British-Cars.org

Welcome to the DMR Site for British Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Carburettor woes

I have followed 'instructions' carefully.

Initially the mix was too rich even with the adjuster nut screwed fully up. Inspection showed that the lids were showing the correct number. However when the shank of the fork was horizontal the closed valve was around 4mm from the lever. Hence the flooding. The float was sound. The float chamber is at right angles to the carb body, and has been removed and cleaned.

Having acquired new lids, the inlet valve just touched the shank of the fork with a 10mm drill under the forks.

I have now reassembled the carb, and set the jet at 0.070in below the body. However the fuel level is no where near at this level. In fact if I pull the choke to its fullest extent the fuel level is still not seen.

Have I got something wrong, or do I have to bend the forks to allow the fuel level in the bowl to rise by around 10-15mm?

Your sage advice and expereince is sought.

IanB
Ian Bowers

Does the car start? Is there actually fuel in the bowls?
Gene Gillam

ian, i do not understand "set the jet at 0.070in below the body." i may be stating something of which you are aleasdy aware.. the jet movement just changes the relationship between it and the needle. fuel level in the body is determined by float level which sets fuel level hence fuel height in relation to the bridge. regards, tom
tom peterson

Ian,

The .070 value is a good starting place for the initial mixture adjustment. The range for the fuel in the bridge is somewhere between .120 and .200 down from the bridge, with .160 being average. You can lower the jet with the choke arm to where you see the fuel level off and measure the depth of the jet, and hence the fuel level.

I just set up a TD where I was unable to get the fuel height closer than about .220 below the bridge, and the mixture ended up being closer to the .070 value (richer) than the .060 to .063 that I might have expected with a higher fuel level.

I always bend the forks to get the fuel higher. The only thing I can surmise is that floats are floating higher, resulting in a lower fuel level, or the needls and seats for the fuel inlet in the bowl lids are longer. If you bend the fork, do so at the end of the straight portion, not mid way as putting a curve in the portion that touches the inlet needle is not very effective. Don't be surprised if you have to bend them a lot.

warmly,
dave
Dave Braun

Dave , the buoyancy of the floats brings up an interesting question, does premium fuel have a different buoyancy than regular fuel ? Is it enough to change float height and hence the fuel height at the bridge ? Which would then affect mixture.
Sorry for hijacking the thread Ian
Cheers, Keith
Keith McKenzie

Thanks for the suggestions, Dave.

Your paper, which is the first one I have seen approach the subject correctly, chimes with comments made about the MGB when I worked at Cowley in the 1960's.

They were saying that a correct carburettor set up took account of the needle being set at the shoulder of the piston and the needle designed to work with a fixed and defined starting gap between the needle shoulder and the jet orifice. On this basis the variable to be adjusted was the fuel level, not the jet nut. However the standard adjustment technique still is to keep the fuel level constant and vary the gap between jet orifice by turning the nut. As a result the needle profile would, in many cases, not start where it should. As a result the mix in the carbs would be away from ideal at all positions.

Your approach, of fixing the needle position at the shoulder of the piston (which is the standard technique), and then fixing the position of the jet orifice at 0.070in below the opening into the carb recognises the absolute relationship between the position of the two. Adjusting the forks so that the fuel level is at the jet orifice is a far better process than moving the jet up or down with the nut.

I note your comments about the amount of movement in the forks to get the fuel level correct, and will go back and try harder!

Many thanks

IanB
Ian Bowers

Ian,

Changes in engine breathing, and the difficulty of getting the fuel exactly at the correct depth in the bridge still requires some adjusting of the jet height. Note that with the correct .160 fuel depth below the bridge you will find the jet adjustment to be closer to .063 below the bridge. I start with .070 because it is easier to adjust the nut up against the jet than down.

I saw a paper by Skip Burns where he suggested that proper depth of fuel was 3/8 of an inch (.375) below the bridge. That had to have been a typo, as I doubt any one could see it down there!

Keith,
Fuel is supposed to have a specific gravity of about .745, and it does vary with temperature. Ethanol lowers the s.g. about .01. Since boyancy of the float is based on the weight of the float and the amount of fuel needed to be displaced (by weight) to support the float, a higher s.g. would require less displacement to support the float. i.e. a float would float higher in water (s.g. of 1.00) than in gasoline, and would float higher in gas without ethanal than gas with ethanol. The higher the float in the fuel, the lower the fuel in the bridge as the float rises and shuts of the inlet. There isn't much difference between premium and regular grades. So the short answer is that our fuel in the bridge is lower in the morning with cool temperaturs, and higher after the heat goes up. But we are talkinga bout 1% increase for every 20 degree F rise. So in an 1.5 tall cylinder of fuel, call it .015 rise in fuel level every 20 degrees.

warmly,
dave
Dave Braun

This thread was discussed between 07/09/2011 and 08/09/2011

MG TD TF 1500 index

This thread is from the archives. Join this live forum now