carb tuning

[arcturus]

B20b with twin HS6 4branch and D cam.KN needles i think. will check when i find paper. Can't get the engine to run below 15 flats an needle adjustment and settled on 20 flats. Is this right or could I have a problem? Normal starting point is about 12 flats
OK, its a senior thing. Can't remember needle. will need to search for spec's later

[v8250]

arcturus, what condition are the SU's in? If a little aged it's worth fitting new springs under idle/off idle screws + check for air leaks [especially on spindles] as these can throw 'off' idle/off idle settings when adjusting base air balance across the carbs. it's also worth checking jet and needle position, and equal spring ratings...and correct oil level [read equal] in dashpots.

Plus, it's near Christmas...so you may ask Santa for a copy of the SU Tuning Bible https://www.amazon.co.uk/Build-Power.../dp/1901295141

[arcturus]

Carbs have recently been completely overhauled, new spindled etc. Funny thing is that the car has been running really well, lots of oomph. I took the car for it's annual inspection today and turned the jets down one flat, just to lower emissions slightly,. when I returned home I thought that I would just check things and discovered this issue. Engine runs great at 20 flats.Am I worrying unnecessarily?
BTW car passed with flying colors, just advisory for high emissions.

[v8250]

okay, if freshly rebuilt all should be good. I wouldn't worry too much if running a little rich providing the right needles are fitted. though the HS6's should still run at idle 600-800rpm with butterflies just slightly open as there's no load on engine [this one of the great things about SU's]. The only draw back with running rich is the risk of washdown of bores into the oil... :. have you a Gunson Fuel Colour Tune to go with your Gunson/other make air balancer? I find setting up carbs with air balancer first, then proper idle followed by off idle at 1500rpm, by which time the engine's nice and warm. Re-adjust ignition timing if necessary...and then use the Colour Tune as this gives a great visual indicator as to what's actually firing within each cylinder.

One thing that's very common with 4pots running twin SU's is that the rear two cylinders are often richer than the front two, especially when the engine's up to full operating temperature. Here, I sometimes slightly lean back the rear SU to get matched combustion colour. This then means all four cylinders are perfectly air/fuel balanced giving known equal combustion. The key here is that most manuals[Haynes and factory] only give basic set up process and miss the most important part, "is the engine running equally in all four cylinders". You may like to tinker more with the set up to ease off the rich[unspent hydrocarbons] fuel.

Lastly, are you running PCV into the SU's? If so this type of archaic system can play havoc with settings. Here, I always running open vented/to atmos' with SU's as it makes no sense re-circulating unspent/heavily oiled HC's back into the SU when what we really need is pure clean air/fuel ratio. And of course, if an engine is combusting the air/fuel intake mix correctly it will exhaust fewer HC's than if crankcase vent/PCV is connected. Sadly, PCV re-circ' is simply a bodge to theoretically overcome exhaust emission controls. Good luck with the setting up.

PS are the floats set up correctly, and/or, could one of the floats be leaking :. over opening 1no jet leading to slight overfuelling?

[Ron Kwas]

Arcturus;

D Cam is normally for the FI configuration, so running it with Carbs may require Idle mixture to be on the rich side to get it to run well the rest of operating range... (and this would be consistent with the high emissions finding...at idle?...or were emissions tested on a rolling road throughout entire range?)

Idle at 20 flats might also be explained by the Metering Needle being "low" in Dashpot...you might verify that shoulder of MN is be flush with bottom surface of Dashpot. Finally, assure Level in Fuelbowls is not low...in the end, proof of the pudding is in the eating, but if the exhaust smells "Rich" you can try bringing up the Jets one Flat at a time, drive it for a few days, and note the results, and adjust to further optimize...

Good Hunting!

[swedishandgerman]

Just use one of these air speed gauges:

https://www.amazon.co.uk/Anemometer-...ir+speed+gauge

You can do everything with them:

• Set mixture - adjust the mixture nut until you get the fastest air speed,
• Balance idle - adjust the idle screws so that both carbs have matching air speed,
• Fast running - adjust the linkages so that air speed through both air speeds match at just off idle, then re-check at different engine speeds,
• Choke - adjust choke cable and screw so that air speed through both carbs match.

Of course, the thing we're talking about in this thread is mixture. If too weak, it's not running efficiently, so not running as fast as it should so intake air speed is reduced. If too strong, the same. That's why the air speed is fastest when the mixture is right.

You can measure correct mixture throughout the rev range using the same anemometer: fastest air speed = correct mixture. If you find you have to re-adjust the mixture screw a lot throughout the rev range, then you know you either have worn out needles or the wrong needles.

The only thing you can't do with the anemometer is check the fuel bowl levels!

Agree entirely with the comments above with crank case breathing of which I am not a fan of oil fumes going back into the manifold. Good air and fuel is what you need!

Personally, I am not a fan of either Gunson kit mentioned above. Okay, they both achieve near enough. The Colortune is alright, but a lot of fiddling about to match up the same colour in front and back. The Carbalancer is a complete no-no for me: as you stick the contraption over the chuff, it restricts air flow, so gives an inaccurate reading. Oh, and the little ball thing in the sliders sticks!

I do not get adjusting mixture on number of flats or height of the piston from the jet. It's good enough to get going, but definitely need incremental adjustment with the engine running.

[arcturus]

Thanks, it's actually what I use now! Didn't know that it could be used for mixture. I'm probably worrying about nothing. It's the first time I have counted the flats so it was probably OK before I re adjusted it. The moral is .If it aint broke don't fix it

[Derek UK]

For emissions testing it's common to raise the jet one or two flats not lower it. At 20 flats I'd think that your nut was just about ready to drop off so watch for that. It will likely feel loose as the spring will be doing less work. At 20 flats the carbs will, or should be, running ultra rich so something is adrift somewhere.
KN needles standard for your engine when it has the C cam. When fitting the large airbox to the B18b, C cam, it is useful to change the KD to a KF but your KN's should be OK, but might need changing due to the D cam. Burlens might be able to suggest needles that might suit if no one here has made this change.
The airbox flows more than most pancake filters so mention that to Burlens if you ask.

[arcturus]

Derek, I have only really noticed this since I changed from the blue filters to the box. Could this have any bearing on the mixture? does fitting the box with filter make the mixture weaker or richer? The filters are the only things I have changed recently. I may put the blue filters back on and see if that makes any difference.

[Derek UK]

http://volvo-122s.blogspot.co.uk/201...b-needles.html

Have a look at the above site. It has all of the needle dimensions. A quick look might indicate that KD needles could suit you. The KD is commonly mentioned when Volvo carb tuning is discussed. It is richer (smaller diameter) across the range. The box filter flows more air so needs richer needles to balance the mixture but in your case it looks as if the D cam needs a bit more juice. Do your carbs have the sprung needles? They have to fitted correctly, should be in Haynes.
Refitting the blue filters and retuning would be useful and give you another set of numbers for the flats down.