450 desmo carb

[graeme]

Or a big Bang !

[Dan C]

Hi Bill,

I've been busy with other stuff and haven't looked in at the forum for a while so have just caught up with your latest "adventures"....can't help but admire your persistence and optimism!! You must have very good malt there in Perthshire......
Anyway there's nothing I can suggest that you haven't already thought of but I suppose you could always try electronic ignition at some point. When i was contemplating the ignition as a culprit Graeme advised me to consider a Sachse unit. I installed their 6v version on the 450 desmo and it has performed faultlessly. Very easy to install and even ham fisted me could set the timing easily.

Good luck - I'm hoping you get it sorted so you can enjoy some summer riding.

Cheers
Dan

[themoudie]

Congratulations Graeme!

Your sixth sense prediction proved correct. I have attached an image of the evidence!

With the MKII attached, with 3.5 slide, needle on the 2nd from top groove, 106 needle jet, 220 main jet, 25 pilot jet, the engine had fired up and been a bit pop and bang through the silencer, until the airscrew was turned in from 1.5 turns off the seat to ~0.75 turns off the seat. Stable tick over at 1,200rpm to 1,300rpm adjusted and when the throttle was blipped, mixture a bit rich, with a touch of black smoke. Making the mixture weaker on the airscrew brought the return of the banging from the exhaust, so returned to previous ~0.75 turns off the seat. Stopped the engine, let it cool and went round all the fixings with a torque wrench. Plug a bit black and wet, but would await a proper road test to further adjust. Fitted the tank and seat, ready for the road test.

Kat came out and asked if it was OK? So, fired the engine up and let it warm through at ~2,000rpm, then setlled to an idle. Blipped the throttle to 3,000rpm to 4,000rpm a couple of times, let the engine settle to a tick over and then blipped the throttle. BLOODY GREAT BANG from between my legs, fuel everywhere, vapourising off the exhaust and engine! Hopped off, turned th eignition off and both fuel taps. No sign of flames, so stood back and threw my rag at it in disgust! Couldn't find any kindling!

So, what to do now Mis'er Rayner? I'll let you know after Kat has kindly fed me and I maybe take the SRX600 for an evening canter and then have an Orcadian malt! It's only a machine after all!

Good health, Bill

[LaceyDucati]

Bill,

Your findings with the AAU are in line with mine. I have tested units at 32 degrees as well.

Regards Nigel

[themoudie]

Aye Nigel and others following this saga,

Thank you for the confirmatory background information on the AAU (AA359). However, I am more perplexed now. Despite setting the maximum advance at 28°BTDC and the resulting 6° ATDC when the AAU is in full retard position, there would still appear to be a propensity to blow the carb off the intake spigot?

Is the AAU the cause of this or should I be changing the carburettor settings? With the #25 pilot jet in the float bowl and the air screw 1.5 turns out from seating, the current mixture is too weak. There is banging in the exhaust when shutting the throttle and this disappears when the screw is turned in to 0.75 turns out from seating; the Gunson plug view conforms this change in the flame colour.

Is using electronic ignition set at an advance of 28°BTDC any more reliable than the current AAU setting? The outcome of setting the AAU to maximum advance 28°BTDC does not appear to have solved the backfiring problem and the MKII carb still appearing to require a rich mixture at tickover that Andy Hunt appears not to require with his engines. Andy is using an AAU set at 28°BTDC, points, #25 or #20 pilot jets, needles with the clip in the 2nd from top groove and #200 main jets, along with a #3 slide, on race bikes that tickover, and win races. He also has ported heads etc that are more efficient at flowing the fuel/air mixture than my head. I am thinking of re-fitting the #3 slide and trying that before giving up on the AAU.

This may be a bad idea, but I am in uncharted territory for me and the engine!

After tea, 60 miles on the SRX600 off into the hills, followed by a mug of 'Gunfire' (not the Orcadian malt! ) has help sort the head out.

Good health, Bill

[graeme]

Maybe a dumb question and I’m not reading your posts correctly?

Did you mention a Boyer ignition? Does this advance the spark at revs?
Then you have AAU points that also advance ?

Can’t understand where the ignition source for the fuel in the inlet is coming from especially now as you said at idle you are 4 degrees ATDC ??????

Graeme

[Jordan]

A Boyer-Bransden analog electronic ignition uses an inductive pickup.
The waveform from such can have an asymmetric shape, one side having a much steeper slope than the other.
I believe it's the basis of the inherent advance-retard feature, as the "slow" slope gradually triggers the spark earlier in the cycle as the engine speeds up.
An added adv-ret device should probably not be used with the B-B in its normal configuration.
However if the wiring is reversed (two terminals swapped over) the steep side is active.
That gives nearly consistent trigger timing whatever the engine speed, so virtually static ignition timing.

Drawing is from a maker of an aftermarket car elec ign that requires a static trigger signal.

pickup_waveform_2.jpg

[themoudie]

Aye Graeme and Jordon,

WOOOAH! Hold your horses!

The Boyer Bransden IDU is indeed an inductive discharge unit (IDU), BUT, RELIES ON THE POINTS AND AAU FOR THE TRIGGER AND TIMING of that inductive discharge. The IDU insofar as my very limited knowledge of electrickery goes, allows the points to act as an ON/OFF switch, with little electrical current and that prevents the errosion of the point contacts, which is the main cause of inaccurate timing. No magnetic fields, infra red beams etc are triggering the spark to be created or the timing of it's creation.

Back in the halcyon days of early electronic ignition (1970's), the Ducati system failed for me and I didn't have the cash to replace it, or to purchase the aftermarket alternatives then on offer. Boyer Bransden and Lucas Rita were the main alternatives and their reliability then was not what it is now and they both had there Achilles heels! Hence my choosing points and an AAU system. At least I could re-set or replace the points at the side of the road and have a running engine. Whereas a bunch of resistors, a thyristor and other incomprehensibles, sealed in a container, are like the record playing in the Sahara and you are in the Southern Ocean. You can't change the record either!

The KISS principle has always appeared the best way to keep my bikes, hence my enjoyment of single cylinder machines and analog workings.

I too am finding it difficult to resolve how, when the valves are both shut, the engine is able to create an explosion in the inlet tract! Pre-ignition on steroids!

Have a grand day and get some miles in on those Ducati singles.

Good health, Bill

[Jordan]

Sorry Bill,
It sounds like you have a "points assist" type electronic ignition.
I mistakenly assumed all Boyer-Bransden ignition systems used no points.

[Celes]

A worn AAU with weak return springs or worn bob weight pins could leave the ignition timing over or inconsistently advanced at low revs to cause the backfire. A big-ish single with a light flywheel will have a lot of torsional vibration in the crankshaft and timing gear at low revs and this could shake a worn AAU sufficiently to lose precision in holding a particular ignition setting.