View Full Version : SU HS6 carb need help

09-16-2003, 12:13 AM
Hi all.its been 4 yrs,on frame off early 1974 TR 6 restro,,finally finishing,,converted to Su carbs as SZ were junked....however having diff getting a smooth power curve,,any one have advice on a needle choice..has a mild cam,K&N filters,stock fuel and DISTributor..eng at 10 BTDC... only 250 miles on engine.still smoking although getting better..Do thease engines take awhile to seat or did I miss something,,, My past 2 rebuilds on my MGBs broke in a lot faster..advice appreciated.

Larry Kronemeyer
09-16-2003, 12:27 AM
What needles do you have now, and what duration cam?

09-16-2003, 01:17 AM
You should start with the standard 2500 S needles - I can't remember but I think BDM or something like that.

A favourite over here for 2500's with a little more go is BFQ - or BQF - can't remember letters.

I can check - 'cos I've got a set at home from a 2500 motor with a PI cam.

Why don't you install an A/F meter and watch the results through the rev range? That way you can measure carb piston position at various revs and translate that to a necessary change in thickness of needle for any given result shown on the meter.

You need to do this if you want a tidy smooth car with good power. Otherwise you're always guessing.

09-16-2003, 04:15 PM
thanks all current needles are BAG,BBW neither seems right any advice images/icons/frown.gif

Dave Russell
09-16-2003, 04:20 PM

Here is an excellent needle comparison program;

09-16-2003, 10:49 PM
Congrats on saving another fine example of a LBC! I, too, have a restored TR6 (73) and enjoy it thoroughly.
Sounds as though you are running cintrifugal advance, I would suggest pushing it more to 8 degrees BTDC. Also, set your points with a dwell meter and don't rely on a feeler gauge. Should be at about 35. Go to a colder plug and lessen the gap a bit and check the valve clearance as appropriate for the cam you have. Yes, if you changed rings, it takes between 100 and 1000 miles for these to seat. Be patient.


09-17-2003, 12:10 AM
Tell me about the smoking - what revs and what situations bring it on.

You shouldn't get any at idle because regardless of needle you can always set the car to have a suitable mixture at idle - it's the profile, the damper spring and the damper oil weight that comes into play at anything over idle.

If it's smoking at idle I'd be worried.

What colour smoke?

Do the carb lifting pin tests at idle suggest a correct mixture?

Do the plugs show correct mixture?

Do the plugs show shiny oil deposits?

There's not really enough to go on at this stage.

09-17-2003, 12:15 AM
Actually - If I remember correctly the MOSS performance tuning manual / catalogue suggests needles for given cams/applications.

Get one of those and have a read.

09-17-2003, 10:24 PM
thanks for all the relpies....still playing w mix..seems better..all indicators are ok,,plugs color corect,not misfireing,plenty of power in 1-3 gears,,top end lags a little,,just not as smooth as I would like....eng smoking less and less...smokes on deceleration only 300 miles on eng...will wait to 500-600 miles before i get agresive and do compession,leak down and other adjustments....everyone says TR6 use a little oil because of lack of valve oil seals..head has all new seats,guides valves etc...althogh oorig springs used..I assume Machine shop tested..maybe a liitle weak and not closing fast enough any ideas would still help.

09-26-2003, 08:39 AM
I checked my spare set of carbs under the house - and yep - when my 2500 was running carbs I was using BFQ needles. That was for a standard mk2 block with PI saloon cam (127 odd bhp version) with twin SU HS6 carbs instead of the injection.

It blew a bit of fuel out near 4-5000RPM but they were suitable.

John Bryan
09-26-2003, 10:44 AM
Hey, JWC, sounds like a beautiful car! I have been trying to find someone who actually did a conversion to SU's. My recently acquired 71 has SU's and I have been trying to find info on recommended plumbing. Right now, they are properly linked, although some prior owner cut one of the 2 choke cables. The vacuum feed off the rear carb wasn't hooked up when I got it, although there was a loose hose to nowhere, presumably for the distributor. I think the best advice I have been able to get on that is to cap them both as the 71 may not even have a vacuum retard! And, does your breather hose feed back into the air filter? That seems really odd to me. Thanks for any info. John in SoFl.