Race Engine Forum UK

Guy, I tried fitting the DCNF 40 earlier this year but couldn't achieve a satisfactory result in that it continued to choke/stall as the throttle was cracked open.
Having worked my way through development jobs on the chassis etc. (boy do the poly bushes make a difference on the live rear end!) I set-to last weekend to fit the 40 using your recommended 225 Neddle-jet, 180 Air corrections,145 Mains and as it turned out the 26 Chokes from the 36DCA. I also invested in a 6" diam. silencer running 2.5" pipework back from the 4-1 collector.
Float height at 50mm by the way.
It would idle happily and run at revs but still wouldn't pick up smoothly when the throttle was cracked open
After much head-scratching I used the accelerator pump from the original 36 DCA as this has a much steeper ramp angle on the cam and eventually bored out the pump jet holes to 1mm (from about 0.4)- bingo - now runs reasonably well.
Two problems though:-

1) As you indicated way back it fluffs on pick-up on right hand bends. Presumably it's the narrowness of the float chamber meaning the fuel is being forced by gravity away from the exit hole at the bottom of the float chamber? The carb is of course being used North/South whereas it was originally designed to run East/West.
Can you offer any suggestion please as to how to overcome this problem?
I wondered if there is any way of baffling the float chamber - if room allows tank foam or maybe a brass baffle?
Fitting a side draught carb isn't really an option given the current set-up sits quite a long way outboard of the chassis.

2) There seems to be a lot of "wuffling" in the silencer on over-run.
Is this a sign of over-fuelling?
Presumably the best option now is a rolling road session?

Thanks in anticipation and regards, Peter.

Hello Peter,

I had an email on this the other day but to my old aol account (the works mail changed to guy.croft@btconnect.com Jan 2008) and never got to replying, sorry, so thanks for posting here.

Re: DCNF 40
145 Main - probably OK with 30mm chokes but really needs prove out with plug checks on 3/4 then full throttle.

210 Air correction too lean go 180

55 Idle OK

175 Needle jet - not sure about that. I've always used a bigger one with a higer recovery rate, say 225. My advice is change unless any experienced reader can remark on that.

30 choke OK

I've dumped the electrovalve on the fuel circuit but retained the heated manifold. OK
The exhaust is a four into one system. Provided long enough - over 28" - OK. The silencer must be a large capacity straight-thru. Any hint of back pressure can really upset the jetting.

The only other mod which I've done is to put a thick alloy sandwich plate between the carb and supercharger inlet as the engine now sits vertically - that means a slightly longer inlet tract but that and the fact that the original manifold sits slightly downhill of the supercharger doesn't seem to have presented any problems so far. No,it won't.

Can you supply suitable carb jets if changes are necessary? No, please get from Troy or Steve at Northampton Motorsport,my Weber dealer, see links.

Also presumably no problems converting from Automatic to manual choke? No.

By the way - I do not know what type of intake you have between atmos and carb. Grave risk of carb icing in this weather if you induct direct from atmosphere. More detail with photos pleasa!

Hope this helps some,

G

Guy, thanks for the info - very interesting especially the carb icing.

When I get round to changing the carb. I'll post some progress reports.
I'll also post some more pictures when the weather improves enough for a decent photo-session.

Regards, Pewe.

Guy, I have now re-jetted and fitted the DCNF carb. (Northampton M/sport were very helpful and delivered the parts quickly).
Tick-over seems to be OK but a major problem is that it near stalls when first applying throttle.
It can be coaxed into higher revs but even then seems unwilling to give much power.
My initial thought was the vacuum advance pipe being downstream of the throttle butterflies but the take-off I've used is up-stream so it's not that.
I haven't been able to find figures for float level height as this may be critical given the larger needle-jet I'm now using.
I know it's difficult to offer diagnosis at arm's length but any pointers appreciated.

Thanks in anticipation and regards, Pewe.

Choke size?

That is characteristic of chokes too big or ex back pressure. I suggested 30mm chokes, you might need smaller.

G