3 replies to this topic

[colinu]

Here's an odd one that's been puzzling me for a while. I completed a full engine/head/carb/gearbox rebuild a year ago and have put about 1500 miles on it (spec below). To be safe when running in I fitted a wideband O2 sensor/gauge (sensor fitted in the Y-collector) to monitor the mixture. Runs very sweet when playing on the country roads (i.e. flooring it in 2nd/3rd/4th gives a great kick in the kidneys!). Under those conditions I see the O2 lambda drop into the nicely rich low 0.8's or a tad lower). However if I'm cruising at motorway speeds then floor it in 4th it goes very lean (1.1 to 1.2 lambda). If the O2 gauge was't there I probably would't notice it as it still pulls strong.

 

Can anyone think why it's running lean only under those conditions? It does't worry me too much as I don't do much motorway driving (plus I can 'compensate' by pulling the choke out a little!).

 

The only thing I can think of is that the head I inherited with this Mini had seen a lot of porting work. The inlet ports seem 'overworked' to me and I'm wondering if their larger diameter could mean lower airspeed through the carb/head hence not enough vacuum developed to rise the carb piston... hence running lean (but why only under those motorway conditions???). If so then would sleeving the inlet ports be something worth considering?

 

... or is there a more obvious cause and a simpler solution to this issue?

 

Another small issue that might be associated is that I pretty much always need to use the choke to get the engine started (and keep it on for the first few minutes driving)... even in this wonderfully warm climate here in California :)

 

 

   Cheers,

   Colin.

 

Engine/carb spec:

1380cc

3.44 FD

AC Dodds ACD-RS+ cam

1.3 Roller tip rockers

Heavily worked head (Stage3+??? ... 35.6mm inlets, 31mm exhausts... no room in there for unleaded seats)

10:1 CR

HIF44 (relatively standard apart from red spring, BCA needle, and LDZ 2085 damper)

K&N 7"x4" pancake filter with MSS44 stub stack inside

Megajolt ignition (still running a base map provided by AC Dodds)

Facet fuel pump + FilterKing... 4psi fuel pressure

 

 

[Guess-Works.com]

From cold, you should always need to use the choke, normal running temperature for an engine is considerably hotter than ambient temperature, wherever in the world you are, however, could indicate a normally lean idle position.

I'm wondering if the needle you're running is actually too rich ( the BCA is a VERY rich running needle, with the bottom 1/4 of it's length being flat ) and to compensate for this the Jet in the Carb is abnormally high, which could then mean the the middle portion of the needle is running abnormally lean for the needle.
 

Unfortunately you're in the position of either leaving it as is, or playing with it and possibly not being able to return to the relatively good running position you have now without putting it on a rolling road/dyno.
 

I could now go into length detail on how to play with carbs, but my initial feeling is, the tuning is setup for full chat running at the expense of mid range performance.

[colinu]

John,

 

Thanks for the reply. Yes you're probably right... trying to correct this 'problem' could create more issues than it would solve (especially as I'm 95% happy its performance :)  ).

Regarding carb needles - yes the BCA is very rich. I've tried BDL (which I ran it it on) and also a BDK, neither of which showed much difference to O2 levels or 'kick in the pants' performance.

Ideally I would get it on a dyno, but that's only half of what's needed... also need an operator who knows SU's (and in my case Megajolt too)... not many of them in my neck of the woods!

Thanks for your thoughts - especially on the carb jet height - I've just rebuilt an HIF44 for a friend so might swap it in to my Mini briefly to verify it works OK... and see if shows any difference to mine.

 

Choke... yes a car will always need a little choke to start but mine needs to be on a lot longer than the other Minis I run with here. Maybe I know why... my engine (water temp) runs really cool. No bypass on the head, no thermostat (sleeved), and a 6-blade tropical fan... water temp gauge is typically 1/4 way between C and H, with it only once every getting a smidgen above 1/2 :)

 

   Cheers,

   Colin.

[nicklouse]

maybe fit a stat. but with a few extra holes around the edge. about the same size as the hole the little wiggly thing is in. about 1/4"ish.