Idle Jets

[Zag]

Having read on here about the standard idle jets being sized from the factory too small due to emissions and having a slight flat spot and minor stumbling at constant throttle below 3000rpm I decided to purchase a set of 4 58s to see if it made any difference.

For the sake of £15 delivered and half an hour ish to fit and adjust the idle mixture screws it is (IMHO) a no brainer.

Car runs smoother than it ever has in my ownership and pulls stronger and cleaner too.

So if you still have std jets and were wondering if you should do it I would recommend you go for it.

May change my mind come emissions test at the MOT but I can always go back to the 55s for a day.

[richardw]

Nice one Zag!

Cheers, Richard

[Lotus-e-Clan]

For forum newbies that might visit these threads later, I'll toss in other options.
Alternative options to idle/progression enrichment to correct a progression stumble, include optimising ignition health and ignition advance tune (weaker, stoichiometric, mixtures need a strong spark with plenty of advance), and, depending upon your Excel model year, eliminate vacuum air leaks first.
A new ignition cap with a quality set of HT leads, and a set of Denso IW22 irridium plugs will restore sparks, assuming the coil and LT wiring are sound. Irridium plugs are an expensive option, but a worthwhile upgrade ... sparks like a sharp edge/needle point to jump a good sized gap.

That said, unlike variable choked/variable jet carbs, DHLA fixed choked/fixed jet carbs can't automatically compensate for degradation in induction vacuum caused by a worn engine. Reduction in compression /vacuum over time drags less fuel through fixed jets, so an increase in idle/ progression jet size is a cheaper option to an engine refresh.

[Marten]

For forum newbies that might visit these threads later, I'll toss in other options.
Alternative options to idle/progression enrichment to correct a progression stumble, include optimising ignition health and ignition advance tune (weaker, stoichiometric, mixtures need a strong spark with plenty of advance), and, depending upon your Excel model year, eliminate vacuum air leaks first.
A new ignition cap with a quality set of HT leads, and a set of Denso IW22 irridium plugs will restore sparks, assuming the coil and LT wiring are sound. Irridium plugs are an expensive option, but a worthwhile upgrade ... sparks like a sharp edge/needle point to jump a good sized gap.

That said, unlike variable choked/variable jet carbs, DHLA fixed choked/fixed jet carbs can't automatically compensate for degradation in induction vacuum caused by a worn engine. Reduction in compression /vacuum over time drags less fuel through fixed jets, so an increase in idle/ progression jet size is a cheaper option to an engine refresh.

not to mention that modern fuels might not be quite equal to the blend the engines were originaly optimized for

[Zag]

There is a saying that 90% of carb problems are ignition. So after replacing the original system with a wasted spark trigger wheel controlled system with tps and a 3d ecu and still having the same issues i concluded that it was not the distributor or the vacuum system as both are redundant.

It was the same running on e10 95, e5 97 or 100 octane stuff i get from planes at work.

Carbs have been ultrasonically cleaned and rebuilt, compression has been tested, all good.

Not saying it can't be but in this instance no, jets were wrong from the factory.

[Lotus-e-Clan]

But the OE idle jets do work on my 89SE and others? (sTD dizzy).

Did you try adding more advance to the map in the region below 50% TPS, especially around the stumble rpms?

I run fully mapped with TPS on my Clan with twin DHLAs. I mapped a lot of advance at part throttle at 'off cam' rpms to get smooth progression.

Engine wear / throttle plate/spindle wear / carb balance procedure method could cause issues with OE idle jets.

[Zag]

Yes I did add advance, helped but did not cure.

Mines an 86my so slightly different as you may know.

Have you tried 58 jets, if not how do you know they won't improve yours?