9 replies to this topic

[BobbyMax]

Hi all I have a 1275 with +30 rebore, hif44, RC40 and LCB. its running a KC286 cam and im pretty sure the heads had some work done to it. Electronic ignition fitted. My AFR Runs around 13.7 to 14.5 at around 1300rpm but after that on higher revs or under load It gets quite lean, Infact sometimes off the afr scale.
Anyways, the needle I have is clearly not letting enough fule through under load or on acceleration.

All suggestions as to what needle to try next would be a great help.
ThankAll
Bobby

[68+86auto]

Note that a misfire will cause high O2 and therefore make it appear lean to an AFR gauge which just uses an O2 sensor.

 

I would also check the fuel pressure during these conditions before touching the carb. The carb cannot deliver fuel it is not getting.

 

 

The carb is the last thing to touch. The mechanical, ignition and fuel delivery aspects all need to be tested first.

[croc7]

Has it always behaved liked this?  Have you made other recent adjustments that could affect your afr readings?  Is your test equipment accurate? As had been said on this site many times, there is no absolute combination of needle, damper oil/spring that you can plug in and expect to work perfectly for you, it takes some experimentation as each individual engine reacts differently. If all other tuning parameters are OK, try this site to help select a needle that is richer is the range that you want.  I've had pretty good luck with it.    https://www.sumidel....gory/su-needles

 

[ACDodd]

Depending on what needle you are using now, it may not be possible to find one that works for you, and it's very likely a custom profile will need to be derived for your engine.

Ac

[PoolGuy]

If all other tuning parameters are OK, try this site to help select a needle that is richer is the range that you want.  I've had pretty good luck with it.    https://www.sumidel....gory/su-needles

Great resource, thanks for posting. That's what this site is all about.

[BobbyMax]

Depending on what needle you are using now, it may not be possible to find one that works for you, and it's very likely a custom profile will need to be derived for your engine.

Ac

thank you Mr Dood as always you give saged advice🙏

[BobbyMax]

So I came across a website that plots su needles on a graph called MintyLamb.
At the moment I'm running the BBC needle shown on the graph. My AFR shows around 13.5 on idle 1100 rpm and runs around the same under load above 4500rpm however, everything in-between runs lean at around 16 to 18 on the afr.Im kinda thinking the BAM needele might be my next attempt?
Any thoughts would be welcome at this stage.

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[Kevybmini]

Classicminidiy on the web also has a good needle chart 

[cal844]

You need a needle change. You need to be around 14 AFR

[Lplus]

Peak power is at 12.6:1, so even at what you aren't achieving full power - that said 13 is pretty good.

 

classicminidiy.com has a good comparison chart system which I've been using and from that it seems the BCA needle would give a richer mixture midrange.

 

The BCG might give you a little more low down but might end up sobberingly rich at peak power