Atlas Fuel Flow Requirements

jms · 2026-09-24T08:00:52-0600

Hi All
So I am aware of past posts concerning Atlas fuel petcocks and associated flow and fuel requirements but this is what I don't entirely get. I may have gotten this wrong but If an Atlas with two carbs, whether it be a pair of Monoblocks or Concentrics, requires 300 ML per carb per minute at full throttle how in the world did Atlas's ever perform with one 1/4" British Pipe Thread tap and a 1/4" hose going to the first carb and then that carb piggy backing on the the second? At best the Id of the petcock and hose would be 3/16". If the piggy back scenario was done away with and the main hose split and individually fed each carb doesn't that mean each carb is receiving half of the petcocks potential? Still far short of the 300ML per carb goal? Or was that meant to be 300 ML total?

jms · 2026-09-24T09:03:55-0600

jms said:
Hi All
So I am aware of past posts concerning Atlas fuel petcocks and associated flow and fuel requirements but this is what I don't entirely get. I may have gotten this wrong but If an Atlas with two carbs, whether it be a pair of Monoblocks or Concentrics, requires 300 ML per carb per minute at full throttle how in the world did Atlas's ever perform with one 1/4" British Pipe Thread tap and a 1/4" hose going to the first carb and then that carb piggy backing on the the second? At best the Id of the petcock and hose would be 3/16". If the piggy back scenario was done away with and the main hose split and individually fed each carb doesn't that mean each carb is receiving half of the petcocks potential? Still far short of the 300ML per carb goal? Or was that meant to be 300 ML total?

Additionally if the main jet is specified to be 220, my understanding is that means 220 ml/ min, how would you ever achieve 300 ml/min??

hybridracer · 2026-09-24T09:20:24-0600

Why worry about it, it works doesn’t it?

jms · 2026-09-24T09:39:07-0600

Well I’m not entirely sure yet as I haven’t had a chance to run the bike yet

hybridracer · 2026-09-24T10:06:38-0600

My featherbed/combat 750 racer worked fine with only one tap, run mostly at full throttle.

marinatlas · 2026-09-24T10:42:53-0600

PETROL TAP, 1/4GAS FOR METHANOL

A 1/4 Gas petrol tap specifically for use with methanol. These have a larger hole than standard taps to allow a faster flow of fuel which is required when using methanol.

It is best used with a copper/rubber petrol tap washer, which as well as giving an excellent seal allows you to position the tap in the best position for ease of operation (see "Related Items" below for details).

This petrol tap is sols by Hitchcock in uk Enfield distributor , or try Pingel but threads are NPT not BSP so need an adaptator or a tap /die .....

Thread 'Flow Measurements on Ewarts Fuel Tap'

May 31, 2016

A thread on the Commando Forum piqued my interest to make some flow measurements on an Ewarts fuel tap as used on the Atlas.

The tap is pull for Main, and twist 90 deg. for Reserve.

I measured the fuel delivery to the float chamber of the Monobloc carburetor, which included flow resistance of the tap itself, plus the nipple (fitting that screws to outlet of fuel tap), 4.5 inches of 1/4 inch inside dia. flex tubing, banjo, filter screen, and finally needle orifice. This was done by removing the float chamber cover, allowing the float to rest on the bottom of the chamber, then...

Triton Thrasher · 2026-09-24T11:10:57-0600

300 ml/minute is enough gasoline for 49 bhp, although not with a huge margin.

That’s the entire fuel flow.

Each carb has to have at least half of that available to it and each float valve must be capable of flowing 150 ml/minute.

jms · 2026-09-24T17:01:15-0600

Oh ok that makes a huge diffetence compared to 600 ml. I did get it wrong

jms · 2026-09-24T17:02:30-0600

jms said:
Oh ok that makes a huge diffetence compared to 600 ml. I did get it wrong

Thanks

Atlas Fuel Flow Requirements

Similar threads