Jetting

[ChrisV]

I have a question that i haven't been able to answer for sure in my searching through the internet

 

The main circuit consists of the main jet, the emulsion tube and the air corrector. The idle circuit has two components, the idle jet holder and the idle jet.

 

I know that if the throttle is lightly depressed and the rpm are low, the idle circuit is working and from a point forward the main circuit is the one that does the job. I get that.

 

The thing is... Does any of the main jet, emulsion tube and air corrector affect the idle tuning in any way ? And which one ? 

 

Let me set an example... If i have a 7772.8 emulsion tube and a certain idle, will a 7772.10 emulsion tube, which is leaner, affect it ?

 

A more certain example is this. I saw it with my own eyes. The main jetting is the same in both cases and the holders are the emission ones. With a 58 idle jet, you have to go up to 7,5 turns on the mixtures and it creates a PERFECT idle with a bad progression. With a 60 idle jet the progression is good, it picks up revs until also 7,5 turns which dictates that it accepts the enrichment in the mixture but the idle is mediocre to say the least. The perfect idle cannot be accomplished.

 

I cannot find a straight answer anywhere because you know how forums are..

 

This might sound stupid to someone, but there are no stupid questions.

 

Thanks

[Triumph-V8]

Both systems, idle/progression and main, work with fuel and air jet.

 

The main system is not in use at idle and with sporty setup still not at part throttle

over a remarkable way. You may try that by yourself with a proper set carb

where you take off the main jet system completely.

You can still drive that engine with care! (and you will find precisely the point

where the main system engages because engine stumbles)

 

So normally one would not make things too compicated and leave two

parametres at idle/part throttle unattended: The progression holes and the idle air jet.

They can be and sometimes must be changed or enlarged to get the perfect setup.

 

As the progression holes are in the carb body and these are spoiled when done wrong

this is normally left as last step and only done from people who know what they are doing.

It also needs a set of precise drillers and gauges to test the jet sizes afterwards.

 

I am only familiar with Weber numbering of the jets where the idle system is numbered

with e.g. "60F8" where "60" is the fuel size in 1/100mm and "F8" is the size and position of air jet.

So if 60 is found a proper size you can still make it weak with big air jet or richer with smaller air jet.

 

I would check where the idle air jets sit at the Dellortos, they are slightly different, not to change idle

fuel jets and air jet sizes unwanted with them and come from that to crazy solutions.

 

Also the Weber provide different idle setup screws with different conical sizes what at last means

you cannot fully rely on the rule that 2 full rotations open get proper idle indicate the correct jet size.

Anyway 5-7 rotations on the Dellortos seem not to be the proper size.......