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CARB

Basically the only common things to go wrong with the SU carb are ;

Blockage of the floatbowl with fine rust particles from the tank

Wrong setting of the mixture adjuster at the bottom of the carb

Setting the gap on the choke lever before the engine is warmed fully up and tickover is set .
Faulty Float or the Needle in the float chamber faulty and not sealing off petrol inlet letting carb flood a little .

VERY worn throttle butterfly valve spindle leak .

Lack of oil in the dashpot

The previous owner might have been trying to adjust the mixture and set it wrong .

SYMPTOM ; CAR RUNS BUT ONLY WITH THE CHOKE ON

POSSIBLE CAUSE ; FUEL STARVATION

Requiring the choke to be constantly on is a classic symptom of fuel starvation .
When you pull the choke on it moves the main jet away from the needle.
Since the needle is tapered this allows more fuel to flow past and into the engine .

The following are some of the potential reasons for fuel starvation ;

{ CAUSE 1 }

The carb is adjusted too weak

{ SOLUTION 1 }

Adjust the carb

Remember to have a fire extinguisher handy when working with petrol  

Get the engine started and FULLY warmed up .

Stop engine .

Remove the air filter assembly

Make sure the choke cable is FULLY pushed in inside the car .
Slacken of the choke cable at the carb and remove the little choke clamping screw and pull choke cable from carb end only .
A wee tip for you ; Get a length of the rubber petrol pipe about 8 inches long and soften one end in hot water then push over the throttle adjusting screw

Start the engine and , using the end of the rubber hose , just screw the throttle adjuster screw in till you get a very FAST tickover .

Click on picture to enlarge

Mark one of the flats on the LARGE mixture adjusting nut at the bottom of the carb with a magic marker .

Screw the adjusting nut fully in by using your fingers { count the number of flats on the nut from where it is set now to the fully up position .
Screw back out about 18 to 20 flats or about 3 full turns out
This should be a starting position .

Adjust {You can often just use your fingers only} the LARGE mixture adjusting nut at the carb bottom IN to weaken and OUT to richen .

Looking at the pic of the carb from the angle above  ;

It is turn nut to the LEFT to RICHEN and to the RIGHT to WEAKEN

Looking at the the carb from the top downwards   ;

It is turn nut CLOCKWISE to RICHEN and ANTICLOCKWISE to WEAKEN

Adjust it SLOWLY a flat at a time and you will come to a point where the engine revs will not be clean and it will be running poorly .

YOU ARE LOOKING TO FIND THE POINT WHERE THE ENGINE RUNS FASTEST AND EVENLY .

Adjust back to marked flat and continue the other way till the same thing happens .
Adjust SLOWLY back and forth till the engine is at its fastest smooth running .

Readjust the tickover down to an acceptable level .

STOP engine .

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SET THE CHOKE GAP

SU CARB showing CHOKE SETUP & BREATHER TUBE

Click on picture to enlarge

Stick the choke cable back on carb and fit through clamping screw BUT do not tighten screw at this point.

Get feeler gauges and set for 40 thou .

Measure the gap between the screw underneath the choke arm and the lever and adjust so that it is a nice sliding fit { take care that the feeler gauges are 90 degrees to the arm or you will get a false reading }

Tighten up locking screw on choke cable .

Recheck gap .

Refit the air filter assembly .

Because you have refitted the air filter assembly you will have to fine tune the mixture just a little .

Restart engine { you should be able to start it without putting your feet anywhere near the throttle } and once again using the end of the rubber hose , just screw the throttle adjuster screw in till you get a very FAST tickover .

Try weakening the mixture by one flat and see if it runs slightly faster / smoother .

If it does , then leave it there .

If not then reset back one flat and try one flat the other way

It should not be far away from your original marker { probably ¼ to half a flat weaker required }

Get a helper to sit in the car and press the clutch pedal right down and you can now reset tickover so that with the clutch depressed it is still ticking over nicely .

Stop engine and Mark the flat on the bottom adjuster nut with a different colour marker and you will be able to fine tune or put it back to the same spot .

NOTE ; You will have to wait till the engine is stone cold to try out the choke .

All you should need to do is pull out the choke fully and turn the key without touching the throttle with your foot .

Remember to have a fire extinguisher handy when working with petrol  

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{ CAUSE 2 }

Dirt in the carb { usually fine rust particles from the petrol tank }

{ SOLUTION 2 }

Remove the carb and check for sediment in the bottom of the floatbowl

Remember to have a fire extinguisher handy when working with petrol  

I would be inclined to remove the carb and check for rusty deposits at the bottom of the floatbowl ,

Before you do this I would advise you to get a new carb floatbowl gasket because they sometimes tear when being removed .

Remove the air filter assembly

Remove the choke cable { Take care not to lose the wee choke cable clamp }

Remove the petrol hose and overflow hose { If fitted }

Remove the carb and work on it well away from the car

Do not be apprehensive about the carb clean

The only parts you need to touch are the floatbowl lid and the suction chamber .

When you do get the floatbowl lid and float off , just get a very thin screwdriver / needle and agitate the brown gunge in the bottom and empty out the bigger parts .
Fill the float bowl with fresh petrol about a third full , swirl around and empty out .
Repeat procedure until petrol runs clear .

