Twin HS6 SU carbs fitted to 1971 TR6 - help needed

[Peterjbay]

Help needed in getting my car running decently, please, lads.

The car: 1971 (CP) UK spec TR6 - standard "150 HP" camshaft.
Ported and gas flowed head, 10.5: Compression ratio.
Single free-flow exhaust system (standar manifold / header)

Twin 1.75" HS6 carbs fitted by previous owner  :B. Black jets - needles unknown, but look very thin. Spring bias needle.
Valve in butterflys soldered closed by DPO.
Ignition system standard - new plugs, points, condensor, coil & ballast resistor.

After market electic fuelpump fitted by DPO (excessive fuel pressure)

K & N type filters fitted.



The problem:
The car runs very rich and has a very poor, uneven idle.

The two carb vent pipes (to rocker cover / PCV?) have been blocked off by DPO  ??) If vented to atmosphere or attached to the rocker cover, the car runs incredibly lean.


If the jets are adjusted to be level with the carb bridge, the car runs very lean.

The car almost feels like it is running rich and lean at the same time, as stupid as that sounds!

The butterfly spindles are fine - no obvious air leaks into the carb / manifold.

The chokes work fine and are not sticking "open".

Not sure about the float level - could be too high.

To add to my woes, the left carb float bowl is just touching the steering shaft, despite a nice 6mm aluminium spacer being fitted that I had laser cut to the gasket profile.

Any suggestions on a starting set up - BAE needeles, red jets and ??? dashpot springs?

Any help will be much appreciated.

[Steve P]

I`m having the same problems with setting up hs6`s on my 2.6 Vitesse,i`m looking at overfuelling,float levels etc.
My only observation on your comments is you said if you adjust the jets level with the bridge it runs lean,you are supposed to adjust them to the bridge and then back them off two full turns as a starting point.
Steve

[andy thompson]

Anyone know a good dyno? I think that would be my starting point. I think the idle will be tricky as the original PI helps the car tickover in a more stable manner if still a bit lumpy unless set at about 850.

Are you sure re. CR? Sound a bit high for a standard cam and the ignition might be horribly retarded to cope which aint going to help.

Short of refitting the Lucas PI I think the best fix will be a good dyno session with a good SU guru

Obviously you need to check all the basics first, correctedly set tappets, good even compression, timing advancing sufficiently

Any other hints people?

[Peterjbay]

Hi
Thanks, Andy and Vitesse 2.6.

Good point about the "initial" jet height setting - I will try again at 2 full turns (= 12 "flats"?) which must be pretty well where it is now.

The CR of 10.5:1 is correct - I measured the chamber volumes when the head was off and did the neccessary calculation.
The very high compression pressure confirms the high CR - the engine also stops very abruptly whenswitched off.

No pinking is evident - I am using 98 RON fuel (the gurus tell me that MON is more imporatnat than RON when looking a detonation / pinking).

I think that I need to put a fuel pressure regulator in the fuel line as well as resetting the float level.

Regarding using a rolling road - these are pretty scarce here in darkest Africa, and as for SU carb gurus, there is one in Cape Town 750 kms away - in this area, I guess that I am it!

[piman]

Hello Peter,

first thing is to get the idle right. High fuel pressure must be addressed so a regulator to get it to about 1.5 lbs\sq inch is needed or a different fuel pump. This is the first step. The floats are non adjustable on HS series carburettors, but is affected by whether the sealing washer is fitted to the needle jet in the lid. With the piston\dashpot assemblies off and the fuel pump on you should see fuel in the jet tube, about 1\4" below or there abouts is fine. The preliminary setting as stated is two turns down from the top. By the way there is only one jet size for the carburettor (I think for a 6 they are 0.100" jets but my manual is not to hand) You should be able to get a good idle mixture whatever needle you have fitted as the idle dimension are virtually the same whatever the needle. (As long as you have the correct series of needle, i.e HS4 needles (or jets) cannot be used as they are 0.090".)

Once that is done then see how it runs.
One person who may be able to advise on particular needles is Kas Kastner as he has a website with a forum and he does participate in that. www.kaskastner.com


Alec

[GT6boy]

Just a quick one- it's my understanding that there are two different threads on some of the jet adjusting nuts. Some have the coarse thread, some the fine thread. Therefore, I'd check that the jet is lowered the correct depth, rather than a given number of turns :). HS6 should be initially set .050" below the bridge- I wonder how many turns that equates to on your set-up?

[Steve P]

Just an update on mine that might help you,
I reset the float level on the front carb by fitting a thin washer under the float valve as 1,2+3 were running really rich.Also i found the biggest gain in performance and running smoothly was to get the air flow as equal and precise as you can between the two carbs before you worry about mixture.
My car runs great and goes like stink now by the way.
Steve

[piman]

Hello GT6,

all S.U.'s as far as I know have the standard two turns from level, This even applies to HD type S.U.'s which have an adjusting screw to alter the jet level. I've never heard of it being specified as a distance?

Alec

[GT6boy]

Hi Alec
The Burlen Fuels SU book lists that the later HS carbs did come with either an option of course or a fine threaded jet adjusting nut. The finer thread was an attempt to make it easier to meet emissions criteria.
I've always been a bit befuddled by the notion of turning the jet nut 'two turns' on initial setting up. Did that mean by two flats, or two whole revolutions of the jet nut :-/?  Given that there are 6 sides on the nuts, that would be dropping the jet by a total of 12 flats!?
It's a while since I fiddled with mine(i've had HIF carbs for a while now), but from recollection, I ended by doing it two flats.
The notion of dropping it a given distance, allows you to work out where the needle/jet oriface start out at, and give some indication as to whether the needle is likely to be correct. If the car only runs with the jet level(or even above the bridge), then that mes the needle is too lean.

