Float height or Fuel Level in a Concentric

[MGriffin]

Hi there,
I have a new 30mm Conc on the VH which has been running fine but I’ve never been able to easily start the bike hot. Cold, no probs, first kick when using the tickler etc. Easy.

The other day after a hot day ride about 31DegC ambient, the bike stood in the sun for about an hour and after i’d run the bowl fuel level down as i always do by closing the fuel off first for about 30 seconds.
After the hour…...Wouldnt start. Infact it never starts first kick when hot. Even after standing at a fuel stop, its a two or three kick event. Anyway, after a dozen attempts we did a rolling start and got it going. After that, no issues. no misfiring or carrying on. Purred all the way home.

I wondered about the float level. So pulling it apart, I’ve noticed when really looking into the float level setting and needle condition etc that the clearance given in the slot of the needle where the float tab inserts seems excessive in and allows the float to move up past the set level. Remember though the needle is technical closed at the set level but it got me thinking that if the float, and its a ‘Stay Up’ type can move up and down quite a bit in the slot of the needle can it theoretically allow more fuel to bleed past the needle/seat and flood given enough time. Yes, i know the tap (brass taper seat type) was and is closed but when watching for fuel leaking past that with the hose free, i noticed that it bleeds. Yes, ok I should replace the tap (3rd time) but the issue i think is the float setting.
Am I wrong and over thinking this?

What i ended up doing in the meantime was adjusting the float probably how it should be (?) as such and that being the float tang is hard on the needle at the fuel level setting of 0.080” below the gasket face.
My only concern there though is the needle now seems limited in its ability to fully lift from the seat.
MG.

[will_curry]

Do you have a heat-insulating spacer between the head and the carburettor?
Heat transfer from the head can cause all sorts of starting troubles with modern fuels.

Once the engine does start the evaporation of the fuel rapidly cools the carburettor
and all the problems go away.

[MGriffin]

Do you have a heat-insulating spacer between the head and the carburettor?
Heat transfer from the head can cause all sorts of starting troubles with modern fuels.

Once the engine does start the evaporation of the fuel rapidly cools the carburettor
and all the problems go away.

yes a 6mm phenolic space is fitted. Thats a trick I well learnt on SU installations.

[chris.shearwood]

Hello MG,
I have never owned a Concentric carb nor taken one apart so know next to nothing about them but I did save this message from the e-mail chat group of almost twenty years ago thinking that it might come in handy some day. I don't know if it is relevant to your problem but thought I would post it just in case. I don't have permission from "Nick" but he seems like the type who would not mind this repost.
Regards, Chris

[cmfalco]

the float tang is hard on the needle at the fuel level setting of 0.080” below the gasket face.

AMAL specifies that the fuel height should be in the range 0.170"-0.240" (4.3 - 6.1 mm) below the edge of the float bowl, which is where I have the Concentrics set that are on two of my Gold Stars. At the risk of jinxing things, both of them start without problem when they are hot or cold.

If your needle is nylon it's a good idea to replace it with an Al needle with a Viton tip. Also, if you replace the float bowl with a later type having a drain, it will allow you to actually see the fuel level when the carburetor is assembled, on the bike, and subjected to the pressure head from the fuel tank above (by modifying a drain screw with a fitting that connects to a length of clear tubing). The drain plug also makes it a simple matter to empty the float bowl when you won't be using the bike for a week or longer.

The photograph shows the fuel level in the Concentric on a DBD Gold Star, where the 15-degree tilt required me to adjust the float to get the level correct in the center of the bowl where it matters, because that's where the jets are.

[MGriffin]

the float tang is hard on the needle at the fuel level setting of 0.080” below the gasket face.

AMAL specifies that the fuel height should be in the range 0.170"-0.240" (4.3 - 6.1 mm) below the edge of the float bowl, which is where I have the Concentrics set that are on two of my Gold Stars.

Good point. I used the wrong words. Float level at 0.080” not Fuel Level. Sorry.
Thanks for the pointers.
MG.

[MGriffin]

Hello MG,
I have never owned a Concentric carb nor taken one apart so know next to nothing about them but I did save this message from the e-mail chat group of almost twenty years ago thinking that it might come in handy some day. I don't know if it is relevant to your problem but thought I would post it just in case. I don't have permission from "Nick" but he seems like the type who would not mind this repost.
Regards, Chris

Much appreciated Chris, thanks.

[keith.mettam]

MG,
This carb tuning guide appears to be at odds with what 'Nick' says about MK1 Amal Concentric float levels and is worth a read :-

tuning_and_using_amal_mk1_concentric_carburetters_.pdf

(289.68 KiB) Downloaded 87 times

I checked the Concentric float levels on my Honda twin by the method Charles Falco uses as the carbs have drain plugs. To me it is the more accurate method.
Regards,
Keith.

[MGriffin]

MG,
This carb tuning guide appears to be at odds with what 'Nick' says about MK1 Amal Concentric float levels and is worth a read :-
tuning_and_using_amal_mk1_concentric_carburetters_.pdf
I checked the Concentric float levels on my Honda twin by the method Charles Falco uses as the carbs have drain plugs. To me it is the more accurate method.
Regards,
Keith.

Thanks Keith. It does mention a few points i’ve not read elsewhere.

[JohnnyBeckett]

hi i use a 1/2" thick spacer and i do not have problem starting hot or cold with my concentric mk1 carb