ignition timing - lightened flywheel

[jimwjarmour]

Hey guys , have just reassembled my 1977 r100s with a sibenrock power kit and new ign ,now i find i cant see the full advance dot with the strobe, seem to remember having the same problem a number of years ago figured last time i had the flywheel off i must have replaced it one bolt hole out, i had the flywheel lightened years ago so doesnt have the timing marks anymore only the full advance dot, which i cant see wirh the strobe, so my question is what are the degrees of full advance from top dead center , i though it was 34 off the top of my head but snowbum says 25 this doesnt seem right to me can some one assist would love to take the old girl for a ride before winter gets here, thanks in advance can get her going but want to set it up right thanks in advance, Jimmy

 

[20774]

I looked at the BMW Repair manual for the R60/7 - R100RS models...basically for the /7 models thru 1980. Took me a bit of time but one page in there says that the static timing is 6 deg +/- 3 deg BTDC. It appears that the full advance range is 25 deg +/- 2 deg. That seems to match Snowbum. I've heard of a 34 deg value but can't remember what that is in reference to. [Edit -- I think 34 deg is for the /5/6 bikes.] You may need to resort to a degree wheel temporarily attached to the front of the engine and manually turn the engine and find out where the S-mark should be then the F-mark.

What "new" ignition are you using?

Kurt

 

[GTRider]

The “new ign”: does that mean new points and condenser, or a new aftermarket ignition like Dyna or Wedgetail?

If it is points and condenser there have been issues with some points, especially aftermarket ones, having the rubbing block incorrectly positioned, such that it is impossible to turn the points plate far enough to bring the ignition into correct timing. The best fit seems to have been the Noris points, which are also sold by some aftermarket vendors.

Not having the OT and static timing marks complicates things a bit, but is still workable. Key thing there is observing the timing mark with a timing light while slowly increasing engine speed—does the mark never appear, or does it move up through the hole and disappear?

Some have been able to get the timing correct by lengthening the slots in the points plate, to allow more rotation, or to narrowing the points gap a tad, or both. But if you have a set of aftermarket points with the rubbing block problem the first step is usually tracking down a set of genuine Noris points—try your BMW dealer.

If your flywheel was off by one hole (72 degrees) I doubt the bike would even run.

Best,
DeVern

 

[jimwjarmour]

dyna 3 electronic ign was fitted for more than 20 years due to its age and mileage ,decided to replace it with another dyna, thats when i realised my dilemma of the timing dot really all i need is to confirm the full advance number of degrees from top dead center

 

[jimwjarmour]

well your right of coarse it didnt start, so i marked the new ot and advanced her from that , am making a degree disc to mark the full advance dot , until i take the time to remove the gearbox clutch and flywheel replace rear crank seal re align the flywheel ,just need confirmation of number of degrees from TDC to full advance , thanks in advance Jimmy

 

[20774]

So are you still unsure of the degrees to TDC? I think you said you still have the full advance dot on your lightened flywheel. Snowbum says 25 deg and I pretty much agree with him per the maintenance manual. The bolt holes are 72 degrees apart. Seems like the position of your advance dot relative to the bolt holes tells you where it has to be installed and it would give you an idea where TDC should be marked.

 

[melville]

Yes, a couple of my bikes have lightened flywheels and the only mark left as a result is the F mark. Which would be fine, except one is double-plugged and gets timed at 28 deg BTDC max rather than 34 deg BTDC. The solution was to make an accurate mark at TDC on the flywheel using a yellow paint pen and to use a dial-back timing light. Set it to 28 deg and it will flash the strobe at TDC.

Edit--lightened like this:

 

[20774]

What year/model is your bike, melville? Max timing changed over time, so 34 might not be right for the OP.

 

[melville]

What year/model is your bike, melville? Max timing changed over time, so 34 might not be right for the OP.

