Re: 450 desmo carb
Posted: Wed Sep 30, 2020 8:42 pm
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
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