450 desmo carb

[themoudie]

UPDATE 30/09/2020 21:10pm

Follow on from previous post.

All the time I had been looking at the carb body and float, but this was not how I had checked the carb before fitting it to the bike after building it.

After building the carb and checking it's operation, I had then bolted the machined aluminium alloy stub onto it so that it could be fitted using the marine hose coupling and be held in place by two 'Jubilee' clips. I had checked these before dismantling and there appeared to be a good seal. After dismantling the inside of the hose appeared sound and no obvious signs of cuts, nicks etc that might cause a leak.

After removing the stub from the intake flange of the carb body, I tried the slide again in the carb body and whilst being a good fit, it slid gracefully the full length of the body, with no hint of sticking or resistance!

So, I put the carb flange on a 'surface' plate and on the underside of the flange a gap of 0.035" was apparent. Whilst, on the upper side of the flange there was no gap. The machined aluminium alloy stub showed no gap on either side. So, when bolted together it would appear that this deforms the body of the carb sufficient to cause the slide to 'stick' at throttle openings up to half throttle at least. (Images below)

This appears to be a potential problem with all MKI Concentric bodies, inherent from the use of a zinc alloy to cast the body, float bowl, slides etc. I have examples of the two screw float bowl showing this warping and also a MKI Concentric body that has a very marked warped flange, but this is really subtle and not readily evident before either fixing a stub to the flange or bolting the carb onto a cylinder head. Needless to say the 4lb lump hammer was applied to the carb body!

"Dinnae over tighten the fixings on these carbs, sufficient to flatten the spring washers, but no wring their necks!" Burlen/Amal does make this comment in their manuals. I would still use a new MKI Concentric 'Premier' carburettor or a known instrument, but the second hand, unknown quantity lot from autojumbles etc. is probably best avoided.

Good health, Bill

[Jordan]

Amal Concentric Mk1 carburettors are not as structurally rigid as others.
This weakness particularly affects the tubular upper section, which is easily warped out of roundness.
Amal Monobloc carbs also had this weakness.

It was common for mechanics of the time to flatten the mounting flange with a single cut flat bastard file, to restore flatness to it.
Then it could stand to be properly tightened again.

[themoudie]

Aye Jordon,

I am sure that it can be rectified with a single cut bastard file and I have used this method on distorted float bowls and finished the mating joint with a fine wet and dry paper with paraffin lubricant to make a good mating surface for the gasket, checked on a surface plate for warping and then fitted, with a smear of vaseline on both sides of the gasket. But, having been carted towards the rear of a bus, due to a stuck throttle slide in a warped carb body, I don't intend to chance my neck a second time, even though I would be confident in making the modification and checking it. Sometimes, it may be better to note the condition and how it occured then move on to a carburettor manufactured from materials less prone to warping and wear. Hence, a new carb for my 450 MKIII.

My observations and comment are not to denigrate Amal MKI Concentric carbs, there derivitive models or those who choose to use them, but to highlight that there maybe faults with carbs of that type, that might damage your health, if you are unaware of or do not see the potential problem.

Good health, Bill

[BevelBitsaBrighton]

I have bought an abandoned project & the rebuilt engine came with a 36mm Mikuni. Photo attached. The chap who rebuilt my Mach 1 motor says the 36mm Mikuni works well on a 450, he checked the carb (& engine) over for me & says its correctly set up. Dont know the settings but I could probably find out?

[themoudie]

Aye 'Brighton',

Thank you for the picture and information on the 'Micky Roony'. However, I have already bought another carb for my 450, but have yet to complete the fitting etc, as I indicated in this thread previously.

Dan C in his posting on this thread, page 3, sep 03 2020 11:36am, lists carbie settings for a Mikuni VM30 on his 450 Desmo, mounted on an angled inlet stub, also a 350 with a straight inlet stub and whilst I haven't gone the Mikuni route, I am sure having your VM36 carb settings on this thread would help others in the future, if they chose to fit that carb to a 450. At least it gives a starting point for the set up, rather than being the 'developement' man!

Hope you can soon enjoy the 450.

Good health, Bill

[themoudie]

UPDATE 15/12/2020

Comments please on plug insulator colour for fuel mixture.

Thank you for your time.

Good health, Bill

[Duccout]

If that was my bike I would say that it was perfect, maybe a touch weak? Hopefully Nigel or Eldert will give their opinion.

[blethermaskite]

Looks ok to me
Cheers,
George

[George]

Maybe a touch weak

[blethermaskite]

Bill,
Are they the same plug chop just taken from different angles or two different chops? if they are the same image mixture is fine, if two images? the top one might be a wee tad rich...but I would leave at that....better a wee bit rich than a wee bit weak.
Cheers,
George