Amal Concentric on 1950 NH

[PeterW]

Hi
My 1950 NH350 came with tired monobloc. It wasn't running well and rather than fiddle about I thought I'd treat it to a new carb and decided to try a Mk1 Concentric 600. That was a while ago and I've done plenty of tinkering with the bike since but it still didn't run cleanly so I thought I'd revisit the carb.

After experimenting with float levels today I've come to the conclusion that it's impossible to get the level correct with the cab inclined at 15 deg. as it is on the NH. The pivot is at the highest point and the bulk of the float (the most buoyant part) is at the lowest point. One of the benefits of the concentric is that if you tilt it left or right the fuel effectively stays at the right level but this isn't true front to back because, for the fuel to get to correct height in the middle where the jet is, the top of the float furtherst from the pivot would need to be above the level of the top edge of the bowl.

The good news is that I put the old monobloc back on and she's running much better :¬)

If anyone is running a modern concentric on an iron head 350 and reckons their float height is correct (0.2" from the top of the bowl) I'm happy to be proved wrong.

[Brian_Walker]

Had some issues getting my 389 monobloc right on my VCH, according to the book the dot on the face is the right float level however because of the carby angle I had to tune my float level with hose barb threaded 1BA fitting that goes into the spot where the pilot jet goes. A short bit of hose is raised and lowered till the level is just below the the carby mouth. I used to run an Amal GP which had no issues with its remote matchbox float chamber. The only way to get my level right was to add a dab of solder on the tang of the brass float to give me more wiggle room. A pity the only adjustment for float height is shims under the needle valve housing, that's fine till you take them all out and the fibre washer and still too high. pity they didn't make a bendable tang to adjust, or have a "Stay upright" float like the concentric.
You can make up a fitting to go in the drain plug hole at the bottom of the concentric and put a bit of hose on it to see where it is in relation to the carb mouth at various float heights if you have stay upright float.

[PeterW]

Thanks Brian. I hadn't thought of raising the needle housing! I'll have an experiment and see if its possible - I suspect its not because the problem is that with the fuel level where it need to be the top end of the float hits the roof of the chamber.
The monobloc runs ok but it seems a shame not to use the newer concentric - I thought about adding a weight to the float so it sits lower in the fuel but think that might make it weaker at shutting the fuel off when it hits the level, not sure if that is true though.

[JohnnyBeckett]

hi i got my new concentric carb from surrey cycles he jetted and set it up for my NH350 1954 and i just fitted it and it starts runs and ticks over very well from the box i just had to adjust the tick over a little bit he is very good, and i think he knows what he is talking about

[PeterW]

Something has glitched - I''ve asked for the multiple replies to be deleted.

In the meantime - I checked the concentric and, as the float needle is below the bowl the needle can only be lowered by adding washers - not raised which is what it needs.
Think I'll raise it with Burlen to see if they have any suggestions.

[Simon.Gardiner]

Something has glitched - I''ve asked for the multiple replies to be deleted.

Peter - I think you can delete the posts yourself (with the 'X' symbol at upper right)?

SG

[PeterW]

Something has glitched - I''ve asked for the multiple replies to be deleted.

Peter - I think you can delete the posts yourself (with the 'X' symbol at upper right)?
SG

Unfortunately I don't see an "X"

[PeterW]

Something has glitched - I''ve asked for the multiple replies to be deleted.

Peter - I think you can delete the posts yourself (with the 'X' symbol at upper right)?
SG

Unfortunately I don't see an "X"

Although I do on this one!!!

[Brian_Walker]

Hi Peter you reply seems to have been replicated multiple times, I meant you probably need to drop the float height if the fuel level is too high, I added a blob of solder on the top of the tang on the brass float in the monobloc it then lifts the needle earlier and lower height to stop flooding. If you get a stay upright kit for the Concentric the float has a bendable tang to adjust height. Washers are if the fuel level is too low.
Brian

[PeterW]

Hi Brian, its the other way round - its running too weak and I want to raise the level. On the concentric the valve comes from below so there is no scope there. Also, as the pivot is at the high end, the float hits the top before it gets to the right height so not much scope for bending the tang.
I'm going to have a chat with Burlen about it and report back.