Technical Questions: 1928 Model C

[cmfalco]

In fact,you will find out all these details and more besides,when you strip your motor ...

Unfortunately, that's not the case. I'll find the current balance factor with whatever piston is in it now, but that isn't necessarily the one that was in the bike when it left the factory 89 years ago. All I'll know when I strip the engine is how everything is now, not how it was originally, which is why I asked some of the question in my first post of this thread.

... clutches way out of balance from wear never even considered.

That's a good point. Everything that rotates contributes in its own small (or large) way to the vibration. However, unlike a, say, Triumph Trident clutch, I infer from illustrations of the Burman clutch that this shouldn't be much of a problem. But, throughout the entire rebuild process I'll try hard not to make any assumptions about anything and will check things even where it appears there's no reason to check.

There's more than piston weight involved in balance factor. ... To be sure of getting a balance factor at an exact percentage you will have to balance the assembly wot you got

Yes, that's true, and I have the equipment to balance the assembly to within a gnat's eyeball. Or, more precisely, to within 0.09 oz-in. But, I'm still hoping someone who knows the original factory balance factor stumbles across this thread and posts the figure.

And do you really need the con-rod length to arrive at the 'working' settings for valve and ignition timing??

It's impossible to calculate piston position vs. crank angle without the connecting rod length, and that's something I need to confirm figures I've been able to find. For example, one source says that the inlet valve should close 50-deg. after bottom dead center while another source says it should be 17/32". Are these the same? The only way to know is to have the connecting rod length.

Timing is another issue. One source says to set the fully retarded magneto to fire at TDC, and another source says the fully-advanced position is "approximately 1/2" BTDC. Knowing what crank angle 1/2" corresponds to would let me set the magneto to fire there when fully advanced and not have to rely on the amount of movement of the (worn) 90-year old magneto points cam to set the fully advanced position. Then, tests on the road would tell me if that was too much advance and would allow me to more quickly arrive at the optimum/maximum timing. A stick down the plug hole would let me do this "pretty good," but having the crank angle vs. piston position let me do this faster, more accurately, and more reproducibly. Once I know the optimum point I can mark a stick with confidence so that should I have to take the magneto off the bike during the two-week run I can quickly reset the timing.

I'm not saying it's essential to have the connecting rod length, only that for several purposes it would be significantly better to have it. Unlike the original factory balance factor, though, this is something I can measure for myself once I have the engine apart.

Had a quick look on Amazon and the 5th edition of the Gold Star Buyers Guide is still available. ... If I had or was looking for A Gold Star I would buy the book

Even if you don't plan to buy a Gold Star I wouldn't object if you bought the book...

[cmfalco]

If you take the con rod length as double the stroke that would normally be pretty close

Thanks for your post. But, as the saying goes, 'close' only counts with horseshoes and hand grenades. I had hoped (and am still hoping) to get the exact length of the connecting rod so I could generate the necessary table in the relatively quiet time before the bike arrives and I become immersed in working on it.

[david.anderson]

For a spark plug an NGK 6 heat range is correct. So for 18mm it should be an A6 or if there is a reducer to 14mm then a B6HS is correct.
Ignition timing is quoted as 7/16 or 35 degrees btdc fully advanced.
The 28 black Ariel is nearly a red hunter having many of the same features. Even the frame on the 28 closely resembles the later VH, particularly the engine mounting. In a 49VH rigid frame I am using a 2/3 or 66.6% balance factor and the bike is vibration free. I would expect the same result with the 28.
The valve timing in degrees and ” is quoted here. http://www.arielownersmcc.com/members ... uide15.jpg
The bore stroke and crankcase of the 28 is very close to the later VH so I would expect the same conrod length. The VH is 7.5” centres
The 6.8VH piston is 320g bare and 427g with rings and gudgeon.
Hope this helps
David

[nevhunter]

I note the Ariel book shown in the link doesn't say what clearance to use on the valves for setting the timing.. It's a little vague if you aren't off the quietening ramps. Conrod length does affect the timing related to piston travel but it's not going to be much unless the rod is very short (it's rod length Vs stroke which gives a lot of anglularity( where you can use a wilder timing in respect of degrees as it's piston travel that has the most effect with the cam. Nev

[john.whiting]

Its doubtful that you will need over 4000rpm ,as the run requires a very sedate average speed.Now as we know,the ignition timing is in fact set by the (un)educated left thumb of the rider.So while you may calculate the balance factor and ignition timing to ten places of decimals,none of this will make one iota of difference on the road.You chug along and set the ignition to where it seems to run best.Ariels aint rocket science.Regards John.

