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PostPosted: Mon Jun 04, 2018 9:51 am 
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Joined: Fri Feb 29, 2008 5:42 pm
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Location: Hertfordshire
Car Model: 1961 96 two-stroke
Some experts favour locking a two-stroke's distributor so the centrifugal advance is disabled, then setting the timing at 18 or 20 degrees BTDC. It will then stay at that figure right through the rev range.

Can anyone explain why this is a good idea? Surely it will be too advanced at low revs, leading to pinking and a snatchy throttle response as drive is taken up in the freewheel, and too retarded at high revs leading to a lack of power and excessive heat in the exhaust ports. It seems all wrong to me, when as standard the timing in my car (early distributor, no vacuum unit) starts at 10 degrees BTDC, advances to 20 degrees in the midrange and then advances further to 30 degrees at high revs. All of which are figures in the ballpark of what you would expect in most engines.


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PostPosted: Mon Jun 04, 2018 1:35 pm 
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Joined: Mon Jan 12, 2015 5:43 pm
Posts: 24
Car Model: 1964 bullnose 96
I've often wondered why people do this too! I rebuilt my distributor with a new vaccuum advance diaphragm & very much kept the centrifugal advance (put new springs in).

Having said all that I also installed an electronic ignition (but you'd never know from the outside!). It still seems to pink occasionally at high revs even though I set the timing carefully with a strobe. I'm not sure if it's to do with the fuel:oil mix being too rich at 33:1.

Aiming to do a bit of tinkering soon although I'm currently in the middle of rebuilding the brakes so that takes precedence!


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PostPosted: Mon Jun 04, 2018 2:17 pm 
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Joined: Fri Feb 29, 2008 5:42 pm
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Location: Hertfordshire
Car Model: 1961 96 two-stroke
Interesting. When coming back on the power at low speeds after freewheeling, perhaps when slowing for a junction then accelerating away, does the power come back in smoothly or is there a snatch? I ask because I tried a distributor with a vacuum unit and the engine became quite snatchy, even though the timing was readjusted to suit the different distributor. So I went back to the original one.

Also, where did you get your electronic ignition? Is it the Swedish Classic Saab Racing Pertronix one?

Meanwhile, we both await wisdom on the locked advance mechanism - which is something that Classic Saab Racing do on their hot engines, I believe. But Tom Donney in the US advocates it for all engines.


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PostPosted: Mon Jun 04, 2018 7:42 pm 
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Because a two strokes ideal curve would start like a four stroke, the dizzy advances, but as you get higher revs it needs to retard. This carn't be done with this dizzy and since most of a tuned engine rev range would be the retarding bit it makes sense to lock it and set more likely at 16-18 deg. Its not only two strokes, but some racing four strokes back in the day ran fixed timing. On a highly tuned engine a few degrees out could cause higher combustion temp for no power gain.


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PostPosted: Mon Jun 04, 2018 9:03 pm 
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Joined: Mon Jan 12, 2015 5:43 pm
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Car Model: 1964 bullnose 96
Hi John. The power comes back in quite smoothly so I don't think that's a common problem with the vacuum advance. The electronic ignition came from Aldon Automotive but it seems to be a rebranded Pertronix Unit. You're welcome to take it for a spin if you want to compare - I know you're sometimes down at Goodwood near where I live (I'm hoping to take it to the classic Breakfast Club later in the summer but am still waiting to hear back from them).

Best wishes

Alex


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PostPosted: Mon Jun 04, 2018 10:48 pm 
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Location: Fliptop Towers, North Yorks...the flat bit.
Car Model: One or two...
Aldon = Pertronix.

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PostPosted: Tue Jun 05, 2018 8:31 am 
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Joined: Fri Feb 29, 2008 5:42 pm
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Location: Hertfordshire
Car Model: 1961 96 two-stroke
rallyv4 wrote:
Because a two strokes ideal curve would start like a four stroke, the dizzy advances, but as you get higher revs it needs to retard. This carn't be done with this dizzy and since most of a tuned engine rev range would be the retarding bit it makes sense to lock it and set more likely at 16-18 deg. Its not only two strokes, but some racing four strokes back in the day ran fixed timing. On a highly tuned engine a few degrees out could cause higher combustion temp for no power gain.


Interesting (again). Why does a two-stroke need to retard at higher revs, I wonder? And why didn't Saab recognise this when designing the advance curve? I'd love to know more about this. Maybe a standard engine, like mine, is best left as designed.

Alex - thanks for the offer. I'll try to get to that Breakfast Club.

John


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PostPosted: Tue Jun 05, 2018 8:43 pm 
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Joined: Fri Jun 16, 2006 9:26 pm
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These days it's quite possible to programme an advance curve to include retard as well.

The Aldon system could possibly be used, not a true 3d system but doesn't need crank sensor or throttle position sensor.
Check with Aldon to confirm it would work with 3 cyl two stroke but don't see why not.

http://www.aldonamethyst.co.uk/


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PostPosted: Tue Jun 05, 2018 9:49 pm 
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Joined: Fri Feb 29, 2008 5:42 pm
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Location: Hertfordshire
Car Model: 1961 96 two-stroke
That sounds a good idea, coupled with a rolling road session to discover what is actually happening in real time and how to improve it. But maybe that's a bit over the top for a standard 38bhp two-stroke. We're off to France in it tomorrow for a few days so I'll mull it over as we cruise the M25.


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PostPosted: Sat Jun 16, 2018 8:08 am 
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Location: West Dorset UK
Car Model: 96TT/900S Cabrio/9-3SW Aero
Interesting discussion. I acquired my stroker from Chris Partington several years ago and he told me that, as it is "mildly tuned" (I assume that means that the ports have been modified) it's running fixed timing at 18° with the dizzie plate locked. I've recently fitted an Aldon Igniter system but retained the original coil; I have a new Bosch coil in reserve "just in case". Slight modification required to the rotor arm so that it sits right down on the rotating collar; everything seems fine & it revs freely & pulls like a train. It's also fitted with a full Jetex system & the carb has been rejetted with a bigger venturi to match. Good results. My only gripe is that I don't get to drive it enough - it's such fun & brings a huge smile to my face.

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PostPosted: Tue Jun 19, 2018 12:59 pm 
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Joined: Fri Feb 29, 2008 5:42 pm
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Location: Hertfordshire
Car Model: 1961 96 two-stroke
Food for thought there, thank you. It ran very well on the French trip with the standard timing settings (and on a 40:1 fuel/oil mix) but I sense there might be scope for improvement.


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