Remove the suction chamber complete with piston and needle and you are left with this 

Click on picture to enlarge

Quarter fill the floatbowl with petrol then GENTLY blow through a small tube into the hole where the needle goes . This will flush any sediment back into the float bowl where you can repeat

Quote:

Fill the float bowl with fresh petrol about a third full , swirl around and empty out . Repeat procedure until petrol runs clear .

Refit the floatbowl assembly .

Refit the suction chamber complete with piston and needle and top up with oil .

Remember to have a fire extinguisher handy when working with petrol  

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{ CAUSE 3 }

Restricted flow from the wee plastic fuel feed pipe from the carb floatbowl to the mainjet

{ SOLUTION 3 }

Check the pipe for kinking / flattening ;
Check the end of the pipe where it disappears into the bottom of the floatbowl .

There is an ” O  “ ring seal in there which sometimes blocks the tube .

If it is not leaking at the brass nut at the bottom of the floatbowl then I would leave well enough alone BUT if you feel competent enough or the problem of not enough petrol to the carb is still there then the following might help .

CARB  ;   FLOATBOWL ; FEEDPIPE “ O “ RING RENEWAL 

Remember to have a fire extinguisher handy when working with petrol  

BEST TO REMOVE THE CARB so you can see inside the threaded hole in the bottom of the floatbowl .

{ I WOULD NOT RECOMMEND THE FOLLOWING  UNLESS YOU ARE COMPETENT AND HAVE A WEE BIT FINESSE / SAVVY ;

You can still inspect it without removing the carb by removing  the passenger side access panel and you will see the brass feed nut . Be aware that petrol will drain from the floatbowl when you do the following

ONLY DO ABOVE INSITU ON A STONE COLD ENGINE  }

Click on picture to enlarge

BEST TO REMOVE THE CARB

Gently grip the outside spring and pull back .

Slacken the nut and you will be able to gently pull it out .

Now this is the part where you need to take great care .

GENTLY pull the nylon pipe out of the hole keeping it inline and do not twist or crush the pipe as you will have to buy a very expensive whole unit .
As you can see from the picture there is the nut , a small washer to compress the " O " ring , and the " O " ring itself .
Often the " O " ring remains inside the hole and has to be dug out with a pin . Take care not to drop the " O " ring or the small washer .
Inspect the " O " ring for cuts or crushing and fit a new " O " ring .
Check inside the hole for little bits of cut " O " ring and remove as required .
Assembly is the reversal of extraction ; Gently grip the outside spring and pull back  , refit the nut , steel washer and new “ O “ ring  .

Just set the assembly up as in the last pic by keeping pressure on the nut so that the end of the tube goes into the hole first . This should ensure that the “ O “ ring stays on the tube  .

Again take extreme care not to twist or crush the nylon pipe .

Tighten the nut but do not over tighten { You can Nip it up a little more if there is a slight leak }

 Remember to have a fire extinguisher handy when working with petrol  

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{ CAUSE 4 }

VERY BADLY WORN THROTTLE BUTTERFLY VALVE SPINDLE

{ SOLUTION 4 }

Replace or refurbish carb

Remember to have a fire extinguisher handy when working with petrol  

Originally posted by chgperformance on the following post #12 ;

http://www.r3w.co.uk/forum/showthread.php?t=21208&page=2

Had a 93 robin LX in the other day. Woldnt run right in the mornings. Weve known the guy with this reliant for a long time so decided to have the carb off for a jolly good clean up. Upon inspection, the main jet was very was as was the needle. THey were changed for new items. The robin would then refuse to tickover without choke. I then used carb cleaner and sprayed around the carb mountings etc to find there was an air leak. THe carb gaskets were replaced where it meets the heat exchanger but still ran with the need for choke. I used car cleaner again but this time with a pipe on the end and aimed it directly at the throttle butterfly spindle. To my amazment it did affect it. I then cleaned and stripped a second hand SU, fitted the new main jet to it along with the needle and hey presto. Robin now runs properly again with some small tweeks here and there on mixture etc.

My conclusion being, the main jet/needle as it wore allowed the mixture to richen up to compensate for the worn spindle bush. When i changed the main jet/needle, it metered the fuel properly and thus leaned it out because of the air leak around the spindle bush.

Remember to have a fire extinguisher handy when working with petrol  

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{ CAUSE 5 }

VERY BADLY BLOCKED INLINE FUEL FILTER

{ SOLUTION 5 }

REMOVE THE STANDARD INLINE FUEL FILTER and

REPLACE WITH A LARGE CLEAR FILTER

Check the inline fuel filter located in the petrol pipe near the petrol tank .

This is usually found inside the clear plastic pipe at or near the part of the chassis which is at a 45 degree angle in front of the petrol tank and back axle

If still fitted , remove the inline bronze filter located in the petrol pipe line just in front of the petrol tank and fit a large clear plastic inline fuel filter or other suitable filter .

Click HERE for pics showing detailed location point of inline filter  


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{ CAUSE 6 }

Air leaks at gaskets

{ SOLUTION 6 }

Fit new Gaskets or tighten nuts up

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NOTE ;

I am not saying that the stuff I have written above is all that could be wrong ,

but it is a start and hopefully some of the info might be of benefit to you . 

ONCE AGAIN I WARN YOU TO TAKE CARE WHEN WORKING WITH PETROL ;

USE COMMON SENSE AND IF YOU FEEL IT IS BEYOND YOUR CAPABILITIES

THEN PLEASE SEEK COMPETENT HELP

Remember to have a fire extinguisher handy when working with petrol