450 desmo carb

[LaceyDucati]

Bill

The inlet stubs on my 450 based race engines are 75mm long, but they need to kick out to clear the frame. The reason for this was a tuned length that I required. Not convinced that will help, but anything is worth a random try.

IMG_5713 (1000x750).jpg

The main reason for the heavy slides and springs in the Mk2's for racing is to prevent scary throttle sticking especially in the wet. When the slide sticks open at the end of the straight on shutting the throttle you learn not to use those light slides and springs.

Maybe It's time to swap the cam just to rule that one out as well.

Regards Nigel

[themoudie]

Aye Nigel,

Thank you for the image and comment.

I was thinking about returning to the AMAL MKI 932 bolted directly to the head, with a 1mm phenolic insulator between them.

Alternatively, making a longer connector between the two alloy stubs and the MKI 932 from the marine fuel hose and trying the same hose with the AMAL MKII 2932 is another option. Your 75mm inlet stubb is going to place the centre of the needle jet of a AMAL 2900 series carb about 127mm - 130mm from the face of the head inlet. This is much longer than the 85mm that I have at present that is shorter than the length that Andy at SCR runs on his 450. My overall inlet tract length from the face of the cylinder head, through the needle centre to the outer edge of the bellmouth is 143mm and this is shorter than Andy's overall length by ~10mm. There is room to fit something like this in the space. Rather than relying on the angle of repose of the hose, I could use the two tapped holes in the upper rear of the 2932 body to hold a bracket attached to the frame to reduce the downdraft angle and reduce the angle of the float bowl to below 15° and keep the pilot jet in the float bowl position.

Finally, I too had thought of changing the cam back for the original soft 450 cam, with less valve overlap. That galls me, but if the head work is preventing the 173A from working properly, then short of re-working the head, it appears to be my only other option to see if that will settle the engine down and make the machine rideable. Ignoring the throttle and controlling your speed using the gear box and brakes in and around urban areas and along nadgery back roads, keeps me on my toes and I enjoy the challenge. But, sitting at traffic lights whilst the engine sits at 3,000 -4,000 rpm and barks through a minimaly silenced exhaust, just draws unwelcome attention. I prefer to have a cordial relationship with 'Traffic'!

Time for some contemplation and a dram.

Good health, Bill

[Harvey]

What if the mixture is so lean and slow-burning, it is still flaming when the inlet valve opens?

Jordon is right. the mixture is so lean it is still burning when the inlet valve opens, to ignite the inlet charge.
She is too lean.

[themoudie]

Aye Harvey,

Thank you for your suggestion as to what is the cause of my ills.

At present I have a 35 pilot jet fitted and a No. 3.5 slide, in a AMAL MKII 2932. I have had the pilot jet up to a 40 that has raised eyebrows all round and still the carb gets blown off and the carbon left on the carbon internals is crazy! See the attached images.

What I haven't tried is a No. 3 slide and the 40 pilot jet.

However, I am increasingly thinking that head porting work done back in 1977, in combination with a cam that has greater valve overlap than the standard 450 cam that I used to run and the exhaust sytem that I used to run are conflicting rather than co-operating when it comes to cylinder filling. Just chucking more fuel down the inlet is not going to sort this self-induced miasma of gas dynamics.

As I said to Nigel Lacey in my post of Thu Mar 18, 2021 10:29 pm, I am going to have to disrupt the current flow patterns and that might involve changing camshafts, so that I can see what is causing the problem, rather than just guessing at it. That's how I ended up with the problem I have at present.

Good health, Bill

[graeme]

Your slide pictures look like fuel and very little air. Black soot.

Bill, when does it back fire?
Throttle on or off ?
After a quick rev?
Revs rising or falling?
Or when ever it feels like pissing you off ?

[Harvey]

Giday Bill, my 456cc 450 was on 11:1 and a V2 cam running a 40 mm Malossi carb, no accelerate pump, always ran clean, would blow the carb off sometimes when kick starting due to the aluminum flywheel instead of the bronze one.

All the soot on the slide shows that the rich fuel is burning in the inlet tract, but the fuel in the engine is lean.
Reading back on your posts, I think the clue is in fitting the restrictive Super Trap.

This has reduced the amount of air that the long overlap cam, is pulling up through the exhaust pipe to fill the cylinder with a very lean mixture, that is still burning when the inlet valve opens to ignite, producing all that soot in the carb.
That cam must have a very long duration, or is not timed right.

[graeme]

As Harvey suggests.

Also, reading back through your posts you said you were using 50:1 two stroke mix. Are you still running that in your tank?
Suggest not using that.

[Duccout]

Bill, I remember back to my Velocette days, when the M17-8 cam would have a continuous spray of petrol coming out of the bellmouth, due to the overlap, and if the engine did not fire after a few kicks, was prone to giving off an explosion from the bellmouth when the collected fuel ignited; your problem must surely be cam related? Obviously, the first port of call is to refit the white cam and see how it goes, it wouldn't take long without removing the head and would eliminate the cam as a source of woes. We are all waiting for the next installment!

