tripple SU carbs

[marcusaurailius]

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

 

[patprimmer]

Opening the tappets cannot retard the timeing. It will reduce the duration and lift. In reducing the duration, it will retard opening, but advance closing, leaving the centre exactly where it was.

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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[BigVlad]

I think you should consider the basic cause of "standoff" - fuel mist/spray coming out of the carbs. The only apparent cause would seem to be the excessively late closing of the intake valve allowing the piston to push the fuel/air mixture back out of the intake tract when the engine is not "on the cam". Is the cam maybe not actually a 280 but but something with more duration? Is the cam still not installed correctly?
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.

 

[bigpaul]

Big vlad the base circle can be a problem we have seen as much as 12th rise on base circle caused by bad grinding, the effect is to close the tappet setting and leave the valve of the seat causing what looks like stand off. Opening the tappet clearance is a quick way of checking
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.

 

[patprimmer]

An easy way to check base circle run out is to get on the start of the base circle,, then with someone slowly turning the engine, keep testing the clearance by constantly sliding the feeler gauge in and out, looking for loose or tight spots.

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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[gt6boy]

Thanks guys.
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.

 

[BigVlad]

103/105 degrees? Where is this measured from? I still suspect that the cam is not correctly installed. "Kerfuffles" are a sure sign of cam or ignition maladjustment. A change of only 2 degrees would normally have little (or no) noticeable effect. So if it was wrong at 105, it still will be wrong at 103 (wherever this is measured from). Installing a cam is a very tricky business once you get away from standard engines with standard cams and timimg marks (and even this can go badly wrong if you are not careful). I have seen quite a few people swear on the bible that the cam timing was correct only to discover later that it wasn't.

 

[patprimmer]

An easy way to check if the cam is close is to check the split overlap. Bring the piston to TDC in the non fireing or overlap stroke and check the valve lift. If both valves are at equal lift within a few degrees of TDC the cam is close enough to run good if not optimum. Optimum can be up to about 6 deg from TDC, depending on cam and engine and track.

Regards
Pat
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[gt6boy]

Hi Vlad, whilst I am not expert at such things- its all been a learning curve, I did try to be very thorough in re-timing the cam in, as it may be that this hadn't been the case by the person who originally timed it in? So, once the head was off, this was how I did it.
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?

 

[BigVlad]

Gt6boy: Not many people are experts at cam timing - I'm certainly not. However your methods and calculations look OK to me. One minor point is that 103 degrees would give (assuming the duration is 280) the quoted 37-63. 105 would give 35-65 - not enough to make any noticeable difference. Your method doesn't give the total duration so I suppose there is a slim chance that the cam has more duration than you think.
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?

 

[patprimmer]

Different cam producers quote duration at different lifts. It can make a considerable difference. Set it up at split overlap as I described earlier, that way you cannot go wrong.

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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[BigVlad]

Pat: The method you describe is actually what I normally do. I work pretty much only with OHC engines where you can usually see which way the cam lobes are pointing. The method has the advantage that at TDC at split overlap etc. you can see the lobes and visualise how they relate to valve opening/cam theory/4-stroke theory in general. If you can see the distributor rotor at the same time (if you have a distributor) you can also verify that the ignition is in phase with the rest of the system. The method does tend to be foolproof - at least as much as such things can be.
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?

 

[gt6boy]

Hi Vlad.
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.

 

[BigVlad]

Gt6boy: I really don't think your manifold changes are very wise and they may be the cause of all the problems.
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.

 

[patprimmer]

I certainly would ensure that there was no exhaust gas entering the inlet air stream. You could use exhaust to heat a jacket sealed from the inlet air or you could use engine water to heat a water jacket on the inlet manifold.

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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[gt6boy]

Hi guys.
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.

 

[PEW]

GT6boy,

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.

 

[patprimmer]

The cold start enrichment on one carby only works. Well at least it does in the climate in Sydney and with a slightly downdraft manifold runner.

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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