Single Carb - Mercury Spec?

Hello Kit - I see that there is some confusion here.  I too bought a new Mk1 Premier Concentric carb from Burlen some time ago for a Mercury.  As you point out they have fitted a new pilot jet (622/502-17 which has two grooves) in place of the bush jet that the original Mk1 Concentric had.  The bush had a size that was equivalent to the older jet it replaced and had a part number of 124/026-25.  So the new pilot jet in the Premier has a 17 thou internal diameter.  The older reference of 25,  I am told,  is defining the maximum flow rate into the airstream and is probably 2.5 gallons/hour but may be in units of lbs/hour.  Sadly it is related to the inlet manifold vacuum and how quickly the pressure increases as more air flows into the engine.  I think you should stick with the 17 thou pilot jet but just in case you've blocked a fuel circuit pathway they can be quickly removed and cleaned out.  Check that the tiny holes in the bottom run in the neck of the carb are free of debris. One is downstream of the throttle valve and the other is upstream. Use carb cleaner  - don't poke a needle down them.   Don't overtighten the carb mounting nuts and ensure that you've fitted the paper gaskets to the manifold to head junction and the carb to manifold junction.  regards, Howard 

Kit - Mine was just about 1 1/2 turns.  It gave a steady tickover at 800 rpm set by the throttle stop.  Have fun but ride safe. You didn't say if you have a magneto or the twin coil system or an electronic version of the coil set-up.  Also do you have an air filter ?  I'd fit the paper washers as well.    Howard

Hi Kit.........I have exactly the same set-up as yourself.
i.e.  a 650 engine running on a newish single Premier 930 Carb.
My main jet is 270 and I have a K&N mesh filter hanging off the carb end. Otherwise, I would fit a 280 main.
The pilot jet is a 17 which other owners tell me is correct for my engine and carb.
I have covered over 10,000 miles with this Concentric carb and noted several issues right from the beginning.
1)  When the carb was new the engine needed full choke to start and catch plus even after several  minutes of warming up needed some choke to pull away.  That lasted about 3 months.
2) I have three separate filters in my fuel line but the float bowl fills up with black gunge every 1000 miles. If I don't clean it out then the main jet gets blocked. I suspect it is coming from the anodised slide.
3)  My pilot screw is 1 3/4 to 2 turns out. It is really difficult to get an even tickover.  My Wassell was much better at giving a smooth tickover plus 60+ mpg.

Photo below shows carb rubbish.  Note the amount!

What is a wire air filter?

I fit a paper waher between the manifold and head with grease on one side, wellseal on the other. The carb O ring should be enough for the other side. These tufnol insulators have a fairly true flat surface.

I don't know which single carb manifold that you're using on your 650SS head but it doesn't sound like any of the ones that I have used.  The ones I have used bring the carb back to an almost horizontal plane.  I once saw a Commando manifold used on a Mercury and the carb was uphill but it ran fine.  The carb on my Mercury was almost horizontal . It would be possible to mount one such manifold upside down but the carb would be very steep downhill (ie > 25 deg).  I did have twin Concentrics on a late 650SS and they were mounted on the parallel alloy spacers (with balance unions) - so they were pointing down at 15 degrees, as you point out,  and they were designed to run like that from the factory.     Good luck, Howard

... the carb on the Electra is similarly tilted down towards the cylinder head. I had a single carb manifold on the Commando and I'm almost sure that was straight, meabing the carb was tilted as well.
I actually think the Concentric is better than the monobloc in this respect as it also doesn't mind side to side tilt.

If you can wait a couple of days when I get home I'll be able to look up the Mk1 concentric spec for you.  I have an original workshop manual for the Mercury and the only difference from other 650's was a small insert for the carb spec.  From memory the 270 main jet is correct and is for no air filter.

I'll also be able to check the original inlet manifold to see if the carb is mounted horizontal or at 15 degrees.

Are you quite sure you are running with the choke off - i.e. lever fully back? I have known a couple of riders caught out by this, one very recently. 

That one you pictured is an ACCESS screw to the pilot jet circuit, so is kept fully screwed in. ( I recently temporarily removed the one on my single carbed Commando 850 MK3, to enable pilot circuit cleaning, for which I had purchased a set of varying cleaning wires and miniature bottle brushes, which worked very well.)
   The 650SS was sold with twin downdraught carbs at same angle as yours, and I have never heard of a problem with those.

Does it run better without the air filter?

Trying it without the filter is a test, to see if the filter is restrictive.  It certainly isn't very big.

I see complaints by Commando owners that the available single carb manifolds seriously reduce power at higher rpm.

I've no personal experience.

... firmly screwed in and its cap on tight with a fibre washer? A loose jet can let too much fuel through.

Skimming through the thread again, have you not checked the fuel level in the float bowl?

Ian - wrong type of jet and carb - it's a Premier Concentric on Kit's bike. Kit - It sounds to me as if the fuel level is incorrect. It's possible to rig up a small piece of clear tubing on to a banjo and fix it to the float chamber at the bottom float chamber plug.  I'm sorry I don't know what the plug thread is but you will probably have to get a longer one. Steve at Burlen will know and no doubt assist you.  Ensure that the float cut-off needle tip is not damaged and it sits firm into the valve.  During your investigations you haven't misplaced the volume screw neoprene washer that seals the screw ?  Similarly there is a washer on the pilot jet.    Good luck, Howard 

Must learn to read carefully!

I have the Wassell evolution which is a close copy of the Premier and it seems to be well made.

Hi Kit,
Whilst having no personal experience of concentric carbs on a Norton, I do wonder if being the 'Premier' carb, they come fitted with a  'stay up' float in them?
I have had issues with these physically binding on my monobloc carb thus making the bike run very rich. The temporary fix for me was a 2nd gasket for the float chamber cover allowing the float to move and do it's job, but that isn't likely to help you 
I understand that others have had similar issues with stay ups in concetrics, but I have no info on how it was cured.
Just because the carb is new, doesn't mean it was set correctly.
Anyone know where the fuel level should be on a concentric? At least it is adjustable with the new floats.
Regards, George 

The drain plug head on my non-Premier Concentrics is 9/16" x 26 tpi Cycle.

You can drill a drain plug or a Monoblock main jet cover and jam a clear hose into it.

Can probably lash up a banjo fitting to the bottom of the bowl with washers too.

If far to rich on idle, you must have something very wrong with pilot screw.  It sounds like it must be screwed much to tightly in.  An approximate start point should be 1.5 turns out.  Have you confirmed this?

Air filter, slide, main jet, cutaway, needle and needle jet all irrelevant at idle.

Some say float can stick on its own gasket.

Also some havd allegedly found swarf inside.  If it won't work you should send it back
 

With bowl on carb, on bike.

That's not 1/4" below top of float bowl!