carb needles and jets

[davepoth]

I got my car back properly, and it works a lot better. Compression check showed an even 145, which I guess isn't bad for the mileage (96K). The only problem now is that the front carb needs a new needle and jet, which I'm told is why it runs really badly below 2000, and a bit rough on light throttle above that. I guess the sensible thing would be to get a set for both carbs, but what should I get? Rimmers' suggest BBT for my setup (twin K&Ns) but I'd like to make sure before I buy anything.<br>
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Any ideas?<br>


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[MWU773V]

I have the similar set up to you (twin K+N's). The BBT needles allow for a richer mixture to compensate for the extra air the filters allow through. I suggest you take thier advice as I did and use the BBT needles. Just be sure to check your carb balance as you'd be amazed how that can affect the running.<br>
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Mark

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[davepoth]

They're knackered. Totally. I managed to get the idle screw completely out of the rear carb and it still wouldn't balance.<br>
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Just out of interest, how big are your filters? The ones on my car seem to be an inch taller than the standard rimmer items. I'd keep them except a PO decided to clean them in what looks to be used 20/50.

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[MWU773V]

The filters must be about 6 inches diameter they're quite compact, got them from Rimmer's actually! Make a nice noise. Must have played for hours tuning the carbs and it's still not right. Balance is good now I've invested in a cabalancer instead of waving a bit of tube around but mixture seems to still be rich whichever needles I have.That said even a new set of standard needles will help as they wear out eventually. If you don't know how long they've been in best to change regardless.<br>
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Best thing i've found to "polish" my K+N's when I get fingerprints all over them is just a regular car polish like autoglym, work's a treat. If greasy though you might have to use the kitchen sink and swill em off and make a nice mess.

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[davepoth]

It's worse than that. They are soaked in old engine oil, and utterly black. I guess that might be contributing, but it still runs bad with them off.

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[BND712V]

I balanced my carbs by removing the dashpot and piston and using a scrap of card to get the jet recessed byt he same amount on each carb. Then it was jus a matter of winding it (mixture) dp / down an even amount on each carb until the revs were highest (optimum mixture) before pulling the idle speed down to what it should be. Better job of it than the garage did, and runs very smoothly now. Didn't take too long either!<br>
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If the needles / jets are shot, you should find worn rings on the needles. Not sure how you tell if the jets are shot though - imagine if one is worn, so is the other.<br>
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Tim

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[BDN712V]

mis-typed ID. Still worked though!<br>
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Tim

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[MaddMart]

I thought that on the early Sprints that the needles were BBTs anyway & the later models had BDQs IIRC. <!--EZCODE EMOTICON START >D --><img src=http://www.ezboard.com/intl/aenglish/im ... s/grin.gif ALT=" >D"><!--EZCODE EMOTICON END-->

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[Jon Tilson]

to tune SU's is to do them individually...<br>
Otherwise you have no way of knowing if one carb is doing all the work...<br>
Ive written extensive bits on pevious threads and at the risk of boring everybody and afflicting you all with my typing and spelling skills here it is again...<br>
Step0...prerequistes...engine at running temp and ignition all pucka..else your are wasting your time really.<br>
Decent fuel...not some old muck thats been in the tank years either. Breather hoses all sound.<br>
Step 1..remove air filter outer bit. BTW I consider K+N's a complete waste of space. They suck in hot air at idle from above the heater pipes and mean you loose the rigidity that the std air box gives to the rubber mounted carbs and throttle linkage. Have a good look. You'll see what I mean. Put them on ebay there are plenty of mugs to buy them. Fit the pipercross element to the std box if you must. There's nothing wrong with the std filter provided its changed every 12k miles. <br>
Step 1...loosen throttle joinng spindle clamp. This is nearest the inlet m'fold..the outer one does the choke.<br>
Step2...Make sure jets are fully up and choke in.. Maladjusted or worn choke can sometimes mean jets dont return home. Push up underneath with mk1 index digit. If you feel it move it needs sorting otherwise you're wasting your time.<br>
Step 3. Make 1 carb do all the work. Lots of hiss from it and none from its mate. Do this with the throttle screws.<br>
You should be able to get a steady idle of 1000 rpm. If you cant there's something wrong. If new jets and needles then start with the adjusting nut fully home and then wind it out(down- anticlockwise)) 12 flats. If it still wont run you have an air leak somewhere.<br>
Step 4. Raise piston a minuscule ammount - 1/32 inch. If the car fluffs and dies its too weak...if it speeds up its too rich. The sweet spot is about 3 flats wide. Raise the speed to 2-3k during adjustment back to idle a few times to see if it picks up and settles consistently. You are looking for a momentary increase of about 150-100 rpm and then a return as you hold the piston up a VERY SMALL AMMOUNT. Its barely a touch, This is where most SU tuners go wrong IME, raising the piston too much.<br>
Step 5. Repeat step 4 for other carb and note differences in behaviour. If there are any then investigate...worn spindles, jets, air leaks etc.<br>
Step 6 Balance throttles. Do this with Mk1 earhole and small bore tube. Its easy with practice...really. At the idle speed that suits you...800 - 1000 rpm both carbs should be hissing the same. Keep any dangling bits like chains and ties well away from rotating bits at this stage.<br>
U dont want your car to eat you.<br>
Step 6...tighten clamps. Make sure both carb pistons raise the same ammount as the throttle is pulled.<br>
Step 7...fast idle setup....adjust screws on fast idle cam part of choke linkage to give 12-1500 rpm before advancing the linkage moves the jets down. A bit fiddly but a small spanner underneath will do the job. Again ensure jtes return home when choke is in. If not fix it...<br>
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Step 8...refit air filter and go for a blast revelling in your extra 20 bhp...<br>
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Step 9...Well earned cup of tea.<br>
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Maybe this combined with other SU'spert tips should be a Dolly Mixture article? Many an SU has been binned for a webber because of incompetent tuning I reckon. On a std road car I know lots disagree but a well tuned SU is a good compromise of power and economy. Get them set right and they stay in tune for ages too....I hardly ever touch mine now.<br>
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Jonners<br>
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