11 replies to this topic

[Neildanks]

Hi ive got a hif 44 1 3/4 " carb . Just got the engine running but when trying to adjust the air mixture below the petrol pipe inlet
it doesnt make any differance

The spark plugs are really sooty and after cleaning become like this after a couple of miutes of running

The choke is fully off and also checked the choke cam it as returned to its home position with the accelorator cam having about 0.5mm clearance on its fast idle bolt.

i thought it was screw in to richen mixture and out to lean it but when doing this it doesnt make any differance


its a 1275cc A+ with electric dizzy it As a BDL needle which i was advised to fit for a good starting point

Any ideas

[jelboy89]

i have just been fiddiling with my carb, what i did was take the dashpot off so i can see the bridge in the carb, then adjust jet to be level with the bridge

[Neildanks]

what do you meem by bridge and do you meen you took your pot that holds the needle out as well then.

[minithirty30]

Hi some people may disagree but I used colourtune the other day to check the mixture because my plugs ike yours were black. I think colourtune is great because you can see into the cylinder and what colour its combusting at yellow=rich, white=lean they say blue is about right. The kit is quite cheap too :)
james.

[Neildanks]

yes i know i had one till sunday and somehow i melted it on the plug

my problem when im addusting the screw it doesnt seem to be making a differance

[ACDodd]

The usual problem is the jet gets stuck due too corrosion if the carb has been left dry for a while. The jet needs to be freed off.
Somtimes the jet can be free'd off by gently pushing on the jet tube with a screwdriver. Be careful not to damage it. Turn the mixture screw up and down to see if the jet then moves.
But you may have to delve deeper. The best way to doo this is get a service kit and clean out the float bowl, change the jet, re set the float level fit a new seal and refit the car. If you do have to goo this far consider a full overhaul.

AC

Edited by ACDodd, 22 February 2012 - 08:56 PM.

[Neildanks]

i thought the jet was the actual tapered screw with a spring behind it

[ACDodd]

The jet is the part the carb needle is pushed into by the piston.

AC

[Neildanks]

ok i just had a look at the manuel from what i can see i have to remove the float and tighten that nut then undo it 3 full turns does that seem right.

I bought the carb second hand so i think someone as rebuilt it and just tightened it upand not reset it. probably makes sense.
or have i talked myself into it

[jon_dec]

Just bought a second carb like yours recently, and I figured out that the previous owner messed around with it and when I tried to adjust the mixture screw, the jet assembly didn't move at all. I removed the carb from the car and disassembled the float chamber lid (the 4 screws in the bottom - beware, you may need a new gasket since the old one can tear apart once you remove it). It turned out that the previous guy didn't manage to assemble the jet properly and it was loose.

Just like other people said - find a nice rebuild manual (I think that Haynes Book of lies covers this part very well), order a rebuild kit and give it a try. SU carbs are not that complicated to rebuild, given that you take some care and attention. Otherwise, get (pay) someone else to do it.

[Neildanks]

I am a maintenance fitter so am used to taking things apart and repairing them but am i right in saying that the mixture jet is set 3 full turns back

[jon_dec]

There is not a exact formula for "how much turns to be ok" in a su carb, and definitively never heard about that "3 full turns back". There are approximations. The best thing to start, is to remove the dashpot and piston from the carb. Look to the whole where the needle fits (this is the jet), and turn the mixture adjusting screw until the jet is aligned with its holder. Then, tighten the screw 2 full turns (it will lower the jet). This is a rough starting point to adjust it.

If, when turning the adjustment screw, the jet doesn't move, then, you'll have to investigate in the float chamber, as I said before.

Edited by jon_dec, 23 February 2012 - 11:10 AM.