Britbike forum Forums British motorcycles in general The competition forum What we think BSA might have done.

I made a video because I got the carbs dialed in pretty well, finally. Added a high tech, phone on a table near my kit, recording of my daughter Nay. Otherwise there are parts with no sound.

I really want to time this bike on a quarter mile. The Firebird as well. I'm curious. There is a drag strip just 6hrs away. Or we can do it with GPs on a GoPro if we could find somewhere. There are probably bits of old highway on farms, if we could find one without cows. Or get permission off council somewhere.

Is this bike a 750 ? Making runs on the quarter would be fun.

734. It has power on for about 1/8th.

Cycle magazine was quite honest, they did a bike comparison test in 1972 and 73. Stock but expertly tuned Commando 750 Combat with lower drag strip gearing ran about 12.8 at 101 mph. It made around 50 rwhp on an inertia dyno...

Sounds great!
Also when the sound was lower I was thinking, " this song sound familiar"
Cool cover of Heart Shaped Box.

I could easily rev this another 1,000 or more, but it seems good just going to 7. Without a measure, because seat of the pants doesn't do it, I don't know if it would push harder than the next gear. I wouldn't bother lowering the gearing it's 3.93-1. The Firebird is Spitfire gearing 4.634 but it has an 18" wheel not 17". The 883 is 3.352 if my maths is right. You multiply the primary ratio and final drive ratio.

Sounds great! Could you give any tips on what needle number and jetting for the 34's please?

There are two types. One has little round mains with a screwdriver slot. I have them on the Firebird. And 160 work on that with around 12.5-1 flat out. I have at the moment Mikuni needles in them. 6fj40 that have new grooves for the clip. these are at the same length as a PWK std needle. The PWK has an upper shoulder which makes a closer fit to the hole in the slide body these do not.

In the 734 I have I think 158mains (which are longer hex type) and 6dh4 Mikuni needles using the original clip grooves. Now the problem is the needle profile needs to give you a progressive air fuel ratio, and displacement and flow change what is needed, same with the main. But basically, the better it breaths the richer it gets. And the 6dh4s go too lean on the Firebird. Having the A/f gauge is good.

It takes around 6.6-7sec for about 1/8mile and is at a speed that will take around 4.5 seconds for the rest to make a quarter. Except it's accelerating and who knows what speed it will pick up. It would all depend on exact distance and getting going. The first std production bike to break into the 11s with 11.9 @ 114mph was a Dunstal Commando 810 in 1971. I don't see why it shouldn't be possible to beat that, other than lack of skill.

Skill and lack thereof. The onboard video with the GPs camera is roll on so even at 30mph where I count from, it isn't right open and pushing as hard as it will, though it's about to, I shift at 60, GPs says 1.95 seconds, though I need it in a program with all the fractions. I was just moving it to get full seconds and power fully on all the time, almost. Rolling on wastes a lot of time before 30 and possibly after a bit but saves wear and tear. And I should rev it more so there is more push time, and the shift could be quicker and second revved out much more, but I was just giving it a hit not trying to max it out. I do that a bit more in another piece of video, but the camera needs to be onboard, and I'm short shifting. I also don't know if the GPs is immediate, but it seems pretty good.

6 hours is a long drive but will be the best thing you can do. Can you take both bikes? If you go, take extra sprockets (and chain links if needed) to get that extra 1000 rpm because when you shift, you'll be at a higher horsepower which should result in lower et.

Those of us on the competition forum are curious.

Tom

With a lesser Head i guess the 6DH4's will work pretty close to what i need., Thanks for that.

The 6dh4s work on the 734 it flows the most through these carbs. They do not work on the 650 Firebird they go too lean. A place in Italy sell 4stroke needles and scooter shops sell them. I got some from Germany marketed as Naraku NK101.95. They are too pointy and go too rich for both these, they are ok for a while but go rich then the main takes over. They may be the thing in your application. The company is called Heavy Tune there is a shop in the UK. The needles are for 28-30pwk but they are the right length and fit the 34mm.

I have some if you cannot get them. I didn't understand the tapper before, but you can feel it when it's ballpark. If it's around 13-1 or 12-1 it's a world of difference to 16-1 or 10-1. On light load 14.7 to 15 seems ok, but when you turn the throttle you need 13-1 or less. The hotter it is the richer it gets.

An article for drag racing a British vertical twin. Found this and it's very interesting. Spec wise the BSA should be better except for displacement. Nothing wrong with the std BSA cam, esp retarded a bit. There may be better but I doubt by much. 11-1 not 10-1. The Branch flow readings are weird as is the gas speed as I get over 400fps in the manifold. I guess it's just comparative and tells you which does better between what's tested.

I'm optimistic the 734 shouldn't be slower, but who knows? The article is showing what gets results, and Branch was on to it. As were the Norton factory. They built themselves a good reputation for high performance despite large Japanese multis.

https://magazine.cycleworld.com/article/1974/5/1/norton-850-hop-up#:~:text=If%20you%20don%27t%20recall%20the%20quarter-mile%20figures%20on,was%20a%20mighty%20impressive%2011.829%20at%20114.21%20mph.

If you don’t already know David Vizard has been posting video on YouTube. Some good information

Thanks, I'll look for it.

I've been comparing video of the 734 and the 883. Second on the 883s 5speed is almost the same ratio as the 734's second which is about 1mph faster at 7,000. They weigh about the same, but in 2nd the 734 does not look to be slower, just listening to the rise in rpm, and the 883 is 83-85rwhp. You can obviously have more torque with a big motor, yet have less hp than a smaller, but usually the smaller pulls more rpm to do it. So this engine is really interesting. You can watch, but cannot feel what it does, it's really cool.

I did a lot of drag racing bikes back in the 70's, as you mention the power delivery is very important.Foward in time to a much older more cautious rider, lol...My 96 Monster 900 is rated by Ducati at 73 rwhp. The 97 Buell m2 was rated the same on non factory dynos.The Ducati feels stronger at high speeds. Both bikes are tuned better than stock with carefull carb and exhaust work....Both bikes in factory tune, with an experienced rider can run 11.9 or so...Both weight about 420 pounds, short wheel base and are wheelie prone...The Buell with the tuned Sportster engine is easier to come out of the hole with wheelspin to control the wheelie. The Ducati was more difficult and clutch slipping was better. Both would still lift the front end all to easily going into second gear.Typical launch was at 5000 rpm,snap open throttle on green using the wheelspin or clutch slip....Find a suitable road for practice on you machines. But less drastic ation is still fun...
Heavier more powefull longer bikes are easier for less experienced drag riders...

If the Mahindra deal had been done a few years earlier this could have been the BSA works team, maybe the basis for a new C15
https://www.mahindra.com/motor-sports/moto-3