywheel and then re mark the tdc mark on the flywheel , 10.5 deg back from the mark i first made on the flywheel ??, im sure this is correct but seem to be suffering brain fade lol.cheers simon

[Adam_Winstone]

Make all of your initial marks in pencil, pen or paint so that you can wipe them off once you've made the marks that you want to keep, knocking those into the mag housing lip with a very fine bladed screwdriver.

With positive stop in place, put 1 mark on the flywheel, which can be anywhere around the circumference. Then do your clockwise and antiCW marks on the mag housing where the positive stop limits travel. Halfway between these is your TDC (or BDC!!!... MAKE SURE IT ISN'T!) mark. I find this by sliding a sheet of paper between mag housing and edge of flywheel, drawing a line along the mag housing edge on the paper and clearly mark the paper at the corresponding marks that you made. I then remove this piece of paper and hold it up to a light source (against a window during the day makes a perfect light box), look through the sheet of paper and fold it line to line and mark to mark, then fold it mark to mark... the crease/fold is your half way mark (TDC). Then slide the paper back between flywheel and mag lip and align with your marks, which will put the crease at TDC. If you keep the paper folded then the TDC mark should be in the same place, regardless of which of your initial rotation marks that you align to/from. When you are happy with this TDC position, mark it (temporary again at this time).

NB: I do this a few times, varying the position/depth of the positive stop as TDC should be TDC regardless, and make sure that I'm getting the same accurate TDC position from repeated goes at the above. Once I am happy that I'm accurately getting the same position, I then mark the flywheel's single mark and the confirmed TDC mark on mag lip (you should now knock these in if you are happy with them). Once I am happy and I have the fixed TDC and flywheel mark (your arrow in effect) I measure back (antiCW) around the flywheel edge/mag lip and mark my intended ignition range... I normally mark 15, 17, 19, 21 (up to 25 if using auto advance/retard ignitions). Then strobe it and rotate stator until you are at your intended firing point. FYI - don't set anything to more than 17 dbtdc these days... but that's another subject.

I hope this helps.

Adam

[a-teamlambretta]

i have tdc marked , its the mark i made on the flywheel i made at the same point as the first anticlockwise mark i made and the clockwise mark , in theory that mark measured back on the flywheel at 10.5 deg should then be tdc on the flywheel aswell ?? . i need to keep the marks i made up at tdc or 12 o clock as i dont have much room to put the marks on the mag housing. i know these can be put anywhere once tdc is found . all the marks i hve made are with a thin marker pen apart from tdc , thin screw driver and a light tap from a hammer.

[Adam_Winstone]

I'm trying to get my head around what marks we've got where so forgive me if I'm a little confused about what we do or don't have.

What I can say is that you can apply the reverse method to keep a single mark on mag housing and put the range on the flywheel edge, should you chose to, which is what Scooter Center have done with their BGM flywheels.

So have you got a single mark on the mag lip and one arrow mark on the flywheel edge OR do you have one TDC mark on the mag lip and 2 rotational marks on the flywheel?

[Adam_Winstone]

Note that if you are working from a single mark on casing that you still need that corresponding arrow for TDC and firing points marked clockwise on flywheel edge so that the firing point is BEFORE TDC.

http://www.mbscooters.co.uk/images/prod ... G_0709.jpg

[a-teamlambretta]

I'm trying to get my head around what marks we've got where so forgive me if I'm a little confused about what we do or don't have.

What I can say is that you can apply the reverse method to keep a single mark on mag housing and put the range on the flywheel edge, should you chose to, which is what Scooter Center have done with their BGM flywheels.

So have you got a single mark on the mag lip and one arrow mark on the flywheel edge OR do you have one TDC mark on the mag lip and 2 rotational marks on the flywheel?

i too am confused lol, yes adam i have two marks on the mag housing one from the anti and one from clock. i measured between the two to get tdc.

[Adam_Winstone]

Si

[a-teamlambretta]

i mrked the flywheel at the anti clockwise stop and then rotated the flywheel clockwise to the stop , using the mark i made on the flywheel to alligne a mark on the mag housing . then measure between th two for tdc. my theory was if i rotate the flywheel back to the anticlockwise mark and then just use the tdc mark on the mag housing to then allign a mark on the flywheel, in my case 10.5 deg back from the anticlockwise stop.

