Float Levels For Concentric Carbs

Obijohnkenobe · Jan 7, 2026

OK. Have completed a few maintenance tasks on my T100R: Replaced knackered throttle and speedo cables, spotted and replaced almost broken wire between the coils, replaced split rubber boot on clutch cable/gearbox cover, adjusted tappets (all of which had no movement at all (previous owner, not me!), and inspected float levels.

The floats I happily discovered to be the newer stay-ups, with new alluminium float needles, and new needle jets to boot (why don't people keep evidence of work done!) HOWEVER, the floats were higher than recommended so I lowered them to what they should be. Result, wouldn't start at all. So I re-set the float levels to where they were and she now starts. Either my measuring was rubbish or something else is at play, or maybe the stay up floats do need to be higher than the originals (although Amal dont say so. Any thoughts anyone?

Eldon · Jan 7, 2026

Obijohnkenobe said: ↑

OK. Have completed a few maintenance tasks on my T100R: Replaced knackered throttle and speedo cables, spotted and replaced almost broken wire between the coils, replaced split rubber boot on clutch cable/gearbox cover, adjusted tappets (all of which had no movement at all (previous owner, not me!), and inspected float levels.

The floats I happily discovered to be the newer stay-ups, with new alluminium float needles, and new needle jets to boot (why don't people keep evidence of work done!) HOWEVER, the floats were higher than recommended so I lowered them to what they should be. Result, wouldn't start at all. So I re-set the float levels to where they were and she now starts. Either my measuring was rubbish or something else is at play, or maybe the stay up floats do need to be higher than the originals (although Amal dont say so. Any thoughts anyone?
Click to expand...

My only thoughts are, Amal carbs are rubbish compared to Japanese.
My Armstrong mt500 started life with an Amal as standard fitment but it didn't take long for the various owners of that model to bin them in favour of a Mikuni.

Obijohnkenobe · Jan 7, 2026

Hmm. Good point. I think both Bonnies and Commandos were also often "upgraded" with Mikuni carbs.

I guess that 50 year old Amals are not exactly "cutting edge".

Iceman · Jan 28, 2026

Obijohnkenobe said: ↑

OK. Have completed a few maintenance tasks on my T100R: Replaced knackered throttle and speedo cables, spotted and replaced almost broken wire between the coils, replaced split rubber boot on clutch cable/gearbox cover, adjusted tappets (all of which had no movement at all (previous owner, not me!), and inspected float levels.

The floats I happily discovered to be the newer stay-ups, with new alluminium float needles, and new needle jets to boot (why don't people keep evidence of work done!) HOWEVER, the floats were higher than recommended so I lowered them to what they should be. Result, wouldn't start at all. So I re-set the float levels to where they were and she now starts. Either my measuring was rubbish or something else is at play, or maybe the stay up floats do need to be higher than the originals (although Amal dont say so. Any thoughts anyone?
Click to expand...

What year is your T100R.

Obijohnkenobe · Jan 28, 2026

Iceman said: ↑

What year is your T100R.
Click to expand...

1972. Unfortunately the carbs don't have the drain plugs at the bottom to facilitate a levelling tube (manometer) to be more accurate with the petrol/float levels, I have to take the bowls off and try and get the levels right that way.

speedrattle · Feb 7, 2026

you can make the amals work if you want to keep them. if you havent already come across it, there may be some extra information here

https://www.ntnoa.org/pdf/Bushmans Carb Tuning Secrets.pdf

if you do ever decide to swap to the VM mikunis they will work well. i dont know the initial settings for a T100 but the carbs are easy to tune. TM mikunis have better throttle response, but i have no experience with them.

Obijohnkenobe · Mar 12, 2026

OK. Been fiddling with the carbs over the last few weeks (well, when the weather permitted). Problem was stuttering and "holding back" when accelerating. Initially I purchased a Colortune as I was going crazy trying to get the mixture right at tick over All done in 5 minutes and ticked over fine. However this didn't help at wider throttle opening of course. In the end I moved the carb needle from 1st to middle position just to see what would happen. Miraculously she now accelerates brilliantly with no hesitation or holding back. Obviously running lean at higher throttle openings. Sods law I have now done my ankle in so can't ride at the minute but in a couple of weeks or so..... Happy days.

