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MG Midget and Sprite Technical - Hif44 needle selection

I'm looking for some help with needle selection on my hif44, it is running on a bdr needle at present but the afr shows the mixture to be leaning out from the mid rpm range and getting leaner at higher rpm. Tickover up to about 3000rpm shows afr at 12 to 13, hard acceleration this figure increases to 15 and at 6000rpm it is up to 17. These figures are general as it is quite difficult to get accurate figures whilst driving, they are somewhere near though.
I have looked at the needle comparison charts and found a bdk needle that has a slimmer profile from the mid point on but i don't know how much of a change in profile will effect the afr.

Thanks in advance
Stuart
S BREEZE

You could try altering the needle you have if its running lean. No idea what an AFR is or what your figures mean but where it's running lean you can file your needle to richen it up.

You are probably looking at the mid to lower part of the needle. No need to remove it from the dash pot, just take a small, fine file and carefully file a flat on it from where you think it starts leaning out down to the tip. Obviously starting with a small amount and experimenting with a few runs.

I did this on my HIF to get it rich enough to run properly then took it to the rolling road. They still had to tweak it a bit but it wasn't too far out. A lot of rolling road tuners use that method rather than swapping needles. Another way is to put the needle in a drill or lathe and use emery paper to slim it down.

Give it a go, the worst that can happen is that you will ruin the needle you were looking at replacing anyway!
john payne

Try a BBC Stuart and see how it goes. What we tend to find is we end up way too rich at lower rpm full throttles when we get the mix richer at high rpms.

Peter
Peter Burgess Tuning

Thanks for that, there are so many profiles i could have taken me forever to find the bbc needle. I will order one of those in and see how it goes.
I will get another bdr needle as well so i have something to file at if needs be.

This forum is a real bonus, i am forever reading through the archive for info, thanks again. Stuart.
S BREEZE

The BDL is usually a good starting point as well as it's the one from the MG Metro, not sure how it compares to the BBC or BDR. A rolling road tune is always going to be the best way to get it spot on though.

john payne

I have a BDL in my HIF44, but the engine also has the Metro head fitted. Echoing JP, I would also say a good start point nevertheless, and personally I would also be tempted to buy the needles either side of the BDL if not too pricey. Yes, profile away with drill and wet&dry/emery cloth accordingly, but take your time and don't overdo it. Bit of trial and error - or in the case of carbs, suck it and see....

Rolling road is indeed the acid test.
Oggers

The BDL is a lot weaker than BDR after position 6 so probably a non starter as Stuart reads weak AFR on the BDR.
BBC a little leaner up to position 6 then a lot richer which means Stuart may be able to tweak the idle position to suit. Stuart has such weak readings it is worth trying a much richer needle to see what happens.
Ideally a good long steep hill to hold the load so you see what you get.
A series seem to make very good power around 13.5:1
A very good website is Mintylamb for su needle comparisons, we use it frequently and have donated to say thanks as without donations the 'freebie' sites we love may fold.
Peter
Peter Burgess Tuning

Thanks for the replies, i have ordered the needle peter suggested and checked on minty comparison, as peter states,richer from the mid point up. Good shout on the long hill test, used to do that years ago for plug checks on the 2 stroke bikes.
I will report back when i have more info.
Stuart
S BREEZE

Peter

Point taken, but then again what has he got under the bonnet that requires considerably more fuel than even my Metro head with the larger inlets?
Oggers

I don't know as I haven't seen it. If the manifold has been polished it would not help, if the carburettor bridge has been modified it would not help.
Valve size in itself does not dictate richer mix required. To a great extent if airflow and efficiency is increased the extra air moving through the carb will increase the pressure drop on the jet and increase fuel supply.
Reducing pressure drop on jet by fitting free flow air filter over std setup requires increased fuelling.
Cam selection can have a big effect on fuelling as can cam timing.
I have seen low CR on a tired engine require richer needles as well.
Ignition timing makes a big difference too.
It shows how difficult it is to guess needle and timing requirements 'off the shelf'.
Also helps explain why modern self learning systems are so good!
Peter
Peter Burgess Tuning

Peter

I appreciate valve size does not dictate mix, but it can dictate volume of fuel used, as can all the performance mods and more that you mention.

