450 desmo carb

[Beach Life]

by the way, I dont recall dad ever having any trouble with the 29mm dellorto, it always seemed to start fairly easily and idle, I know when I rode it (back in the '80s dark ages) it never gave any trouble. Nor from the intake tube from airfilter compressing as some have mentioned, perhaps it was a rock hard type, it certainly was when I removed it after sitting for 30+ years! Haha.
I have also read some discussion on 750gt's with carb problems from phf's, I have one yet to be restored that Iv had since '82 with 32phf's. It also d 1st or 2nd kick, idled from cold with a bit of choke and with biv pistons in it ran really well, I think top speed on it was around 130mph on a good run.

[Beach Life]

also started 1st.....

[themoudie]

Aye Bernard and Dan C,

Ran my '74 450MKIII, with a Amal 932 32mm MKI Concentric for many years attached to a pair of milled stubs ~30mm long each and 38mm length of reinforced marine fuel pipe to keep them apart. As mentioned the tank needs to be lifted at the rear by ~60mm and the frame webs needed relieving. However, the MKI slide wears pretty fast due to the induction pulses and the time has come to replace the MKI ( worn carb body and slide (heavy brass type)).

I thought about a Mikuni VM32, but was informed by the proprietor of 'Motocarb' http://www.motocarb.com/ that the Mikuni could only be used up to a maximum downdraft angle of 15° and because the Ducati's was greater than this it would require a manifold to accommodate this. This maybe why Dan C is having rich running problems? My 450 MKIII head has a downdraft angle of 20°.

I ran my MKI 932, with the float level 2mm below the upper surface of the float bowl, NOT THE FLOAT BOWL GASKET, and was happy with the slow running, pickup and mid range through to the top end. No air filter, short carb trumpet, lengthened inlet tract, standard valves, cleaned up head, standard exhaust pipe and a gutted Gold Star pattern silencer body.

I have been persuaded by price, £176 + cost of inlet stub for the Mikuni VM32 and advice from several friends to use a Amal 2932 32mm MKII Concentric that I have 'in stock', as the cold start is better, the carburetor performs better than the 932 MKI and they can be used up to a downdraft angle of 35°(The latter info from Burlen (Amal) directly) [ADDED_07/09/2020 DO NOT VIEW THIS BURLEN ADVICE AS DEFINITIVE.] As the 2932 has a stub fitting, the inlet tract will be shortened by ~30mm (single stub on the head instead of two) and rather than setting the float level, as recommended by Burslen, at "level with the upper surface of the float bowl", I am setting it at 2mm below the float bowl upper surface and will see if I get flooding problems and amend if necessary. I am using a black 'Stay Up' float with stainless tangs, not the white float that requires the valve seat to be adjusted to achieve the correct fuel level. As the valve seat in the 2932 MKII is offset this can lead to all sorts of machinations!

Another advisory about the Amal MKII is that the choke plunger end seal must be seating, or else the mixture will be enriched throughout its settings. Worth checking, all for the want of a wee disc of viton.

I'll be interested to read how the Mikuni VM32(?) kit fits and performs and will post how the Amal 2932 32mm MKII performs for me and it's settings.

Good health, BillR

[Dan C]

Just an update on Mikuni VM 30 jetting.

On 1974 450 Desmo.
Pilot 35, Needle 6F5 on bottom notch, Needle Jet P5, Main 180, Air jet 2.0, Slide 2.5
These settings on this bike are giving nice tan coloured plug readings throughout the rev range. Bike starts easily, idles and revs cleanly at all throttle settings. Maybe not the carb size for maximising power but I'm very happy with everyday ride-ability. At my age power isnt important....!

On 1969 350 Desmo.
Pilot 30, Needle 6F5 on middle notch, Needle Jet P5, Main 160, Air jet 2.0, Slide 2.5
Similarly these settings on this bike are giving nice tan coloured plug readings throughout the rev range. Bike starts easily, idles and revs cleanly at all throttle settings.

I'm very happy with the Mikunis.

Cheers
Dan

[themoudie]

Aye Dan and Bernard

Sounds very good Dan. Do you have your carbs on both the the 350 and 450 mounted onto a straight or curved manifold? The latter to retain the float bowl in a horizontal plane. Or are they working well on a straight manifold, so that the venturi and float bowl of the carb are not horizontal?

