Spitting & Banging

[sgrant]

(more of a blog for myself than neccesarily an invitation for replies, but all advice welcome)

Since converting to a 45 DCOE, I've never quite got it jetted correctly. Having spent many hours getting it working at all (the Vizard settings never worked for me), I finally got it running with the following;

idle jets 55f8
emulsion tube f2
main jets 150
air corrector 180
aux venturi 4.5
choke 34mm

Le Mans last year was fine, but it drank fuel like a fish (appearing to have 20-odd miles less range than the other elevens) and the lift-off banging and popping in the exhaust was rediculous; it would bang for 20-30 seconds. Having said that, it was stable and got me there and back and didn't stall in traffic or anything else nasty.

Over the winter I changed the head to a Slark Stage 3 head with suitable valves and high-lift rockers. Oddly, the same jetting worked straight out of the box. Switching back to the Vizard jetting, I find that it runs like a train at higher revs, when it's switched over to the main jets (perhaps the mains switching up to 160 from 150?), but horribly lean on the idle jets, requiring choke to keep it running even when warm. Hoping to combine the best of both worlds, I went back to the 34mm chokes and 55f8 idle jets but left the mains on the Vizard setting but, oddly, then found that the revs were very slow to die after coming off the throttle (perhaps just running massively rich?).

So, back to the settings above. All runs well. Except 1) The idle speed does move around a bit, although never enough for it to stall and 2) I do still get a small amount of banging and popping on the overrun. In heavy traffic it sneezes as well, which everything i've read suggests it's running lean. Perhaps TADTS, but perhaps a tweak ricker on the idle mix might cure it. I did think there might be an air leak somewhere in the manifold, but then again I have been advised that air leaks make the idle horrible, and it's not particularly horrible....

[erictharg]

Popping and banging on over run is often due to air leaks on the exhaust side. The engine throws a bit of extra fuel down the exhaust and all it takes is some nice air with oxygen in it to bang.

[sgrant]

Yup - I haven't excluded that possibility, although i've sort of waved a little piece of tissue paper around the manifold to see if it flutters, but it doesn't seem to. I suspect there may be a leak somewhere, although i'll be darned if I can find it....

So, over the weekend I ran the car up to temp and then adjusted the idle mix until the revs wouldn't climb any more on a richer mix, indicating (I think) the ideal idle mix. Unfortunately this was quite a way richer than I had been running the car and I ended up 3.5 full turns off the stop on the idle mixture screw, and left me idling at 1200-1300rpm, with the idle throttle stop screw completely screwed out, ie, there was no way to bring the idle revs back down without leaning the mixture back out again. Whilst I accept that this is largely irrelevent for driving (since I guess it's the jets and not the idle mix that provide the richness when on the move), I think it does suggest that my 55f8's are too lean. I also tried various other jets - 50f9, 45f9, and so on, but every time I step down the idle jet size (ie, from 55 to 50/45), I find that the revs refuse to die for several seconds after blipping the throttle. Why this is, I have absolutely no idea because I would have thought that indicated that the carb was running rich, but all I did was to reduce, not increase, the fuel orifice. But there it is; the revs refuse to die and they flare up and down randomly, like there's excess fuel being burned somewhere in the barrels. I don't understand this. I also don't really understand why my car runs on a choke and an idle jet that is so large for the car; according to the formula on the midget register website (which reads like a reasonably well-informed article), I should be on ~32mm chokes and 45f9 jets, but this just doesn't work at all for me.

Next up, I'm going to order some 55f9's and 55f11's and see if, by just tweeking up the mixture on the idle jets, this allows me to bring the idle mix screws back down a bit and get the idle speed below ~1000rpm at "ideal" mix.

And then just take it to someone who knows what they are doing.....

[sgrant]

It's time to take it to someone who knows what they are doing

This Friday -> http://www.aireytuning.com/

I'll let you know...

s

[sgrant]

Now jetted as follows;

idle jets 60f8.
main 130
main tubes f9
air corrector 160.

Dyno'ed at 110bhp @ 6500rpm. Tom Airey is sure it has a semi-race cam and has been professionally balanced (nothing to do with me). No spitting, no banging, sneezing or popping any more and feels much more powerful, especially around 3500 and up. Diagnosis was that it was running lean on the idle jets and massively massively rich elsewhere.

s