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#703652 - 08/02/17 2:37 am Re: 1968 bsa a650 gearing [Re: COP242]  
Joined: Aug 2001
Posts: 4,095
Mark Z Online content
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Mark Z  Online Content
BritBike Forum member

Joined: Aug 2001
Posts: 4,095
Owego, NY, USA
I ran without gauges on either of my A65s for...oh, around forty years. And then I got me an electronic tacho (which, I'm sorry to say, is no longer working - GPS speedo soon to come though). But having the tach work for a little while allowed me to do some benchmarking. Now I can go back to "what feels right".

My riding is a combination of 15 miles to and from work, mostly on an interstate, where the average speed travelled is 70 mph, and on the surrounding secondary highways and rural roads, where speed varies from 45 to 60 mph. I have 20-47 gearing, and Mikuni VM32 carbs (FWIW, because I'm going to include throttle openings). Whenever I go on "outings", I stay off the interstates and stick to the secondary highways, which I find more pleasurable.

On the interstates, I find I have to kind of "work at" maintaining 70 mph (4400 rpm), especially if it's windy, whereas 65 (4100 rpm) feels like an easier gate, requires no effort and has better throttle response. With the MIKS, with no wind, 65 mph is about 3/8 throttle, and 70 mph is about 1/2. Don't get me wrong; the engine runs strong at 4400 rpm and will go way faster; I'm just talking about what's comfortable. I don't know if it's me or the bike, but like I said above, I'm going back to what feels right, and getting over the stigma of being passed by Smart cars.

Riding on secondary highways, the A65 (and I) will run happily at 55 (3500 rpm) to 60 (3800 rpm) all day long without breathing hard. I've also been staying in 3rd gear more on the back roads - this way I can just use the throttle to speed up and slow down for curves without shifting or braking. You just don't reach maximum speed on the straights, but oh well, for me at least, it's not always a race.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
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#703666 - 08/02/17 8:18 am Re: 1968 bsa a650 gearing [Re: COP242]  
Joined: Sep 2004
Posts: 644
gunner Online content
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gunner  Online Content
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Joined: Sep 2004
Posts: 644
Farnham, Surrey, UK
When I first built my A65 Firebird, I was using 19/47 sprockets and found the bike would accelerate very well, however when cruising at on the highway at a steady 60mph, the engine seemed to be revving more than I expected. I don't have a tacho fitted so couldn't tell you what revs I was doing.

Subsequently I decided to fit a 21 tooth gearbox sprocket. It was a few hours work removing the primary cover, clutch etc to get to the sprocket and if I remember correctly the new sprocket only just fits through the hole on the inner cover. I think I read somewhere that to get a 22 tooth sprocket to fit, the hole can be filed to clear the teeth, so no need to split the cases, maybe someone can confirm.

Back on the road with the new 21/47 sprockets, I found the acceleration not quite as good and the 60mph cruising was a bit more relaxed, the revs having been reduced by a few hundred so was a worthwhile mod.

The engine still feels buzzy and I guess this is just the way A65's are, nevertheless I wouldn't mind trying a 22 tooth sprocket to see how that feels.

One of the issues with raising the gearing is that take-offs in first gear need more clutch slip and riding around town means you're almost always in 1st or 2nd.


1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
#703700 - 08/02/17 6:11 pm Re: 1968 bsa a650 gearing [Re: gunner]  
Joined: Jul 2001
Posts: 11,813
Lannis Online content
Life member
Lannis  Online Content

Life member

Joined: Jul 2001
Posts: 11,813
Central Virginia
Originally Posted by gunner
When I first built my A65 Firebird, I was using 19/47 sprockets and found the bike would accelerate very well, however when cruising at on the highway at a steady 60mph, the engine seemed to be revving more than I expected. I don't have a tacho fitted so couldn't tell you what revs I was doing.

Subsequently I decided to fit a 21 tooth gearbox sprocket. It was a few hours work removing the primary cover, clutch etc to get to the sprocket and if I remember correctly the new sprocket only just fits through the hole on the inner cover. I think I read somewhere that to get a 22 tooth sprocket to fit, the hole can be filed to clear the teeth, so no need to split the cases, maybe someone can confirm.

Back on the road with the new 21/47 sprockets, I found the acceleration not quite as good and the 60mph cruising was a bit more relaxed, the revs having been reduced by a few hundred so was a worthwhile mod.

The engine still feels buzzy and I guess this is just the way A65's are, nevertheless I wouldn't mind trying a 22 tooth sprocket to see how that feels.

One of the issues with raising the gearing is that take-offs in first gear need more clutch slip and riding around town means you're almost always in 1st or 2nd.


Yep, exactly my experience with my '69 Firebird. You are correct in that a 21 tooth fits through the inner primary as is ... a 22 tooth would need some machine work (or a lot of handwork) to fit.

Me personal, I wouldn't go higher than a 21. Sometimes I think 21 might be a little too high, although it's the way I've been riding it for a long time. With 21 teeth on the front sprocket, I'm almost to 70 MPH before the bike is at 4000 RPMs, and at 60 MPH it's at 3400 or so. With an engine that is literally lugging at 3000 RPM under load (despite how bad some people wish it would chug like a V-twin), you have to manage it carefully to make sure you don't ruin it with that gearing.

I found that getting the crank dynamically balanced and installing an EI made the biggest difference in smoothness. Lowering cruising RPM can only go so far .....

Lannis


"To learn who rules over you, simply find out who you are not allowed to criticize."
Voltaire
#703735 - 08/02/17 10:58 pm Re: 1968 bsa a650 gearing [Re: COP242]  
Joined: May 2013
Posts: 4,494
Allan Gill Offline
Allan Gill  Offline



Joined: May 2013
Posts: 4,494
Rotherham - S. Yorkshire
I'm another one that started with 19:47, then switched to 21:47 (now on 19:42 we'll close)

21:47 is the most you want to run on a stock bike. I used to run a thunderbolt that had 22:47, it would go some too but there was a lot of clutch slip setting off, which resulted In a lot Of clutch issues. When I attended the Danish Rally i was running 20:42 which is a tad taller still with a close ratio box. My bike was setup to give a lot of torque at lower revs so it worked ok, it would cruise at 75 all day long but on the ride outs where the runs are slower it wasn't much fun. So I dropped to 19:42 and returned to the standard gearbox. Still running a small port head... and that makes a big difference in tractability.


beerchug
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