Did 'Road&Race' supply the carb, manifold and an air filter to suit as a package or as individual items? What was the cost, please?

Bernard "Beach Life" on Sat, Aug 08, 2020 wrote:

Update: thanks for all your advice, so in the end I'm getting a 34mm Makuni from Road and Race, comes with manifold and airfilter and fits better without having to raise the tank to accommodate the bigger Dellorto. (And cheaper!)

Have you managed to fit this kit now Bernard? If so, how does it perform?

Thank you both for your time.

Good health, BillR

[Dan C]

Bill,
I purchased both Mikunis from MikuniOz. Cost of the carbs were AUD $154 each.

I also purchased a Flange and Boot set 32mm ID, 40mm Spigot 51-58mm stud centres from Mikunioz. The set comprises 2 flanges (manifolds), 2 boots, 2 gaskets and hose clamps. Cost is AUD $90 https://www.mikunioz.com/product-catego ... c98ba2045f

For the 350 I used the Mikuni manifold which is straight and this means the carb is not horizontal. I had to raise the rear of the tank about 10mm to get aircleaner clearance. I can't remember whether the angle Mikuni say not to exceed is 15 or 20 degrees. Whichever it is, this is very close to the maximum. As best as I can estimate anyway.....!

For the 450 I used a curved manifold I had in a box of bits. Internal ID is 29mm which is a bit smaller than I'd like but it sits the carb perfectly horizontal and also allows the aircleaner to tuck behind the frame gusset (which can be problematic on the 450's). Don't ask me the angle of the curve, its just what I had on hand....

I had no idea where to start with jetting so I purchased some extra pilot and main jets when I bought the carbs they were about AUD $8 each from memory. Mikunioz weren't able to advise me on Ducati jetting, so I rang Phil at Road and Race and he gave me some starting suggestions which proved pretty right, not far from where I've ended up. Interestingly he suggests removing the air jet for the 450 if required. This will allow you to get a bit leaner while still using the 35 pilot jet. I haven't found that necessary.

Hope that helps Bill.

[Beach Life]

Hi Bill, I have ordered this as a complete package from R&R with airfilter and manifold, still waiting as some parts not available as yet. I am a long way from getting the 450 on the road, as just started delving into the box of gearbox bits to sort everything out. ( its been in bits for about 34 years) The frame has just been powdercoated and the engine cases vapour blasted.
Cheers
Bernard

[themoudie]

Aye Dan and Bernard,

Update! Spoke with a friend who has successfully tuned and raced Ducati singles since the '70's and other one off bits of kit including ex-GP machines. Having acheived nothing more than sporadic explosive outbursts using both a MKI and MKII Amal, this included blowing the whole MKI carburetor from the stub with a large blue flame! Pogoing on a 450 is hard work and having spent Saturday and Sunday trying to fathom this out, my friend related that a 'silencer' could screw up the whole carburation. He had re-stripped a 250 race motor twice, having stripped, refurbished and rebuilt it, to check timing etc and in desperation removed the 'silencer'. Gave it a bump and the motor chimed in. To prove that this was no fluke, the 'silencer' was fitted to his own known 250 racer and it then replicated the starting/running problems of the original. "Go back to the little basics!" were his words.

So, using the MKI, with the 'Stay Up' float height 2mm below the upper edge of the float bowl (I have also used the white plastic 'standard' floats in both MKI and MKII carbs) and the air screw open 11/2 turns from seated and the throttle stop screw resting on the slide, not lifting the slide. Checked the points condenser and the coil. I also replaced the NGK resistor plug cap, with a rubber trials cap. Removed the gutted Goldie and pogoed again. This time there were less explosions and it fired most times I turned it over. So I unscrewed the air screw by 1/4 turns and with the air screw 31/2 turns out from seated I managed to get the motor to run at ~1,500rpm, with much popping and banging and bogging if I tried to open the throttle. Same results with the NGK resistor cap in place. Thought that I should try the MKII on the same set up. Whilst removing the MKI from the rubber hose connecting the two mounting stubs, the inside and outside of the throttle slide was wet with fuel and trapped against the cylinder head stub in the bottom of the hose was a puddle of fuel. Ureka! Fuel level in the float bowl too high and pouring neat fuel down the inlet. No wonder I could blow the carb off the head!