[a-teamlambretta]

this is how i did it lol

20mb image hosting

[Adam_Winstone]

Providing you are rotating the flywheel ACW back the 10.5 degrees from the CW stop then this should allow you to mark the flywheel opposite your mag TDC mark. Don't just measure 10.5 degrees without rotating the flywheel, otherwise you'r just marking the position of your positive stop.

Personally, I think the whole 'measuring' and quoting of degrees (especially as 21 is an original firing point) just confuses things, whereas the positive stop method is great for finding TDC without figures... a sheet of paper, a pencil and a stop is all that's required. It doesn't matter where the marks are how many degrees between them... simply a case of 2 stop marks and a sheet of paper folded to show you where the mid point is all in relation to a single mark or arrow on the flywheel. Only then do you need to look at degree figures for your fining mark.

Adam

[CHRIS in MARGATE]

It's very confusing the first time but once you've done it successfully it will all be very easy.
I use the piece of paper cut around the circumference and the folded in 2 AND I also use my degree disc to find the midway point. Once you are happy that after several adjustments on your positive stop tool, that TDC keeps coming out at the same point then scribe a good mark in the magneto housing; a good mark that you will NOT confuse with the myriad of marks that I often see.
Remember your firing point is to the LEFT of this mark. It will be just over an inch away.
Good Luck. Don't rush; it's cold out there anyway.

[a-teamlambretta]

thanks for the help.


just to clarify , i have TDC marked up. im not inerested inthe firing point yet. its the marking up of the FLYWHEEL thats confusing me lol.

[a-teamlambretta]

think ive got my hed round it thanks for the help. next is the firing point . right, ive marked ou the mag housing for my desired firing point of 24deg allowing for 8deg of retardation . if it does not retard this much i will alter the base line firing point and yes its a varitronic ignition , so when on full chat i need 16deg . i cant get enough rotation on the stator to achieve this. do i just elongate the slots a bit more ?? or ave i done something else wrong . cheers simon

[johnnyXS]

I have never used the positive stop method myself as I always use a pencil or screwdriver in the spark plug hole and rotate anti clockwise until the piston hits the highest point . (I'm lazy) then I always use a degree wheel to mark and set advance

I'd be interested to know how much differnce in accuracy between the two methods .

I suspect the greatest potential improvement in timing and performance is likely to be in the final choice of advance setting.

[a-teamlambretta]

I have never used the positive stop method myself as I always use a pencil or screwdriver in the spark plug hole and rotate anti clockwise until the piston hits the highest point . (I'm lazy) then I always use a degree wheel to mark and set advance

I'd be interested to know how much differnce in accuracy between the two methods .

I suspect the greatest potential improvement in timing and performance is likely to be in the final choice of advance setting.

you are using the positive stop method , im on a smashed out spark plug with a bolt through the middle , then screwed into plug hole to stop the piston going all the way to tdc.

[CHRIS in MARGATE]

Yes you do have to elongate the slots.

[LateAgain]

you are using the positive stop method , im on a smashed out spark plug with a bolt through the middle , then screwed into plug hole to stop the piston going all the way to tdc.

Out of interest what did you use to smash the plug internals out? I started to make a +ve stop a few years ago, but gave up when I couldn't get the plug hollow. Been meaning to revisit it every time I see it sitting on the shelf !!!

[Strummer10]

Incidentally (and I've checked a number of times) a screwdriver slammed down the barrel gives exactly the same results lol...........

[a-teamlambretta]

you are using the positive stop method , im on a smashed out spark plug with a bolt through the middle , then screwed into plug hole to stop the piston going all the way to tdc.

Out of interest what did you use to smash the plug internals out? I started to make a +ve stop a few years ago, but gave up when I couldn't get the plug hollow. Been meaning to revisit it every time I see it sitting on the shelf !!!

an FBH lol

[bsso78]

I have never used the positive stop method myself as I always use a pencil or screwdriver in the spark plug hole and rotate anti clockwise until the piston hits the highest point . (I'm lazy) then I always use a degree wheel to mark and set advance

I'd be interested to know how much differnce in accuracy between the two methods .

I suspect the greatest potential improvement in timing and performance is likely to be in the final choice of advance setting.

Is that the positively imprecise guesswork method then?