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RPW, that looks suitably hooligan. Time I had another BSA methinks... (C15 @ age 17, followed by a B25 / C15 bitsa - which was a lot of fun and passed my test with)
 

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Thanks Rossmick, I'm building another one from bits. It will have my tricky small port head with 32mm Amals and this nice home market tank and covers, you've seen on the movie. The bike from the movie was sold 3 years ago as original Thunderbolt and still is used in Calgary, Alberta.

 

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Discussion Starter #44
paint a mark on the floor and a correzponding mark on sidewall. air up the tire and sit on the bike. then roll the bike forward until the mark on the tire is against the floor again.meazure between them with a tape measure. that iz the circumference of whatever tire you have

my avons are 81.4 inches in circumference
Mate , it says 4.00 18 on my back tyre

That's good enough for me

But thanks for the advice

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sure, whatever works. depends on how exact you need to be. i have to pay close attention to mine. i run dunlopz and avons and continentals with slightly different aspect ratios, and i use tbe diameterz to pick sprockets. one tooth makes a big difference.

i have17 × 2.5 tires an an old bonneville, and to get into the powerband where i want i have to run 21/38 sprockets, lotz different from the 19/43 it came with.
 

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G,day

I,d like some advice on gearing please

I have 1971 Firebird scrambler that I've bought back to life after it had a fourty year nap

Unceazed the clutch , drained all the water out of the gearbox and got it moving again

Seezed oil ring and big rust Mark on the bore , so new 3 size over piston and rings

It goes like a champ and I love it to death but to me it seems to rev way to much

Standard it was doin 6000rpm to do 60mph , which seemed to me way to high , I didn't want to blow it up just keeping up with traffic , not that there is much where I live

I changed the front 18 tooth sprocket for a 20

It's way better , 5000rpm at 60 mph but it still feels to high

The back sprocket is a standard 47

Can I change it for a smaller one , do they even make one ?

Or should I just put up with it , smile and wave ?



CHEERS Mick


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The teeth on your rear sprocket appear to have the curved "wave" look. That means the sprocket is worn out. Since you've done the front sprocket, it would be prudent to do the rear and replace the chain. ebay is the best place to find vintage NEW parts. There are other online parts sellers specifically for Brit Iron. Don't use another worn sprocket or chain. You might be able to go down a couple of teeth on the rear. Slower, but fewer revs.
 

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Hi, 47t is the smallest that will fit on conical hub. Smaller sprocket the chain hits hub/bolts. 46t sprocket from earlier bike has smaller ID & different bolt pattern. So rear sprocket is out of the equation.
Don
 

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Discussion Starter #48
Yeah , I have a new chain ready to put on once it's all sorted

A new rear sprocket makes a lot of sense

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Discussion Starter #49
Ok

So I checked my Speedo against an app on my phone

The Speedo reads exactly the same as the app

I did Speedrattles clutch test , as I originally thought, there is no clutch slip

So it must be the tacho reading wrongly

My mate across the paddock has is building a 52 beeza

Would his tacho work on my 71 , so I could check his against mine ?

Cheers Mick


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An original tacho will have 3:1 on the face, the ratio that an A65 uses. As far as I know, BSA did not fit a tacho as standard in 1953, but a drive if fitted was a bolt-on gearbox driven from the magneto drive nut. That gearbox could be 1:1, 2:1 or 4:1, but the camshaft turns at half engine speed, so the tacho would be to suit. The late pre unit BSA twins had a similar drive to the unit engines driven off the oil pump (usually blanked off) and that is 3:1, but that early inner timing cover can be swapped onto early engines. If your mate has his tach drive from the lower front of the inner timing cover, then it would work on your bike, and if not, not.
HTH
Mick
 

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Discussion Starter #51
Ah Ok

Thanks Mick

Might be better off getting in contact with an old bike club to see if one of their members could help me out with a tacho loan

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Discussion Starter #52
Don't know if this would work- at least to confirm your tachometer?
A phone/satnav can check speed as can a mate following

Know anybody with a strobe that reads HT pulses?
Or maybe this way , like Dave suggested

But I use my fingers and toes when I'm counting

So I'm gunna need help from one of my kids or their friends , one of them might be tech savvy enough

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Some mechanics shops used to have a handheld diagnostic tool that hooked up to the coils and gave revs.....I don't think they are used any more - there are better things out there and it is rare for a car not to have a tacho standard these days - but somebody could have one kicking around. Its worth a shot to ask maybe?
I'd also get onto Britbike and the BSA section and see if there are any aussies on there in the 'hood' that may be able to help?
 

