SU Hif4 Carburettor removal, refit setup and balance

Removing hif4 Carburettors

Having rebuilt the two spare carbs it was then time to swap them for the originals. Before removing the old ones I took several pics of the hoses to ensure that I can refit them correctly.

Front cross hose, position of linkage clamps

Throttle cable and choke cable 

Fuel in to front carb

Vent hose from rear carb

Choke and throttle cable routing

cable connections

(The following is how I removed the carbs for a full strip down. However if you do not need to disturb the choke or throttle, its easier to tie the carbs together with string, compressing the spindle and cross carb fuel pipe. This holds the carbs together as a pair and saves disturbing the spindle and its settings. The carbs can be removed, worked on, and eventually refitted  as a pair.)

To remove the carbs I first compressed the clamps and then slipped them along the link hose before easing the hose off the carb cross connections.

 ... and disconnecting the vent hoses

 ... and fuel inlet hose- raise it to prevent fuel spillage.

 Remove the carbs by unscrewing the 4 nuts on the mounting studs. The lower stud on the front carb is both blind and blocked and a bit of a pig to get off. The tool of choice seems to be a 1/2" AF stubby spanner. Disconnect the choke and throttle return springs, slacken the throttle cable.

 Ease the carbs forward as a pair separating the linkages. The old gaskets are pretty tatty and were removed.

 ... and replaced with new ones. Before fitting the carbs, its necessary to set the jets so I screwed the jet adjustment screw fully anticlockwise and then screwed it back in until I could feel the jet start to move with an inserted finger (!). The adjuster should then be screwed a full further two turns from this point to provide enough fuel to start the motor.

New gaskets!

I slipped the carbs on, engaging the linkages as I did so. Devote at least 30 mins to doing up the hidden nut lower front carb! This is the work of the Devil to get at. I found that
1. Clean the stud and nut threads thoroughly before you try to fit the nut as doing as much up with finger pressure is ideal. If threads are clean then this should go almost all the way with finger pressure alone.
2. Use a magnet pick-up tool to feed washers and nut down to the studs one at a time and press the carb back or use a screwdriver lever to move them onto the stud. Fingers (as above) should then seat the nut.
Loosen the throttle actuating levers on their common shaft so that each carb can be adjusted independently. Screw up the throttle stops on each carb until they are free of the stop pads, then tighten then again until they just make contact. Finally screw each two full turns down to provide enough throttle (air) to start the motor.

 carbs installed.

Well there was good news and bad: The good was that motor started pretty easily... sadly the bad news was that the motor ran very lumpily and unevenly and  no amount of adjustment made any difference! I discovered that the front carb was simply not working. Opening the rear throttle led to a nice increase in revs and associated motor sound, opening the front throttle saw the piston rise and an obvious sound of increased to air intake- but no response from the motor- if anything it faltered and threatened to stall. Since fuel is clearly reaching this carb (it has to pass through it to reach the rear carb which is working) it appears that there must be some internal blockage in the fuel channels. Sadly I did not have enough time to remove and strip the carb again to sort this out since "Dads taxi" duties were rapidly approaching. I had no choice but to remove these carbs and refit the originals since I need to be mobile in fairly short order. However, before refitting them I now had the opportunity to examine their cold start systems which had been implicated in their problems.

Cold start mechanism, origin front carb

 I removed the column nut, tab washer, upper plate and spring. The cup washer beneath lifted off to reveal a brittle and broken shaft seal

 The broken part of the seal was found in the lower plate. I removed the two screws...

Broken seal fragment found, centre above lower plate.

... and lifted the lower plate off. In this carb the outer shaft on the cold start mechanism⁷s was integral with the lower plate and unlike the other carb, was not removeable. The O ring seal was also brittle and worn

I replaced the O ring, carb gasket and shaft seal before rebuilding the shaft. Examination of the second carb revealed a similar situation although the seal had not actually broken. I renewed these parts too, before refitting the  carbs as above.

