Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board BSA 750

I haven't been on line for quite some time but with a recent move, setting up a new work space and this whole Covid thing, I am starting to look at my projects again. I am going to build up a 750 motor, nothing radical, just an SRM big bore kit with the iron barrel which I have had for some time, using the stock crank and timing side bush and maybe a camshaft with a little different profile from stock, possibly a Bob Newby Belt Drive and a 5 speed cluster. My goal is to have a nice running sport/touring bike with enough grunt to tackle the hills here in British Columbia occasionally.
So my questions to this knowledgeable group is which camshaft profile is recommended, from which supplier and which 5 speed cluster? I know of at least 2 suppliers of 5 speed, Nova and Performance Engineering. Is one better than the other? Is there another option I am unaware of? I am not going to be racing this bike so looking for the best ratio for the road.
I may try and get the head flowed and ported as SRM recommend but for me to send it over to the UK and back is a pricey undertaking. Would the standard head suffice?
Any experience/input appreciated, thanks.
Forgot to mention that I also have a pair of billet rods to go into this motor. Not steel but should be better than the stock ones.

The 750 kit is a great bolt-on, it works very well with the standard cam.
Any twin carb head will be fine, a set of slightly larger inlet valves is a
fair plus PM do some good ones. Do some work on the top of the ports
and open the seats and bowl for the valves, no need to go bananas.
The PES 'box is a lovely bit of kit. Don't know about the other one.
Rather than a belt drive i'd spend the money on a set of steel rods and
a 7 plate clutch conversion, but that's just my opinion.
Unless you plan to ride at 6k+ all the time, 30mm carbs will be fine.

Nick

Considering a Nova gearbox is $3500 sending head head back and forth across the Sargasso appears cheaper.

Hi

Plus 1 on what Nick said. I no longer have any involvement with the PES 5 speed cluster. However people may try and tell you its CR cluster and not good for the road, which is untrue. You can adjust your final drive gearing to allow it to work well on the road. Of the ones I was involved with, most are in road going bikes.

regards

John

I run my 750 with the following mods.
In order of importance IMO.
Boyer EI
End fed crank.
7 plate clutch,
5 speed John Hill/ PES box. With outrigger bearing in primary seal plate.
Big valves , ported head.

Still stock, con rods , cam, carb size. 9:1 pistons. No pinking on regular fuel.
Like this the bike is V reliable, end feed increases oil flow to big ends and deletes the TS bush using a bearing to control end float. The bike has much more stomp through the mid-range than a stock 650.
I have nothing against belt drive except you may want to change engine breathing through the primary, the stock triplex chain wont mind this, belt drive will. The stock 650 overpowers the timed breather if kept over 5K rpms, a 750 is worse,
I keep meaning to delete the timed breather and go to breathing through the main bearing , until then I will live with a bit of splatter from the timed breather after a caning.
The NOVA 5 speed has a re engineered selector system which could be a good thing, I have no experience of it, the PES 5 speed is a good bit of kit with needle rollers everywhere and works very well on the road.
Big valves are not absolutely necessary, mine ran for years on stock valve sizes , if you look at Mark Parkers port work, he gets much improved flow by tweaking the ports using standard size valves.
Regarding con rods I have a set of steel rods on the shelf, the reason rods fail is not because they are weak or alloy, its because the furthest big end ( drive side) gets starved of oil when the timing side bush wears, the results are generally catastrophic,, the DS big end seizes then the rod lets go, often taking the cases with it, one day I will fit the steel rods , but so long as the motor is end fed i dont think its a priority.This comes up over and over , if the timing side bush is done right and the oil is kept clean its fine,, two big Ifs,finding a shop familiar with the timing side bush tolerances might be difficult, the stock oil filtration system is crude , a paper cartridge type is a very good mod.
. the end feed needs no pressure to the timing side bearing, once its done its done and is a far better solution IMO.

I am going away from the breathing through the main bearing as the clutch is best using an oil designed for wet clutches and the debris coming from the primary into the engine is no doing any good filter or no filter.

So for the next Unit Single I do will keep the timed breather but augment it with a reed valve sump breather. On the Unit single if you replace the steel sump plate with a thick piece of alloy there is room for a reed valve plus extension to reach up above the returning oil to catch the air in the crankcases. Here is my Commando version on its screw in sump plug which is even smaller.



and screwed into the case.



as the Unit Single square sump plate is the same as the Unit Twin it should carry across, if the reed valve sticks out too far then just add a banjo to the sump plate and run a tube to the reed valve a bit further away.

Thats V natty John. Keep us posted on how it works out.

Could you still protect it with the skid plate?... great idea, but that is a mighty vulnerable position.

That is why I mentioned the Banjo if space is tight. then it will be the thickness of the alloy sump plus the banjo of maybe 20mm with the outlet running backwards and the reed valve being of the Yamaha YZF125 2008 2014 air valve type that fits in a straight tube say going up behind the gearbox.

I have a Yamaha XS650 breather valve on my B31
20170211_100110 by Sigma Projects, on Flickr

20170211_100055 by Sigma Projects, on Flickr

I like the primary case used as a vented breather chamber.
I just use SG oil, it's fine for the clutch.
I've only ever removed the sump screen once in 20k miles
(that was because someone on here said i should)
and it only had a small amount of debris on it.
You do get a bit in the primary drive area but i only have a
3mm hole drilled about level with the chain tensioner rod as a
drain back to the sump, it's drilled at an upward angle. I doubt
it is necessary at all really, the bearing vent is probably all that's
needed.
Don't have to mess about checking primary oil level, one type of oil, great.