[cmfalco]

Hope this helps

Indeed it does. Thank you very much.

7/16 or 35 degrees btdc I would expect the same conrod length. The VH is 7.5” centres

Just for the record, using that con rod length, 7/16" is nearly 36-deg (35.97-deg.) and 35 deg. is measurably different from 7/16" by 0.56 mm. I won't argue whether or not 1-deg. would make a significant difference in performance, or whether gasoline from two companies of nominally the same octane require the same advance to within 1-deg., but there is a measureable difference of ~1-deg. between 7/16" and 35 degrees.

I fabricated a setup for my Gold Stars that lets me set the timing to within an absolute value of 0.2 degrees of any value I choose. It's at least as fast for me to do this as it would be to wrap a piece of wire around a stud to get the degree wheel to ~1 deg. and then use cigarette paper to end up with maybe ~2 deg. absolute accuracy. It's not that the ~10x increase in accuracy is essential to performance, it's that knowing that the timing is correct removes it from the other factors and uncertainties one has to deal with.

doesn't say what clearance to use on the valves for setting the timing.. It's a little vague if you aren't off the quietening ramps

I'll be a bit surprised to find quieting ramps on a cam that old; quieting ramps weren't added to Gold Star cams until 1952. But, as you say, if they are present on the Ariel's cam the numbers in the Owners' Guide aren't well defined without knowing at what tappet setting they are to be measured. Luckily, even if there are ramps the "correct" tappet adjustment can be determined by trial-and-error using the listed values. If a too-small clearance is used the angular difference between opening and closing will be found to be too large, and vice versa.

So while you may calculate the balance factor and ignition timing to ten places of decimals,none of this will make one iota of difference on the road

Hmm, I was only going to calculate it to five decimal places...

But, seriously, it does make a difference on the road. I have my Gold Stars equipped with tight-wire-advance cables and with the fully advanced positions on them set at the proper BTDC timing value. Under most riding conditions simply pushing the lever fully forward against the stop gives the best result and, unlike a tight-wire-retard setup, vibration does not move it from that position. If conditions require an adjustment it's always in the same direction of the lever (retarding), and if conditions return to the usual ones the lever only needs to be returned to the same hard stop position. This minimizes the time spent fiddling with the lever and, more importantly, minimizes the distraction. So, knowing the correct ignition timing allows for a 'set and (almost) forget' adjustment allowing me to focus on all the other things I need to be aware of. That's why it does make more than an iota of difference on the road.

Of course, it's certainly not impossible to ride with a magneto whose lever requires more adjustment than described above. Neither is it impossible to drive and talk on a mobile phone at the same time. People do both all the time. But, thanks to hundreds of millions of mobile phones in use there's enough data on them show the distraction results in significantly increased odds of getting into an accident. If 10% of your concentration is diverted for 5 sec. doing something you otherwise wouldn't have needed to do, every time you do that you only have 90% left to notice the car about to pull out in front of you in time to do something about it.

Now, where did I put my ten-digit calculator?...

[cmfalco]

Oops, I meant tight-wire-retard on my Gold Stars.

[Ian.Taylor]

Great to see an early Ariel entering the Cannonball Run

[cmfalco]

Great to see an early Ariel entering the Cannonball Run

Thanks for your comment. The bike is (finally) en route, due to land in the US in under an hour. Given how long I've been told it will take to clear Customs, plus ground transportation to me, the soonest I expect to see it is Friday, although early next week seems more likely.

[nevhunter]

I understand tight wire retard is used in competition machines predominantly so if the wire breaks, or nipple pulls off, you still have the full performance of the engine and finish the race. Nev