450 desmo carb

Ducati single cylinder motorcycle questions and discussions, all models. Ducati single cylinder motorcycle-related content only! Email subscription available.
Moderator: Morpheus

Moderator: ajleone

graeme
Posts: 938
Joined: Sun Mar 25, 2012 12:00 am
Location: Tasmania Australia

Re: 450 desmo carb

Postby graeme » Sun Sep 20, 2020 9:16 pm

Some thoughts and what works for me with 3 x 450’s
RT with phf32 jetted as per Darmah but 262 atomiser and Mallosi rubber tube mount.
Scrambler with phf32 same jetting but bolt on manifold.
450 Desmo yellow bike with some breathing mods etc phf36.
All 3 need choke/enricher about half on, 2 kicks with ignition off, choke off, ignition on, a bees dick of throttle and kick.
They usually start and are held at about 2k rpm for a minute or so before they will idle.
RT has Sachse ignition, the other 2 have the original ‘74 red box.

None of them will start without choke, and none of them will idle when the choke is on.

Also Mk3 commando with one VM34 Mikuni, choke on full, kick and it usually starts straight away and will idle

Graeme

themoudie
Posts: 649
Joined: Sat Jun 14, 2014 12:44 am
Location: Scotland

Re: 450 desmo carb

Postby themoudie » Mon Sep 21, 2020 11:57 pm

UPDATE 21/09/2020 23:20

After threatening to put a 3mm drill through the integral 20cc Pilot jet, I went ahead. Cleaned out the swarf, flushed and blew everything through, then filled the holes with epoxy metal and left to set in a warm place. Rebuilt the carb, a MKI 932 fitted with 220 main, 31/2 slide, needle on No.2 groove from top, 106 needle jet, cast in pilot jet removed and a 25cc Pilot jet in the back of the carb body. The SuperTrapp exhaust, with 16 diffusers fitted and managed to achieve a reasonable tickover of 1,000-1,200 rpm. BUT, would only start if well flooded on the tickler, even when hot! Fitted a new NGK B6HS plug, gapped to 0.035", this improved the tickover, but still black, but not wet after ticking over for 1minute+. Checked the ignition timing again, all appeared satisfactory. I also contacted Boyer Bransden, asking why the condenser needed to remain in circuit across the points, inspite of them acting as a switch and carrying no current. I will post the reply on the forum, in a separate thread, when I have it.

Took the bike for a road test and after 4 miles steady riding up through the gears and held in 4th in a queue of traffic, went into a 30mph built up area, down into 3rd gear and the engine was surging on small throttle openings. After a mile the road is 60mph limit again, so opened the engine up through the gears, clean all the way and pulling strongly up to 60 - 65mph. Had to slow for a bus and shut the throttle, but the revs continued to climb past 6,000rpm (at last glance :shock: ), changed down the box and applied brakes but revs refused to stop rising, so cut the ignition and coasted to a halt. Still behind the bus! :evil:

It felt as though the throttle was stuck wide open. Yet no sign of icing; the throttle working correctly and snapping shut, with no sign of the slide sticking or slack in the bore either laterally of fore and aft. Almost the same as a diesel engine sucking oil from a blown turbo and on wide open throttle. I would love to know the reason behind the behaviour and suspect an air leak somewhere?

Thought I had started to get the hang of it, and had ordered some bits for the MKI from Burlen to try to tune the bottom end and starting (pilot jets and a choke kit), based on comments posted about needing a proper choke with ethanol fuels and the fact that I was having to repeatedly tickle and kick until I had some heat in the motor before it would idle. Now I'm inclined to junk the MKI carbs and try the MKII again, with a deep float and a No.3 slide. I have been reminded that the MKII is a superior carburettor when compared to the MKI and cleans up the carburation throughout the rev range. Also they are the instrument of choice for competitive Manx bikes. I have tried the 'Stayup' float, but on another thread on Motoscrubs I read that the vibration can cause problems as the float is not sufficiently boyant to seal the float needle on it's seat?

