Carb needle / Jet Tuning

[Skinner_87]

Hi everyone,

Might be a daft question but im starting to tune my carb setup and am wundering what am i looking for with regard to AFR... as close to stoich as possible accross the whole rev range? or should there be a slightly richer mix at the higher rev range?

Cheers

Paul

 

[frank]

rich on acceleration (13ish ?), and stoich @ cruise paul

 

[Skinner_87]

rich on acceleration (13ish ?), and stoich @ cruise paul

Awesome thanks Frank (now i just need to get those figures )

 

[frank]

(now i just need to get those figures )

assuming you have variable venturi carbs ? the acceleration enrichment is down to the dashpot response to the thottleplate eh paul (there,s a thread on here somewhere)

 

[Skinner_87]

assuming you have variable venturi carbs ? the acceleration enrichment is down to the dashpot response to the thottleplate eh paul (there,s a thread on here somewhere)

Hi Frank yep they're Keihin CV variable venturi carbs just found this article on them http://www.gadgetjq.com/keihin_carb.htm

It may just be a case of raising or lowering the needle to get the best results then.... il have a good look for that thread

 

[frank]

good article eh paul if the needle adjustment still leaves compromised area,s you can file a flat on the needle @ any leanspots eh (i used to use a stanley knife to scrape a flattened area) that way you can keep track on where you have altered the needle

 

[Skinner_87]

good article eh paul if the needle adjustment still leaves compromised area,s you can file a flat on the needle @ any leanspots eh (i used to use a stanley knife to scrape a flattened area) that way you can keep track on where you have altered the needle

Il going to give it a good few reads make sure it all sinks in! Thanks for the tip's Frank hopefully start giving this a go on Thursday as its way rich all over at the minute

 

[tomwest]

whatever ratio you end up using as stoic, stay within 10% if you can. max fuel efficiency at 10% lean and max power at 10% richer.

 

[l_jonez]

I'm in the same situation, from speaking to a few local bike suppliers/tuners they have said to start with the main jet then the needle.

As said around 14.7:1 when cursing and then 12.5 - 13.5:1 when accelerating.

 

[Noddie]

whatever ratio you end up using as stoic, stay within 10% if you can. max fuel efficiency at 10% lean and max power at 10% richer.

I dont think this is always the case Tom otherwise mapping would be easy :L

 

[tomwest]

im pretty sure it is the case. ive read a number of fuel chemistry books and indeed dissertations, they all seem to agree on that one point.

mapping is easy. it just depends how far your start point is from your end point as to how long it takes.
its also a case of experience though, so the more you do it the better you get at judging where to go.

 

[l_jonez]

I think 10% is to narrow for the performance and to wide for economy. let's assume stoic is 14.7 that only gives 1.5 (rounded up) each way.
So at its richest would be 13:1. Many cars run richer than that at WOT under load. Also 16.2:1 is starting to get a bit to lean especially on non standard engines with longer duration cams etc.

Things are good in theory but in practice can be very different.