450 desmo carb

[ducwiz]

I believe the Lucas system has a wasted spark on the exhaust stroke?

No! There is definitely no spark at the exhaust TDC.
The Lucas reluctor has only one "nose" (see figure below), hence triggers a spark via the pickup coil once per turn of the points shaft. This in turn rotates once per two turns of the crankshaft. There is no differnce to points or other electronic ignition systems (Ducati, Motoplat, etc.)

[LaceyDucati]

Hello Bill and all

The Lucas Rita doesn't have a wasted spark as it has one point on the rotor and is driven at the same speed (half engine) as the points. Many ignitions used on the singles do run a wasted spark without issue. Some race singles I've built have fixed ignition, a wasted spark and a Mark2 Amal without this blowing of carbs issue....in fact even the 450's would start by a good pull on the back wheel from hot.

However I will add that anyone attending Classic race meetings a few years back, will be familiar with the calls on the Tanoy, "Has anyone got a Boyer Branson ignition that they can lend a fellow competitor". In fact what was so amusing was the anouncement would usually be "Boyer Branston"......another fine pickle I thought

I've only ever fitted two Boyers to Ducati singles (both race at the customers request) One failed in the first practice and the other never worked.....just saying.

I am still thinking if two different carbs give the same issue, then it's likely they are not the issue....the quest continues.

Regards NIgel

[blethermaskite]

I stand corrected sorry for any misleading info on the Lucas rita system, however I would still be questioning the boyer ignition in this instance.
Cheers,
George

[themoudie]

Good evening George, ducwiz and Nigel,

Thank you all for your thoughts and suggestions. This being a Boyer Bransden Induction Discharge unit, means that I am using the points and mechanical AAU to control the spark, All the IDU does is take the current away from the points and potentialy give me a bigger voltage across the spark plug electrodes. The electrickery of that is lost on me!

However, having described my problem to Boyer Bransden yesterday, I have received today a detailed list of checks to make from Boyer Bransden and intend to make a start on them tomorrow. I will keep you posted.

This engine has been run on Ford Escort points, Escort distributor cam, with 3 cams removed and a modified Escort backplate, with no AAU. Starting on the kickstart was "tentative", but brought back onto compression in 2nd gear, then a run and bump was the preferred method of starting and with the standard compression piston (~9.0:1), no problem.

I too had heard of "problems" with Boyer Bransden CDI, but had hoped that this IDU would be a straight forward addition. Pazon used to make a similar IDU, but they had stopped production of them some time ago.

Good health, Bill

[Jordan]

In fact what was so amusing was the anouncement would usually be "Boyer Branston"......another fine pickle I thought

Hey, Branston Pickle is my favourite condiment!
Very droll, NIgel.

[ranton_rambler]

I have a RITA on my Guzzi 950 and it has a wasted spark but this is because it has 2 x 6V coils in series so it fires both plugs every time.

When I bought the bike I got the distributor innards and the original 12V coils in a box - one of those coils is on my little Ducati.

[themoudie]

UPDATE 04/03/2021

Ran through the checks sent to me by Boyer Bransden and could find no intermittent shorts or bad connections that might short to earth and cause the ID unit to spark the coil. So disconnected the ID unit from the points and the coil, but left the power supply and earth in place as suggested and reconnected the capacitor and points to the coil. Checked ignition timing and spark at plug, all appeared to be in order, spark at 0.5°BTDC and a good white/blue spark across the electrodes.

Re-fitted the AMAL MKI 932, without altering the settings. After an initial carb tickle and single priming kick, turned the ignition 'ON' and the engine chimed in on the next kick and then warmed through, whilst holding 1.500 to 2,000rpm. As the engine became hotter, so the reluctance to return to an idle became more pronounced. After about four (4) minutes running, the only way to drop the engine revs back to 1,000 to 1,200rpm was by sticking a finger at the bottom of the slide to impede the airflow, yet not actually disturbing the slide from its position in the venturi. The revs would then sit happily at ~1,000rpm and if the throttle was opened slightly, immediately climb to 2,000 to 3,000rpm and not fall when the throttle was released. The throttle cable is brand new, nylon lined, free of kinks, with no tight bends and routed between the tank and frame spine tube. I can wiggle the cable about, this does not affect the revs and there is 2mm of free play in it.

With the engine still hot, I decided to try altering the air screw and slide screw to see if I could prevent the reluctance to return to a stable tickover. Started the engine and returned the airscrew to 11/2 turns out from seating (5/8ths of a turn out from its previous position) and without touching the slide screw the revs rose as expected. I then blipped the throttle and with a blowback, the carb and inlet rubber stub were blown off the inlet mount!

