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#863228 11/15/21 3:03 pm
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Winter has arrived here in the canadian west so it's time to contemplate the next project I'm wondering what the best options are for maximizing the midrange grunt of an a65. I am contemplating a big bore kit and or cylinder head work but would really like advice from those who have experimented with engine mods. I'm sort of inclined to see what is possible with the single carb head. Thanks.

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What year?

The 66 - 71 head, IMO, tend to have a flat spot mid-range with the stock head & 68-473 cam.

The early dual carb head (64 - 65) if you can find one unmolested are the bomb for riding on the street.


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Search for Mark Parker's threads here to see what can be done with heads.
My 71 has great middrange. Heres the recipe,John Hill big bore Alloy Nickasil barrels, B44 pistons, stock stroke , end fed crank,2 off 30 mm mk 1 Concs,stock airbox , stock pipes, big inlet valves. Seven plate clutch with mainshaft outrigger bearing. The head was flowed by following MP's advise, flattened D ports, deeper bowls.put more stuff in than I took out


71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
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You can fatten the midrange easily by reducing the tail pipe outlet to about 1 3/8. A lot of aftermarket pipes have the same size tail section as the sleeve pipe that goes over the down pipe. If this is the case you need two reducing pipes shoved up the tail pipe. Makes a big difference!

You can make changes to the head and cams after that and have an even better bike, but the above will help a standard bike run pretty darn well.


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71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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Many thanks all! How critical is the outrigger bearing?

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The outrigger is not essential for road use, I used my bike with the big bore kit ( Originally Devimead barrels) for years without it, but in the great scheme of things while the motor is down for work its not that hard to fit and helps stop the mainshaft flexing. PES in the UK have something that might fit or you can modify an existing inspection plate.


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Single carb head is better in midrange than double carb head, so you are ahead from the beginning in that respect. With a change in exhaust pipe diameter Alan mentioned you will have a great starting point for further tries with head work ( if you really need more grunt in the midrange ). Nick L. is using single carb head and as a former racer perhaps have something to add what worked for him. Most people tried to improve double carb head, and in this respect early A65 double carb head wasn't bad at all..For a bike used as a daily rider I wouldn't go crazy with high compressions pistons which mainly make vibration level uncomfortable.

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I'm hoping we might soon get a dyno graph on engines with heads like I run on my std Firebird. It's hard to qualify the response it has without that. Other than the fact it's hard to get the 3 spring clutch to hold it. I was of the opinion that 34mm twin carbs on a 650 would tend to push power delivery into higher rpm. And though that is no doubt true the flow through the port is having a big effect right through the range. From when you move the throttle. It's just nice and very willing, if you twist it right around it goes nuts.

The idea it might need a 750 kit doesn't cross your mind really. it would add to what's there no doubt but it's already not normal.

I use 34mm pwk because they look nice and are extremely cheap and well made, with a chrome slide and should last. Less than $100au a pair. Nick's outfit he looks after uses a dual throat Weber, which would be great if you could fit it on a solo. What carbs are bolted to makes a big difference. And the A65 has a great casting.

These 34mm look big in this shot but are quite small and light. The throttle action is light. A smooth bore would flow more. But flow through this head and carbs is already up by around 50%, and weirdly that comes across as midrange punch as well as fierce top end. We could build a 744 but it seems a fun idea to use a 654 in P4. The main challenges being beefing the clutch, outrigger bearing behind it and on the crank, and balance. Otherwise stock cam and maybe higher comp. The crank outrigger means the rotor will not run into the stator on a flexing crank. Just build bullet proof with rods and bearing conversion.

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Last edited by Mark Parker; 11/18/21 6:06 am.

