Author Topic: Timing off idler  (Read 939 times)

Offline ShaunMac

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Timing off idler
« on: 19.04. 2014 15:47 »
Well after an almighty cock-up trying to find tdc with a pen which subsequently broke, I needed to take the head off again. Argh!
Thought if time her up with heads off so got tdc on right cylinder and stuck my degree dial on the idler. Am I correct that as it's on the idler it will be half of btdc (35 recommended by SRM) therefor 17.5 on my dial?
Thanks,
Shaun
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Online bsa-bill

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Re: Timing off idler
« Reply #1 on: 19.04. 2014 17:36 »
no I wouldn't the idler moves the cam pinion one tooth every time the crank pinion moves one tooth that's all it has to do but it doesn't revolve in any useful way for timing as it moves in relation to the other pinions ( has an odd number of teeth so it doesn't mesh with the same other pinion teeth all the time )
Your method would need you to use the cam pinion which does turn half engine speed, most folk use the cush drive to attach the dial to or someone here reckons to use the crank pinion (timing side)
Sorry not what you want to hear I know
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Offline ShaunMac

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Re: Timing off idler
« Reply #2 on: 19.04. 2014 18:00 »
Thanks for that. It's fine, need a full gasket kit after today's cock-up anyway so a new one for the primary cover won't hurt.
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Online RichardL

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Re: Timing off idler
« Reply #3 on: 19.04. 2014 19:05 »
Regurgitating some old arithmetic we were playing with some time back, you might look here for an explanation of the crank/idler/cam gears relationships.
http://www.a7a10.net/forum/index.php?topic=789.msg5039#msg5039


I gotta say, I think I would have gone to great lengths to clean out pen parts from the cylinder before rocker and head removal. Anyway, I just did my timing a half hour ago, but it took a lot of tries to get it within about 1/64". At one point I just wanted to use the rolling paper for its intended purpose. I had intended to post the attached photo to "Home Made Tools" or maybe just to show the wayward magnet from a previous topic doing what I had intended when it went walkabout (get that Muskie, Brian, Trev, Trevor, Wozza, Richard, and all the Aussies I missed? *smiley4*) So, what you see is a 1/4" aluminum tube with a paper graticule wrapped around it. The graticule was printed in AutoCad with 1/32" increments. The indicator is obviously just a bent piece of sheet metal held in place with the magnet.

Richard L.


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Online bsa-bill

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Re: Timing off idler
« Reply #4 on: 19.04. 2014 21:25 »
I've used all the assorted methods to find TDC but got to say if you can stretch to it a dial gauge is tops in my book, Shaun you'll find when you get the head off and rotate the crank that when the pistons reach the top of the cylinder you can turn the crank quite a bit without feeling with your finger any movement of the piston, this is going to affect your timing to a degree or two or three a dial gauge will register movement your finger never will, even down a plug hole at an angle it matters not in finding TDC.
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1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline muskrat

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Re: Timing off idler
« Reply #5 on: 19.04. 2014 21:28 »
Nice work Richard, but you know what I'm gonna preach!
Shaun, while you've got the primary cover off make a disc out of 1mm alloy sheet marked in 5 degree increments and mount behind the cush spring. See here   http://www.a7a10.net/forum/index.php?topic=6011.msg41207#msg41207  or here  http://www.a7a10.net/forum/index.php?topic=8226.15
Then you can time with a strobe light on a running motor. Much more accurate.

As a side re gaskets. For the cost of one kit I can make at least 5 kits, bar the head gasket. I use a kit gasket as a template to cut out of oil jointing from Repco in 0.4 & 0.8mm. Great job for rainy days.
Cheers
Just saw Bill's post. I agree and you mark it on the disc and work out your timing marks.
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Offline ShaunMac

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Re: Timing off idler
« Reply #6 on: 19.04. 2014 22:40 »
I have quite a long digital depth gauge at work so I may try and see if that'll get me tdc better.
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Online RichardL

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Re: Timing off idler
« Reply #7 on: 20.04. 2014 02:08 »
Musky,

I'm trying to read between the lines of your comment about timing with the engine running. I take it you mean folks with electronic ignitions or slotted holes in their K2fs could time with the engine running. If you had to have the timing cover AND the primary cover off to time using a disk, it really seems a hassle for when you want to re-time after assembly. One question I have is how the disk holds position. Is it just by cush-spring pressure?

