Upgrade Navigator to Concentric carb?

At the risk of alienating other lightweight twin owners, I wonder if it's worth the cost to change my leaking, smelly Monobloc 375 for a new Amal Concentric MkII?  I have just bought the bike from another Noccer and although it's not in all-original condition it's a reasonable runner (it started first kick!) with potential for a "tidy up".  ("All it need is a spot of Solvol and a tune-up" as Real Classic magazine says!)

Unlike my other Norton (a 1959 99), which I intend to restore to concours, or as best as I can get, this one needs to be on the road soon, or my wife will be cross that I bought it!  She's already checked the pillion seat for comfort!  It needs a few parts but the most dangerous areas are the electrics - which include domestic terminal blocks, insulation tape and crimp-on terminals - and the carburettor which leaks badly even though a receipt for a new washers/gasket kit is in the useful pile of documents that came with the bike.  But then again, so is the receipt for a wiring loom!  The petrol leaks over the output terminals from the alternator, which were wrapped with dissolving insulation tape!

Bear in mind that a Major Repair kit for the Monobloc would cost £29.32 and a new MkII Concentric is £104.58.  The Monobloc might not be much better after a new kit - it still leaks after new washers & gaskets!  I might as well bit the bullet and go for a much more reliable (?) new unit that might outlast me and the bike!  The problem is that I don't know what the technical spec should be.  Mine's a De Luxe so I'd have to rig a remote tickler lever, but that won't be Rocket Science.

Has anyone done this conversion?

Not entirely relevant, I confess, but for what it's worth my Jubilee's got a Concentric (same size jets as its Monobloc forerunner) and it works just fine.

Many thanks Howard - that's just what I wanted to hear!  I now think I will go for the Mk1 600 series rather than the Mk2.  It's a bit cheaper at £99.88 (+VAT) from Amal.  At least I know it's got 2" flange hole spacing, so it should bolt straight on.  The only slight prob might be the tickler extension lever.

Previously wrote:  The only slight prob might be the tickler extension lever.

Ah, yes, the tickler lever. Mine's got a finger-sized porthole (rubber-grometted) next to the lever slot and, as my digit ain't long enough to reach, I use a custom-designed probe (oh, all right, a pencil) to depress the tickler. Well, it works for me.

Why muck around with Amal whatever; fit a decent carb - a Mikuni VM24-473 with the cable choke mod from Allens Performance of Nottingham - 01949 836733. Just tell them the jet sizes etc and they do the rest. you will then find out what decent performance from a carburettor is like. It will only cost around £80.

You will need to adapt your inlet to match the 24mm of the Mikuni, but that is not very difficult, and you retain your existing choke cable - very necessary with the Deluxe panels on the Navigator.

Thanks Derek - the phrase "You will need to adapt your inlet" does put me off a bit!  If it takes longer than a day to swap carbs then it's not on - according to The Boss!  I spoke to the Surrey Carbs expert at Uttoxeter last Sunday (their Concentril MkI is £101 inc. VAT) and he reckons the Concentric "breathes" about 10% more than the Monobloc so it needs down-jetting a size.  However, I intend to add an air filter which will probably balance it out if using the original spec jets - easy as the jetting numbers are the same as Monoblocs.  As for a Mikuni?  I have no idea about jetting specs so wouldn't be able to tell anyone what I need!

As to 'performance' - my own opinion is that people often make the mistake of upgrading old bikes beyond their capability.  My Dommie is 51 years old and has the original crank with reground (i.e. smaller) big-ends and rebored barrel (+.030") - no-one should "soup up" a bike of that age and expect it to survive for long!  The same goes for the "family tourer" (my words) Navigator.  It's a 350cc which, on test when new, only managed 83mph so the word "performance" doesn't mean much - it just needs to start easily  and run reliably, reasonably economically and of course, look good!  (My old Dommie used to get around 70mpg in the late 1960s)

Generally speaking, the higher the spec, the less reliable a bike (or car) becomes.  I have a fair amount of experience with the Jaguar XK series of engines as well as Dominators so I speak with at least little experience.  Incidentally, the Dommie's valve guides are the same diameter as the Jag's so I was able to use the same guide drift!

I Intend to fit s petrol filter and am considering an oil filter but haven't seen one for less than £50 so far.  As I'm expecting to do less than 1500 miles per year that might not make economical sense!  (Bear in mind I already have 2 cars and one other Norton - plus a BSA C11G which someone was throwing away, so my wife rescued it!)

Howard - I'm hoping I'll be able to adapt the tickler bracket to suit a new Concentric or make one myself.  Derek - I WILL check out the Mikuni seeing as you've been kind enough to respond to my query but isn't 24mm a bit overchoked?  An Amal would be 22mm.