My question was based on the fact that my engine essentially has all the mods you mention and more, yet seems to run happily on a BDL needle. I am interested to understand why Stuart's engine should require a needle which pulls in considerably more fuel. It could be he is curing a symptom of the problem rather than curing the root cause.
Oggers

Interesting comment, the engine spec is fairly mild, it's an osseli 1380 what they used to call a stage 2,having a 266 cam and high lift rockers,metro style manifold re angled,free flowing air filter with short bellmouth,lcb exhaust. Alden dizzy.On aldens (sorry Peter, will come to you if i can't sort it out) rollers it put out 97bhp running on twin hs4's. Ignition timing has not been altered since the rolling road set up.
Stuart
S BREEZE

Have you had it on a rolling road Oggers to see what fuel line you are getting?

Peter
Peter Burgess Tuning

Stuart

Fair enough! I would concede then there is a possibility it may pull in more fuel than mine - though has Peter says may be a symptom of ignition timing or poor compression.

Peter - No to rolling road, and again I would concede that it is something that is well overdue but I did undertake the long hill test to verify my choice of needle.
Oggers

Oggers, how did you verify choice of needle if you do not have an afr monitor?
Peter
Peter Burgess Tuning

Peter

Essentially trial and error driving the thing and reading around a bit on what would be the best needle to use for my particular engine c/w the uprated cam, Metro head etc - which on reflection, it could be a BDK in there not a BDL! Mini websites are also very informative, as indeed is the Mintylamb website you mention. The engine seems happy enough, pulls very well, and the mix seems about right - used a gunson thing but never got on with it, so used the method as per the SU manual for HIF carbs.

However, that is not to say it is a good as it could be. As I say, rolling road would refine further, but have just never got round to it - really should do....

Oggers

Oddly enough we had a well modded Mini 1328 with Omega flat tops in today, it was really flat and despite having jets set rich by owner and some very rich needles in the HS4s it gave an odd fuel line which matched horrendous torque dips!
On quick inspection assuming cam fairly symmetrical the cam was around 13 degrees advanced at TDC!
Pic shows fuel line and power curve.
I reckon the guess of the spec should show a good 90+ at the wheels.

Peter



Peter Burgess Tuning

Peter

Indeed. Carb sucks in as much as the engine can take. Sticking a bigger carb on or fitting a richer needle when the engine is not set up to make use of it is somewhat pointless. It is what I was alluding to with regard to Stuart's problem, but perhaps given his modifications, and provided everything else is set up correctly, he may indeed find his engine can make good use of a richer needle.

Oggers

When the cam is dialled in correctly and I get to dyno the car again I will post the end results.

Peter
Peter Burgess Tuning

Update, I have now tested the bbc and bdk needles. The bbc was very weak at the bottom and good at the very top.After some adjustment of the jet,the engine would run ok but the bottom was still to weak. When i got to the upper rpm, the car really took off,i think better than with the twin hs4's, but who can tell?
The bdk in profile is between the bbc and my start point bdr and seems to be performing that way. Afr at the bottom end is around 13 and at the top 14. I am in the right sort of range with this needle,that is to say i am not getting an overly weak or rich mixture anywhere except at initial acceleration. The afr goes to about 15 before settling back. I am guessing that is because the su has no dedicated pump jet. What effect does the spring have on this,if any?
I think what i need is a live feed for damper position with an overlay of rpm,afr and throttle position!!!
Stuart.
S BREEZE

Update more. I have a clearer picture now, tickover afr 11-12, running at constant throttle 12-13,this doesn't seem to alter too much with engine speed or throttle position. Acceleration, going up the gears to 6000rpm,there is a leaning off moment,afr 14-15 then it clears a reverts to 12-13. From low rpm say 2500 the afr runs at 14-16 for much longer,it can be clearly felt.then the engine picks up and the afr goes back to 12-13 and off we go.
Damper oil in 20/50.should i try thicker oil?
The damper opens fully but i don't know at what point, I also don't know the spring spec, what do you guys think would be the spring to use?
S BREEZE


S BREEZE

Text book is to have the piston at the top of its travel at 3/4 revs WOT(we measure this at times on the rolling road). Thicker oil will make it richer on acceleration(this is the dedicated acceleration jet in an SU). A heavier spring also increase richness on acceleration and at steady throttle openings.

Peter
Peter Burgess Tuning

Thank you Peter, i will try thicker oil.
S BREEZE

This thread was discussed between 03/07/2016 and 09/07/2016

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