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By TexasChris
#87070
Super useful info guys. I have one of these carbs on my 500, and have been really struggling to dial in a nice consistent idle, reliable starting etc. I was suspicious that the idle jet was too lean... sounds like I should look into changing it. Thanks
By Daiwiskers
#87072
So pleased I went for a used monobloc i was in two minds at first but decided against the mk2 concentric or premier due to the fact I used monobloc's back in the 70's

A word of warning to everyone with old monobloc's the new side's won't fit the old carbs !
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By Presto
#87074
Not good Andy to have these problems with a new carb – but a few points mentioned here aren’t too clear.

When you speak of not using the choke do you simply mean not operating it or have you removed it from the carburettor body. If the latter the cable drilling must be blanked off or the carb will run weak at all settings.

In general the 17 pilot is the correct size - of course in some applications this may need to be changed, which is why various options are supplied. But the standard 17 jet suits most applications.

Nothing has been said about the main jet. That needs to be big enough for the ‘thirsty’ 612 engine.

Failure with starting is unlikely down to the pilot system, which is overridden at starting. The starting mixture needs a lot more fuel than the pilot system would supply. The pilot system is about tick-over not starting.

Final word - which will not find universal agreement! - the Mk1 Concentric is a well-proven carb and a good choice!
By Jools G
#87076
I honestly wonder why people buy new carbs to replace their old ones when it's so easy to get the original ones reconditioned.

I had the concentrics on my '72 T150V triple reconditioned and was astonished by the drastic improvement in smoothness and MPG. I bought a brand new set of original Amal carbs when I rebuilt the bike in 1984, and the bike would regularly return a touch over 41mpg while touring. They were worn out after 20K miles. After reconditioning, I was getting 50mpg at a sustained 80+mph and 55mpg when going more gently, way better figures than when they were new!

I've had several of my old Monoblocs reconditioned in the same way, and my Constellation now returns nearly 70mpg.
By Andy C
#87078
Reply to Presto - in my original posting I said "The premier carb is setup exactly as per the MK 1 that came off." so jetting is identical.

Re not having the old carb re conditioned, although I did not look into it, I assumed that the costs would be pretty much the same.

I am waiting a reply form our hosts to see what they say, but it looks to me that I will need to fit the choke slide.

Cheers.
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By stinkwheel
#87080
How do they recondition them? Do they sleeve then rebore the body where the throttle slide runs? Or is there some other way of dealing with the wear and scoring that happens in that area. Because that's usually where they fail.
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By Presto
#87082
I read that the set up was the same as your previous carb but the performance isn't. So changes seem to be necessary. Which is why I made the suggestions I did.

Fitting the choke slide may help starting but won't do anything about the spitting when you snap open the throttle or when you pull away from a standstill.

Hope you get it sorted.
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By Wheaters
#87085
When you speak of not using the choke do you simply mean not operating it or have you removed it from the carburettor body. If the latter the cable drilling must be blanked off or the carb will run weak at all settings.
Good point, I blanked mine off using a metric screw and a blob of silicone sealant, although it is possible to buy an "OE" blanking plug.
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By Presto
#87086
With all due respects - what beats me is why anyone would dispense with the choke slide in a Mk 1 Concentric. There are no advantages and a number of disadvantages to doing this.

What baffles me even more is why with no choke you'd be surprised if starting is a problem! ;)
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By Wheaters
#87088
In my case, as per my earlier post - my bike doesn't actually need the choke to start from cold. The OE Mikcarb didn't have a choke fitted, either, only a device to enrich the mixture for starting.

In forty nine years of owning bikes, the only one I've owned that does need a "choke" is my present Honda CB750 Nighthawk and only because it's set up for the Japanese home market where it needed to run as weak as weasel water. But the carbs on that don't actually have "chokes" either, only mixture enrichment. ;)

The choke flap on a concentric is spring loaded "on" by default, it's separate from the normal air slide. If you don't want to fit the cable and lever to a bike which wasn't designed for one (mine wasn't), the choke flap has to be removed.

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