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Wondering if I might get some feedback from those that have done it. A lot smoother running? Or better to just get the stock 360 degree crank dynamically balanced. I know what is required cam and ignition wise and it's expensive but might be worth it for me anyway.

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Yes, it's worth it, performance and vibration wise.
Stronger engine is another benefit.
BUT an expensive procedure these days.
Must use a twin carb head for best results.

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Smooth running is dependent on a few things. Std stroke and big bore is best. Aim around 50-52% don't make the crank too light. Don't gear too low, it pulls better, fix the head. The big bore isn't necessary but pistons are lighter by a little usually and good forged ones from Ed V have a modern ring pack and run tighter. Use good rods.



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Originally Posted by Mark Parker
Smooth running is dependent on a few things. Std stroke and big bore is best. Aim around 50-52% don't make the crank too light. Don't gear too low, it pulls better, fix the head. The big bore isn't necessary but pistons are lighter by a little usually and good forged ones from Ed V have a modern ring pack and run tighter. Use good rods.

What gearing do you recommend?

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It depends, if it's 650 or 744. 744 20-40 is good out on the highway. The one above is 20-38 with 17" wheels which is similar. Some riders prefer lower first gears and are in cities. A stock 650 with just a good head is a bit low with 21-47, but is ok in a std bike and if it was smooth to rev would be fine considering the brakes. It could pull quite a bit taller for out of town depending on preference.

I would not do bigger than 744 again. And even 650nis really nice.


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Sounds great!

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Originally Posted by Mark Parker
.

I would not do bigger than 744 again. And even 650nis really nice.

Interesting statement Mark, why would that be?


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Fair bit more stress on the old lumps when you stroke them more than about 85mm.
Stroking always makes balance harder too.
For 95% of the use of these, 750 is adequate.
Just my 2c.

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Same as Nick, it's hard to exactly define, probably the ease of kick starting, smoothness, friendly. A 744 doesn't lack hp it's possibly the same with a few more rpm. Rod length is good. Everything fits easy. Parts are cheaper and easier to get. $450us for a pair of good steel rods. The long stroke has custom and the length is a bit short with high side load. And though it may pull past 8,000 it cannot do the rpm of the shorter stroke engine. I think I did a calculation on piston speed and volume and the 744 would win, as in pump more air.

The big one has more grunt but if it had an edge it wouldn't be significant and racing it would be down to the rider, and how well it was tuned. If it was going to show up it would be with an outfit, and a 750 can get that job done, and even a 650.

It's like an Aprilia 1000 my son had, it goes fast easily, a smaller engine can work harder which you can use more. On the road there is a limit, and also top speed isn't usable on either, even in RGV frames. Both would cruise beyond what you need, or could use.

Displacement is in the equation cubic dollars or cubic inches, cubic dollars is really just getting it set up well with stuff that works together. Nick and Damien and Matt have a 650 set up that can win against much bigger outfits.

I'm seeing breathing as being more influential than displacement, and 744cc matches what the head can easily do, so I'm spending time experimenting.

But big engines definitely have a place and can be really great, if loaded up two up etc. I'm just talking about mine, and a 750 90degree, I like the sweet little one.


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I still haven’t got round to riding the 823 yet (still in the middle
Of a house move so priorities change) I’ve always liked the 650 and whilst I’d like to do an A70 esque 750, what I would like more is to build the 500 motor with full race cam. I’ve got 2x twin carb heads for that. An early cyclone/wasp? Head (68-465? Like the A65 466 head in that it tapers down, but for an A50) and an ex works Daytona head with 30mm ports to match up with the GP2 carbs. Though the works head will probably be more of a pig than a blessing on the street.


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Has anyone any input to how difficult (or not) it is to kickstart a 650/750/850 270/90 degree twin?

I don't think the time honored way of easing one cylinder over compression, then giving it a good swing would
work well on a 270/90. You are either going to hit the next compression stroke almost immediately, or spin the
crank for some distance before the next, depending on which pot you first eased over compression.

My last road bike was a square case '77 Ducati 900 desmo which had an electric starter, but was a royal pain
to start via the kickstarter. However, once running, it was surprisingly smooth over 2k rpm.
I only mention that because I perceive the Ducati to be a 90 degree twin.