speedrattle · Mar 23, 2026

one easy way to evaluate wider throttle jetting is to chop the throttle a little bit while running under load.

maybe pick an uphill section, then open the throttle fully for a few seconds to stabilize the mixture through the main jet. then back off the throttle 1/8.

if the bike speeds up for a few seconds that jet is too lean.

correct jetting will have the motor slow down a bit, not speed up.

this works at smaller throttle with needle height changes too but is more ambiguous.

the reason is that fuel has greater momentum than air. when you get the fuel moving through the main and needle jet and cut the throttle, it will continue flowing into the venturi for a few seconds, while the airflow suddenly decreases. same fuel but less air richens the mixture a bit until the system re-stabilizes, so if the bike speeds up th eexisting jetting was too lean.

Obijohnkenobe · Mar 23, 2026

speedrattle said: ↑

one easy way to evaluate wider throttle jetting is to chop the throttle a little bit while running under load.

maybe pick an uphill section, then open the throttle fully for a few seconds to stabilize the mixture through the main jet. then back off the throttle 1/8.

if the bike speeds up for a few seconds that jet is too lean.

correct jetting will have the motor slow down a bit, not speed up.

this works at smaller throttle with needle height changes too but is more ambiguous.

the reason is that fuel greater momentum than air. when you get the fuel moving through the main an needle jet and cut the throttle, it will continue flowing into the venturi for a few seconds, while the airflow suddenly decreases. same fuel but less air richens the mixture a bit until the system re-stabilizes, so if the bike speeds up th eexisting jetting was too lean.
Click to expand...

Great bit of insight. I'll try that and see what happens. Many thanks

Pegscraper · Mar 23, 2026

speedrattle said: ↑

one easy way to evaluate wider throttle jetting is to chop the throttle a little bit while running under load.

maybe pick an uphill section, then open the throttle fully for a few seconds to stabilize the mixture through the main jet. then back off the throttle 1/8.

if the bike speeds up for a few seconds that jet is too lean.

correct jetting will have the motor slow down a bit, not speed up.

this works at smaller throttle with needle height changes too but is more ambiguous.

the reason is that fuel greater momentum than air. when you get the fuel moving through the main an needle jet and cut the throttle, it will continue flowing into the venturi for a few seconds, while the airflow suddenly decreases. same fuel but less air richens the mixture a bit until the system re-stabilizes, so if the bike speeds up th eexisting jetting was too lean.
Click to expand...

A cracking piece of old school tuning knowledge, lost to the digital age of EFI.

speedrattle · Mar 24, 2026

i confess that i am a confirmed disciple of the Old Ways.

theres lots of specialized knowledge like this that just gets lost as time moves on. with motorcycles in particular, there is lots of lost knowledge about how people fifty years ago made their racing machines go so fast.

its because the fastest tuners didnt tell anybody what they did, in order to stay competitive. and their knowledge died with them.

Forums

Float Levels For Concentric Carbs

Obijohnkenobe Well-Known Member

Eldon Crème de la Crème

Obijohnkenobe Well-Known Member

Iceman Crème de la Crème

Obijohnkenobe Well-Known Member

speedrattle Senior Member

Obijohnkenobe Well-Known Member

speedrattle Senior Member

Obijohnkenobe Well-Known Member

Pegscraper Elite Member

speedrattle Senior Member

Speed Four Brake Discs - Fully Floating Or Worn Out?

Ebc Floating Rotors For Water Cooled T120

Float Bowl Drain Cap Screw

Worrying Levels Of Support By Triumph

The Ditch Pump Taken To New Levels

Share This Page

Useful Searches

Float Levels For Concentric Carbs

Obijohnkenobe Well-Known Member

Eldon Crème de la Crème

Obijohnkenobe Well-Known Member

Iceman Crème de la Crème

Obijohnkenobe Well-Known Member

speedrattle Senior Member

Obijohnkenobe Well-Known Member

speedrattle Senior Member

Obijohnkenobe Well-Known Member

Pegscraper Elite Member

speedrattle Senior Member

Speed Four Brake Discs - Fully Floating Or Worn Out?

Ebc Floating Rotors For Water Cooled T120

Float Bowl Drain Cap Screw

Worrying Levels Of Support By Triumph

The Ditch Pump Taken To New Levels

Share This Page