Making an A65 punch above its weight.

[Mark Parker]

This is just about tuning an A65. Working out specifications for using these neat little engines in competition.

This illustrates reverse porting and seeking to have that fine BSA casting functioning near to its potential. This simple two valve BSA head now flows more than a new Triumph 8valve. I know the quoted flow for the 8valve, but not the all-important gas speed.

I can measure speed on the bench and it varies depending on the port entry. It also varies down in the port. I also have the problem my graph to convert a probe's inches of water to foot per second only goes to 40" and around 400fps which this exceeds. F1 engines have that sort of speed, and the basic advantage is eliminating reversion and flat spots at low rpm, delivering more punch right through. So what it delivers is quantity of charge and energy pushing it into the cylinder. Either one on their own is not as effective.

This is a very drivable smooth and sweet engine. It's tall geared, pulling against 20-38t final drive with C/ratio 1st and 2nd.

https://www.youtube.com/watch?v=ZGeIK0gXB1o

[DAVE BRADY]

Hi,

A very interesting project.  The porting mods look very worthwhile and another member of the Forum has done similar work on his A65 head.  One of the benefits also seems to be better fuel consumption which can only be good thing with running costs these days.  I can see that lot of work goes in to this modification but how difficult is it to do?  I am not an engineer and would dearly love to be able to get a modified head or a 'press in' insert to achieve the same end.
I am currently running an A65 with an A70 crank and single 30mm Mikuni.  The engine pulls very well and is usually two up with camping gear and a trailer so I am after low to mid range torque/power rather than top end. 
Any suggestions?

Dave.

[Mark Parker]

I have no experience with a single carb Dave. Only with a dual throat carb on a Thunderbolt head on a similar 750 in sidecar racing in OZ where for period 3 racing they must use a Thunderbolt head. They pit it against Vincents and bigger Nortons and Triumphs. It won the Queensland championship in 2020 against its bigger methanol burning opponents dropping a class lap record by over 4 seconds in the process, with 745cc A10 crank engine using a Weber on the modified Thunderbolt head. 8 wins from 10 starts for the old A65. They had many raised eyebrows over its small displacement running on petrol. Competitors possibly don't realize they use a max of only 6,800rpm because of the fragile old crank. It's reported to simply pull like a train to redline.

[DAVE BRADY]

Hi Mark,

An interesting racing outfit.  I acquired a twin choke Webber with some other bits but there is no room on a standard bike so I passed it on to a classic car owner.
The Thunderbolt does benefit from a longer inlet tract but again there is no room for much of an extension.  The use of a Mikuni helps a bit due to the spigot fitment with just enough room for a thin 'pancake' filter.
On my 650 A65 I have managed to fit in a 1.5" spacer and the standard filter can just about be fitted.  This spacer helps to smooth out a bit of a flat spot at 50/55 mph.  I suppose a custom oil tank with a cut out for a longer inlet is possibility though.

Dave.

[ferretjuggler]

I'm not into racing or any type of competition, I'm just building an OIF Firebird replica as a bit of fun.
Something flashy to pose around town on in the summer months.
If I can make some mods which will unlock some of the A65's true potential without spoiling it's tractability I will do so.
The design of the cylinder head, I belive, was one of the last assignments of competition shop foreman Len Crisp, before he retired.
Much better than the design of the Triumph twin IMHO, in fact I don't think that Edward Turner ever designed anything on the BSA twins.

[EDDIE SIMPSON]

the japanese bike makers  opened out the ports of their 4 cylinder bikes and fitted c.v carbs in the 80s but needed huge airboxes to run properly. space would be an issue.
my personal preference is the smoothbore monobloc and a k&n pod filter on a bsa. little tuning required.
for an a65 the priority has to be quelling vibration and frame strengthening around the front downtubes for 100mph speeds. i see you solved the frame issue mark but i bet the crankshaft is not standard?

[Mark Parker]

Yep the vibration needs addressing. They could have rubber mounted the engine in a nice frame if they didn't put all their eggs in the triple. They spent massively racing the triple. They didn't like the guys with outfits racing the old twins and wanted them on triples. But given a factory engine one racer tried, that team was not used to being down in 9th before, then it blew up.

I like the triples, but the twins could produce grunt without being peaky. In 1970 they had a 760cc twin making 79hp with a brilliant power curve. A Norton F750 won the IOM TT in 1973 with 76hp. That BSA was long stroke with limited rpm. The short stroke big bore can rev further and drive as well. The twins also were easy and simple to build, giving better returns. But that vibration was handicapping them.

The blue 734 has a std crank cut in half and bolted at 90degrees. That's the other way to smooth the motor. It also means over 40% less stress on the crank, like it being 40% stronger in a way. People buying a billet 90 degree crank usually go long stroke because it increases displacement and costs the same. But it also vibrates. Even a 90 needs rubber mounting. And that big cc then gets difficult to kick start. I have an 883 with 97hp 73ftlb but the 734 can probably approach that and is more practical.
If BSA had flow bench equipment, and I think they did, it was early days. The Japanese were taking notice of it and Yamaha spent around a million $ developing a new head for the 650 twin they had out to 750 for flat track. From that they gained a lot of knowledge. It wasn't just a matter of opening up ports but getting high flow for the size.

