450 desmo carb

[themoudie]

Thank you all for your comments.

George NI

Aye, both images were taken at the same time, but from differing angles to show the variation in colour across the insulator.

Pulls like a train with the greater valve overlap of the Piper 173A cam when compared with the 'standard' 450 MKIII cam that is quite soft.

With a gas flowed head, an Amal MKII Concentric 2932, with a short bellmouth and the gutted Goldie exhaust, the breathing is very different from what I was used to with a gas flowed head, the old 'standard' cam, MKI Concentric 932 and gutted Goldie. I started far too weak in all the settings and compounded my fo paux by not getting the ignition timing set correctly! Hence, I was fighting 5 variables, rather than just the 4 carburettor settings. However, with the timing set up at 0.5°BTDC, rather than the incorrect 3°ATDC that I initially had set, set up has become more manageable.

So, the carburettor settings that are giving that plug chop colour in my post of 15/12/2020 1:32pm, using a NGK B7HS plug with 0.032" (0.8mm) gap are as follows:-

Amal MKII Concentric 2932, with standard short bellmouth (the gussets on the 450 frame do not allow fitting of the trumpet, without their removal). Fitted to the cylinder head using an alloy stub extension bolted too the head and then a 38mm length of 38mm ID reinforced marine fuel hose, with two broad stainless steel jubilee clips. The attached image shows what happens to Amal's standard carb mount rubber when there is a 'backfire' through the carb, it ruptures and at £15+ plus P&P a time, it's no cheap! This was the second to fail. The half metre of marine fuel hose with steel wire and double fabric reinforcing appears more able to withstand the occasional explosion! And at just over £20 appears more reliable.

WARNING SINCE PUBLISHING THIS POST I HAVE YET AGAIN CHANGED JET SIZES AND THE NEEDLE CLIP POSITION!

Throttle slide: 3.5
Main Jet: 280
Pilot Jet: 35 fitted in the float bowl, not the carburettor body. Even though the downdraft angle is 20°, exceeding the 15° max used by Amal!
Needle Jet: 107
Needle: 2A1 on the second notch down from the top of the needle
Air screw: One and half turns from seated and then a 'touch' more out.
Idle screw: Two full turns from lifting the slide off the venuri seat and then a 'touch' more in.
Cold start jet: 40

Hope this info helps others, with later 450 MKIII's.

Good health, Bill

[Duccout]

Very good Bill! Where did you get the mounting stub, is it home made?

Cheers,

Colin

[themoudie]

Aye Colin,

The mounting stub was purchased as one of a pair, for mounting my old MKI Concentric 932 onto the 450 MKIII, from John Wittmann in the late '70's. John is still about, but retired and no longer dealing with bits for Dukes, so far as I am aware.

Maybe if you give Laurie Richardson an email: laurie37@ntlworld.com he could turn you up a couple, as I see that Nigel doesn't have them in his stock. If Laurie requires a pattern I am happy to supply him a pattern, if you will let me know.

For goodness sake do not buy the cast stubs from Wassell they are very nasty cheap wonky Chinese blobs of metal that appear to have been sandcast in something from Outer Mongolia. I was offered one when trying to sort out the MKII Concentric!

You could also try using the Amal 32mm flange adaptor: Amal_MKII_32mm_flange_adaptor Whether this will withstand a healthy backfire, I am not sure?

The marine fuel hose I use was obtained from Hoseworld and here is a link to their very useful site: Hoseworld_38mm_Marine_fuel_hose I would also recommend using the wide type stainless steel jubilee clips rather than the narrow ones as the grip a larger surface area and are less likely to cut into the rubber.

I hope this helps. Good health, Bill

[blethermaskite]

Bill, delighted to here you have now got the "beastie" sorted out, as I always say "if it was made by a man it can be fixed by a man" thank you for your carb setting info, I will file that away......you just never know when that might be needed, pity it the cold and rainy season for us up in the north, sorta spoils the chance to get out and play on our toys.
Best regards,
George

[Duccout]

Hi Bill,

Thanks for the info!

Regards,

Colin

The weather is awful down in Essex too! I have two newly rebuilt engines to road test and every day I look at the roads and wonder if I can get out, but they are covered in muck with rivers of mud running across them, and it is just not worth it. Maybe tomorrow......

[George]

"The weather is awful down in Essex too! I have two newly rebuilt engines to road test and every day I look at the roads and wonder if I can get out, but they are covered in muck with rivers of mud running across them, and it is just not worth it. Maybe tomorrow."

Same here Colin nine months restoration and a ride of one mile up and down my road. Still don't know if it is all OK or not.
Not taking my shiny new toy out in this weather.

