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Jon W. Whitley
Jon W. Whitley
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Thread Like Summary
Allan G, BSAonAnyDay, BSA_WM20, gavin eisler, Hillbilly bike, Kevin E, pushrod tom
Total Likes: 9
Original Post (Thread Starter)
#839855 02/12/2021 10:09 PM
by Bruce
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.
Liked Replies
#840338 Feb 17th a 11:03 PM
by NickL
One of the tricks used when tuning the a65 engine is to cut a large section of the crankcase out in
front of the left hand con-rod, thus allowing free passage when it breaks.........
4 members like this
#839897 Feb 13th a 12:59 PM
by kommando
Originally Posted by gavin eisler
I run my 750 with the following mods.

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.

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.

[Linked Image from]

and screwed into the case.

[Linked Image from]

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.
1 member likes this
#839969 Feb 14th a 12:17 AM
by NickL
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.
1 member likes this
#840143 Feb 15th a 06:54 PM
by kommando
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.

[Linked Image from]
1 member likes this
#839876 Feb 13th a 03:46 AM
by DMadigan
Considering a Nova gearbox is $3500 sending head head back and forth across the Sargasso appears cheaper.
1 member likes this
#840212 Feb 16th a 05:06 PM
by Hillbilly bike
Hillbilly bike
Originally Posted by Allan G
Im going to play dumb here (ok, half play it)

When you look at the breathing system on a car, it lets in air as well as it expells toxic blowby,

[Linked Image]

So, apart from the Bunn breather system which has an inlet and outlet. why is it suddenly a great idea to fit a system on a bike that will give a negative pressure in the crank case? Before PCV valves started getting added on our old heaps, an open breather was quite satisfactory. But if its also such a good idea why don't the boffins that design car engines use the same principle? It should work better surely as not all the engine displacement volume is being evacuated on a 4 or 6 cylinder car engine?

If you are going to fit a reed valve then what is the benefit over the timed breather?

(I've got no sway either way on this, just asking a question in a critical way)
The closed crankcase breathing was adapted as a result of the first emissions laws in the US in the early 60's. Before that it was offered as an option on slow moving delivery vehicles to limit oil dilution from water vapor. US engines used a road draft tube for crankcase ventilation that needed air flow from road speed to pull vapors from the crankcase. The PCV system does cut down water vapor and sludge in the engine.
I notice my 96 Ducati has only one breather, Don't know if newer bikes have a system to pull in fresh air...
Drag race cars use vacuum pumps or a tube hooked to the exhaust to keep a high vacuum in the crankcase. I belive this is primarily for ring seal purposes...
Is there any proof, lets say on a dyno, that the vacuum one way valves offer any advantage over a proper sized open breather?
1 member likes this
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