Cheers, Lionel

The main question is "Where does your Monobloc leak".  If it's from a distorted side cover, a bit of judicious rubbing on fine emery on a piece of plate glass might stop the leaks for next to no cost.  If you can provide more information, I might be able to help you out of your troubles for hardly any money.

Previously wrote:

The main question is "Where does your Monobloc leak".  If it's from a distorted side cover, a bit of judicious rubbing on fine emery on a piece of plate glass might stop the leaks for next to no cost.  If you can provide more information, I might be able to help you out of your troubles for hardly any money.

Thanks Colin, I'm wise to the plate glass tricks!  I used to restore Jaguar XK engine oil pumps that way!  I don't think it leaks from the cover but it had been serviced with a new set of washers and gaskets so I couldn't be bothered to mess around.  Unfortunately it was obviously reassembled with ignorance!  Too much red gasket compound everywhere!  Evenm on the manifold joint which shouldn' have ANY!  People seem to think you have to chuck compound everywhere and tighten up the manifold to within an inch of its life!  Thay have an O-ring jint which is designed to just pinch up the faces LIGHTLY!  No wonder people are always warping the flange!

My new Concentric's on order from Amal - but taking a bloody long time to put together!  I've been waiting 4 weeks now! I might play around and check the carb body to se if the face would benefit from treatment but I think it was coming out of the tickler so the favourite is the float level being too high.  It's brass and well dented - gawd knows how! But it doesn't seem to leak anywhere.  I'll test it off the bike sometime and see if a new float would help before I sell it.  I've no intention of spending too  much time or money on it.

Unlike my Dommie, I intend to update a few nuts & bolts with stainless as well as fit a fuse and decent ignition isolating switch.  I've bought a Triumph-style on/off switch that operates with a real key so I will sell the Lucas 88SA ignition switch that someone had fitted to isolate the battery.  This bike will look the part when finished but it's a road bike so can't be off the raod for long periods like the Dommie (37 years!) - wife's orders!  I will probably get the wheels rebuilt as the rear has a good chrome rim on rusty spokes and the front has a rusty rim on stainless spokes!  I might just run the wire wheel on the rear spokes though and zinc paint them as a rebuild is so expensive.  It's the front rim that needs replacing as it stands out so badly.  Snag is that it's stainless nipples on stainless spokes which a wheel builder has told me is bad news!  He reckons to only ever use brass nipples on any spoke as the stainless on stainless tend to seize!  That's a surprise to me and I intend to post more about that elsewhere on the site.

Cheers, Lionel

Previously wrote:

Previously wrote:

The main question is "Where does your Monobloc leak".  If it's from a distorted side cover, a bit of judicious rubbing on fine emery on a piece of plate glass might stop the leaks for next to no cost.  If you can provide more information, I might be able to help you out of your troubles for hardly any money.

Thanks Colin, I'm wise to the plate glass tricks!  I used to restore Jaguar XK engine oil pumps that way!  I don't think it leaks from the cover but it had been serviced with a new set of washers and gaskets so I couldn't be bothered to mess around.  Unfortunately it was obviously reassembled with ignorance!  Too much red gasket compound everywhere!  Evenm on the manifold joint which shouldn' have ANY!  People seem to think you have to chuck compound everywhere and tighten up the manifold to within an inch of its life!  Thay have an O-ring jint which is designed to just pinch up the faces LIGHTLY!  No wonder people are always warping the flange!

My new Concentric's on order from Amal - but taking a bloody long time to put together!  I've been waiting 4 weeks now! I might play around and check the carb body to se if the face would benefit from treatment but I think it was coming out of the tickler so the favourite is the float level being too high.  It's brass and well dented - gawd knows how! But it doesn't seem to leak anywhere.  I'll test it off the bike sometime and see if a new float would help before I sell it.  I've no intention of spending too  much time or money on it.

Unlike my Dommie, I intend to update a few nuts & bolts with stainless as well as fit a fuse and decent ignition isolating switch.  I've bought a Triumph-style on/off switch that operates with a real key so I will sell the Lucas 88SA ignition switch that someone had fitted to isolate the battery.  This bike will look the part when finished but it's a road bike so can't be off the raod for long periods like the Dommie (37 years!) - wife's orders!  I will probably get the wheels rebuilt as the rear has a good chrome rim on rusty spokes and the front has a rusty rim on stainless spokes!  I might just run the wire wheel on the rear spokes though and zinc paint them as a rebuild is so expensive.  It's the front rim that needs replacing as it stands out so badly.  Snag is that it's stainless nipples on stainless spokes which a wheel builder has told me is bad news!  He reckons to only ever use brass nipples on any spoke as the stainless on stainless tend to seize!  That's a surprise to me and I intend to post more about that elsewhere on the site.