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Never started a twin or triple in that "time honored way". Always brought one cylinder up to compression then kicked it through.
Maybe that is why you see people jumping on the kickstart lever. The cylinder that was just on compression is past the ignition point of any point or EI so you have to kick it hard enough to bring the next cylinder all the way around, up to and through compression to fire.
On a B50 you cannot kick it through compression. You have to bring it up on compression then using the decompression lever move it closer to TDC then kick it through. You would never get it to go through compression kicking from just after TDC.

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Originally Posted by DMadigan
Never started a twin or triple in that "time honored way". Always brought one cylinder up to compression then kicked it through.

Maybe we are describing the same thing, but in different ways.
However, my interest is in how difficult (or not) it is to kickstart a 270/90 twin.
Not a standard twin, triple or single.

My experience with the Ducati was that it wasn't easy. In fact, without the electric start option, it would
have been unfeasible for me to have used it as a daily rider.

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When he had his firm going, my older brother carried out many
conversions to road going bikes, fitting norton offset cranks into
750 triiumph engines and several a65's. None of those bikes
needed or had electric starters.
Starting various engine types is largely a matter of sorting out a
technique rather than brute fore and ignorance. My sidecar outfit
for example as a 880-920 offset twin with high comp and hairy cam,
could be started by myself in five or six paces, when racing the pair of
us used 3 paces to push start.

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My 360 Firebird is 650 and easy, 1 kick usually hot or cold unless you leave it sit for a month or more. My 883 is a pig, bounce off that kick starter. I park it on a hill, with that backup when warm one kick, without it no way. At home I made a roller started, it's instant but my right boot lets water in on a wet day because the kickstart cracks the sole. Otherwise boots last years.

Ben's 750 can be good or not. It has two 12v CDIs and a trigger for each I made. It can stand you up. The 360 with Pazon never does. I've read that SRM recommend removing the carb balancer on a 90degree. I do mean to try that. The head I made to try on Ben's has no balance fittings. The Firebird doesn't have them, or need them, but the carbs were set up that way. The tube will effect it when removed why we haven't done it yet.



This is with the old mufflers, the new ones go through at 51mm not 38mm. A 750 big bore can give very good power and even 11 or 12-1 is better to kick than two 441 singles with no valve lifter. On the 883 you do not open the throttle with your foot on the lever. Put chokes on 3 or 4 kicks no ignition, turn chokes off, don't touch the throttle and kick and see if the lever moves fast enough. PWKs on the 650 have chokes that work perfectly, so we will see how using a pair of them effect the 750. carbs and fuel and ignition all play a part, what the balancer may do I don't know but with a single cable and a splitter near the carbs it's not necessary.

I'm hoping we can get the 750 to match the grunt of the 883, it isn't bad really, it's just that it may be possible to get most of the grunt, and it's a better configuration for rpm, easier to do.

Head ready to go. The ports flow about the same as what's on it, but are high speed.

[Linked Image from i.postimg.cc]


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Originally Posted by Mark Parker
My 360 Firebird is 650 and easy, 1 kick usually hot or cold unless you leave it sit for a month or more. My 883 is a pig, bounce off that kick starter. I park it on a hill, with that backup when warm one kick, without it no way.

I guess I am not surprised at that. Part of why I thought that the 900 Ducati would have been unfeasible without the electric starter was that if you stalled it in traffic and had to kickstart it, it's a push it to the side of the road, put it on its center stand and start heaving on the kick starter kinda proposition. The warmer the engine, the more likely it was to kick back at you.

I expect that an 883 360 twin wouldn't be much fun, but not as hard to start as a 90/270.

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A 650 or 750 90degree should be easy. My brother's was easy. Ben's starts ok most of the time. I doubt the configuration makes much difference. But maybe the triggers are a bit suspect at low speed. Displacement isn't all it might seem. It gives more power in an obvious way if you have no other way to find it, but there is another way that delivers. Which is the intriguing part.

A 650 can be brilliant. A 744 gives you a bigger bore and more displacement and I think is the optimum configuration. I was going to say nothing sounds so good from a distance, but it does, if you have ever seen an old Elvis movie where he's driving a 426hemi stock car on a big oval track, that at 6,000 + is what it sounds like.

