SU set-up and tuning

[boggie]

Evening all,

 

Our car has H6 SUs which I completely stripped and restored but I am struggling to get them running as well as I would like. They appear to be running rich as the plugs are sooty and there is a puff of black smoke when we set off. However when I lift the pistons using the lifter button the revs drop and the engine dies which, according to the Haynes book of lies, indicates a lean mixture but using my trusty ColourTune to watch combustion the colour appears as a Bunsen blue with the occasional white flash, indicating a good mixture.

 

HELP!!!

 

Now I can spend hours tinkering with the twin Weber DCOE45s on the Seven and don't get me started on the twin ICT34 setup on my old Vdub camper, that can take days....but SUs should be easy to setup. Aside from centring the jets and setting the jet height there is bugger all else to tweak (jet / needle size for a standard 2L motor?).

 

What am I missing here? Anyone able to walk me through a simple setup or perhaps recommend a decent guide?

 

Cheers,

Ian

Edited September 27, 2014 by boggie

[Motorsport Mickey]

Firstly check for inlet manifold leaks, spray brake cleaner (or other flammable substance) around the inlets (not near carb mouths obviously) and see if the engine picks up, if it does it is sucking it in through gaps and they need refitting. Then check for float and fuel heights being correct and report back.

 

Try a search for www.classiscarhub and check out their setting up SUs guide.

 

 

 

Mick Richards

Edited September 27, 2014 by Motorsport Mickey

[boggie]

Cheers Mick.

 

I have checked for leaks with easy start and the float height as listed in Haynes (can't remember what it was). What should it be in your opinion?

[Peter Cobbold]

My guess. The piston(s) not rising fully into the dome when the engine is being driven under load.

Lift the piston with finger through the mouth of the carb with the dashpot plunger removed. It should fall back rapidly. If it sticks anywhere then loosen the three screws that hold the dome to the carb body and retighten until the piston goes up and down without sticking. The dome and piston are paired assemblies, if they might have been interchanged in the rebuild swap them round. Check again with the plunger in place but dry of oil -if bent it will hinder piston motion.

 

Here's a start:

http://www.zparts.com/zptech/articles/mal_land/ml_sucarb2/SUcarb_111601b.htm

its for Hitachi SU copies but the principles are the same.

 

The choke tube depression that lifts the piston, and pulls fuel out fo the jet ( aka 'constant depression') is only around 0.5psi. So that gives an idea of how free moving the piston should be.

 

Peter

Edited September 27, 2014 by Peter Cobbold

[saggy]

I used a 3.5 to 4.0 millimetre drill bit between the float lid and the float itself (when its turned upside down so the float arm is resting on the needle valve)

Would air leaks make it rich? I would've thought lean. Either way any air leaks would spoil the mixture.

[Peter Cobbold]

Cheers Mick.

 

I have checked for leaks with easy start and the float height as listed in Haynes (can't remember what it was). What should it be in your opinion?

Take the dome off with the piston in it, and isee f the fuel level is within a few mm of the top of the jet. If so you're OK. If its overflowing its too high.

The 'constant depression' that pulls fuel out of the jet is enough to lift fuel about 1 foot so the odd mm wont matter, but it must not be overflowing.

Edited September 28, 2014 by Peter Cobbold

[Lebro]

When lifting the pistons, either using the built in pin, or a screwdriver blade, you only need to lift it a tiny ammount,

too much, & the engine revs will fall.

 

Bob.

[Peter Cobbold]

The piston lifter does'nt get to the level of lift that running a loaded engine will. The piston will be about one third up the choke tube at cruise load, typically. And further lifted at higher loads. So the piston lifter only tells you if the mixture is about right around a very few horse power, a couple or so. Likewise Colotune - tells you nothing about the engine when its making more than a hp or two.

 

To amplify my post above. If the piston can only rise say an half inch or so and then sticks, then at higher airf lows the carb is no longer working at 'constant depression'. Now the air velocity rises and the 'suction' ( aka depression) at the jet rises. It does so in such way that a doubling of the air flow leads to a more than doubling of the fuel flow. So a stuck piston gives a richer and richer mixture. And a rich mixture when driving but not at tickover is the symptom described. Hence my guess.

