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MG MGB Technical - Needle advice for HS6s on modified head

I need some ideas for new needles. I have a 5-main GK engine bored +.030 with a 9.5:1 CR. The carbs are HS6, rebushed and rebuilt. The dizzy is also rebuilt. The car ran fine with the HS6s and stock head, and I was using SY needles.

Just recently I installed a head with extensive port work, large diameter intake valves, and 3 angle valve grind to improve flow. I'm still using the SY needles. Since then the car runs great until 4000 RPM. If I hold the car steady on the highway at 4K and above, I start to get signs of fuel starvation- a light balk or a few barely detectable misses. Increasing the speed increases the secerity of symptoms. I also get a balk and loss of power around 4K when accelerating at wide open throttle.

I shut the car down on the highway and pulled the plugs- they were all white so I know it's leaning out at speed. I've richened the mixture to the point that I'm almost fouling the plugs at idle but it still leans out at speed. I've swapped fuel pumps, fuel filters, and checked the float bowls. The supply end of it is fine. I'm figuring that with all the radical head work I'm going to need to try new needles. Based on the SU needle comparison engine, I'm guessing my best choices are the SZ, KK, SS, and TN needles, but I'm not very knowledgable on this subject. Can anyone offer any advice?

Thanks,
Mark
Mark J Michalak

Hi Mark- an Innovate A/F , dash gauge, and assorted wires can measure your fuel mix from idle up. A freeware program -"SUneedle" I think ( Goggle Su Needle chart) can compare off the shelf needles with your "SY". With the innovate, it is easy to find a needle that will be rich or lean at the 14 stations ( every 1/8th inch) of the SY. Good Luck Vem
vem myers

Hi Mark

I ran a B on the rollers yesterday with 1 3/4" sus and K&N filters, with one of our heads a Piper 270 and a nice ex system it was happy on TF needles.Maybe a good starting point?

Peter
peter burgess

MY MGB with 1930cc, ported head with big valves and HS6 runs with RR-needles. They were choosen on the rolling road, years ago and worl really good
Joern-M.

If you want a quick check on top end lean condition, just put in some readily available needles that are richer on top end. I think TR3/4 would do - SM (but check the charts as I am going from memory)
Bill Spohn

This thread was discussed between 17/05/2009 and 19/05/2009

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