[Duccout]

No George, I can't see things improving anytime soon, even when we get a fine, windy morning like today it is not enough to dry the roads. I suppose the next stage will be frost, snow and layers of salt. Oh well, it is almost Christmas, look on the bright side!


Colin

[themoudie]

Aye George NI, Colin and George,

After going out for the umpteenth time this month trying to get the plug chops in, my plug chop runs are a 20 mile there and back canter between lay-byes to get the engine up to temperature, yesterday's sun and 10°C temperature was balmy. The only problem is the slick of soil, diesel, and tyre rubber that is in the middle of our main road lanes at present after all the rain, luckily the volume of rain has washed what little salt/brine solution that has been spread this year, away. The back roads, C and U classification are increasingly potholed, flooded and covered in mud/leaf mould splattered by passing tractors taking to the verges.

However, after coming up with the reasonable plug colour, I went for a cross country canter of 50 miles and came home looking like Vic Eastwood after a round of the BBC Grandstand scrambles coverage, with a big grin on my face!

With 30 minutes of light left (15:15pm), I got the hose out and gave the beastie a hose down, wash down with hot water and shampoo, followed by another rinse and then a dry-off with the leaf blower. OK, it wasn't the most fastidious wash down and my bikes have never been concours queens, but it is now clean and tidy.

If you get the chance, I would always say "Get out for your canter, the throttle works both ways and there is plenty of engine braking from a Ducati single". If you want to keep the spares suppliers in business, you have to wear them out!

Good health, Bill

[LaceyDucati]

Hi Bill
My thought is the mixture is to rich and maybe you have more than one issue here. What Piston have you, have you raised the CR? My thoughts are as the bike is non standard the plug is too hot and that maybe clouding things. Other thoughts are: the plug gap is larger than I always run (25 thou) and the manifold is straight, where as I always run a cranked out manifold, how close is the bellmouth to the gusset?

Just one other comment from what you said, I've never found the long trumpet on the mk2 worked for me, the standard bellmouth that you have always was the best for me.

Maybe call again to see if I shed any light on the backfiring, now I've seen a little more.

Regards Nigel

[themoudie]

Aye Nigel,

Thank you for taking the time to contemplate this saga. Much of it no doubt of my own making!

The piston that I fitted during the rebuild is a Borgo 86.8mm flat topped piston with small valve cut-outs Part No:615-47-700 I specifically didn't want to raise compression at the time of the start of the rebuild and a subsequent injury to my right leg has made me more grateful that I didn't buy a piston with a raised crown and increase the compression ratio.

If you think that the NGK B7HS is too hot then I can easily revert to a NGK B6HS or it's Iridium electrode equivalent and see to what colour the electrode porcelain changes to. My reason for using the larger plug gap is because I am using the Boyer Induction Ignition module, so that the points are just a switch and carry no current. I will try reducing the gap to the tried and tested 25 thou. as well.

I never used a cranked inlet manifold with my old MKI 932 set up, since I installed it in the late 1970's. I must admit that I was using smaller jets in all settings with that set up, with a standard 450MKIII cam. The only cranked inlet stubs that I have are some 2nd hand, but sound, Norton Commando 32mm inlet stubs for a MKI Concentric, or else the Chinese abomination offered by Wassells! The former are too long and result in the carb leaning backwards at ~10 ° from the inlet port; whilst the Wassells casting is just too rough to use, but the carb has a more upright stance and hence the level float bowl, which is the preferred position. As an aside, you should also note that I am using a white float and not the 'Stay-Up' float, it appears to be less affected by vibration and more bouyant, in spite of Burlen's reports.

The bellmouth and carburettor cap are both close to the gusseting and I have removed some of the top tube gussetting to accommodate the top cap and enable the carb to be near vertical when viewed from the rear.

Having viewed your wee black Nigel Palmer 350cc many times, I see that you used the short bellmouth with an enlarged diameter flange, but what appears to be a straight inlet stub and extension. Hence my thought that a straight inlet should work with a MKII as it had with my MKI and on countless other Ducati single engines.

I agree richness appears to have been part of the cause of the backfiring, However, when starting from cold the bike will run on the 40 jet in the cold start. But even when warm, a single kick with the cold start 'ON' is often required to prime the cylinder before the engine will start, just kicking the engine over produces a kickback as though there is a weak mixture and a refusal to catch and run.

There is part of me wondering if the 3.5 slide has too much of a cutaway, but the engine picks up and pulls well without spitting back with it in. I can try the 3.0 slide again, but do not have a 4.0 slide that is used in the Velocette MSS set up.

I also have a Gunson see-through plug that might help with tuning the pilot jet and air screw, but any further up the rev range involving the slide, needle and main jet, I am not sure.

Thank you and good health, Bill