 

Peter

 

And.

Make sure the atmospheric bleed at the mouth of the carb is not obstructed by a filter or ram pipe.It feeds atmospheric air under the piston and if blocked will impede pistion lift.

The bleed holes are above the two bolt holes in the filter-mounting flange, or similar position:

http://www.carburetterspecialists.fsnet.co.uk/Hs6.jpg

Edited September 27, 2014 by Peter Cobbold

[dingle]

I used a 3.5 to 4.0 millimetre drill bit between the float lid and the float itself (when its turned upside down so the float arm is resting on the needle valve)

Would air leaks make it rich? I would've thought lean. Either way any air leaks would spoil the mixture.

The float level on H series SUs should be set using a 7/16" (.4375" drill) or 11mm. That may account for a rich mixture. Also, make sure the jets aren't sticking when the choke knob is pushed in.

Berry

[boggie]

Great info, thanks chaps!

 

I will have a tinker when I get the chance and report back.

[Dave Larnder]

The piston lifter does'nt get to the level of lift that running a loaded engine will. The piston will be about one third up the choke tube at cruise load, typically. And further lifted at higher loads. So the piston lifter only tells you if the mixture is about right around a very few horse power, a couple or so. Likewise Colotune - tells you nothing about the engine when its making more than a hp or two.

 

To amplify my post above. If the piston can only rise say an half inch or so and then sticks, then at higher airf lows the carb is no longer working at 'constant depression'. Now the air velocity rises and the 'suction' ( aka depression) at the jet rises. It does so in such way that a doubling of the air flow leads to a more than doubling of the fuel flow. So a stuck piston gives a richer and richer mixture. And a rich mixture when driving but not at tickover is the symptom described. Hence my guess.

 

Peter

 

And.

Make sure the atmospheric bleed at the mouth of the carb is not obstructed by a filter or ram pipe.It feeds atmospheric air under the piston and if blocked will impede pistion lift.

The bleed holes are above the two bolt holes in the filter-mounting flange, or similar position:

http://www.carburetterspecialists.fsnet.co.uk/Hs6.jpg

Peter

 

You photo shows an HS6 and Alan´s are only H6´s. Does that make any difference to all your comments.

 

Dave

[Ian Vincent]

I set my jets to 38 thou below the bridge using a depth mic and then adjusted them with a pair of colourtunes. I had to raise them slightly to get to a Bunsen blue flame.

 

Rgds Ian

[Peter Cobbold]

Peter

 

You photo shows an HS6 and Alan´s are only H6´s. Does that make any difference to all your comments.

 

Dave

Dave,

No difference really. The constant depression works on the same principle in all SUs.

The atmospheric bleed might be in a different position on the mouth of the carb.

Peter

[Ashley James]

I set my jets to 38 thou below the bridge using a depth mic and then adjusted them with a pair of colourtunes. I had to raise them slightly to get to a Bunsen blue flame.

Rgds Ian

.

 

This is the correct jet position and provided the float heights are correct (7/16" on H6) the butterflies are synced and the Pistons aren't sticking, the car will run as it should. It's that simple.

[Peter Cobbold]

PGG Knight's article:

http://sucarb.co.uk/technical-su-carburetters

He shows that:

The fuel level is not critical, with provisos he makes.

The .038 inch setting of 'jet drop' is used as datum when comparing needles with different profiles - which few owners will attempt.

The jet drop is set routinely according to the mixture strength, tested with the piston lifter at tickover.

 

The article is in the liitle book on SUs:

http://www.amazon.co.uk/SU-Carburettors-Tuning-Tips-Techniques/dp/1855202557/ref=sr_1_1?ie=UTF8&qid=1412116169&sr=8-1&keywords=SU+carburettors

The older edition with a brown cover, if you can find one, is more useful for more detailed SU specs engine by engine.

 

SU wear will of course alter the ideal SU/Triumph needle specification, as will many engine mods, and possibly fuel changes (eg ethanol).

So its reassuring that AFRs from around a lean 16 to 12(rich) will burn OK, although perhaps not optimally !!

 

Peter