tripple SU carbs
tripple SU carbs
(OP)
A friend has been trying for months now, with the help of others and some rolling road work done,to try and get the tripple Su,s he has put on his car to work
the car is a 2.0,litre , six cyl with a fire order of 1.5.3.6.2.4, and a 280 deg cam,
the prob he has got is blow back, /fuel stand off of the carbs.
some folk have come to the conclusion that because of the fireing order, then 3 sets of carbs of the SU type will not work
the standard set up is twin SU/Stromberg carbs
he has tried putting in a bigger balance pipe, but is no better
3 x twin side draughts, or FI will work, but these are different from the constant depression carbs like the SU
any one any experiance with tripples on an engine with this fire order
regards Marcus
the car is a 2.0,litre , six cyl with a fire order of 1.5.3.6.2.4, and a 280 deg cam,
the prob he has got is blow back, /fuel stand off of the carbs.
some folk have come to the conclusion that because of the fireing order, then 3 sets of carbs of the SU type will not work
the standard set up is twin SU/Stromberg carbs
he has tried putting in a bigger balance pipe, but is no better
3 x twin side draughts, or FI will work, but these are different from the constant depression carbs like the SU
any one any experiance with tripples on an engine with this fire order
regards Marcus





RE: tripple SU carbs
Street machines always had an air filter and I assume the standoff problem was not as critical at lower rpm. On the race cars adding the trumpets made a big difference.
Perhaps you could use a K&N type filter over them.
Some things that come to mind (wow, that's a stretch!)...Using different viscosity oil in the dashpots and or springs could and sometimes did help. The richer the mixture the more standoff (well, DUH) and on these little engines the needles looked like "sewing" needles!!!
Rod
RE: tripple SU carbs
but the stand off is different on the end to carbs so he says, , and going by the pic and his discription, its still standing off at higher revs
this is whats leading folk to think that they wont work with SU,s due to the fire order
its 1.5.3.6.2.4, and the exhaust header is 6.3.1. with 1-6, 2-5, 3-4, linked together.
thanks any how, , Marcus
RE: tripple SU carbs
One was a 3.3 litre (202CI) with 3 by 1.75" SUs.
We had a longish manifold, small balance tube and about 1.5" big bell mouth ram tubes. The tubes were mounted inside a plenum and there was a Unifilter at the inlet to the plenum.
We set the carbies up exactly as per the BMC manual for these carbies as twins on an A series engine. I think twin 1.75s were stock on a Cooper S, but memories are now faded.
Some people ran as small as 1.5s and as big as 2". all worked, depending o engine size and rpm range sought.
1.75 worked well from 1000 rpm to 6000 rpm in the 3.3 litre push rod engine with 285 deg cam, Rhodes lifters and 12 port head. There was some stand off, but it never did any harm. Valve float limited the top end. It was a daily driver and my brother taught his 70 year old mother in law to drive in it. We never dynoed it, but it probably made about 150 hp. It outperformed the 5 litre V8 available in the same chassis. The V8s typically dynoed at about 140 to 145 HP and were slightly heavier
Regards
Pat
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RE: tripple SU carbs
RE: tripple SU carbs
Bill
RE: tripple SU carbs
E types, as far as i can remember they all had tripples,
the fire order is what some folk are saying, cos there are a few Cars over here, that dont seem to go that well with this combination,
Pat, was the engine you had tripples on, the same fire order !!, because it seems to be the middle two that is giveing the trouble
will try and get some pics for you,
and thanks again for your help lads
Regards Marcus
RE: tripple SU carbs
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this may help you Guru,s better!!,
regards Marcus
RE: tripple SU carbs
Hi Guys
Thanks for the input- I will try to put something detailed together for you over the next few days.
The car is off the road at present due to the cylinder head failing after I skimmed it too much.
Not withstanding the fuel stand-off, it was running so well when I broke it!
RE: tripple SU carbs
Marcus
RE: tripple SU carbs
First off, Bruce...You need to get some of the heat wrap paint, the silicon/heat resistant, whatever, stuff they sell. Use it and it will save you countless hassles down the line with the wrap coming loose and "stringing". Even comes in colors...smokes and stinks a bit on first startup but it goes away quickly. (I use the stainless steel ty-raps, but safety wire works just as well. Nice job, though).
Second...You REALLY need three DCOE's (OK, so I'm biased)!
Third...Like I said to start, a nice three inch trumpet is what the setup needs...betcha'!!!
Had a close encounter with a "flying" GT-6 at Riverside many years ago (flown by none other than Don Devendorf). Good story as far as close calls go.
Rod
RE: tripple SU carbs
There was some stand of, but it never created a problem.
