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MG MGB Technical - needle selection

can any one help me, ive had some mods done to my car (4 branch man fast road head 285 cam k a n air filters etc) the car producers another 35 bhp but it seems to use alot more fuel on normal driveing. the needles were not changed but ground down when being tuned ,is this normal or should i try and get the correct needles.the exhaust is very black is that normal,its doing about 15 to a gallon thanks
daz

Daz-
Its normal only for carburetors that are way too rich. Although most people are intimidated by the prospect, the truth is that selecting appropriate needles for your particular enhanced performance engine is not as difficult as it might seem as long as the matter is approached systematically. Once all of the appropriate modifications have been performed in order to improve the volumetric efficiency of the engine, and the appropriate ignition curve established, the correct needle profiles become identifiable. Using Burlen Fuel Systems' "SU Reference Catalogue", find the section that covers 1800cc four cylinder engines equipped with dual carburetors. Noting the various spring and needle specifications, use the SU Needle Profile Chart to chart the needle sizes from the first diameter to the end of the operating range. After selecting a pair of needles that should be slightly lean when compared with your original needles, test their performance in your carburetors at each metering stage of the needles. The first and second metering stages of all SU needles for a given size and series SU carburetor are either identical or very similar, so regardless of which needle you have selected, the engine should start and idle reasonably well. With the damper rods removed from their respective dashpots, open the throttle very gradually until the butterflies are half open (5th or 6th metering stage on 1 1/2" SUs, 7th or 8th metering stage on 1 3/4" SUs). Note if the engine hesitates while doing so, as well as at which metering station of the needles the hesitation is produced. If it does, using a drill chuck spinning at 200-400 RPM to securely hold the needle and #200 Wet & Dry sandpaper to remove material from the needle and #600 Wet & Dry sandpaper for the final polish, polish the appropriate metering stage of the needles which produces the hesitation to a .0005" smaller diameter in order to richen the fuel/air mixture slightly. Bear in mind that the needles are almost always made of brass and are very easy to remove material from. Repeat the polishing and testing procedure until the engine no longer has the slightest hesitation when the throttle is very gradually opened. At this point you will be ready to do the final sizing of the needle profiles. Taking 1/2 second to open the throttle butterflies to the 1/2 throttle position, again note again note any hesitation. Polish the appropriate metering station of the needles at which the hesitation occurs to a .0005" smaller diameter in order to richen the fuel/air mixture slightly. Repeat the polishing and testing procedure until the engine no longer has the slightest hesitation when the throttle is very rapidly opened. Using the exhaust gas analyzer, aim for a CO rating of 5-5.5% at each needle metering station. At that point you will have a correct 12/1 fuel/air mixture. Having accomplished this, you will have noticed that there are more metering stations further down the needle that are never exposed to airflow. These are present in order to effectively channel fuel up the jet. Be sure to incrementally size these remaining metering stations .001" to .002" smaller as they progress toward the tip of the needle.
Steve S.

Daz,

I have done these modification too and found the BDR needles to suit best after some testing with other sizes.
The engine runs fine and, ignition and carbs set right, consumption is between 9 and 12 liters for 100 Kilometers, depending upon how the car is driven.
I have done the selection of the needles by driving with a CO - Meter installed, reading +/- 5 % CO under load.

Engine specs: 18V780, cr 10.5 : 1 (County flat top +0.060), lightend and balanced crankgear, Piper 285/2, HIF 6 with red springs and BDR needles (think blue springs will even work better or will give better respose - but never tried), Minispeed substacks, K&N conical filters, Abingdon ST dizzy, ported and flowed rally head, opened up cast manifold, modified downpipe and PECO HDR exhaust, 4 speed od, 3.9 rear end, 175-80/14 tyres.

Ralph

Ralph

hi do you no what bhp you have at the wheels
daz

Daz,

I've got a GT running a similar spec with 100bhp at the wheels. I've never worked it out before, but it'll use more than half a tank to do 100 miles. If the tank is 10 gallons (??) then that's 20mpg tops. Exhausts are black. If you give it some stick, it'll use more. It does better on Optimax plus Valvemaster octane booster, worse on ordinary unleaded (probably because you need to use a lower gear to stop the pinking...). Also, the seal between the fuel filler and the fuel cap is rubbish so if you really giving it some stick you may be losing some fuel there! Also check the floats as too much fuel in the chambers will make it run rich. Rich is way better than weak. Also check you have the right oil in the dashpots.

You can buy a book which tells you how to profile needles, but it seems like a black art not a science. By the time you've bought a book, plus a needle chart, plus a SU catalogue plus a couple of sets of needles then you've got the best part of the price of a rolling road session. There are so many needles, you are bound to find one that is okay.

You could try polishing the needles but I bet you'll never, ever get it right. The biggest problem is that you're not setting up the engine with any load on it, which kind of spoils the point. I know lots of people do it and have had no problems, but using a drill and a piece of emery to polish something is stupidly dangerous - you'll lose a finger and be sorry.

Ralph: 10.5:1 compression ratio? You must have better petrol over there than we have in UK!

Neil
Neil

Daz,

BHP at the wheels is allways different, dependig upon the rolling road you use.
So you aught to be careful with comparing the fugures.
Tested at 200ft. over sea level the local rolling road read out 96hp at the wheels. Driving with works hardtop fitted, the max. speed is 200 km/h., with hood down it is hard to exeed 185 km/h.
I think with a smaler wheel circumfance (175/80 are larger than the most often used 185/70 size) the top speed would be a little higher as the engine would be able to reeve out better in od.

Ralph
Ralph

standard mgb tyers are 165 80 14 my tyers are 185 70 14 less than 5 mm smaller than standard not alot of difference
daz

@ Neil

we have 100 ROZ fuel in here, so picking is not a problem.
It is critical to find the right dizzy and set it propperly. If in doupt, have yours tested and do some comparisons to the curve given in the BL Special Tuning Manual.
Driving some tests with one of those cheap CO meters on board, you will find out the emission under load. It should read +/- 5 % CO when the carbs and ign. are set right. It took me one afternoon to do the mesures and another to check out three different needles, starting with BBU and checking the stations on the listing that you can find at www.teglerizer.com

@ Daz

with the smaller tieres, you will put less load on the drivetrain, helping to reev out better at topspeed. If you have a look at the Abington's Le Mans cars, they did a very high speed on the straight, beeing fitted with a 4.5 or so axle, and HS 8 carbs for high reevs in 4th. top. There was no OD fitted to these cars!

Ralph
Ralph

This thread was discussed between 23/04/2006 and 28/04/2006

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