Cheers, Lionel

hello lionel  why dont you build you own wheels like I did  on my 1954 Domie 88 and my Norton Manxman 650 If a Woman can rebuild wheels so can any Guy ?  and I do not like stainless steel spokes  As some Stainless spokes can be britle and bust in the wheel and you could be riding it at the time   ?   Your anna J Dixon 

Previously wrote:

Previously wrote:

Previously wrote:

The main question is "Where does your Monobloc leak".  If it's from a distorted side cover, a bit of judicious rubbing on fine emery on a piece of plate glass might stop the leaks for next to no cost.  If you can provide more information, I might be able to help you out of your troubles for hardly any money.

Thanks Colin, I'm wise to the plate glass tricks!  I used to restore Jaguar XK engine oil pumps that way!  I don't think it leaks from the cover but it had been serviced with a new set of washers and gaskets so I couldn't be bothered to mess around.  Unfortunately it was obviously reassembled with ignorance!  Too much red gasket compound everywhere!  Evenm on the manifold joint which shouldn' have ANY!  People seem to think you have to chuck compound everywhere and tighten up the manifold to within an inch of its life!  Thay have an O-ring jint which is designed to just pinch up the faces LIGHTLY!  No wonder people are always warping the flange!

My new Concentric's on order from Amal - but taking a bloody long time to put together!  I've been waiting 4 weeks now! I might play around and check the carb body to se if the face would benefit from treatment but I think it was coming out of the tickler so the favourite is the float level being too high.  It's brass and well dented - gawd knows how! But it doesn't seem to leak anywhere.  I'll test it off the bike sometime and see if a new float would help before I sell it.  I've no intention of spending too  much time or money on it.

Unlike my Dommie, I intend to update a few nuts & bolts with stainless as well as fit a fuse and decent ignition isolating switch.  I've bought a Triumph-style on/off switch that operates with a real key so I will sell the Lucas 88SA ignition switch that someone had fitted to isolate the battery.  This bike will look the part when finished but it's a road bike so can't be off the raod for long periods like the Dommie (37 years!) - wife's orders!  I will probably get the wheels rebuilt as the rear has a good chrome rim on rusty spokes and the front has a rusty rim on stainless spokes!  I might just run the wire wheel on the rear spokes though and zinc paint them as a rebuild is so expensive.  It's the front rim that needs replacing as it stands out so badly.  Snag is that it's stainless nipples on stainless spokes which a wheel builder has told me is bad news!  He reckons to only ever use brass nipples on any spoke as the stainless on stainless tend to seize!  That's a surprise to me and I intend to post more about that elsewhere on the site.

Cheers, Lionel

hello lionel  why dont you build you own wheels like I did  on my 1954 Domie 88 and my Norton Manxman 650 If a Woman can rebuild wheels so can any Guy ?  and I do not like stainless steel spokes  As some Stainless spokes can be britle and bust in the wheel and you could be riding it at the time   ?   Your anna J Dixon

Has anyone actualy seen stainless steel spokes break?    regards nick

Previously wrote:

At the risk of alienating other lightweight twin owners, I wonder if it's worth the cost to change my leaking, smelly Monobloc 375 for a new Amal Concentric MkII?  I have just bought the bike from another Noccer and although it's not in all-original condition it's a reasonable runner (it started first kick!) with potential for a "tidy up".  ("All it need is a spot of Solvol and a tune-up" as Real Classic magazine says!)

Unlike my other Norton (a 1959 99), which I intend to restore to concours, or as best as I can get, this one needs to be on the road soon, or my wife will be cross that I bought it!  She's already checked the pillion seat for comfort!  It needs a few parts but the most dangerous areas are the electrics - which include domestic terminal blocks, insulation tape and crimp-on terminals - and the carburettor which leaks badly even though a receipt for a new washers/gasket kit is in the useful pile of documents that came with the bike.  But then again, so is the receipt for a wiring loom!  The petrol leaks over the output terminals from the alternator, which were wrapped with dissolving insulation tape!

Bear in mind that a Major Repair kit for the Monobloc would cost £29.32 and a new MkII Concentric is £104.58.  The Monobloc might not be much better after a new kit - it still leaks after new washers & gaskets!  I might as well bit the bullet and go for a much more reliable (?) new unit that might outlast me and the bike!  The problem is that I don't know what the technical spec should be.  Mine's a De Luxe so I'd have to rig a remote tickler lever, but that won't be Rocket Science.

Has anyone done this conversion?

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can you write me at <lighttwins@nortonownersclub.org>

I can take you through it in detail.

Have not time to read all the threads at present - unsure where you have got to