450 desmo carb

[Duccout]

Hi Nigel,

Your mention of breaking through the top of the port jogged my memory. Years ago, myself and two mates totally exhausted ourselves trying to bump-start Roger Moss's 750 Desmo at Cadwell; it fired up no problem, but wouldn't run, and after about an hour of this we gave up. When I saw Roger Moss a few weeks later he told me that he found that he had broken through the port and hadn't noticed, but the tiny hole was enough to stop the engine running. Worth investigating Bill.

Colin

[themoudie]

UPDATE 19/03/2021 18:40

This afternoon's run without any "silencer" did not change the situation and I have uploaded a short video to YouTube for clarity.

Link: 1974_450_MKIII_Overun_vid

After this run, I took an image of the No. 3.5 slide used. You can see the build up of soft carbon on the slide inlet face.

Next plan was to fit the SuperTrapp with 8 diffuser plates fitted (maximum "back pressure"), to see if this would stop the 'running on'. However, after blowing the carb off the inlet stub twice, without even firing, I have cried "ENOUGH!". There are more lumps out of the frame, the carb bellmouth and fuel spilt over the floor on both occasions, this time the pot belly stove wasn't lit and all the workshop doors were open, so less of a panic.

The carcase is getting moved to the "naughty corner", where it can be disemboweled, without obstructing something that runs and I will see what else I can find. So far ~400 miles and 18 months of "fettling"! I don't think that there is anything wrong with any single component, but the combination just doesn't work, due to conflicting physics, in this particular instance.

Good health, Bill

[graeme]

Hello Bill,

I do admire your persistence.
Sorry to hear of your continued issues. You will get on top of it.

One thing I thought of last night, to rule out at least.
Is it possible for you to fit a degree wheel and strobe the timing ?
Just to rule out a wandering spark ?

Graeme

[themoudie]

Aye Graeme,

Thank you for the suggestion to strobe the timing. Unfortunately, I don't have a OME or pattern pointer to enable me to carry out a running timing. I have a steel disc that has a locating tab on a shaft and a 'V' notch on it's periphery that I could use and an Accuspark SP8000 timing strobe that I have used on a Morini V twin, a 125H single and Japanese bikes, but never on the Ducati. I think it requires two people to do the adjustment properly on the Ducati as the timing marks and adjustment are on opposite sides, until confident, when I would tackle it on my own. My current inability to control the engine revs, or keep the carb on the cylinder head makes me wary of trying this procedure. With the current Covid lockdown here, I am unable to ask a friend, who is a professional vehicle electrician to give me a hand. If I had a "wandering spark" I don't believe that I could have ridden the bike or started it for the past 18 months, but would still give it a looksee.

George's (blethermaskite) experience (Wed Mar 17, 2021 9:59 pm page15 of this thread) rings very true. Whilst 32mm ID inlet bores are more prevelant on modified 250 and 350 heads, the 450 heads are usually at least 34mm ID plus and racing 350's start at 34mm ID and go on up. My having the inlet bore relatively narrow to other gas flowed heads and with a polished finish would facilitate, rather than exacerbate an elevated venturi affect. Apparently, the same affect achieved by George, with his small diameter exhaust header.

So, a cam swap, as a last resort looks the likely outcome, as it is the only individual component that has yet to be swapped from my original rebuild. I will see what happens? Shame, as the motor appears mechanically smooth. I am also not so sure about Colin's comment "....., it wouldn't take long without removing the head .... ". Knowing my luck!

Good health, Bill

[Bob_Matthews]

Hi Bill

Was interesting to see the video you posted up.
The engine sounds to be running very nicely so I don't believe it's a cam or ignition timing problem and as you say it performs well on the road - apart from the rising revs.

To me its smacks of a carb related issue (sorry), I know you have tried various carbs and the mk 2 amal is new. From my experience (with monoblocs) and seeing your video it seems to me to be a loose slide or oversized/warped body that is causing the problem. Is it possible to borrow a carb from someone that is a known good runner?

