450 desmo carb

[blethermaskite]

Bill, I too have been running Ducati singles with Mk1 concentrics for years at the standard downdraft angle with absolutely no problems with richness right through the range, I don't like rubber mountings on concentrics and instead use various lengths of tufnol spacers between carb and head from 20mm up to 60mm in length, on the 350 mk3 I am still using the original hard white plastic float needle and white float, on the 250 desmo I am using a viton tip float needle and a stay up float. Sounds like something else is wrong??......I.m sure you have checked this but that is the behavior of a duff condenser(that's if you are on points)
Cheers,
George

[themoudie]

Aye George, thank you for your suggestion.

I have put a digital meter on the 20M Ohm setting across the condenser - to the wire and + to the body casing. The resistance rises and then stops, with no fluctuations or drops in resistance. So, I thought that it is OK? However, even Halfords these days no longer stock condensers and in Perth, Scotland, there is now only one auto electrical garage where I might obtain another. I haven't yet been able to visit to see if I could get a couple in stock, will try later today.

Good to see you at Stafford last year and pity that the Bob Mac wasn't on to get the craic, see what 2021 brings.

Good health and my regards, Bill

[Duccout]

The official site for Mikuni states that the max angle is 20%, so subtract the 10% that a Ducati's cylinder sits at, and that leaves 10% downdraught for the port. Has anyone measured it? It looks about 10%......

[themoudie]

Aye Duccout,

I have used a plumb bob line and a timing disc aligned through the central line of the inlet tract to obtain an approximate angle of 20 degrees. This is different from percent (%) of slope which converts to a 36.4% slope! Just in case anybody interprets the "%" incorrectly.

Update! I have for the second time been back into the engine and checked the the cam and ignition timing, the valve gaps and all appears to be in order.

However, when re-assembling the points and back plate into the timing cover, the angle of the points and the position of the moving point against the base plate point was misaligned, so that only half the points overlapped and they faces weren't flush! I have tonight modified the points so that I have a better fit, overlap and nearly flush faces. The latter is as good as it is going to get. I then retimed the ignition and the base plate is now approximately central within its range of adjustment, whereas before it was to the left end of the adjusting slots or ADVANCED further than it appears that it should have been.

I shall see come tomorrow, as it is a bit late to fire up, even though I have a tolerant neighbour and live in the sticks!

A set of known quality points from Nigel and the blister pack of 5 'Ducati' Italian points purchased from Comerfords in the early 2000's will be consigned to the bin. The carbs may work away fine on the straight stubs after all! I will also see if the curved stub arrives and likely give it a trial as well. It may make location of the carbs less cramped, even though some cylinder filling efficiency could be lost and this is not a race bike, but one to be ridden and enjoyed.

Regards, BillR

[themoudie]

UPDATE 17/09/2020

Well! Nigel's points arrived and, yes, plastic blister packed the same part number and manufacturer as my 5 sets purchased from Comerfords! So, I didn't bother changing the modified set for the new set. Checked the timing through for the umpteenth time and persuaded myself that I had this part of the spell correct.

When asked for a points ignition condenser/capacitor the auto electrician looked at me blankly and suggested that I should try a motor factors, as all their stock of points ignition related items were thrown away at least 5 yeras ago when they moved. Not surprised, but! The factor came up with the item, but as it tested the same as the one fitted, they weren't swapped.

Rebuilt the Amal MKI concentric 932, with a white plastic float (1mm below top of bowl casting when sealing), viton tipped aluminium float needle, 3.5 anodised slide, 2 banded 4 stroke needle, clip in the middle groove of three, 106 needle jet, 220 main jet, the pilot jet is the drilling in the carburettor body, not a replaceable brass jet (all used before the rebuild ,with no problems and in good condition) and the fibre washers replaced with homemade viton 2mm depth washers (no more weeps and seeps). Air screw at 1.5 turns out from seated, slide screw resting on the bottom of the slide, as it rests on the bottom of the carburettor body venturi. The carb is mounted on two alloy stubs, with 38mm of marine reinforced fuel hose between them and secured with 'Jubilee' clips.

After a lot of kicking managed to get the engine to run and achieve a high tickover ~1,500rpm, with the air needle at 1 turn out from seated and the slide screw 3/4 turned in from rest. A slower tickover can be achieved but this is not lasting and the engine stalls and 'spits back' through the carb (sign of weakness) From cold, the engine requires a 'good tickle' and will then fire up and run, but any whiff of throttle results in a spit back through the carb and stall. After 3 or 4 of these short bursts the engine can be held on the throttle at ~2,000rpm and warmed through ( less than 100 miles on rebuilt motor using Morris SAE40 oil) factory exhaust pipe, combined with a gutted 11/2" (38mm) inlet I.D. Gold Star pattern exhaust, with a total length of 271/2" (700mm) and outlet I.D. 17/8" (48mm). There is nothing inside the shell (blown away by the engine, many years ago!) and used in this condition before the rebuild.

Went for a good cross country run today and after ~10 miles stopped to adjust the carburettor. The engine refusing to settle to an idle or run below ~2,000 - 2,500rpm even when running in the bottom 3 gears. Reduce slide screw lift, adjust air screw to stop spit back through carb and then readjust slide to achieve a steady tickover ~1,300 - 1,500 rpm. However, once underway again the revs rise and the whole scenario is repeated. Rode the last 30 miles home, through town and country with the engine revving at ~2,000rpm. That is now 200 miles run with the condition. An oil change after 50 miles, showed no slivers, flakes, or lumps of metal and nothing picked up on the magnet.

