I am begining to hate su carbs!

F

fett

Member
so after buying a set of refurbed HIF 6 carbs which wernt cheap, paying a clown to fit them and tune them ,and then another garage(2) to tune them they were fine for 50 miles, they then went off balance a bit and needed the choke out a just a little to stop low idle-stalling when very hot.

then one float stuck so I ended up taking it off my self, sending it back to be fixed under geuarnetee and then re-fitting it my self. when went well and I was back to it running well and just needing a little choke when hot (I know obviously that that isnt right lol)

So garage(2) said ok we will have it back for another tune up foc as it still wasnt right, I saw an oppertunity to learn a bit here with no concequences and try to tune it my self as I have two times colourtunes and a balencer but am not of the carb generation.

so ran it up to temp on a 10 mile trip so the oil temp was right up and all how it should be , found one carb was pulling 17 and the other 29 , so rebalnced them after being very very thouroughly cooked with under bonnet heat in the garage , boy do those stianless headers kick out some heat. anyway did it properly , link bar off , revving it between each adjustment and letting it settle etc, adjusting the link bar before clipping it back and I would say they were darn perfect!

switched off to set up the colourtunes whioch I did with a few minor burns lol , ran it up with them in and it didnt sound right, plus the colour looked lean but the plug I took out looked very slightly rich ?????? I dont like the colourtunes now :p

anyway I thought it may have been rought because they were in so I didnt make any adjustments , just put the real plugs back in and it still ran crappy, few back fires after a rev and wont idle without choke again.

so can anyone tell me why after getting it spot on , just turning it off and swapping plugs in and out and then turning it back on again would make it run like crap?

I am getting very fed up here! :shock:
 
it might be but the bonnet was open during the whole process and cooking me in the garage instead lol

I ran it for about 5 mins and gave it a few good revs but the idle was still bad :|
 
What sort of condition are your plugs in , are they the correct plugs , how old are they ? Might be worth a new set , if one looked rich was it firing ? Only a thought have had plug trouble recently .
stina
 
Have you possibly mixed up a couple of plug leads ?
Introduced an air leak (are the carbs bolted down properly, something trapped ?)

Backfiring (out of the exhaust) is usually unburnt fuel burning in the exhaust, either over-rich mixture, failure to spark, or really retarded ignition timing.

If it's popping back into the carbs, this is more likely a weak mixture or air leak.
 
The garage should know what the problem is TBH if they are familiar, I'd whip it back to them and they might spot the fault, although you may have given them a further challenge :)
 
plugs are about 100 miles old , the short reach ones correct for th engine , bpr5es from memory os soemthing like that . ngks.

there is now missfire and leads are correctly fitted and new magnecores. the igntions side is perfect. the carbs are bolted back down nicely and the back fire is only a little thud in the manifold no huge bangs. with that exhaust on it has always done it a bit after a hard rev but I wasnt reving it very hard. more worryingly its the idle that concerans me , do go from perfect right back to square one so quick. very odd.

I need to go aon a training course for these carbs cos I really want to know how to do it right!

it will go back to the garage, although I am up to 6 cars atm so it will have to wait its turn lol. I am going to start thinning some out next week!
 
I'd recheck the link rods between the carbs.

The HIF has a rod for the choke and a rod for the throttle going between the carbs. When you pull the choke it should open the throttle a touch before it actually increases the fuel supply.

When you set the rods up make sure that they are tightened up properly, if they arent then you will get the throtles opening out of sync.

Make sure that the choke on each carb goes off fully when you have the lever in.

There is a couple of good guides from SU (burlen fuel) on how to set the carbs up. They also have a book (how to tune su carbs ??) I think once you have read them a few times and it makes sense in your head you will be fine.

Not sure if it is true but the colourtune gives misleading results thanks to modern fuel addatives or so I have been told.

You might be better sticking with it yourself rather than let a garage who didn't do it right first time make a hash of it again.

Colin
 
also when setting the choke cable and before you tighten the cable at the carby (with chokes off) have the choke knob inside the car pulled out a bit (about 1/4") then tighten at the carby so when you push the choke in you know it is home (and chokes are fully off) by the gap you can see as opposed to the knob butting up (with no gap) and chokes remaining slightly on, (a tip my old man taught me for all SU carbed cars)
regards,
Scott
 
I'm unclear why you adjusted the rod between the carbs after doing the balancing? The balancing really only affects the situation at idle and just beyond.