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I saw a YouTube video on lathes and the guy had a hand held non contact tachometer.
Like an IR thermometer?
 

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i have an excel spreadsheet that takes primary gear tooth counts, final drive gear tooth counts, gearbox internal ratios, tire circumference, rpm, and road speed, and lets you generate any of these from the others. if you want to know how fast your bike is going at 6000 rpm, you type in 6000 and press enter. you will get road speed in all gears. or if you want to know how fast your motor is spinning at 100 mph, you just type in various rpms until 100mph shows up in the speed cell currently it has columns for BSA A65, T120RV, T150 five-speed, T120 4-speed, T120 4-speed close ratio, T140, and my daughter's 2007 ninja 250. you can edit it for any internal gearbox ratios, tires, rpms, sprockets, whatever. you can add any bike you have the numbers for.

i haven't been able to make it work reliably for tire growth, wheel slippage, and rpm drop between gear changes, but i'm working on that. the solution fields look like his:




i don't know how to make it generally available, but i can send it to anybody who is interested.
 

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Kevin, since the end result of the calculation is speed, to factor in tyre growth you have a cell with the growth equation based upon RPM of the tyre then multiply your speed with the growth. Put in a macro that recalculates the result. Hit recalculate until the speed number stops changing.
This is what I do for the oil pressure calculations which are recursive, from the Machinery Handbook.
I have one of those old tach and dwell meters that hook up to the coil. Still works.
 

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i have an excel spreadsheet that takes primary gear tooth counts, final drive gear tooth counts, gearbox internal ratios, tire circumference, rpm, and road speed, and lets you generate any of these from the others. if you want to know how fast your bike is going at 6000 rpm, you type in 6000 and press enter. you will get road speed in all gears. or if you want to know how fast your motor is spinning at 100 mph, you just type in various rpms until 100mph shows up in the speed cell currently it has columns for BSA A65, T120RV, T150 five-speed, T120 4-speed, T120 4-speed close ratio, T140, and my daughter's 2007 ninja 250. you can edit it for any internal gearbox ratios, tires, rpms, sprockets, whatever. you can add any bike you have the numbers for.

i haven't been able to make it work reliably for tire growth, wheel slippage, and rpm drop between gear changes, but i'm working on that. the solution fields look like his:




i don't know how to make it generally available, but i can send it to anybody who is interested.
Hello Kevin,
That looks ever so useful for specials I build from time to time! I'm pretty poor on Excel, but I am about up to modifying existing if push comes to shove. I'll swap you for my write-up on Triumph cam timing if it's any use to you on [email protected].
Cheers,
Mick.
 

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Hi folks.
Long time A65 owner here.
The notches for 21 tooth gearbox sprocket were stock from at least 1968.
If you read 5,000 rpm at 60 you have the wrong tach fitted ,as others have said you need the 3:1 tach. Thats if the clutch isnt slipping.
As Don said 47 tooth is the minimum you can fit on the rear.
Things to pay attention to.
Carb balance, I use the one drop method, which involves setting each cylinder for optimum pilot mix with the inlet balance pipe temporarily blocked off, set each to same revs , recouple, then synch cables, for me this gives best results. Ignition timing, modern fuel can provoke pinking at stock max advance, retarding a degree or two will help a lot.
I use EP90 in the gearbox.
ATF trans fluid in the primary , 20 -50 in the main tank.
Always check oil immediately after a ride, they tend to wet sump, its easy to overfill the tank if it has wet sumped, this will make a real mess.
 

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Kevin, since the end result of the calculation is speed, to factor in tyre growth you have a cell with the growth equation based upon RPM of the tyre then multiply your speed with the growth. Put in a macro that recalculates the result. Hit recalculate until the speed number stops changing.
This is what I do for the oil pressure calculations which are recursive, from the Machinery Handbook.
I have one of those old tach and dwell meters that hook up to the coil. Still works.
thank you, dave. i'll see what i can do. th eprobblem i have is mostly mathematical, as the tire growth macro i wrote up works backwards. so far i've just been setting the multiplier to 1.00, and ignoring it.

i'm not sure how much they actually gro. tom schaeffer says his tires grow by an inch sometimes on his turbo BSA, but there are lots of variables and he clocks 160-something. i put 50 psi into my tires when i run, and i suspect that they don't grow much at that pressure.

tire slip is similar, only a problem on the salt, but there it can be serious,
 
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