The engine picks up, once hot, without any spitting back through the carburettor and revs up to ~4,000, without any hesistancy, I suspect that I may need to increase the main jet size, but until I can get this idle sorted out there seems little point.

The plugs are either a NGK B7HS or a BR6HIX, I have also used a B6HS, the latter two heat ranges recommended by NGK and the first by Nigel. They appear to have no effect upon the reving and all have a light tan colour on the insulator.

QUESTIONS?

Is engine vibration causing the fuel to froth in the float bowl, due to a harmonic caused by it's remote mounting?

Should I use a carburettor with a replaceable pilot jet (25), rather than the drilled gallery (20?)?

Should I try the Amal MKII 2932 again, with the same settings as the MKI? I am not sure if a 31/2 slide in the MKI has the same effect in the MKII, they being very different instruments.

Should I substitute the Goldie for a short 'Supertrapp' meggaphone with removable disc diffusers to increase or decrease the exhaust pressure waves.

Should I substitute the Goldie for a gutted short reverse alloy cone megga.

My apologies if this bores the pants of anybody, but if it helps another at some later date, it will not have been in vane.

Good health, Bill

[Dan C]

Having been there it doesn't bore me Bill. I share your pain.... Its much better to be listing all the variables rather than leave something out.

All I can say is that I had similar issues, firstly with the original Dellorto VHB 29 Carb and then with a Dellorto PHF 30 (which was better but I still couldn't get it right at idle and just above). With both those carbs you'd think you had it idling nicely at, say, 1,000rpm , then the next day it wouldn't. At every intersection you'd be listening and blipping to makes sure it didn't stall and you didn't want to park where anyone would be watching you kick it for 5 minutes to start it, not cool at all.......

I tried all the usual fixes, changing pilot jet sizes, changing needles, needle jet and needle position etc. Also fitted new gasket set. Always changed one variable then tested, then moved on to the next one......but could never get it right.

Graeme Moloney in Tassie suggested I try new Mikunis. I resisted a bit because in my head a carb is a fairly basic bit of kit and provided you have the jetting right it should have a bit of tolerance and work reasonably well. But eventually he persuaded me and since switching to new carbs the problems have gone. In my case they were Mikunis (much cheaper) but I suspect new Amal or new Dellorto would have been just as good.

I have no experience of Amals. But a mate of mate rebuilds pommy bikes for a living and apparently he wont undertake to rebuild any pommy engine unless the owner is prepared to fit brand new carbs. But if you are going to persevere then I think having a variable pilot jet would help with the low rev issues.

I can't comment on exhaust variables, I never tried to change anything. The 450 has some sort of stainless steel replica of the original but i dont know what the internals are. The 350 has a Conti probably from a 750 twin. It sounds superb!


Cheers
Dan

[Bevel bob]

A low fuel level will make starting very hard and require an artificially high slide just to keep running ,the motor will stop all the time. You could also try backing off the ignition timing a bit. I too have several sets of new points that don't work at all Gone back to old worn set with Pazon /Boyer EI booster which works well and unloads points.

[Duccout]

Bill, it won't be vibration causing the problem at low revs. Is the carb old? Amals are notorious for distorting the mounting flange when tightened up, allowing air in through the gap. My guess is that is what is happening, so you may be better going back to the Mk 2, although I'm leaning towards the Mikuni solution!

[themoudie]

Good morning Dan, Bob and Duccout,

Bright eyed and bushy tailed, my thanks for all your positive suggestions.

I have checked the mounting flange against the machined alloy stub and both surfaces are fine when checked against the edge of a steel rule and I am using a new branded 'O' ring on the body of the carb. I have sprayed the whole of the inlet tract with WD40 when the engine is running and this doesn't produce any change in the running, as I would expect if there was an air leak.

Low fuel level in the float bowl could be a possibility as the float sits 1mm below the top edge of the bowl face I shall drop the float bowl and replace the white float with the adjustable 'StayUp' float that is new, rather than tapping at brass inserts after heating the bowl in hot water! I'll have the float level with the bowl face and sealing the float jet at that point.

The points required the inner piece of the nylon bearing to be reduced in length and the inner side of the arm to be filled away so that clearance from the points adjusting screw could be achieved. I too have fitted a Boyer Bransden Inductive discharge box that appears to be working and the red LED timing light aids the timing adjustment, as it illuminates when the circuit is broken at the points (the reverse of the festoon bulb method).

A glorious Scottish autumnal morning with clear skyes, dense mist clearing and a heavy dew on everything, with the sun starting to warm things up to a comfortable 18 - 20 degrees C. Off to the workshop and then to put some miles under the wheels, even if it still isn't "right" at the bottom end. I'm not worried about the 'cool', life is too short and I don't have that sort of mobile phone! Mines a clamshell with big buttons.

I'll keep posting results, Bill

[Duccout]

Have a good ride Bill. Another thought: have you considered fitting the Electrex World ignition kit? It would at least eliminate ignition problems from the equation.