Spec. for the MKII 2932 at present is:
Main Jet: 220 (180 - 280 available)
Needle Jet: 106
Needle: 2A1 (Middle groove)
Pilot Jet: 25cc
Starting Jet: 50cc (assumed, as this is standard fitting and it is siezed in the bottom of the float bowl!)
Throttle Slide: 31/2, but fiddled with by previous owner, cutaway is much taller than that on my MKI 31/2 slides. (Purchase a new No.3 or 31/2 slide?)
Modifications/upgrades:
'Stayup' float, replace with deep float for extra bouyancy.
Float jet appears sound as does the seat, both viewed through a x10 watchmaker's loupe/lens.
Purchase a new rubber adapter, rather than my home made reinforced marine fuel hose.
May need a new 105 needle jet and 27cc and 30cc pilot jets?
Gasket and washer kit (just in case!)

I have read these board threads as well: Motoscrubs_Amal_Concentric_MKII_R2934/3 and Motoscrubs_450_which_carb??? but the end never seems to come or a different carb from a MKII 2900 series is eventually chosen.

Maybe it'll have to be a 'Micky Rooney'? ;)

'Nough from me and a wee dram to steady the nerves afore sheet alley!

Good night, Bill

graeme
Posts: 938
Joined: Sun Mar 25, 2012 12:00 am
Location: Tasmania Australia

Re: 450 desmo carb

Postby graeme » Tue Sep 22, 2020 2:10 am

Bill, you are a persistent bugger !!

Does your slide have a closing spring to shut the slide ?
If it has a big cutaway and no (or weak) spring the inlet air could hold it open.

If as you think it is an air leak keeping the revs up it would have to be a big air leak

Good luck

Graeme

Dan C
Posts: 75
Joined: Wed Feb 12, 2020 12:52 am
Location: Armidale, NSW, Australia

Re: 450 desmo carb

Postby Dan C » Tue Sep 22, 2020 4:31 am

Bill, you are a persistent bugger !!


Ain't that the truth Graeme! But I understand where Bill's coming from - a carb is a relatively simple bit of kit and after you've been through everything 10 times, and its all within spec, you tell yourself "it should bloody work.....!" All I can say is I'm glad I finally took your advice and tried a new carb!

I don't think it matters which brand. Just that's its new.

Bill, in my case that surging at low revs was due to the pilot being too rich.

Cheers
Dan

PS I hope it was a good malt!

graeme
Posts: 938
Joined: Sun Mar 25, 2012 12:00 am
Location: Tasmania Australia

Re: 450 desmo carb

Postby graeme » Tue Sep 22, 2020 8:18 am

Yes and yes !!!!

Duccout
Posts: 1285
Joined: Wed Feb 05, 2020 1:20 pm
Location: Essex UK

Re: 450 desmo carb

Postby Duccout » Tue Sep 22, 2020 10:34 am

Bill, apologies if you've covered this, but I can't be bothered to read all through the thread again. I have just cured my dodgy low-speed running by raising the float level in my Amal carb, is this something that you have looked at?

Colin

themoudie
Posts: 649
Joined: Sat Jun 14, 2014 12:44 am
Location: Scotland

Re: 450 desmo carb

Postby themoudie » Tue Sep 22, 2020 11:02 am

G'day! :D

Thank you for the compliment!

The man from Boyer Bransden has replied and he says:
"A faulty condenser can affect the switching operation of the points.
The I.D. unit can be run without the condenser, try disconnecting it to see if the running improves.
Also check for smooth operation of the mechanical advance mechanism."

Secondly, I have bitten the bullet, new carb, but will put my posts on this subject on hold until I have the instrument fitted and see if I can get it to perform!

Yes, it was a single malt. Whether it was good is down to your palate. I enjoy a 'Leapfrog' as much as an Orcadian drop, Speyside sherry, or Lowland snifter. The 'Leapfrog' was the one of choice last night!