I was then  able to start the motor- and immediately it was a much smoother tone. I equalised vacuum using an airflow meter and then installed Colortunes in cylinders 1 and 4. I adjusted the mixture until both burned Bunsen blue- although this is very difficult to see unless you have the car in a darkened garage! I found that the recommended method of raising the piston either with a screwdriver or the piston lifting pin simply did not work. The theory of seeking a temporary rise in rpm followed by a drop is clear, but it simply didn't happen! I found that raising the piston always caused the motor to falter regardless of the position of the jet adjustment screw*. This meant I was pretty much forced to rely on the Colortunes.
* Note added later- this problem with the traditional method was probably caused by poor timing adjustment. I later checked timing and found the car was set at 30deg BDTC at 1000 rpm. This large advance was probably needed to burn the very (very) rich mixture coming in from the carbs with leaky choke mechanisms. I'm guessing it needed longer to burn. I reset it to 10deg BTDC at 1000 rpm. This gave 32deg advance at 4000 rpm but revving the motor this high required a lot of nerve and I'm not guaranteeing that this is correct. However the cent advance does clearly work. In any event timing should be set better now and although it drives much more smoothly  and pulls better, its still not a smooth idle.
** note added later... Idle was eventually sorted by swapping for new carb butterflies without poppet valves see below

I then rechecked air flow and re-balanced with the individual throttles. When these seemed optimal ("even" motor rhythm and blue colouration) I was able to re-tighten the throttle levers back down onto their common shaft. This has to have a certain amount of free play between the lever actuating pin and the fork on the throttle plate into which they fit. The method is to insert a 12 thou feeler gauge between the throttle shaft stop lever and the cold start mechanism lever. Holding the  feeler in place, each throttle lever is then pressed gently onto its stop and held there whilst the bracket screw is tightened with a 2BA socket. When both were tightened the feeler can be removed. This will raise the throttle pins away from the lower fork of the throttle plate as it drops onto its own stop on the cold start lever. Finally the throttle cable was readjusted to take up slack but making sure that this free play was preserved. IE the idle speed is set by the carb throttle stops and not by the cable holding the throttle partly open.

I then removed the Colortunes in favour of the plugs and  refitted the air filters before road testing the car. Fortunately the motor ran much more smoothly and although it did seem to have a bit of a flat spot, it was running better then before, and most significantly the rough and choke-y idling had been cured completely! It still seemed that idle speed came down somewhat slowly after a throttle blip but whilst driving this wasn't a problem and. The spontaneous elevation in idle speed is still present but see on for fix!

I should add that examination of the plugs 24hrs after this setup showed that cylinders 3 and 4 were still running rich. I repeated the colortune setup  and discovered that the mixture on this carb could be adjusted considerably weaker with the beneficial effect of  improving the idle rythym.

I will examine the two rebuilt carbs and see if I can identify the blockage** in the forward carb; but if my original pair are now running in an acceptable manner then I may not refit them immediately.


**Stripped the offending rebuilt carb. The problem was quite simple in that the float had been pushed upwards during reassembly and had jammed in the closed position. All passages and the jet were clear. Consequently  fuel was simply not entering the float chamber and thus not exiting thru the jet. I freed it up and checked that it was opening and closing by blowing thru the inlet (finger over outlet to carb 2) whilst inverting and righting the carb. It seemed to be working fine now.

UPDATE- well I still haven't fitted these recon carbs. The reasons being that fitting new seals to the choke circuit of the old carbs was a cheap and easy fix for the horrible lumpy running and allowed me to set the timing back to normal- it was well out having been adjusted to burn Heaven knows what sort of mixture. However, this still left the spontaneous increase in idle speed that I experienced, idle set at 900 rpm would start to rise when the motor got hot and stick at 15- 2000 before falling back to 900. This seems to be cause by leaking poppet valves- I fitted new solid butterflies and carb behaviour is greatly improved, setup was more consistent and idle speed now steady!