Regards
Pat
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RE: tripple SU carbs
The engine original spec was as follows:
2x 1.5" SU constant depression carbs.
Balance pipe fitted to heated manifold
2 litre straight six engine
CR 9.5/1
Cam 25/65 65/25
As now:
3x 1.5" SU constant depression HIF38 carbs.
Was an in-appropriate balance pipe arrangement.
Large bore cold air fed to air plenum box housing bell mouths.
Was an unheated manifold arrangement.
2 litre straight six.
CR 9.5/1
Cam 37/63 74/28. Correctly intalled now!
Has shallow depth MGB style bell mouth 'stub stacks.
6-3-1 exhaust manifold with un-evenly matched primary pipe lengths.
Mapable Megajolt ignition.
Gas-flowed cylinder head.
Unless I go for Alfa Romeo Spider style bent stub stacks, there is no space available to fit longer stub stacks due to the wheel arch being in the way.
I have been very careful to ensure the carbs are balanced at and off idle properly (i.e. gauges prove they simultaneously draw the same volume of air) and the float heights are correct to book. The carb pistons are all weighed and evenly matched. This includes the piston in dashpot drop time test and they all rise/fall at the same rate.
Where-as twin carbs draw evenly through each alternate carb, triple carbs exhibit a longer drawn out cycle where-by the outer two carbs draw a different sexuence to the middle carb. My outer two carbs(1+3) exhibit pronouned fuel vapour stand-off throughout the rev range, to the point that RR sessions are a definite health hazard!
I can't help but think that as the twin carb set-ups don't do this to anywhere near this extent(if at all), it frustrates me that the current set-up is busy pushing fuel vapour out of the carb throat when it should be drawing it in.
These cars put out an alledged 105bhp in the 60's. The car was putting out 120bhp at the rear wheels on the last session. It did drive much better than had been the case post build a few years ago.
Initially the camshaft wasn't timed in correctly.
The mapable ignition also greatly improved the torque and drive-ability of the car- far more torque.
The gas-flowed cylinder head was inadvertantly left with a CR of only 8.3/1! This was corrected with skimming it to 9.1/1, but that was ll it would do due to an inability to skim the head any further. The improvement to the CR also improved the performance incrumentally.
Getting better needles and lighter carb piston springs improved the situation too, to such an extent that after the last rolling road session, it was a dramatically improved car to drive.
A recent Trackday session at Cadwell Park showed the car to be very nice to drive- a good spread of torque, though even though the cam is only mild, it does have a marked powerband after the initial low down torque.
I've ditched he exhaust wrap. Unless there is an issue with under bonnet temperatures(there isn't at present), i'm not convinced of its merit for my installation.
I can see the relative merits for outright performance of side draught carbies, but the installs on GT6' are fraught with dreadful after market linkages and then there's the question and cost of finding the correct fuelling and getting a knowledgeable RR operator to achieve that throughout the rev range and not leave me with a gas guzzling monster that's got a miniscule power band at 4000+rpm!
I am happy to fit twin 1.75" carbs and a gas flowed inlet manifold in due course, but I want to just try to sort these out to a point i'm happy with before then.
My initial plans are a new cylinder head with a CR of 10.5/1.
Trial the much enlarged (and improved) balance ppe arrangement, with the newly installed heating arrangements for the inlet manifolds.
Try longer ram pipes on the RR- irrespective of them not being able to be fitted with the bonnet down!
The, have a power mapping run to maximise the power throughout the rev range.
So, any thoughts on my plans?
Regards
Bruce.
RE: tripple SU carbs
RE: tripple SU carbs
Rod
RE: tripple SU carbs
Of course I am only referring to fueling an internal combustion engine in this instance.
1. get it in - as fast as you can
2. squeeze it - as hard as you can
3. get it out - as fast as you can
RE: tripple SU carbs
The only purpose I see for so called balance tubes is if the carby is to small and each cylinder can pull a little from the nearby carbies, or else mainly for allowing fuel enrichment on one carby for cold start and to a smaller extent for closed throttle.
So long as the idle throttle settings are reasonably close, and the linkage is not ridiculously bad and they all get full throttle when you stomp on the pedal it will run OK. It may not be optimized but it will be 95% of optimum.
Maybe you are trying to solve a problem that does not matter and overlooking one that does.
I have seen a lot of standoff problems along with poor performance created by valve train problems.
Regards
Pat
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RE: tripple SU carbs
The bigger balance pipe is actually a balance reservoir in effect. The previous install wasn't correct in its design- or even my implementation of that design. I haven't tried the new install yet, as the car is awaiting its new head at present.
I understand and agree with the sentiment about OEM spec's. The guys who designed these set-ups were after all full-time Engineers.
