Anyone have a good SU tune up article for BN2

[bighealeysource]

Hey all,
Have the Healey shop manual and a couple of other sources but thought I would see if anyone has a good article on getting the mixture set correctly on a BN2. I just can't seem to get away from one carb a bit too lean and other just right.
Thanks,
Mike

meant to ask, should the jets start out flush with the bottom of the carb as was common with T series MG's ? Both of my jets are about 1/8 recessed and it's my front carb that having trouble getting it to match the back carb re rich vs lean

 

[57_BN4]

Hi Mike,

12 flats down (two turns) is a pretty good starting point for the jets.

If you can't get the mixture right then something else is worn or not adjusted correctly. Float levels, worn throttle spindles, worn needles and worn jets will all affect the idle mixture to some degree. I always start by replacing the needle/seats in the float bowl with a modern rubber-tipped ones as this gives much more reliable control over the fuel level. Then take the needles out into sunlight and look at the area immediately next to the top, if there are any score marks at all then replace them. That should give you back the mixture adjustment- assuming the jets are centred ok, the throttle spindles are tight enough and the throttles are opening equally.

Andy.

 

[RandyHicks]

Make sure you disconnect the linkage between the 2 carbs. I start with the jets flush to the base and turn 9 flats down to start.

Randy

 

[bighealeysource]

Thanks guys, that's exactly the info I needed !
Regards,
Mike

 

[EV2239]

The manufacturers used to set the jets at 35-38 thou below the bridge with a vernier, it's how I've done it for around fifty years and it's usually spot on. Just as important is the float height and you need to check the book for that because it's different with different versions of SU.

 

[Randy Forbes]

The manufacturers used to set the jets at 35-38 thou below the bridge with a vernier, it's how I've done it for around fifty years and it's usually spot on. Just as important is the float height and you need to check the book for that because it's different with different versions of SU.

I wish I had read this forty (actually, 41...) years ago, could've saved a lot of time over the years!

Like Andy & Randy, I too set the jets flush, then turn the adjusting nut down 1-1/2 to 2 turns (counting the flats). After that, and sync'ing the throttles, I set about to fine-tune the mixture using the lifting pins.

All this assumes you've already checked and set the valves, points and timing beforehand.

 

[MikeAH100M]

Norman Nock's book of technical articles has a very good tune-up/mix description.

 

[EV2239]

All this assumes you've already checked and set the valves, points and timing beforehand.

Absolutely, otherwise the time is wasted.

 

[bighealeysource]

Thanks y'all for the advice and recommendations. I have just set the valves, using a good Petronix and timing is set so think I'm good to go to get the carbs fine tuned. Will pass on the results after I have a chance to work on it next week.
Regards,
Mike

 

[bighealeysource]

Hey all,
Okay, ran into a electrical problem which took me a few days to figure out. Turned out the small 'button' in the distributor cap that makes contact with the rotor had somehow got itself recessed into the cap and was not making contact with the rotor. Just a fluke I found it after trouble shooting every other electrical part and piece. Anyway, as far as the carbs, have them both at 12 flats and running very nicely at speed but a little rough at lower speeds, like 20- 25 or so. Know one carb is not synced with the other from using the unisync tool as little ball not even moving on back carb but right in the middle on the front carb. Plus using the lifting pin on front carb will stall it whereas lifting rear carb pin does pretty much nothing. Based on that would seem front carb is still too lean ? Any suggestions or advice welcomed .
Thanks,
Mike

 

[steveg]

Hey all,
Okay, ran into a electrical problem which took me a few days to figure out. Turned out the small 'button' in the distributor cap that makes contact with the rotor had somehow got itself recessed into the cap and was not making contact with the rotor. Just a fluke I found it after trouble shooting every other electrical part and piece. Anyway, as far as the carbs, have them both at 12 flats and running very nicely at speed but a little rough at lower speeds, like 20- 25 or so. Know one carb is not synced with the other from using the unisync tool as little ball not even moving on back carb but right in the middle on the front carb. Plus using the lifting pin on front carb will stall it whereas lifting rear carb pin does pretty much nothing. Based on that would seem front carb is still too lean ? Any suggestions or advice welcomed .
Thanks,
Mike

Some suggestions:
loosen the link between the carbs and set each carb individually. Use a dial caliper to verify the main jet drop is the same in each carb and that both are set to the range specified in the previous post. Make sure both idle screws are set the same. Do all that then re-tighten the linkage.
Get the SU tuneup kit (Item B):
https://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=31169&SortOrder=20

Install the pointers and see if they match as you rev the engine.

Some folks here have had good results with the Colour Tune.

A comprehensive tuneup pdf download:
https://www.spanglefish.com/thursda...ing-SU-Carburettors/Tuning-SU-Carburettor.pdf

 

[bighealeysource]

Thanks everyone - Steve, that pdf tune up is great. Think best one I have seen.