Lack of power at 4000rpm

[abandonearth]

Morning folks. Some of you may have seen my travails in the ignition forum. Solved that now, but I do have a new issue...

Context for the car - mkIV 1300 with a Dolomite engine, rebuilt with a Kent TH2 fast road (at least it has the invoice for). Running 1500 SUs with ABT needles. K&N filters and an extractor 4 branch manifold. New fuel pump from Rimmer's. 123 distributor set to mkIV curve. 

Starts nicely, idles around 800. Dynamically timed to 10° BTDC. Goes well under load all the way up to 4000 rpm, then it feels like the ignition is breaking up, or there's not enough fuel flowing.

Short shopping for richer needles, does anyone have any suggestions of things I can check/test first?

All the best

[abandonearth]

Here's one of the plugs for someone that can read these things. 

[yorkshire_spam]

Plugs indicate a lean mixture. My fast road spec. 1500 runs on AAU needles, but obviously they may not the right for you on a 1300.

Interestingly on my previous rolling road session we found similar to the symptoms those you are describing. Changing the coil helped eliminate some of the issue. The only change I'd made between tuning sessions was the fuel filter - so I've changed that again. Not had the chance to re-try on the rolling road since then though. Went well on the RBRR though.

If I were you I'd drop the jets by a couple of flats and see if that helps the running and plug colour. If not start hunting other issues.

[Clive]

Pulling the choke out when hitting the 4k should help diagnose weak mixture.

I think the 1300 uses richer needles than a 1500, and I think ABTs are std 1500 needles. I am sure somebody like Canley Classics or Burlen can suggest a suitable needles.

[abandonearth]

Thank you for the feedback. Ran out for a test this afternoon and pulling the choke out when it stumbled picked it right up and away it went. 

Happily I've just found a set of AAU needles in my box of bits. I'll pop them in and see what happens. Certainly richens up accordingly to mintylamb. 

[abandonearth]

Thought I'd share this here before refitting tomorrow. The difference between ABT and AAU needles(!)

[BriComp]

....and what was the outcome?