Jetting query Weber 40DCOE

[alistairnz]

Hi Guy , i have got a set of side draft Webers 40doce that i want to fit to my 124cc with a 1800cc motor . the carbs are secondhand . what size chokes and jets will need to get thank you Alistair

[Guy Croft]

Hi Alistair

40DCOE - first tell me what you have in them now:

chokes
main jets
emulsion tubes
air correctors
idle jets
pump jets


GC

[alistairnz]

main jets 115
idle jets 45
chokes 30mm
air correctors 170
emulsion tubs f16
pump jet 35

[Guy Croft]

Thanks Alistair:


main jets 115 - go to 125
idle jets 45 - OK
chokes 30mm - go to 34mm. If they were 32mm I'd say try them but 30 is a bit small for an 1800.
air correctors 170 - might run OK but 180 probably better (leaner)
emulsion tubs f16 - might be OK but if too rich go to F11
pump jet 35 - might be OK but if hesitant in acceleration go to 45

Some notes on S/H DCOE:

Before fitting the carbs go over them carefully. Clean throughout with a toluene aerosol carb cleaner like Saftey Kleen or Wynns, STP. Use an airline to blow out but be careful the high pressure doesn't the carb parts across the room. Best to strip everything down first. Check that the throttle plates close fully, hold them up to the light to check. If they don't you wil never get the carbs set right.

Check float needle condition, level and shutoff with fuel in and operation of pump jets before fitting. They should produce a laminar jet of fuel that sprays a good couple of feet.
Check the pump jet bypass valve in the floor of the cloat chamber - its ball valve can get blocked with calcium over the years. i would not use the cold start mechanism, always start DCOE with a pumps on the pedal to prime the manifold, go easy till it warms up because they spit back very readily.

You have to carefully set idle mixtures and balance. I advise use of a Colortune device to do the mixtures - at 750-850 rpm tops, remember idle the speed will climb as the mixture is perfected. Run it too fast and the engine will draw fuel from the progression drillings - which you cannot adjust. The CO on an engine like that - if you're checking with a gas analyser- should be 2.8-3.6% CO. If the HC is higher than 400ppm then you have a misfire - plug/HT system or bent valve, and neat fuel is going out the ex valve.

If you cannot get any flame with the Colortune in a particular cylinder the throttle is closed or the jet is blocked and if you cannot lean it out it's running too fast or the jet is too far out - or the carb is flooding. I use human ear for balance but there are all sorts of balance tools around these days.

Check the jetting under load by road testing - my jetting will not lead to engine damage even if it is not 100% right for your motor. An awful lot depends on your exhaust system - if it has high back pressure that has to be fixed, no amount of jetting will compensate. You do a plug check after a 1km run at full throttle in 3rd/4th gear. You are looking for plug colour light brown-grey.

As for fuel pump pressure you want 3.5psi max, and this is best achieved by running a pump like Facet Silver Top that produces that pressure without a regulator, as for running high pressure pumps like Red Top and others I have never found fuel pressure regulators sufficiently sensitive on these low pressure fuel circuits, much over 5psi will lift the float needles off their seats. The prodcution mechanical pump is plenty powerful enough if it's in good condition.

Hope this helps,

Gc

[alistairnz]

thank you guy as l live in a very remote part of newzealand its hard to get this sort of imformation Alistair