Ignition timing and fueling interaction. where to start?

[SirYun]

HI GC.

I'm tinkering away at an engine that has great potential but is a bit finicky setting up.



i have a few problems.

Fueling : for some ,as of yet unfathomable reason, the SU HIF44 carburettor is not able to supply a rich enough mixture above 4000 rpm regardless of needle profile. Adding full choke (which is a seperate 'jet'' in a HIF44) helps a lot

the unit is fully rebuild and should work properly.

A pressure regulator has been added and a high capacity pump is due to be installed ( the mechanical pump should be enough for the job though)

Timing: while the engines i have build so far where extremely unsensitive to timing (they more or less produced the same bhp across the rev range even with rather large adjustments) this one is totally different due to a longer and higher lift cam.

The designer(s) of the cam suggested a 8 degree centrifugal distibutor advance all in by 3000 rpm. (giving about 27 degrees total advance)

but suggested as an alternative to lock the distibutor fully as this had yielded good results as well and only run static advance.

idle is prettty bad in my setup (a lopey idle is to be expected with this cam) and starting the engine is somewhat dodgy as it it advanced quite a bit too far, but it does pull a lot better in the midrange and top regions.

I know that it is impossible to diagnose the problems from across the channel.

but the fueling and ignition timing interact in a way that causes me to go around in circles.
with x amount of advance the engine seems to want a lot of extra fuel. changing the timing will reduce this effect but affect performance.

:what would be the proper way to go about setting it up?

fueling first , then advance, then reset fueling ?

The ignition will be replaced by a mappable/programmable unit as getting a distributor recurved once here, is more expensive than a new system.

i would like to be able to understand it better as spending a lot of time on a dyno is quite expensive and that should be used for setting it up rather than trouble shooting.

suggested reading would be great

I hope this is worded clear enough
thanks

[Guy Croft]

Some details of the engine spec and a decent photo showing inlet setup Joost please, I can help,

GC

[SirYun]

BMC A+ series
1310cc
Ported head 35mm in./29mm ex. Static Compression Ratio of 1:10.5
RE13 Profile cam 276 degrees 0.290 lobe lift LCA 104.5 deg timed at 105 deg
1.5 forged rockers (that are nearer to 1.56 ratio when measured)
RC40 exhaust, Maniflow Stainless ''cooper freeflow'' exhaust manifold 1.75 inch I can't measure primairy lenght
hif44 with a donut substack and ITG filter. Mini Spares Centre medium size intake manifold
Lucas 65D4 distibutor, locked to give no curve, no vac advance.
Filter king Fuel pressure regulator/Filter.
New mechanical fuel pump. (electric pump is to be installed)
spark plugs NGK BP6ES (The BP7ES's seemed to run too cold)

exhaust manifold

127-2788_IMG_resize.jpg (138.3 KiB) Viewed 11616 times

manifold-side.jpg (34.66 KiB) Viewed 11615 times

manifold-underside.jpg (50.87 KiB) Viewed 11617 times

hiff-stack.jpg (25.83 KiB) Viewed 11615 times

[Guy Croft]

Try it on a 40 SU

GC

[SirYun]

a smaller version ?

like a 38mm?

[Guy Croft]

yes, if you can, 38 or 40 - because the problem sounds maybe as if the air velocity thru the carb is way too low and you're not getting decent fuel signal (insufficient depression).

GC

[SirYun]

Hi Guy.

I think I found the problem (I have been at it for about a month now, it was about time I made some progress) and you where right (in a way) at least it got me thinking about a certain aspect of the setup.

The modified needle was such that it needed the jet to be fully down to give anywhere near a decent mix. This pretty much puts the jet orifice about 8-12 mm down in the bridge and i think this plays havoc with the signal/atomisation/fuel delivery (choke is seperate and is not affected thus explaining the large and positive effect ) . The SU's constant vacuum design should keep depression more or less contant.

SU-jet-to-bridge-full-down.jpg (41.34 KiB) Viewed 11343 times

So far i have drilled the float jet to the larger size (0.097 instead of 0.070) in my case 2.5mm (this has been reported to be a restriction when you try to make a lot of power from a single HIF, this combo should make about 110-115 bhp). This turned out to be quite easy and with a litte lapping in the seal was fine as well.
after installing set the float level to 1.00mm exactly.
Swapped a standard MG metro spec bell housing and needle and damper assembly. Thus basicly returning it to standard

now the jet was pretty much in the starting position as it should be (only slighly below the bridge).

this is the jet in the normal position. as this is a spare carb it is dirty. the one on the engine is qweaky clean

SU-jet-to-bridge.jpg (41.74 KiB) Viewed 11340 times

the engine is now pretty happy at higher rev as well ,idles reasonably and i am confident that the further fueling should prove fixable on a dyno.

I now think that all needle swaps and modification should be first made without altering the basic jet position at all

[Guy Croft]

Some very useful SU tuning tips there, well done indeed!

GC