The first thing to do is to set the linkage so that both carbs reach full throttle together - this obviously with the engine off and looking down the intakes. Check that the choke mechanism is definitely allowing the whole action to come off the carbs or disconnect altogether. Then have another look down the carb throats with the throttle closed. Have the throttle discs truly shut? The discs can be centered in the opening by releasing them from the throttle shaft and refixing, but don't do anything unless there is an obvious error. Next up, take the pistons and dashpots off the top of the carbs (three screws each). Make sure that the mating faces of piston and cylinder are both spotlessly clean. Check that the springs are of equal lenrth. Then pop the pistons back into the cylinders with out the springs and see if they both have the same "fall time". ie the take the same amout of time to fall out of the cylinders (dashpots). This without the littlke dampers or any oil in the top. If they are not the same time then swap the pistons and see if they are any closer. Have a further cleaning efort at the one that is the slowest and reassemble with the springs and dashpots when you have the best match. Then check that the needle valves in the fuel reervoirs shut off at the same level of fuel in each carb. Do this by blowing down the fuel line whilst operating the float. (yes, I know, that was easily said and not so easily arranged, so if you are reasonably confident of the floats and needles go on with the rest of the set up and only try this if the carbs are really difficult to set)

That's the fundamental setting of the carbs done! Now on to set the idle mixture and balance.

This procedure really only affects things at idle and at very low throttle openings. Insert the colourtunes so that one is in a group of four cylinders fed by each carb. First set the balance as you've already done before. Then adjust the mixture in each carb in turn. Have the revs sufficiently high that the engine doesn't stall as you adjust the mixture. Make some prety wild adjustments at first so that you get to see what really rich and really weak misture looks like in the colourtune. Then try and find the exact changeover point between orange and blue. You won't be able to get it spot on, so set it just on the rich side. Now go back to the balance, which will have changed. Reset that and go back to the mixture again. Keep going round this sequence until nothing changes with each circuit. There, job jobbed!

If you now feel confident and want to show off, you could now set the choke mechanism so that the throttle starts to operate on both carbs simultaneously and the mixture enrichment starts to operate on both carbs simultaneously.

All of these settings will change if you alter the ignition timing. So if you are feeling really competent now, try driving the car down a known stretch of road and just shading the timing each trip and choosing the setting that gives you the best performance, measured with a stopwatch or mph up a hill, anything that measures how much power the engine is producing in a way that allows you to compare one setting with the next.

After this go back and reset nbalance and mixture. Then repaeat the ignition timing test, etc etc.

You have now completed a rolling road tune. But you didn't have to rent all that equipment and the knowse of the guy on the machine. It won't be quite as accurate, but pretty close. And you won't have been able to alter the needles to suit a changed engine configuration, such as large exhaust manifolds.

Chris
 
thanks , yeah the first bit had been done by andrew turner and the second but was exacly what I did and it was perfect.

turned it off and then back on and it runs like crap-well not crap but how it did before I started.

btw I adjusted the link rod so as to not move one throttle off the stop slightly but hooking the rod back up. it was only one turn of the cup though so about 3/4 of a mm at a guess
 
I think you might be missing the point slightly - the link rod is the setting that allows both throttles to open fully at the same time - if you set it so that they both pick up together when opening then they wont reach full throttle together.

Chris
 
chrisyork said:
I think you might be missing the point slightly - the link rod is the setting that allows both throttles to open fully at the same time - if you set it so that they both pick up together when opening then they wont reach full throttle together.

Chris
Click to expand...

Reading what you say makes it sound that you would accept that the carbs would open from idle at different points, and if that were the case, then you would be sacrificing having them opening together all through the rev range that you are using almost all of the time, in order that they are both reaching full throttle, which you don't use nearly as much, at the same time?
If they are set up and balanced correctly, then they will open from idle at the same time, and reach full throttle at the same time as well.
 
Sorry to contradict, Harvey, but this is one where I am very certain. Ideally you would be right, and it is is also possible to set the inter carb link to achieve a compromise between full throttle being achieved together and throttle opening being achieved together.

In the real world, though, you need to start by setting for full throttle achieved together - otherwise the engine will be out of balance at all throttle openings except idle. If there is a huge mismatch once idle balance is done then I might be tempted to alter the link rod slightly, but I'd probably be more tempted to find out why.

The key thing is that the balancing procedure only applies at the engine conditions at which it is done - usually idle. The principle of the SU is that balance through the rest of the range is determined by the matched actions of the throttles plus the identical reactions from both pistons / needles. The idle balance is really only there in the procedure to cope with small arrors at very low gas flows caused by minor differences in the air leakage past a closed butterfly. To truly set an SU you will be changing piston springs and needles. The setting up procedure really only copes with a pair of carbs that already have needles and dashpot reaction that is suited to the engine condition and fuel type.

Chris
 
I know when I rebuilt my carbs in 2007 I made a point of ensuring that the butterflies both opened at exactly the same time. From there they track each other perfectly both being fully open at full throttle. I checked the suction at idle along with at 2000rpm and they remained in sync.

Ron.
 
That's how it's supposed to be Ron! But very few cars escape some sort of very minor issue with the butterflies or oivots, so setting the linkage at idle can lock that error in all the way up the rev and throttle range. Hence my preference to set full throttle first and then investigate any error that emerges at low throttle.

Chris
 
interesting debate guys :p

I only moved mine one turn so I think its probably ok, it could have moved more then that when it was off lol

as soon as I have moved another car on and broken another for parts I will send this one back to the garage for him to try again, if it works he will have saved me being baked alive in the garage again trying to reach the screws and see colourtunes in the dark, if not I will be back to that and back on here crying for an expert to visite :mrgreen:
 
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