[EDDIE SIMPSON]

hi mark,
a special crank and camshaft sounds expensive but rubber mounting the engine someway might help the vibration issue. what frame are you using? and what rubber mounts are you using? someone in a blog mentioned mounting the engine in bearings to allow the engine to float a little in a rigid frame which sounds interesting. however my development will be rubber mounting the footrests and the handlebars like the t160 . also i hope the b44 pistons in my john hill big bore kit will take the edge off the high speed  vibration.

[Mark Parker]

Both the frames above are RGV250. Rubber mounting isn't simple. If you are using John's Big bore kit on a std 74mm crank it will likely be smoother. You can balance better to suit the rpm you use. BSA used 70% to have it smooth down low, recommended for racing was 56%. 60% is meant to be good, it depends on the frame. I have a std Firebird I need to change rods in and balance as it vibrates a lot over 6,000, changing bars lessened that as most I felt was in the bars.

I have a modified Thunderbolt head on it, so its fast enough for me in this frame. It's quite surprising how a std 9-1 650 can go. The Firebird pipes and mufflers work well.

https://www.youtube.com/watch?v=a2Q4ZmcC5FA

[DAVE BRADY]

Hi Mark,

That sounds good and seems to rev easily.  Is there an 'idiots'  guide to doing these head mods?  As it is a modified Thunderbolt head will it work with a single carb?

Dave.

[EDDIE SIMPSON]

all your bikes are very impressive mark. i like the rgs frames. however i dont really understand why 165 main jets cause clutch slip unless you mean there is more power? dont you have a commando clutch?
30 years ago i sold my spitfire and spares and swore i would never go back to riding a vibrating self destructer [ 10/1 pistons didnt help] but the wheel turned full circle and my latest pogo is a 65 rocket. like dave i would like to do some motorway touring on the a65 and for me tackling vibration at 70 - 80mph is the key. the standard a65 piston has too much weight above the pin for my liking , causing a lot of vibration at high piston speeds. so i hope the 750 kit with better designed pistons will help.
i have just discovered ED S offset and nitrided crankshafts and impressed with his website. for the motorway speeds i m after it sounds like the way to go. also geoff recommends the standard bush set up for road so an alternative to a needle roller setup and less outlay.
when i have the basics sorted i will try some porting /reverse flow.

[Mark Parker]

These are Thunderbolt and Lightning inlet ports they flow close to 170cfm they are 34mm. with a 42mm valve. I'm now getting close to 180cfm from some. They are oval and the manifold transitions from round to oval. The top and bottom work far better if they go straight or just curve over. If the top goes down from a high entry, down near the guide then back up into the bowl the air actually doesn't, because it will leave the roof. If it's a smooth turn the air runs into the roof and follows it into the bowl which directs it at the back of the valve. If it goes wide out and around the guide and stem it flows. It has to turn squarely onto the back of the valve, so the valve flows properly, std the flow is across the valve and only uses part of it. A std 68-701 head is 17-18mm wide at the guide, a ported one I measured to compare is 39mm. And flows 179cfm not 109std.

It's not that easy and you don't want to be breathing or getting what you are cutting in your eyes.



You can see how it becomes oval where the manifold meets the head. The manifold needs to aim up if anything so the roof doesn't go down then up. The whole point of being oval is because height is limited so it's wide.

[DAVE BRADY]

Hi Mark,

This is way beyond what I could attempt.  I take it that you have built up with some weld and then reshaped.  Did you use any kind of template and how difficult was it to get both ports the same and at what RPM are the benefits felt?

Dave.

I have just found this by Paul Henshaw.  It illustrates quite well the bias of flow past the inlet valve.  Maybe not a perfect example but it makes things a bit more clear for me.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjx4YD2yfv6AhUIXMAKHVECBDcQtwJ6BAhGEAI&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DNO3PzpkCAAk&usg=AOvVaw2v8tDV0etGWGTnG7umJjal

[Mark Parker]

I use JBWeld in the floor to lift it a bit. The blue port is a std '71. It's one of BSA's better ports I think, I never looked closely or measured a '71 head I did a few years ago, or one for my son Ben's race project before porting it. I have a std broken one and it flows 139cfm, its factory ported by the look. I don't know if they are all the same but it would have really improved them. Crazy the UK government let that company go bust.

I can get ports pretty much the same, I measure as I go and if one flows a few cfm different I try to find out why and generally they are within 1 or 2cfm. This blue port doesn't even try to turn onto the back of the valve at the floor, so i's loosing flow it could have with that valve.



The one on the left is std. The one on the right is the same size, same valve with 143cfm.




The benefits with the 34mm ports are better response from about 2,000 and it builds as it revs. 34 isn't that big if it flows enough and the engine can use it. 167cfm or so through a 34mm means it's high speed. Compared to 34mm with 125cfm which is slow. I'm just trying to find out what stuff does. The 34mm with high speed and lots of flow means throttle response. People can think it has a very light flywheel blipping the throttle.
I asked Matt who drives the 745cc outfit what the power felt like, as I wanted to get an idea how it ran, he just said 'it pulls like a train to redline.'

This is the head for on the 734 Blue bike, it uses 34mm carbs, but these are 44.5mm inlets. It had 38mm ports and carbs but flowed no more air. The 34mm port flows the same cfm so it goes so much better because the port speed is faster and it reads over 190cfm.



The std exhaust has no flow problem.

[DAVE BRADY]

Hi,

I might have play with JB weld on a spare head.  Do you think the use of a blowtorch gives an idea of the behaviour of the inducted mixture as in the YouTube video?  I suppose the JB weld will take the heat for a short time.

Dave.

Just found JB Weld Extreme Heat with max. of 538º C.