All my junk is 1979 and earlier. Snowbum, bless him, says the change was for 1978 models. Here's his thing on flywheel marks:

https://bmwmotorcycletech.info/F,OT,S,Z.htm

I swear I've seen a chart with all the 'S' and 'F' specs year by year but I can't seem to find it right now. 34 deg max shouldn't be too terribly wrong at this point so long as premium fuel is used. Wait! It's in my Haynes--static or idle timing should be 9 deg BTDC for /5 and /6 and maybe some /7, 6 deg BTDC for all others. The ATU should have 25 or 26 deg of advance for a final result of 31 to 34 BTDC depending on the starting point.

 

[jimwjarmour]

hey melville ,now were getting somewhere so 34 degrees may be correct thats what i have in my head could you please check that from ot to full advance is 34 degrees or 25 for my single plugged 1977 r100 s thank you so much only need to set ign replace fairing lowers and take her for a ride before winter steps in very cold this morning , thanks so much Jimmy

 

[melville]

The amount of advance in the ATU should be 25 or 26 degrees so if you end up at 34 deg BTDC your idle/static timing would be 8 or 9 deg BTDC.

 

[20774]

melville -

On Snowbum's page that you linked to, he states near the bottom of the page that the point of maximum advance on the flywheel should be 25 degrees before OT through the 1977 machines, and that was increased to 26 degrees for the later bikes. No where on that page does he mention 34 degrees.

 

[jimwjarmour]

ok thanks guys when i mark tdc or ot , on my modified flywheel and via a home made degree disc go back 25 degrees there is my F dot so will adjust ign to that ,interesting though kurt , looking at past posts like this , you have advised that full advance is at 34 degrees before tdc !! sorry for all the confusion, Jimmy

 

[jimwjarmour]

well guys, set ign at f mark full advance around 3500 rpm i have now fitted a new digital boyer ign to do away with advance and retard unit , which i belive has been holding high revs slow to return carbs have been rebuilt numerous time over the 400,000 km have ordered a set of 34 mm mickuni,s so getting closer to this last ride on her this season thanks for everything, looking forward to next season ,had big plans this year but my son mark hit a deer totaled his r1200rt so this season shot ,theres always next year, but at 76 wondering how many more next years i have ,bye for now Jimmy

 

[20774]

jim -

I guess you have better search skills than I do...or a better memory! I don't recall mentioning the 34 degrees before and have only just dove back in on the research as a result of your post. I thought that the early bikes might have had 34 degrees of full advance, but with tightening EPA requirements it got changed. Snowbum probably has some discussion about advance units and changes made over time. The advance unit is what controls the full advance on the original points set up. Looks like he discusses versions of advance units near the bottom of this page:

Ignition operation, ignition Timing, Automatic Advance Units, coils, BMW Airhead Motorcycles

BMW Airhead motorcycle ignition systems

bmwmotorcycletech.info

So I see you mention the Boyer...up to this point you had only mentioned the Dyna III system.

Edit -- I did find where I mention the 34 degrees...it's for the later advance units:

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[jimwjarmour]

hi Kurt, sorry didnt mean to contradict you, I have been confused lately , regarding the ign timing i had a boyer in her originally replaced it with a dyna 3 i purchaced at the oshgosh national, you tell me how long ago that was, so every service i only had to lube the adv bob weights and check full advance with the strobe job done, really got it into my head i had reinstalled the flywheel on one bolt out so was busy setting the ignition via new marks, which i didnt need to do reinstalled the dyna lined up timing marks on pc board and engine caseing bingo all back to normal, reassembled her with the sibenrock power kit and stupidly removed the dyna again installed the boyer nothing would line up back to the flywheel one bolt out therory, alls well now thanks everyone for their patienta ,happy thanksgivening ,Jimmy

 

[jimwjarmour]

hopefully sending a picture of my 1977 bmw r100 S/RS/ now RT much loved 400.000 km also have 2000 r1100rt 203,000 km