Quote Gavin
"Big valves are not absolutely necessary, mine ran for years on stock valve sizes , if you look at Mark Parkers port work, he gets much improved flow by tweaking the ports using standard size valves."

Yes if you have a flow bench etc for testing, you can get very good results with port work.

For an amateur a set of larger valves and a little work with rotary files/emery will get some
extra flow and a noticeable improvement on a road bike.


Also 90% of the blokes who successfully raced these things never saw a flow bench.

Here are my thoughts on your 750 conversion:-
- as you already have the billet rods, use them together with a new oil pump and OPRV and add an oil filter. These are the most important factors in maintaining reliability IMHO.
- I would avoid using the primary case as a breather area as it will blacken the now shared engine oil rapidly and tiny bits of swarf from the chain & clutch will get sucked back and forth through whatever breather hole you add. Maybe you could add a breather to the rocker box or elsewhere and use a reed valve.
- The standard A65 Spitfire cam is considered as about as best as you can get, not sure how well it works on a 750cc variant but presumably as good.
- use Pazon, Boyer or Wassel Vape electronic ignition, they are all fairly similar and should serve you well.
- Don't know what carbs you are going to use but consider new carbs like the AMAL Premiers or maybe the Kehin PWK replicas like JRC, OKO etc.

Thanks all for the replies, lots to consider. I don't intend to do an end feed conversion as there is no one local here with any experience with it and given the amount of miles this motor is likely to see I should be fine with standard timing bush. I will fit an oil filter. The sump breather is not something I have ever heard of before and sounds interesting if the 750cc proves to be an issue with excess pressure. Might be able to find someone local to add larger intake valves. That would mean seats as well? Not sure what size valves as looking at the head there doesn't seem to be a lot of extra room.
The motor will eventually find its way into my Spitfire mark IV project with the idea that I could remove it and re install into any other project as this is being built up using NOS unstamped cases.

John,

I didn't realize that was your 5 speed cluster. What final drive would you recommend for road use? In the past with the regular 4 speed I have gone up a tooth on the gearbox sprocket from 19 to 20 keeping the rear drive at 47.
Are you still involved with Alloy Barrels?


Cheers,

Bruce

Yes that's probably true, I should contact SRM and see what the turn around time and shipping would be.

Cheers,

Bruce

Nick,

That's good to know. I am still looking into cams but I know the Spitfire cam is supposed to be very good as is, just didn't know if it was suitable for the 750. I have put a 7 plate conversion into my current Lightning build but won't know how it works out until I am finished and on the road but if it proves good I may just add one to the 750.

Cheers,

Bruce

Having had the 7 plate clutch and the Bob Newby... fit the Bob Newby. In my every attempt to destroy it.. I cannot. Also if you have the PES 5 speed box, Bob will make you a clutch centre to fit the spline on the mainshaft.

The standard cam is useful, it can be advanced or retarded and still give a good power spread but move the peak torque up or down the revs. If you fit the SRM race cam it give you a lot more torque throughout the range, it makes for a very tractable yet quick bike. I currently have the X12 in my bike and it’s not bad but I think I should have saved my money and stuck with the SRM item.

PM Kibblewhite do the valves i was on about. You will need to cut the seats
back to get the correct fitted height. May as well fit new guides at the same time.

If you do decade to fit a different cam the SRM 'race' one works well with the 750 kit,
it's not radical, it's quiet, and the motor will idle. It does increase torque above around 4k.
You will have to mess about with carburation/mixture a bit though.
You will also need different springs.
(personally, for the road i'd leave the spitfire cam in, guides will last longer, use standard springs etc)

As i've run primary type breathing for around 30 years on the track and on the road, i
suppose i'm rather convinced it works well. Believe me, i wouldn't run/put up with it
if it caused the problems others say will happen. I'm a lazy sod and if it made more
work i wouldn't do it. As long as you have a filter it's fine, Triumph did it for years, i know
that's no recommendation but it's not a unique setup.

Hi Bruce

For clarification regards the 5 speed. It was Mark at PES who made/makes the clusters, it was I who got it kicked off and had the first 12 sets made.
I think 28 x 58 & 20 x 47 (26" OD tyre) will work fine for road work, bit of traffic and twisty roads will be Ok. Although my gearing is slightly different it works out more less the same and it'll hit 85 at 5k RPM in 5th and can still work traffic in 1st.

I don't have any barrels at the moment. I have been looking for a foundry, but so far the ones I've tried are just doing their own in house work. Or their prices have been scary in comparison to what the last batch cost.

regards

John

John, sent you an email with link to a guy doing B40/C15/B25 alloy barrel castings.

Hi Kommando

Thanks for that.

John

A friend of mine is using one of their barrels on a very much used B40. It seems to run quite well!

Ive been thinking about the reed valve in the sump plate, i doubt if it will work on a twin with the central flywheel, there should be more free space in a single so long as the conrod clears the intake.

It works on a Commando, mine is just my cheap copy of a 300 USD, the objective is to get as near to the high pressure air.