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MG MGB Technical - From FX to needle #6

Hy you all
I have "fixed" the engine in my 69B as Peter Burgess says (K&N, unleaded, cleaned the admission).
Running in is almost done. Now P. Burgess recomends changing the needle from FX to #6. Whatīs going to happen ? Increase in power, better gas/mileage or just nothing at all ?
I am getting the needles next week but if any one out there has tried it I would be happy to know.
Thank for your coments
Miguel
Miguel

When I did exactly what you are about to do, my 1967 MGB no longer hesitated from idle, ran smoother steady state, and required less choke time when cold. I don't have a mileage check,yet.
Tom Wright

I have found that No 5 needles are OK with K & N filters. No 6 are a bit rich at the top - checked on a dyno with a gas analyser. The FX needles were a disaster from the start - too lean at light throttle which caused surging. No 5 were fitted up to about 1970 when they changed to FX & these stayed until the change to HIF
Garth.
Garth Bagnall

FWIW, I have had the same experience as Garth on my cars.
Bob Muenchausen

Thank you for your coments.
I think that what I wanted to read is something more like Tomīs opinion. That is exactly what is hapening with my car now. Lotīs of choke time and it hesitates when I acelarete.
I am geting #6 needles īcause thatīs what P. Burgess recomends for my "new" tuning and now Bob and Garth got me woried.
Iīll post my experience as soon as I have the new needles on.
Thank you again
Miguel
Miguel

I also have the same set up as Miguel. My understanding is that the K&N filters are so much more free-flowing than the originals, i.e. so much more air can now enter the engine, that the mixture is weakened. This gives the hesitant running. The #6 needles give a richer mixture to bring the air:fuel proportion back to where it should be. Mine runs great with #6 needles.

Mike
Mike Howlett

Does anybody know whether the #6 neddles are suitable in HIF 4 SUs running with K+Ns?
nik

Interesting, Mike and Miguel, because both of my engines use K&N filters. Perhaps there is more to this equation than just the needles given the difference in our experiences? I do use 45D "euro-spec" dizzies on both engines, and perhaps the dizzy curve contributes its fair share to the difference. I will let the induction geniuses weigh in on that one. What dizzy are you folks using?
Bob Muenchausen

5's were standard until emission regs required a clean up of the exhaust. FX arrived and for a period 5 and 6 were then incremental richer needles recomended by the factory.

FX was usually a cause of many hesitation and flkat spots even with standard cars. So many cars of the late 1960's and first couple of years of the 1970's simply reverted to the original fit of 5 for smoother and crisper motoring with actually better fuel consimption. (me included)

The better consumption coming from the simple fact that with a proper balance mixture the combustion process was more complete and more efficient. A more complete burn led to more power being produced that before and so to achieve the same speeds in normal driving required less throttle. In addition with FX there was also some misfire and under these circumstances any fuel in the misfiring cylinder is completely wasted.

Now a simple transposition of these historical circumstances with standard engines shows that with even minor modifications such as the K&N, which is light years ahead of the efficiency of most 'sports pancake filter' of that period, there is usually a need for more fuel to be available to match the greater airflow. the 6 needle fits this bill and applies to probably 8 to 9 out of every 10 cars following this route.

The 5, 6 and FX needles are fixed and as such do not apply to any of the later carbs which use spring biased needles. The HIF series only using spring biased needles.

Nik for these later carbs you should consider a move to ABD needles which again applies to 90% of all K&N MGBs. If this proves too weak then try AAA needles.

Finally there is one further point to consider and this is the accuracy of machining for the valve seats in unleaded conversions. Many shops now by default use a 3 angle seat cutter that provides a very efficient seat cut and this usually manages to increase airflow. The result is that when fitted to an otherwise standard engine up to an extra 5% power is not unusual. I mention this as this will also help to underline a need for more fuel.

Rog
Roger Parker

Rog thanks v much for the info im off to get some ABDs
nik

I am geting my #6 needles tomorow.
Now I also found out reading the archives on this subject that I have been using the wrong oil in the carbs. I have thin (10) oil instead of the factory recomended engine oil ( for my climate) 20w50. Now I wonder how this might also have helped with the not so good performance of my engine. In any case I hope to have everything good tomorow.
My distributer is the original 25D. It was overhauld together with the engine. I also have the pertronix circuit breaker.
Thanks for your help.
New posting as soon as I have news
Miguel
Miguel

First impressions.
No misfire on acelaration and sligthly more response. I only changed the needles and off I was in town driving. Iīll fine adjust it later today.
By the way Bob my air filterīs I copied from your home page.
Thankīs
Miguel
Miguel

Miguel, Thanks for the endorsement. It does work and well. Luck is sometimes kind to us in our experimentation. Enjoy. I will revisit my needle situation given what Roger and the others have said. I may have been running too lean all these years.
Bob Muenchausen

Consumptions
I did my first highway outing and these are the results:
At a average speed of 125km/h (78miles/h) carefull highway driving (in the sense that the engine has only done 1200km (745miles)) my consumption was 9liter/100km
(26.12miles /USgallon)
(31.36miles/UKgallon)
wich I find OK.
I never went over 4300 revs.
Engine runing fine.
Miguel
Miguel

This thread was discussed between 16/11/2001 and 24/11/2001

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