Consider a very hot modern hemi V8 drag motor, no blower, twin 4 barrels 500ci NA with 1,275hp. So a 45ci 750 would translate to 115hp at 8,700rpm if it was possible, if the head would stay on at 15-1 comp, but aiming at 85hp+ with a nice power curve isn't unrealistic at all. There is a lot of area for exploration.


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i've built my own ignition system for the 650 (and LM II). the ignition reference-point is set at MaxAdvance. The BSA is easy to start as the cylinders are only 325cc each. Starting a big single like the B33 is different. No way you can get such capacity started with the reference at MaxAdvance. So I made two reference points, one at TDC and one at MaxAdvance+ and the computer switches at 300 rpm between TDC and MA+. B33 starts and runs reliable.

a morini 350 can be started at MAxAdvance (MA), a Morini 500 not. although only 250cc per pot, the kickstart-lever turns the crank insufficiently fast to use MA. so again I used TDC and MA+ as reference positions. starts and runs reliable.

a 270 BigBore BSA can be made to start in the same way. the ignition is built in such a way that kick-starting it will produce a spark at 1~2drg BTDC reliable. so starting without kicking back.

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You could have changed the pre-scaler up tp 300rpm and done it that way
rather than employing 2 sensors.

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Originally Posted by NickL
You could have changed the pre-scaler up tp 300rpm and done it that way
rather than employing 2 sensors.

it is not the prescaler or timing by processor, it is the irregular crankshaft-speed at low rpm. if you use MA as rference then at kickstarting the 32 drg BTDC reference point passes by but due to the different crankshaft speed (every kickstart is different) it isn't said that timing by processor and crankshaft position line up at TDC. when the crank is slow you end up with a spark at some degrees BTDC and have the engine backfire.

if you use TDC as a reference then kickstarting is easy but when you want to shift to the advance curve the spark has to wait 690 drg before firing. even at 1000 rpm crankspeed is measurable unstable to have a correct ignitionmoment. so I use two sensors. as the processor has >4 inputs -> no problem.

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Fair enough 720 degs is a long while at low rpm.

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If the ignition timing starts at maximum advance, crank slowing will not be a problem. Plus, if the firing at starting is constrained to the minimum advance then it cannot backfire. At the stock 12 degrees BTDC you have to be kicking very slowly for the ignition to be a backfire.
The RPM has to be filtered somewhat because the RPM from one cylinder's firing can be different from the other so the timing calculation has to be taken from at least two revolutions (for a twin cylinder).

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Ducati 90 degree two valve OHC belt drive engines for many year had two separate waste spark ignitions, one for each cylinder... Don't know if it's just different engineering or useful...Of course they are electric start.
I have a 89 Honda 650 NT Hawk...400 pound 52 degree V twin with an offset crank, usual weird Honda engineering...It's a 238 degree engine rather than the usual 270 of a 90 degree crank..Not highly developed for the best power.
.It's rated at 58 crankshaft hp @7800 rpm but realistically you need to use an after market exhaust ,pods and jetting to get that 58 hp...Set up this way the bike can turn 12.7 1/4 mile times with gearing that has the engine spinning only 3800 rpm at 60 mph.The engine has a wide powerband and the pleasant wide angle v twin vibration...This sort of performance will smoke most any modified Brit pushrod twin...As Mark says, 650 can be ok..


79 T140D, 89 Honda 650NT ,61 A10 .On a bike you can out run the demons
"I don't know what the world may need
But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"
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There are many approaches to this, myself i'd use a wasted spark as that
means only a single sensor with 2 magnets. Load the maximum retard
value in when starting, do the speed calc during either dwell or spark time.
That way when starting you always apply max retard on the first cycle and
you have plenty of time for speed and retard calcs etc which can be done
every 360 degs.

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@hillbilly bike: broke my right leg starting a XS650 using this apporaoch.

@NickL: good solution with a 360-crank, a 90-drg-offset-crank doesn't work properly with one sensor and two magnets. then the coils have to be in series. at high rpm one coil isn't energized correctly, to little dwell on that one.

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