Xflow Jetting

[alcal]

She's running rich. Plenty of power but a bit fluffy. Had 108 bhp at wheels when built. Carbs have :

33 chokes

F16 emulsions

130 mains

165 airs

50 F8 idles

I'm thinking about changing the idles to 45 F9s as I've seen several references to these in posts. Could anyone advise if this is likely to help ?

[7 wonders of the world]

Certainly ditch the F8 idle jets for F9's - never once found a car that ran properly on F8's) but you really need to check the fueling under load on a set of rollers.

 

Many thing to check on side draughts to, jet seating, jet sizing - actual drilling size compared to that marked on the jet, many get drilled oversize some get very badly drilled too!, cold start enrichment, needle valves, seats, float and heights etc, whislt your there check the return springs too, air leaks, mountings also tension on these too, butterfly alignment, balance, fuel flow and pressure that's before you get to the base settings and adjustments!

 

All the above assume the frest of he engine is correct, compression, valve clearance, timing etc etc

 

 

[George C]

As said above there are many things to check etc etc etc but you could drop to 45 idle jets which will lean the low speed running. 50 seems quite high compared to the main jet you are running. If its too lean you will know quickly as it will cough and spit back through the carbs whilst driving slowly. Once at higher revs it will run on the mains so will have no effect. You need to be certain with the jets as they are often drilled out. If you are near Suffolk I can lend you some 45 jets and have a jet measuring thing.

[alcal]

Thanks 7 wonders. I haven't checked the timing yet but everything else seems o.k. Sadly I dont know of a reputable rolling road nearby.

Thanks for your kind offer George C. I'm in Canterbury so a bit of a trek. The jets are not expensive so I think I'll risk buying a set of 45s. Should these be F9s as older posts have referred to ?

[Roger King]

I'd be very surprised if it's running rich. I'm assuming this is a Supersprint to standard Caterham spec.

 

Jetting is about right, although you might benefit from bringing the air jets down to around 150 so that it doesn't lean off too much at the top end.

 

The real problem is the idle jet. 50F8 is way too weak for this. The official setting was 45F9, but even that would often lead to light throttle spitting. Most Supersprints run better on 50F9 and that would be my first port of call.

 

If this fluffy running is occurring on large throttle openings, the problem will not lie with the idles so you would need to look elsewhere.

 

Do check that your jets are actually the size marked: there is a breed of rolling road operator that will drill out a jet and leave the original markings in place - they should be shot!

 

I would also advise that the compressions are checked and the ignition timing re-set if necessary. And don't forget that you will need to make sure the carbs are balanced before you can assess any changes. Even more important, you need to realise that changing idle jets will usually mean that you need to re-calibrate the idle screw settings to suit - if you don't do that you won't get any useful results at all.

[George C]

Most people seem to use the F9 and the change to 45s will be noticeable - its trial and error and may not work. You will need to adjust the idle mixtures after and best to balance too. If its still too rich then there may be a problem with something else - out of adjustment (ie float levels?), something leaking (ie pump jets) etc etc. Good luck.

[alcal]

Thanks All. I'll try some 50 F9s in it with BP7ES plugs and see what happens. Could wrongly set float levels cause the sooty plugs ?

[Roger King]

They could, but they'd have to be way too high to make much of a difference.

 

But even if all the jetting is weak it's still possible to have the idle mixture screws set too rich and that will give you sooty plugs.