...and All,

 I think there is a question as to how precise timing really needs to be. With the method in my picture I can gauge TDC within better than 0.015". It seems to me that any number we have for inches or degrees BTDC is based on historic fuel. Fuel is different season to season and region to region. So, accuracy within a couple of degrees or, say, 0.015" may make us feel good for achieving it but seems far more precise than the fuel combustion time can be predicted, considering the fuel variables.

Even though I feel like there may be legitimacy in what I'm thinking here, I will not be surprised or upset if someone chooses to put me straight.

Richard L.
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Online bsa-bill

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Re: Timing off idler
« Reply #8 on: 20.04. 2014 08:50 »
Quote
I will not be surprised or upset if someone chooses to put me straight.

Wont be me then Richard, lot of sense in what you say, the other Richard ( Ora in Oz ) has provided lots of data on where to time (degrees).
You have to start somewhere though so the ideal is set to a known to work point then as you mention have an easy adjustment employed like the slotted Maggies many members have or electronic (the circuit plate is slotted), but you always need TDC.

To fix a degree wheel a good option in the SRM cush nut it has two tapped holes in it
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1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline morris

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Re: Timing off idler
« Reply #9 on: 20.04. 2014 09:00 »
I to think that timing doesn't really have to be correct to the degree Richard (although others may disagree on this)
It has been suggested before on this wonderful forum that our bikes where designed to be run and maintained by Joe Average, who had nothing but a pencil and a fag paper to time the ignition.
When I converted to electronic ignition I looked for tdc with the aid of a vernier caliper, fixed the timing disc, and turned backwards 33°. Performancewise no issues to report, except the fact that on the (very) odd occasion I have a kickback when starting hot, so it may be timed a tad bit to early
However, on the first occasion I will be opening the primary, attach a disc, and have a look with a strobe light, even if it where just to satisfy my curiosity. Keep you informed.
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Online Brian

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Re: Timing off idler
« Reply #10 on: 20.04. 2014 09:13 »
Just for interest here is how BSA suggested to set the timing.
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Offline muskrat

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Re: Timing off idler
« Reply #11 on: 20.04. 2014 11:40 »
G'day Richard L. Yes the spring pressure holds the disc very firmly, mine hasn't moved in a few years.
 The strobe/timing light can be used with a normal mag with auto advance or manual. Pop the timing cover on with just a few screws, run motor and check timing. Note how much and which way Adv/Ret. Stop motor, adjust points (if it's only a couple of degrees out) or reset the AA unit or fixed pinion. Retest and adjust if needed. I would guarantee doing the old fag paper and pencil method a fair way out when checked with a strobe. These old bangers (when new/ish) running on the petrol of the day could probably handle 5 or more degrees out either way. With the wear and tear (gear backlash, sloppy bushes) on our motors and the fuel designed for modern vehicles of today. The ignition timing is a bit more critical. A few degrees advanced will give pinging or retarded causes overheating. I try to use the same brand and type (98) if I can. 
 Shaun, the depth gauge is probably better than a spoke but a dial indicator is better. Another method of finding TDC is to use a tool like in the first link (a modified spark plug). Put plug in hole, wind clockwise till piston touches, mark disk. Turn back till piston touches again and mark disc. The center between the two marks is TDC. REMOVE TOOL Mark disc at 30 and 35 degrees before TDC and away you go.
 *fight* *lol*
Cheers
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Online RichardL

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Re: Timing off idler
« Reply #12 on: 21.04. 2014 16:55 »
G'day Muskrat (I'm allowed to say that, thanks to the 6 weeks I spent in Sydney in 1976),

I looked into gasoline combustion time and it gets pretty complicated. I didn't go into a lot of depth, but one number that came up a couple of time was 2ms. So, say we use that number to start with. At 2000 RPM, 2ms represents 24 degrees of crank rotation. If that number is off by 1/2ms, the difference is, of course, 6 degrees. If 4000 RPM, 12 degrees, etc. If using the dip-stick method and you're off by 0.015", that represents 8 degrees. The two sort'a equate for 2000 RPM and the 1/2ms burn-time tolerance I used for an example. It seems setting within one or two degrees of a target value assumes your fuel will be extraordinarily consistent.

OK, I am in deep trouble for debating this point, given my well-known dumb moves in other areas.

Richard L.
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Offline muskrat

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Re: Timing off idler
« Reply #13 on: 21.04. 2014 21:46 »
G'day Richard. 1976 the last year of school at Heathcote, Sydney.
Greater minds than ours debate the subject, so who are we to banter. Most of my observations are concluded through the seat on the pants.
If I forget to take the octane boost and have to fill up on a trip the bike (all bar the '51) pings. Knock a couple of degrees off the timing (easy with electronic) and away we go.

Cheers
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