Today, I contacted a UK 3rd party supplier of Amal spares and relayed the sorry saga. His comments were pithy and blunt. Amals like the Mikunis from which their design borrows, do not like being mounted at downdraft angle greater than 15 degrees from the horizontal. When I mentioned that it was Burlen whom had told me the MKI and MKII could cope with up to 35 degree downdraft angles, there was a long silence. He had supplied a 'Deep float' to a Ducati single owner, which had cleared the float bowl flooding problem on that particular machine and offered one to me as a "possible" solution. However, I have decided to go back to the curved inlet mounting type used by Ducati for the Dellorto carbs.

This is a cast aluminium, curved stub that bolts to the cylinder head and then the carb is mounted, using the rubber connector in a more horizontal position, similar to that illustrated by DanC's image of his 450 Desmo Mikuni mount. As I have a stub mount for the MKI it enables me to try both carbs and select the one I think best.

I am not sure "How" or "Why" DanC is able to get his 350 to run with a Mikuni on a straight down draught inlet, or how I was able to run the MKI for the last 40+ years on my current straight downdraft set up that now is not playing ball, as do many other track and road Ducati singles. It always ran a bit 'rich' and tended to blacken the plug, which I noted appears to be worse with the 'greener' fuel that we are supplied these days.

Once I have received and fitted the new stub etc, I shall post the outcome of the saga.

Good health, BillR

[Duccout]

I dunno Bill, I have run all four of my Ducati Singles with a concentric with no issues, and most UK racers use them too, although racing is very different because the engine is flat out most of the time. I wonder if something else is going on if the inlet is collecting fuel? My Velocette (without air filter) would spray petrol out of the inlet trumpet as a consequence of the valve timing, so have you changed the cam to something racey? Could the valve timing be out possibly? Apart from that maybe the carb is simply flooding due to a problem with the float or needle? The curved manifold will give you some comparisons though.

BTW, it is common today to see Velo Thruxtons and BSA Gold Stars fitted with concentrics and Mikunis, and both those engines have very steep downdraughts on the inlet ports.

[themoudie]

Aye Duccout,

I agree with all that you say. It just seems very peculiar.

Having gone for a spin on the Morini K2 last night and stopped the repetitive strain syndrome of "I wonder?", I think that the timing may get a wee peek, to settle that brain worm.

Yes, it has a better cam fitted than the worn 'standard' cam that was in it for one of Nigel's 173 profile and I do not believe that the cam is causing the problem. I too always had fuel vapour come back through the inlet trumpet.

My understanding is that a Thruxton has a downdraft angle in the port of 10 degrees as standard, I am not sure of the Gold Star measurement. However, both their barrels appear to sit vertically on the cases. Whereas the Ducati's sit at 10 degrees angle on the cases and then have additional angle from the downdraft angle of the port.

If you view this link to the 'Motocarb' site you can see the type of inlet supplied with a Mikuni specifically for a BSA Gold Star. http://www.motocarb.com/uploads/1/6/9/6/16966648/vm38-gold-star-2_orig.jpg

I quote verbatum Steve's (Motocarb) written comment to my enquiry about using a Mikuni:
"Thank you for your enquiry.
I don’t have a specification on file for any Mikuni carb for the 450 and have not to my knowledge ever supplied one.
Based on your photos, there are a couple of problems with the Mikuni, the first is the severe downdraft angle the carb is mounted at, the Mikuni would not run properly mounted like that. Mikuni state a maximum downdraft angle of 15 degrees."

I have not contacted Allen's http://www.allensperformance.co.uk/ and they specifically list VM28 and VM34 carbs for Ducati 250 singles, so I do not know what they might do to enable them to enable Mikuni carbs to work on Ducati singles.

I have attached an image to show the fitting of the MKI that I have used for 40+ years and now find I have problems with.

I need some Manx dust!

My apologies if this boring, but it might help somebody.

Good health, BillR