[LaceyDucati]

Bill

On looking back on my post last night I see brain fade led me to type 75mm whereas the correct figure I used for the manifold length was I think 90mm. Not that it really matters as I don't think this is relevant to your issue. I was only responding to your enquiry about manifold lengths used. The length is longer than most run, but is what I believed was the correct length from the valve seat to the end of the belmouth for making maximum power at 7750rpm for those race engines. Considering my customers successes of race wins, various championships, Lap records including the IOM for Ducati singles, dyno charts etc, i'm thinking I'm not too wrong.... but others may have other views.

I know we've discussed the cam several time, but for the benefit of those new to your woes, The 173 cam is not radical with long duration, it's one of Phil Joys profiles and is a "development of the Green and White. It's not to dissimilar to the V2 cam for duration but has less lift and overlap. It's been used in loads of road and race bikes.....without the issue being experienced. My suggestion of trying something else is purely to just discount it with a stab in the dark to stop those nagging doubts you are having and others are suggesting. Personally I don't think it's the cam unless it's not timed correctly...but that's been checked. That said I always have an open mind and I'm always open to being wrong! The carb/ mixture suggestion has been round and round and I'm of the opinion that lightening doesn't strike 3 times, unless you are very unlucky

The suggestion that the mixture is igniting in the port is just a statement of fact, as you know knowing theoretical the answer to a problem is of limited help and comfort, once you've exhausted the standard checks. I'm getting the answer 42 pop into my mind here. ( Some will understand what I am saying here!)

Unless there is an obvious answer/ fault then you may be correct that this is a bunch of unhappy parts not working in harmony. So then my usual solution is to start changing everything until it disappears then reintroduce the suspect parts until the issue returns. I've seen many a racer waste a whole weekend racing changing one thing at a time. It works to an extent, but whole ignition system/carbs is a quicker diagnoses with less frustration.

Some quick questions.

If this is the usual 70's head job, is the port roof breached where the rocker cover face meets the manifold flange. This is a common fault on many heads done in that period and a source for air leaks. I will add the breach is not always that obvious....unless it's the usual glob of araldite.

Is the head twin plugged?

Have you tried advancing the ignition to say 4 or 5 degrees and see what effect it has on this tickover situation? I ask this as the race bikes are ticking over nicely on full advance at around 28 - 30 degrees. I fully understand the issues that may arise from kick starting and the fact that it will be too far advanced to ride the bike, just curious to know the effect. I dont have personal experience of kick starting 450's but it seems to me they are a long way retarded and whether this is a necessity for starting or whether its a fudge because the 450 needs 28-30 degrees max advance and the use of the advance unit from the smaller bikes was a compromise. The ignition advance needed to start the bike may not be the ideal for a steady tickover. My thoughts on this is, if a race bike ticks over on full advance that is a significant difference between my race experience and your experience.

What sort of size port is it? bearing in mind my race engines will still tick over on the stand at just under 2000rpm with much larger ports and longer duration cams, 38mm Mark2 .....

I don't think I will add any more to this post, as without having the bike in front of me, I've probably exhausted all my useful input. Unfortunately, long posts tend to suffer from the Facebook issue of suffering with repetition of questions and answers. Of course if you want chat through any findings/bounce ideas off someone, feel free to call me.

Lastly I do have two Standard 450 spring valve heads in good shape if you eventually decide the issue is with the head in some way.

Regards Nigel

[themoudie]

My thanks to Graeme, Harvey, Colin and Nigel,

I have read through all of your suggestions and earlier today Friday 19/3/2021 I had a short blether with Andy SCR on the phone.

In answer to your questions Nigel:

The inlet port roof has NOT been breached/rewelded/araldited and is polished. Apparently this rules out it being a MEZ porting job, but possibly an Ainslie. BIG CAVEATS? The porting was done in 1977.

The head is NOT twin-plugged.

Ignition advance can be a VERY SORE problem as I know of people who have had their ankles/soft tissue broken and kickstart levers come apart and cause serious injury. The memories of the 1976 Escort points set-up are also still very clear, so I know that at a fixed 28°BTDC the engine will run and be rideable, just the riding in traffic can be interesting! I am not saying I am feart, but my tib and fib already have a titanium pin in and I don't wish to bend it! I have tried 1° BTDC and the kickback can vicious, but not constant. I have been adviced to time the spark with the AAU jammed at full advance (28°) and then let the initial spark be wherever the AAU's repose is, in my case 3°ATDC. I had thought this too much and consequently used Tassie Graemes 0.5°ATDC as a compromise.

Inlet port size is 32mm, I purposely kept it smallish as I was looking for mid range rather than top end power. I suspect that this relatively small port diameter is contributing to the elevated veturi affect that appears to be occuring.

So, I am going to waken the dead!

This afternoon the new AMAL 2932, with No. 3.5 slide + 30grammes of lead + AMAL strong spring, 35 pilot jet, 2A1 needle, with clip on 3rd groove from the top, 106 needle jet, 240 main jet, 'StayUp' float and a fuel level 2mm below the machined surface of the float is going on the head with the TSS angled inlet rubber. Ignition remains at 0.5°ATDC and the Boyer IDU remains. The exhaust will be just the standard factory exhaust pipe, with no exhaust on the end and see what happens.

Thank you for your time and expenditure of grey matter energy. Mine is becoming slushy!

Good health, Bill