As for the throttle slide spring this is a brand new 'Genuine' Amal replacement from Burlen, as are all the replaceable parts that have been involved in the saga to date.

Colin, yes I have raised and lowered the float level in the MKI and MKII carbs, using the 'StayUp' float. I have also used the original white 'Standard' nylon floats in both carbs. For some raising the float level 1mm above the top edge of the float bowl appears to cure their woes, whilst for others lowering the float level by as much as 5mm below the upper surface of the float bowl as cured their problem. I have had the 'StayUp' at both extremes and at 1mm intervals between these extremes, in my case to no positive effect! :? Note: float level is not the same as fuel level and I suspect that could be my "bete noir"! This may be varying rather than being stable. :?

Off to strip out the offending MKI carb and see if I can find anything "not right" that I may have done. If there is, I will report here on this thread. Then tackle the Morini K2 rat's nest or go for a canter on the SRX. Autumnal today, wind a cool WSW 20mph plus, temp ~12C and overcast with cunim cloud.

Good health, Bill

Duccout
Posts: 1285
Joined: Wed Feb 05, 2020 1:20 pm
Location: Essex UK

Re: 450 desmo carb

Postby Duccout » Tue Sep 22, 2020 11:20 am

Brave man Bill! What carb have you gone for?

Colin

Dan C
Posts: 75
Joined: Wed Feb 12, 2020 12:52 am
Location: Armidale, NSW, Australia

Re: 450 desmo carb

Postby Dan C » Wed Sep 23, 2020 12:24 am

Leapfrog....Nothing like a bit of peat in your malt..... Maybe run a bit of that through the carb and see what happens....:lol:

Looking forward to the next installment!

Cheers
Dan

themoudie
Posts: 649
Joined: Sat Jun 14, 2014 12:44 am
Location: Scotland

Re: 450 desmo carb

Postby themoudie » Wed Sep 30, 2020 8:10 pm

UPDATE 30/09/2020 20:30pm

Other jobs have got in the way and the weather has on occasion been too good to 'waste' in the workshop and so we put 430Km under the wheels instead. :D

The new carb has arrived and some 'surgery' of those 450 and some 350 frame gusseting is required that's as far as that has gone. More to follow, later.

Firstly, out with the plug and it looked as though it had been hot! :oops: This is a NGK B6HS plug gapped at 0.035". I know it is a 'cool' plug, but thought OK to use to run in and then go up to a B7HS or the iridium equivalent later. (Image below)

Removed the MKI Concentric from the marine hose between the stubs and again tried the throttle. It snapped shut from fully open, but if only half open there was a nasty juddering of the slide as it tried to close under the pressure of the half tensioned slide spring (new Burlen). The greater the angle toward the horizontal the carb, the worse this became. Producing a noise not dissimilar to dragging a finger nail down a blackboard! :twisted:

Removed the bowl and checked that the needle and needle jet were unmarked (new items) and under a 10x magnification watchmaker's loupe, no sign of scoring on either. Removed the slide, from the carb body looking for obvious scoring of the anodised finish. Areas were definitely more matt in colour than others and there were areas of black residue around the front and rear of the slide where it sits in the body open to the venturi. This residue did not resist being removed with a finger wipe, but appeared to be anodising rather than a carbon build up.

Put the bare slide back into the body of the carb and it would happily sit at half throttle opening in a vertical body. At angles toward the horizontal, as when mounted at the 20° angle on the cylinder head this resistance to closing increased. (Image below)

An ovoid carb body! :oops: But why hadn't I seen, felt or heard that when building the carb up, with this body that appeared to have a good sliding fit, with the anodised slide? :?: :?

To be continued in next posting with images of the carb body. Bill
You do not have the required permissions to view the files attached to this post.


Return to “Ducati Singles Main Discussions (& How to Join)”

Who is online

Users browsing this forum: Google [Bot] and 42 guests