However, this firing order appears on other straight 6's which seem not to suffer with the stand-off mine has- something is intrinsically wrong here, I'd like to put that right, rather than run the white flag up the pole at the first hurdle, as it were.
Carbs are pretty much sorted for state of tune. I haverebuilt them very carefully (new throttle bushes, plain throttle discs, thinned down throttle spindles etc. I break outhe micro-meter when I set the jet heights(all of which are new). The fitting of lighter springs and alterations to the needle were fine tuning of a non-standard set-up, which improved the engines behavior and performance considerably.
The throttle linkage is all new and works well(no slop or lost motion between the carbs) allowing uniform full travel/opening of throttles.
The valve train is entirely standard- albeit the 'go-faster' cam has a higher lift than standard. I set my valve clearances to .017"
I will try longer stub stacks on the rolling road- as the bonnet/wheelarch will be raised and out of the way.
"Maybe you are trying to solve a problem that does not matter and overlooking one that does".
I suspect there maybe something in wht you say!
Regards
Bruce.
RE: tripple SU carbs
we had similar poroblems with older 4 cylinder engines
when you hold the power on the rollers @ a costant
speed you get stand off. If you do a sweep off at least
300rpm per sec no problem. One other thing springs to mind
check camshaft base circle run out as this will also cause the problem if it is cronic, try opening out the tappet setting on the offending cylinders then do a run
RE: tripple SU carbs
Forgive my ignorance- but what is cam shaft base circle run out? Opening the tappets will in effect retard the valve timing if my understanding is correct?
Regards
Bruce.
RE: tripple SU carbs
Base circle run out is the base circle of the lobe not being perfectly round and or concentric with the cam bearing axis.
Regards
Pat
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RE: tripple SU carbs
Having said this I have seen helical cam engines demonstrated idling with the intake valve closing at around 110 degrees ABDC with no sign of fuel spray coming out of the carbs - so maybe "standoff" can have other causes.
What is the cranking-speed compression pressure? This should give some indication of how "big" the cam is or if it is installed correctly.
I don't think base circle runout affects cams with tappet clearance much - if at all. Runout does affect hydraulic tappet cams very much though.
On the subject of balance tubes - I have never seen a one-carb-per-cylinder motorbike engine that had a balance tube (or any interconnection) at all - so it would seem that the size etc. of the balance tube is not very important.
RE: tripple SU carbs
if this is the case if you are at the rollers. beleve me it
not as uncommon as you might think, its just it gets missed.
RE: tripple SU carbs
One cause of stand off is if the cam starts to lose a lobe. I can't explain why, but I have seen it happen.
Regards
Pat
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RE: tripple SU carbs
A little more info for you:
OE Triumph cam: Dur 270. 25/65 65/25 lift .232"
Triumph tune: Dur 288. 37/63 74/28 lift .288"
The Triumphtune cam is instructed to be installed at 103 degrees. But mine was found to be dialed in at 105 degrees! The car had a very noticeable lack of low end torque which was improved considerably when the cam was re-timed in at 103 degrees. It also had a bizarre comedy 'kerfuffle' out through the exhaust valve, moments after killing the engine. This was also eradicated on re-timing the cam in.
RE: tripple SU carbs
RE: tripple SU carbs
Regards
Pat
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RE: tripple SU carbs
Having established the genuine piston travel TDC with a pre-loaded DTI, and setting the firmly mounted cranksaft timing disc to a fixed marker: this gave n accurate indication of TDC.
I then set about establishing the cam lobe centre angle on inlet valve 1(valve 2 of 12 on engine).
By using another DTI carefully set-up to operate vertically on the pushrod sat in the cam follower for No.1 inlet valve, this allowed me to establish an idea as to maximum lift on this pushrod. Zeroing the pre-loading DTI there, the crank degree position on he timing wheel was noted .020" before and .020" after the inlet valve No.1's DTI's zero indicated.
So this gave me an indicated 62 degrees and 148 degrees respectively on the timing wheel. That makes 210 degrees in total: divide by 2= 105 degrees.
I then adjusted the vernier timing wheel to give the recommended cam instal position of 105 degrees.
The valve springs are nice and 'gentle' OE items, not super strong and trying to flatten the cam lobes.
What do you think?
RE: tripple SU carbs
With 120HP at the wheels there can't be too much wrong with the engine.
Maybe all GT6's with your carb/cam setup behave just like yours?
RE: tripple SU carbs
Once you try it at split overlap, if you want to move the power band down the rpm range, advance the cam a few deg. 4 deg is normally enough to see a few hundred rpm difference. If you want more top end, retard the cam. Yes I know that's counter intuitive, but that definitely is how it works
Regards
Pat
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RE: tripple SU carbs
Peter Waggott (of Waggott cams fame) recommends this method if you doubt the timing marks etc.