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For a road bike with good overall performance upto around 85-90mph the a65 t'bolt
is pretty good if it is well put together and running correctly.
Allans comment on the exhaust is relevant as some aftermarket gear is far from correct if they are
used without other modifications.
These engines always responded well to larger inlet valves, i'm going back to the 60's here
literally everyone who tuned the a65 always fitted bigger inlets as a first up step. NOT always
enlarging the ports but just larger inlet valves.
The single 28mm carb is fine upto around 6k on a reasonably tuned motor. The small ports with
large valves work very well in mid-range and make for a very flexible bike which requires few
gear changes.
My own bike differs little from standard, it does run pm large inlets and i did do a little work on
the inlet ports and manifold. Aim for and measure compression ratio, 9.2-1 is good with standard
cam and fuel, take all the edges off the piston crowns. A decent ignition system is a must also.
If you want a growling beast of a bike, look at Marks road thing, if you want a nice overall bike
for plenty of miles, I'd be looking at Gavin's as a guide. The 750 kit is a magical bolt on tweak for
these engines, it transforms them without anything else. If you go that way, yes an outrigger is essential.
My favourite a65 engine is the 750 big bore one, it's just right for balance and everything. Yes, you
can build 900cc+ versions etc .but the 750 big bore is what the factory should have produced.
The gearbox is the Achilles heel on the a65, spend plenty of time getting it right, it needs it.
There are a few guys here that run a65's on the road and love 'em, they are normally happy to
share experience etc.

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Thanks for the vote of confidence Nick. good things about the 750 big bore , 6kg weight saved, lighter B44 pistons ( less material from crown to gudgeon pin) and a dynamic crank balance make it smooth up to about 5.5 k with the stock balance factor, the stock chamber combined with the B44 piston tops gives a squish effect which helps the fuel burn.
at 9:something below 5 ish to one its an easy starter , the heavy flywheel really helps. On 21 tooth gearbox (One tooth more than normal) /47 rear sprocket gearing its in the sweet spot from 60 to about 90, stuff doesnt fall off or crack too much, the big downside is you need more brakes. Having thought a bit more about the outrigger bearing, maybe it is essential, before I fitted it I had a lot of bother with the clutch, ( it was shagged , new bits were fitted at the same time). Going up through the gears changing up at 5.5 gives enough poke to get past most stuff indecently quickly. I sometimes rev to 6 + but its not really any faster, the stock 30 mm carbs probably hold it back a bit at full chat, the flip side is the high gas speeds in the carb make it perform better, I had to fit 3 sizes smaller main jets , and a weaker needle jet it breathes harder faster gas lifts more fuel.Hardly ever needs tickled, starts of choke down to about 12 C. It does about 55 mpg cruising at 4- 4.5 K, seldom gets overtaken. At its best on hills, with twisty bits. It deals with A road tourist traffic without any issues, its my daily transport when the rain isnt falling, a very fun / practical bike. theres an overlong thread here if you want the gory details https://www.britbike.com/forums/ubb...els-5-speeds-a65-750-oif-1971#Post541665, theres about 10 years of evolution, the pics might not work until the end, some were hosted by phot =£$ cket, who went all wrong, the ones near the end should be OK.


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56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
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Best bang for the buck is with the exhaust. The 69-70 pipes with the connector work pretty good or, a bit cooler, is the 'Siamese' two into one with the correct silencer or Goldie style. I think it is THE place to begin a quest for fatter mid. Cheers

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The big valves work Nick, for sure. But they are letting more air through because the port is so flawed and just using part of the valve. So the bigger that part the better. I was putting 44.5mm inlets in 650s and they really made a difference. But the smaller valves can flow more than they were, by using the whole valve circumference. And the big ones so much again. Though an overkill on a 650 probably.


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Nick could you clarify your comment about the 750 kit being just right for balance? What are you using as a balance factor? Do you mean that no adjustments to balance are necessary?

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When i first used one on the race bike i just bolted the 750 kit on and
did nothing else. The balance /vibration was better straight away.
I then used a couple of static factors starting at 68% then 65% which
is where the engine was when i sold it.
Mark, i understand about the head/ports but without either experience with
these heads or a flow bench, most blokes are looking for an immediate gain.
Sticking large inlet valves in them works straight away. 30 mins with a porting
tool can make them better or worse depending on who's wielding it! There is
no risk in just putting a pair of inlets in.
Tom, i too am a believer in the siamese exhaust setup, certainly for mid range.

Last edited by NickL; 11/19/21 11:50 pm.

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