HIF38 mixture too rich at WOT and 'spirited' acceleration

[John Gervais]

Hoorah - I took the mini out for a little jaunt today! (I haven't driven it more than a handful of times in the past few years (busy, illness, etc...)).

This is probably going to ramble a bit...

But this has been bugging me for years now, and the more I read, fiddle around, try this, try that, the more confused I get - and in the absence of like-minded, knowledgeable and physically available friends, I would greatly appreciate tapping into some long-distance brain power.

Years ago, I'd installed a brand new HIF38 (FZX3004) carb (it still has fewer than 3K miles on it) and I can get the idle and various steady cruise mixtures comfortable at between 14½-16(ish) as displayed on a dash-mounted Innovate Motorsport MTX-L wideband AFR meter (which jumps all over the place - would be nice if was slower and held a steady reading...).

However - when I accelerate vigorously, there's a huge cloud of sooty smoke behind me - the AFR drops to 10-ish or even down to the 9's, which leads me to believe that there's too much fuel going through, either from the engine sucking too hard and the oil & damper is causing the piston to rise too slowly, or the piston is rising too quickly, exposing too much needle ...   I've also read that shortening the damper piston increases the rate of piston drop, and reducing the diameter increases the rate of piston rise - or something like that - my notes are in the garage...

Here's the basic engine:

998 engine
997 Cooper camshaft
C-AHT87 cylinder head
3-in-1 maniflow exhaust
1:3 rockers (roller tip)
HIF38 carb
AAA needle
Red spring
K&N in drilled std. plastic housing
10W fully synthetic oil in the carb suction chamber
LZX1511 damper

(Ignition is Luminition optical, Aldon Yellow with vacuum advance (set at 13°@1000 rpms, vac advanced disconnected & plugged)

At idle, manifold vacuum is steady 19-20"hg.


The needle is the same as Keith used in the article; if you've got a copy of Keith Calver's 'Mini Expert' book/magazine, or if you've read the '998 Further Tuning Options' articles, this is my engine (I bought it from Keith) back in the day.

So - I also have a yellow (8oz) spring and a LZX2985 damper in a box o'parts, 5W, 10W, 15W, 20W synthetics & genuine SU carb oil, which I've tried various combinations of over the years, but I've never been able to get it to run 'nice' - and a dyno session isn't an option, as I've never been able to find one in this country. 

Maybe I'm missing something obvious, or maybe someone's got experience that I can tap into in order to not affect the steady cruise mixtures at various speeds, but to weaken the 'spirited' and WOT mixtures.

Thoughts?

[MPlayle]

My thoughts (and they may be way off base):
- At steady cruise, the piston should basically steady out at the appropriate level for the vacuum.
- When steady, the weight of oil in the dashpot is not having any affect.
- The weight of oil affects the rate of rise/fall of the piston in changing vacuum.
- With the lighter 10W oil in the dashpot, the piston is more likely rising too fast and going to rich.

I would recommend trying the 20W oil (by itself - change one thing at a time) and see what the results are.  Gather readings for idle, cruise, and WOT to compare against the reading you are currently getting.

[John Gervais]

Thanks - I was kinda thinking along the same lines, but then started thinking about 'well, the piston will rise slower, so it'll suck harder on the jet and pull more fuel...' 


I'll give the 20W a try, and if it's a tad better, maybe try to find a 25W or 30W and see what happens.

Thanks -

[94touring]

Faster rising gives a leaner mixture on initial tip in fyi. It stabilizes in a matter of seconds. Lower piston drop (more weight) gives a richer mixture.

Edit: due to venturi effect even though leaner needle stations are exposed.

[John Gervais]

That's what I've understood, and why there's 10W in there now - but the AFR is still going sub-10 when the pedal is floored.

And why I mentioned the damper & spring as a possibility. 

I wonder if there's a dimensional difference (barrel height, external and internal diameter) between a standard AUC8103 HS4 damper and the LZX1511?  I think I have one, so it might be worth measuring. 

[94touring]

If it's holding the piston down at full throttle, yes, it will read rich. Lots of fuel being pulled in by the engine sucking in and not enough air with the piston blocking intake air. If it was an airplane wing you'd be climbing at a max rate.

[MPlayle]

When going to WOT, does it drop to the 10s and stay as you hold WOT?

If the oil were too thick and holding the piston down initially to draw too much fuel, the sustained vacuum of WOT should raise the piston into the WOT portion of the needle and I would anticipate a change in readings from the initial drop to a sustained reading.

If the oil is too thin, the piston will rise too high, too fast.  You would get a brief lean then the drop to way rich and stay in the way rich while at WOT.

As stated in my first reply, my understanding may be totally wrong.

"Baby-step" debugging seems the best way to characterize the behavior for various configurations.

[MiniDave]

Maybe a quick call to Keith?

[94touring]

Another thing to check is the choke O ring.  If it's broken it will allow fuel to be pulled in as well.

[John Gervais]

Thanks guys - I'll try all of the above.  I really like the o-ring idea, since setting the idle mixture has always been a very fine, whisker's bredth adjustment.  Being a new carb, there might have been a small burr that could have nicked the o-ring.

And, it's been a while since I've spoken to my 'brother from a different mother' in the UK - we're due for some catching up, but he's been so busy that I haven't wanted to bother him.

Afterwards, I'll start at the beginning and revisit oils and stuff.

On a personal note, thanks for having me back - I've been away, life's kicked me pretty hard lately, and it seems like I've forgotten just how much fun everything is, and I'm truly grateful for y'all.

[John Gervais]

I spoke with Berlen Fuel Systems today, and while I wasn't able to really get much feedback beyond what I've already figured, I did receive a swell exploded assembly/component part number .pdf image of the FZX3004 'service replacement' HIF38 carb.

Yesterday, I was in the garage and found the remnants of an old service kit which happened to have a pair of O-rings, so I pulled the carb off and fitted them - they're not the correct ones, but better than what I saw when I looked at the crushed and longitudinally cracked (but still soft & pliable) green-colored outer O-ring.  The ones I installed are both black and seemed slightly small and thin, so I also installed a gasket between the retaining plate and carb body.

The car fired up and the idle mixture 'seems' more stable; I haven't tried driving it quite yet.  I have a new gasket pack ordered and will replace the O-rings again before I test drive it - hopefully before the weekend.