Gt6boy: On a slightly different topic - the photo of the manifolds on the windowsill. I wouldn't have thought that this type of manifold would have any form of heating at all - and the copper pipe fittings look distinctly odd. The balance pipe is not connected to any exhaust heating is it?
RE: tripple SU carbs
The manifolds have a very pronounced downward slope from the head to the carb, in order to obtain bonnet clearance. This leads to the potential for fuel vapour fall-out to gather in the carb throat/inlet manifold area. The heating of the inlet runners is effected by .5" pipe drilled through the underside of the intake runners. This should assist minimising the collecting of fuel fallout: albeit it at the potential cost of reducing the benefit of a cold intake of petrol/air mix.
The copper pipe affair is being chromed- even as we speak! The tee-piece affair is the take-off fashioned for the brake-servo.
Regards
Bruce.
RE: tripple SU carbs
What does the half-inch hole do in connection with heating? What goes through the hole? I don't think you need to worry about petrol vapour etc. gathering at the bottom of the slope of the manifolds.
Also it looks like the stock rocker cover breather hose goes to atmospheric pressure in the air cleaner. Your photo and words imply that the breather hose now goes to full manifold vacuum on the new copper pipe fittings balance tube.
I personally would return all the manifold pipe arrangements back to standard.
RE: tripple SU carbs
I would block off any tube that fed air into the ram tubes apart from the carbies themselves and just possibly the output from a PCV valve.
An upward slope from the carby to the port is not conjunctive to neither high performance nor quick resonse.
If you could fabricate a manifold to turn on the horizontal plane and retain the carby at a slightly higher position than the port with the carby facing more fore and aft (like maybe about 45 deg bend) you might do better. That might leave a lot more room for ram tubes. The linkage could be a real b***ch.
Copper tube and swept bends with the same or slightly larger ID as your throttle bore can be purchased from your local plumbers supply. Flanges can be flame cut or milled from steel plate or even cut from OEM cast iron manifolds and brazed together.
Regards
Pat
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RE: tripple SU carbs
We may be at cross purposes here.
The .5" pipe through the bottom of the inlet manifold runners will have engine coolant running through it via a restricted size oriface union, as did the OE twin carb heated manifolds.
The OE UK market cars had a closed loop crank case breathing system to minimise emissions, by taking the crank case breather pipe to a 'Y' connector which then fed to an oriface on each carb, just the engine side of the throttle butterfly, there-by putting the potential pollutants into the incoming petrol/air charge, and burning them. I have a very similar system in place.
The balance arrangement again is very similar in practice to that used by Triumph on their later manifolds. On these they had a much larger balance pipe- more of a chamber than a pipe, between the inlet manifold areas immediately behind the throttle butterfly. Then, immediately on top of the balance pipe,chamber was a single take-off for the brake-servo. This is what I have achieved with the copper-pipe.
When I pick up the balance pipe from the chromers this week, i'll try to get a picture of it in situ on the head for you.
Regards
Bruce.
RE: tripple SU carbs
I had similar trouble with my twin SUs on my tuned Reliant 850. No-one else seemed to have done this conversion; the manifolds looked very similar to yours (except that mine were semi-down draught and not semi-updraught like yours seem to be). Try disconnecting the crankcase breather assembly (run it to a catch bottle with a small filter to atmosphere) and blanking off the breather inlets to the carbs. It may be that pulsing crankcase pressure from your breather system is affecting the barometric pressure in the manifolds. If you have a couple of cylinders with more piston ring blow-by than the others it may not affect each carb in exactly the same way. I can't give you a mathematical reason why as my brain isn't up to it tonight but it might be worth a try.
Regarding the difficulties of setting up the carb linkages, you can convert a single throttle cable from the pedal to a double or a triple cable at the carb end by using a 1960s - 1970s motorcycle cable "splitter". Basically this is a free-floating plastic or metal piston in a cylinder with a screw end cap. One end of the piston has a single fitting for a cable from the throttle pedal and the other end has a multiple fitting for two, or three individual cables. This allows you to fit carbs at completely different angles - as long as the individual cables to the carbs are of the correct length the relative angles at which the carbs sit doesn't matter.
As you are in UK like me, look at Vehicle Wiring Products web catalogue, in the motorcycles section.
If you want to use the SU mixture enrichment (or jet holder lowering system, I am reluctant to call it a choke), you can try using it on just one carb, or if not, a second cable splitter for all three.
Regards, Paul Wheatley.
RE: tripple SU carbs
I have tried the motorcycle cable system, but had trouble getting anything like synchronization and the throttle was VERY heavy and sticky. We ran the cable to the centre carb and used its shaft to mount arms on each end that linked to the outboard carbs.
Regards
Pat
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