Hope you get it fixed soon although I do enjoy all the updates on this thread

[Duccout]

Bill,

Playing the video and listening to the exhaust, the engine sounds very flat and unresponsive, and after you put your finger against the slide - very rich and lumpy; could the slide have too small a cutaway?

Colin

[themoudie]

Good morning Bob and Colin,

Thank you both for your further thoughts and suggestions.

Bob, I have tried the alternative carb test (RE: Wed Mar 17, 2021 10:27 pm, page 15 of this thread), with two other carb bodies, so I think that I have put that to bed.

Colin, my camera is being held in against my chest and the open exhaust pipe is hidden below the side of the motor, so the sound is not directed towards the camera microphone. Hence the slightly muffled bellow, I can assure you it was LOUD!

I initially put horizontal pressure on the side of the slide each time, but as this does not reduce the revs, I then slide the end of my finger over the gap at the bottom of the slide, sufficient to 'choke' the engine back to a tickover, rather than gagg it dead.

I agree the engine does sound "rich and lumpy" and it is, with the 35 pilot jet, the air screw at 11/4 out from seated and a No. 3.5 slide in place. when the throttle was either slowly opened or blipped to raise the revs, I thought that I had caught the black soot exiting the pipe, but hadn't due to supporting the camera on my chest. Needless to say the reek was noticeable. But, when 25 and 20 pilot jets are in place the engine was very reluctant to start, blew the carb off the spigot and even with the air screw in at 1/2 a turn off seating still no sign of yellow in the flame when observed using the Gunson plug. All indicating a larger pilot jet is required.

All the above has been tried with a No. 3.0 slide in place, with similar results and more reek when using the 35 pilot jet. I have tried 20, 25, 27.5, 30, 35 and 40 pilot jets in combination with both the No. 3.0 and 3.5 slides.

I haven't changed the air jet on the face of the bellmouth and have the 'standard' 3.5mm jet in place. Only smaller jets are available (2.0mm and 1.5mm ID) and fitting these would INCREASE the depression across the jets through the venturi and I already have TOO MUCH of that depression in pressure.

Maybe a No.4 slide is required as per Velocette Venom Clubman? But that wouldn't alter the gas flow properties downstraem of the carburettor.

I am off to pull it apart before the rugby this afternoon and I will certainly be examining everything as I open it up.

I still believe that there is nothing "wrong" with any of my component parts. It is my combination of these individual parts and the laws of physics that are causing the problem. The porting job works well with the OME cam, but appears to be add odds with the increased duration and lift offered by the 173A cam. There is nothing inherently "wrong" with the 173A cam.

Let's see how I get on!

Good health, Bill

[Duccout]

Good luck! We are all being entertained.


Colin

[LaceyDucati]

Bill

Thanks for your reply to my questions, just checking the obvious and seemingly no issues there. Contra to my not posting.......

In your last post you stated "with a polished finish", this set off alarm bells, do you actually mean polished? A polished port may well explain spitting fuel globs and potentially your issue. I wouldn't recommend a surface finish better than that achieved with 120 grit emery, probably courser, I use 60 or 80 grit in the exhaust port.


Regards Nigel

[blethermaskite]

Bill, I know from the mention of it in previous posts that you are not keen to try out your original cam, but it seems to me you have exhausted (sorry for the pun ) many of the possible causes of your problem, regardless of minor setting issues with ignition or fuel delivery (all of which are ultimately important) the one thing you have not altered is your cam, for the sake of a couple of hours work and a few (for the moment) home made gaskets if it were me I would do the swap. I'm sure your sporty cam is ultimately fine if its what it is supposed to be, but it would not be the first time that I have encountered a rogue cam grind that is not doing what it said on the box have you put a degree wheel on it and checked what the figures are? .........just a thought.
Cheers,
George