So, from this latest pantomime I think that I can deduce that the problem does not lie with the Boyer Bransden IU unit, but with a weak pilot mixture? Increased engine heat reduces the density of the air/fuel mixture. Hence, the climb in engine revs due to a weak mixture/fuel starvation and the blip of the throttle resulting in backfiring in the inlet tract, with the increased valve overlap of the 173A cam over the old soft (less overlap) OME 450 MKIII cam timing.

Because the AMAL MKI 932 has the cast in pilot jet (20?), I am going to revert to the AMAL MKII 2932 and use a 35 pilot jet in the float bowl, with the air screw ~11/4 turns out from seated, a 260 main jet, a 106 needle jet, with the needle clip in the 3rd from top groove. From my extensive notes this might result in something that will idle, run and pick up without hesitation and hopefully not blow its carb off. But the plug colour may still appear weak (no ocherous to tan staining on the plug insulator). Should I worry about this, as long as the insulator isn't looking like bubbled glass?

If anybody would care to share their actual AMAL MKII 2900 series set up on a Ducati single (preferably a 450) that actually runs, even if it is a race bike, rather than a road bike, I'm willing to give it a try. Thank you.

Good health, Bill

[graeme]

Hello Bill

Good to hear you are keeping notes as you go.

I use 32mm phf dellorto on my 450 (RT and Scrambler)
Scrambler has a mk1 cam, RT has a Desmo cam
SCR is original Ducati electronic ignition
RT is Sachse.
Both (RT Sachse anyway) is set to half a degree ATDC
RT has the rubber Malossi manifold.
Never had an issue with back firing through the carb on either 450.

Sorry I can’t give you any advice on Amals

Don’t know what overlap your cam has, but the mk1 grey cam and the desmo cam profiles are available to check against the cam you are using.

Both engines, and my ‘74 yellow 450 do pick up revs at idle once they are warm.

Can’t see why you are getting pre ignition as you do.??
Boyer?
Inlet valve sealing?

Good luck

Graeme

[Duccout]

Bill,

Is it possible that the carb body is distorted and that is stopping the slide from fully closing when hot? It may be ok when cold, but is there a possibility that the heat is affecting the carb body? The other thing that I'm wondering is whether the cam overlap could be causing the backfires? Is the cam too wild at low revs, allowing sunburnt fuel to ignite in the inlet port? If the gas speed is very slow through the port at low revs, fuel may be building up in it and then igniting, causing a backfire.

Colin.

[LaceyDucati]

Bill

The 173A cam is only really a modified Green and white, the inlet lobe is pretty much identical. It is far from a radical cam in a 450 and my race engines run far more overlap and they will tick over no problem and don't spit or bulk around the paddock.

All the race engines I've run (regardless of size) run similar fairly standard fourstroke settings for a MK2

2A1 needle
106 needle jet
3 to 4 slide. 3.5 being the most used. Slides are mostly Brass chrome plated (heavy) and use a stronger return spring.
20 to 25 pilot jet (generally weaker 20)
Needle clip either one up or even two up from centre (weaker)
35 choke jet (weaker)

250 using 32 or 34mm carbs main jet from 200 to 240. Have even used as small as 160 in a 32mm with one set up.
350's on 38mm carbs from 270 to 310 main jet
450's on a 38mm carb from 300 to 320 main jet

That's running on 50/50 avgas and super unleaded

Just a few thoughts:

Have you checked for air leaks on the manifold and head face?

What sort of piston to head clearance are you running, wondering whether you are getting unburnt fuel left around the edge....unlikely but just looking for differences to maybe what I've run.

That silencer probably needs some sort of alternative experiment, anything straight through short term should eliminate that from the search.

I've run 450 race bikes from 10:1 to 12.5:1 and never experienced any of these issues, so I'm not thinking it is the CR that's you problem either. Generally speaking the 450 race bikes run like tractors and pull hard from as little as 3500rpm, much as a very strong road bike even on far "wilder" cams.

I'm still not convinced two carbs can both be faulty, unless you are just unlucky! The offer to loan you my 34mm Mk2 from my 350 still stands as I know that works fine, cold starting, idling and running. Certainly never gets blown off! The engines in bits for some eventual improvements so it's not going anywhere soon.

Regarding your previous comments re "Dyno testing" I have never thought it in any way different to normal use. If it goes Bang on the Dyno, then it would anyway, certainly as far as a race bike is concerned it would. At the end of the day you should only be simulating what you do in reality and with the sort of testing you would do on a road bike the same would pretty much follow. I've been using Dynos for 30 years now and so far so good.....

Regarding MK2's and racing over the last 30 years in the UK, I would say that on Ducati singles they have been more popular than anything else and I've never in all those years seen a MK2 blown off... fall off due to not being tight.... yes

Good luck,

Best Wishes Nigel