Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board Decades long search is over

It needs that because the trumpet otherwise doesn't do much. Where the slide sides are might be thinner but isn't helping. Pwk are not smooth either nor VM nor TM but better, Lectron are very good, but old school GP2 no needle in the way. Not Big but on a 654 they could kick butt. If the quirks are under control.

It's not the 32 concentics on the MKIV so much that are the key to it's equal hp with the lower 9-1 compression, it's the altered port, as using that port on a Firebird with 30mm gave a similar increase at Umberslades on their dynos in 1970. 32s would help but not on their own. That head with GP2s and 10-1 would have been nice.

Or.........It could have been the marketing dept. Sure they would not try to sell less hp. Kind of like the Hot Rod Mag test of a new Lightning Rocket where they did something like 132 with an out of the box machine!

Whoo! On stock gearing, what was that then....10,000 RPM?

Oh, the good old days when Bob Green and Floyd Clymer were holding the stopwatches .....

Lannis

In 1970 Umberslades built the engine for the '71 prototype photo shoot Firebird and did the intake ports to the drawing - probably the special '68 MKiv head drawing, which was listed under the wrong old part number for that bike. It's blanked and the new number stamped I think, people have them. On their dynos they reported around 8hp extra over the production line engines for that Firebird. The riders for the photo shoot reporting how lively and responsive it was. I have one '71 head like that, so they may have incorporated that and not said? Production engines wouldn't have had the dyno testing and fine tune. And the '71s performance wasn't going backwards with the new quieter mufflers though Firebirds didn't have them.

Some old things had speed. A 1950 plunger A10 was rated at 105mph top speed. Later Super Rockets with all their specs were not claiming better. What was faster in LSR an early iron A10 or a later Super Rocket based hottie? In old drag racing there was an iron A10 plunger with a blower that was super fast at the time. And a spitfire in the 12s. Factory reps even went to try and workout why.

A Lightning Rocket would have the gear ratio, they just needed the big tail wind or the hill. Sounds like they had the optimism.
That show Harley and the Davidsons is really funny in a few places, but the speed testing and excitement at the new Nucklehead's 105mph, 'fastest motorcycle in the world' is really great, the guy probably turned around in the next state to come back, unless brakes worked on that model. The designs are beautiful. New ones sort of lost it. Worth watching if you haven't.

Yeah, The early LR's had a 42 tooth sprocket as part of the brake drum. I have that with a 22 front on my Speed bike which recorded a best of 166+. A bit overgeared for street use :}

Don’t forget that apart from 68, firebirds have 1 3/8” pipes and the 71 model even has a smaller gearbox sprocket so revs shouldn’t have been a problem.

Having ridden Shane’s Firebird, it has fantastic midrange compared to a stock Lightning.

The oif megas are also a fantastic way of knocking a good 15 mph off the top speed of an oif compared to a set of pre oif T120 silencers. So there was plenty of reason to not mess too much with their flagship A65.

Interesting Allan. Certainly how they feel. Hard to tell on mine with this head but it's just very strong. Purists would wonder what the hell was wrong with it when they turned the throttle, until it shook them off I guess. The race bike's getting Firebird style pipes with an X connector. Ben's ordered some mandrel bends they will be stepping bigger for that set up. We sort of know what they can do.

When I did my spell fabricating exhausts, we had our own mandril which helps a lot, but instead of directly crossing the pipes over (your 883 will be different from my way because of how the pipes connect, this way is only for pipes that are parallel) we would make 2x sections, each having 3 bends, then mate the two together… I’ll do a sketch then upload the photo.

My freehand drawing isn’t what it used to be, but I hope this explains Better than I can.

You don’t need mandril bends to make it either, just and angle grinder and a MIG welder to make the bends etc the middle bend you make, then cut it out, 50% of the pipe OD is an ideal amount to cut off to get a decent cross over.


In effect the two exhausts are then mated together over a longer distance and you get a better sharing/splitting of the gasses. The factory Siamese pipes worked in much the same way and have a big performance difference against the aftermarket crap.

This is Paul's 744 with exactly that type of X Allen it works really well, internally really nice. The problem with this system which was big bore is the head. I sent this system to Chris for his LSR because of its good top end.

The head has 38mm carbs and ports and because of lack of gas speed it had reversion and wasn't good till around 5,000. I shifted at 8,500 in second. It probably flowed 160cfm. What I know now is that the pipes are probably good at lower rpm with the new 34mm head which flows so much more cfm. So speed is good and no reversion, like Ben's 734 which uses the same size headers. It needs a big bore really as the inlets are 44.5mm.

With this head riding on the road was pretty useless, it needed rpm to function. That smaller port head will I expect remedy that and have more punch and more top end.

That engine had +0.060 10.5- 1 Venolia pistons in it and a reasonable head, it had been worked/flowed and
had SRM inlet valves. We were clocked on it at 108mph at one circuit, about 15mph too slow but for a 650 it
wasn't bad. We both weighed about 180lb and the old outfit was no lightweight. I put it together to learn to ride
the old crate again in 1991 after a 15 year lay-off. It rekindled my dormant lunacy.

After sitting for 6 months, I charged up the battery in my ‘66 Spitfire and it started on the third kick. Let it warm up for a few minutes and it idled at 8-900 rpm on the GPs.

Can't be a fast one then..........

Doesn't matter. Idle is important for normal riding.

It runs pretty fast. The idling comes from the cable kit described earlier in this thread. I also have the mixture a tad rich to get rid of the flat spot when the throttle is opened from idle. I have all stock settings and jets, except the needle clip is in the next higher position. When I was originally putting the bike together, I got the BSA shop manual, the owner's manual, and the parts book. They all listed different jets! I don't remember which stock setting I have, but it is one of them! A further complication was that I had a bad cam from the factory. The intake on the primary opened late and closed early, which I didn't discover till after the bike was together and I was trying to find out why it had no pep. I then put in a new Megacycle cam (don't remember the number, but its the one that is just slightly hotter than stock), which transformed the bike. That wasn't the end of it, though. With the cam and stock jetting, it was still lean in the mid-range. That was when I discovered that the previous owner, presumably in an attempt to deal with the cam issue, had put in the lean GP needles. Changed to the proper needles and now it runs like a top. Although after 20 years of no aircleaners, it is now starting to smoke a bit (although not constantly).

Ed from NJ

Sounds pretty good.
Why charge the battery? spitfire had ET ignition didn't it?
How do you know that the megacycle cam 'livened it up' if you never had
a good standard cam in it?

20 years......how many miles? 50k? That's very good for 'once around'.

No ET ignition on the Spitfire. I do have a cap on it, so it should have started on the cap, but didn't. I don't know that the cam livened it up, I just know that the cam I bought was MegaCycles' slightly hotter than stock cam. Probably 10-15 thousand miles. I ride recreationally and have a number of bikes.

Ed from NJ