[99176]

Yep, 12 flats Bruce, but surely if it only runs with the jet high that makes the needle profile too rich (thin).

[GT6boy]

Doh! Too rich indeed!! :B
So 12 flats at a fine thread would be markedly different to 12 flats on a coarse thread, meaning it wouldn't be a surprise if it needed to be dropped more turns than usual?
I'll dig out my old HS6 carbs at the weekend and measure how far the jet drops for each flat and 12 flats in total, just out of curiosity.

[piman]

Hello GT6boy,

a point many people seem to miss is that at idle it doesn't matter what needle you have. All needles (of the same series i.e. 0.090", 0.100" or 0.125") have virtually identical idle dimensions. So if you can't get a good idle mixture, at around the recommended setting, then there is something else wrong.

I must say I hadn't come across the coarse versus fine thread on the jets. Mind you as my nickname implies I am not running carburettors :-)

Alec

[GT6boy]

Couldn't agree more Alec about the similar initial needle diameters at idle :).
I wonder if there is something in the fact that the crank case breather from the rocker is blanked off? Perhaps excess crankcase pressure is leaning the mixture when the rocker cover pipe was connected to the carb 'breather'?
I hope Peter comes back with either the outcome or a little more detail, i'm inruiged! :)

[Dale]

HS6's can be fun (not) to set up if the rest of the stuff under the bonnet isn't right, but if it is they should be fairly straight forward provided there's nowt fatally wrong with them.

Disconnecting the breathers usually means trouble somewhere else or needle choice badly wrong, add to that the fact that you say the head has been ported and gas flowed and it could just be (especially if it has a lumpy bumpstick fitted or the old one is worn) that the intake charge is separating (i.e. the fuel isn't staying atomised evenly throughout the charge) possibly due to low gas velocity at idle (what speed is it set at?) which will effectively mean that it can be running both too rich and too lean by virtue of being unable to burn the fuel which it is being given.

Although the actual sizes of .100" biased needles at stage 1 are similar they are not the same. Also at idle, stages 2 and 3 will affect mixture too. When working out mixture profiles on carb needles you need to pick needles with similar sizes at those 3 stages otherwise no accurate direct comparison can be made.

I can tell you from experience that in order to achieve the same idle mixture with needles of different sizes at stages 1-3 it may be necessary to adjust the mixture setting by more than half a turn just to get the same datum value (idle CO reading) so it's easy to fool yourself into thinking something incorrectly.

Be prepared to raise or lower the jet adjuster by several whole turns before deciding whether something is too rich or too lean particularly if the fuel delivery pressure is excessive, anything over 3 psi will cause flooding with those carbs even if the needle valves, floats and pins are in good shape, so drop the pressure don't start packing washers under the needle valve as that will change the level of fuel in the float chambers and also necessarily in the jets thereby b*llsing up your mixture settings all over again.

Don't worry too much about piston springs, they don't make as much difference as you might think as they tend to affect the mixture throughout the whole rev. range thereby cancelling out more than half of the net change you might expect - they're for fine tuning and it sounds to me like you're stuck back with the basics.

Make sure you really know what cam you've got, if it's been changed a few times has someone got it timed up a tooth out? Are the valve clearances correct? Has someone been over-enthusiastic removing material from ports and throats? If so you'll never get it to run right below around 1200 rpm or so. Make sure that the ignition timing is set correctly to within a few degress at least and don't bother trying to get it to run below 850 rpm - an early TR6 on carbs just won't do it smoothly full stop.

If you've done all that and your fuel pressure is OK then get on to the carbs, find out what the needles are, make sure that one of them isn't bent (this is amazingly common with very rich (thin) ones and will completely b*gger the thing).

After that be prepared to muck about a lot (a gas analyser is really useful here though not essential) and find the point at which you get the highest STABLE idle speed then back the jets down a flat - that's where it wants to idle - and if your needles are right it should work.

If it still won't run properly at higher speeds/under load etc then the needles are wrong and you need to change them but at least you should know which way they are actually out and that's a start which should allow you to get closer next time, even if it takes several goes to get it right. Failing that drive the 750 klicks and get it to the rolling road.

Hope this makes sense, sorry for the essay, hope you're still awake!

[piman]

Herllo Dale,

my point was really that the needles are blamed for poor running when there is something else fundamentally incorrect, so the needle change still leaves whatever else is wrong. Significant deviation from the 'standard' setting at idle indicates that it is not the needle and further investigation is required. Once a good idle is established then running tuning needs to be done, especially with a non standard engine. But get the idle right first at or near the basic setting.

Alec

[shedmonkey]

I know this sounds really stupid (wibble wibble) and basic but I had a similar problem on the shed when I put it back on the road after a six month lay off (cheers Devon and Cornwall police)
It has a similar set up - PI engine HSU6 carbs electric fuel pump unkown lumpy stick etc
I just couldnt get it to tick over smoothly / constantly randomly going weak /rich etc lumpy slight misfire etc
First thing I found was faulty Tacho buggering up the ingnition circuit this improved things slightly when I took it out the loop
Second thing that cured the problem was clearing out all the white rubbery gunk that had built up in the fuel lines! they looked like the  arteries of a 90 year old Glaswegian, these little deposits where breaking up moving around the fuel lines and effectivly blocking one or the other carb randomly -might just be worth checking for deposits etc -just to make sure you have all the basic checks ticked off before you go full tilt on on the carbs

And for the record I would go with Dales advice - ive seen him polish the worst turds into smooth running mean machines-normally my sheds!

[GT6boy]

Nice one Dale, tip-top info :).