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MG MGA - carb synch vacuum port adapters
|I just made these to try - they are vacuum port adapters that replace the carb spacers and allow you to then use a set of vac gauges or mercury sticks to tune and balance the carbs.|
i am going to try them today. They should work great.
If so, I will then want to know if anyone else is interested in a pair as I can make a short production run using the water jet at work.
Let me know (email) if you would like a set - probably in the $25 range - have to figure that out yet.
|here is one more...|
I had a pair on my 48 TC but did not know where they came from. I have just been doing a search and it seems APT over your side of the water does them for about $16.
|That is $16 each...thanks.|
I didn't know anyone was making them - I had looked online using Google and didn't find any - so you are a bit better at it than I am I guess...
|Jim's thread got me thinking about the precision of using his system. Much better than my handheld device I have to hold over the carb intakes. It also allows the balancing to be done with the filter boxes in place which must also be a bonus.|
After balancing in the time honoured fashion I always then use the SU balancing rods in the carb pistons as a final check; see here: http://www.mgaroadster.co.uk/su_carburettor_tool.htm
Having just read the instructions again, I note that it says the rods can be used as the prime means of balancing, although it adds: "For precision synchronizing, a 'Unisyn' should be used along with the (SU) tuning tool".
I was wondering if anyone had actually used these rods as the ONLY means of balancing. If so, had the results been satisfactory?
I believe Barry Gannon may have gone down this route, so may be he could comment.
|Wouldn't the balance tube on the inlet manifold effect the result even though it is down stream of the tapping point. |
Jim, is the water jet a way of cutting out these parts from sheet plastic/fibre?
|Now I have seen the ultimate to the SU balancing rods. I found a video on the web showing a Jaguar tuner with a dial gauge in each of the carb dash pots.|
|I use the Factory Service manual's procedure, a piece of hose to balance the carbs. I use the rods to verify smooth chamber-piston action and their relative action. The wire rods are IMO not to be used for much more.
|And the 2nd part.
|yes, the parts are cut from a sheet of material using the water jet...very clean and fast.|
JIM in NH
I've always had good results using the SU tool balancing rods as my only means of balancing the carbs.
|k v morton|
Good to hear. An added bonus with the SU rod set-up is that when doing the fine mixture tuning (raising the pistons slightly), instead of using the almost inaccessible lifting pins, you can lift the rod shafts slightly as they are a good interference fit inside the damper shaft. This is especially handy with the the filter canisters in place.
|This thread has got me checking my carb balance and mixture for the first time since putting on my prototype stub stacks last October. Prior to fitting them I believe my mixture was pretty well spot on. On the checks just now they showed up as definitely running weak. Makes me wonder if the smoother air flow into the carbs has caused this. Has anyone else who has fitted the stub stacks found the mixture needs richening up a tadge?|
I had to increase the mixture after fitting stub stacks in association with K&N filters, I've not gone as far as changing the needles yet but it is something I'm considering.
I remember the wow factor when I first put the stub stacks on. I have just richened up both carbs by about 3 flats each (surprised it was that much) and my first drive out gave me that similar wow. The car is also so much smoother and less lumpy at and a little above idle.
I thought the carbs were ok on mixture before, but I had to take the canisters off to get at the mixture nuts, so maybe I did not have them that good before. Unless of course the crap fuel we get nowadays has an impact?
Just in case anyone is wondering about my poor access to the adjuster nuts, especially the rear, I have my water expansion tank sited right behind the rear carb.
Did you make a note of which needles you are using?
I must admit that needle types are a weak point in my knowledge and what constitutes changing them around. I have the standard GS needles from my 1500 days even though I am running the 1800. I have always been able to adjust the mixture accordingly to get the right plug colour etc. I seem to recall asking Bob West if I needed to change to a different needle when I converted to the 1800 and he said to only change if I could not tune it. When correctly set up, like today, the car goes like greased lightning.
How would I benefit from other needles? Please instruct me.
The correct needle profile can only be arrived at through real world testing, preferably on a rolling road. The factory did that, for standard engines, but any changes are just guesswork, educated we hope! Even at the factory with dead "standard" cars, there was a wizard who set the ones up that were "not quite"; he took them and filed the needles to the proper "corrected" profile - we sometimes used to come across these - the needles have XXX or similar stamped in along with the standard designations. Of course these are mostly lost now, or irrelevant as the engines/fuel/conditions have changed.
Only after the right profile is found for your combination of bits can you "set" the carbs. This is always based on the off idle profile being right when the idle mixture is right. In fact, nearly all needles are very close to identical at idle, but there are huge differences above. While an "educated" guy might get fair results in road testing, even the best and most experienced CANNOT get it "right". Even Vizard said they once got 30% more power after putting a competition Cooper S on the RR, and "it sounded no different than it had before". The most experienced and educated are the most insistent that you use the RR for setup; every one of these people say that the RR setup is the cheapest, most cost effective, and most important part of any engine build from stock to full comp. What can you get from needles? More power, better economy, better driving, melted pistons, cracked heads... All of which means get your car on the RR with somebody like Peter Burgess, who can actually get the profile correct. You are unbelievably fortunate to have RR readily available, and especially when you get the few like Peter who know what it all means - use it!
IF the vacuum chambers on SU are correctly matched and in spec, the position of the piston is a precise measure of the airflow through the carb - it is in fact an analogue air flow meter. No other meter is needed - you can use the rods, dial gauge, or on many cars just look down the bore to verify that both pistons are the same height, at ANY throttle opening (which you cannot do with a UniSyn or similar). MGA is hard, as you need mirrors or gymnasts!
The proposed vac adaptors are a complete waste of effort, since the balance tube means that the vacuum behind the throttle plates will be the same even if one carb is completely shut off and the car is running only on the other. Works great on non balanced systems, like 4 independent carb bikes. These are just a manifold vacuum pickup, and if you wanted that you could just drill the existing spacers or the manifold.
A little complicated for me to explain but here goes.
The SU has a fixed jet and a tapered needle which moves up and down in the jet.
There are different shapes of taper, the GS which you have is one shape, but you could choose a differnt needle such as an RO which would have a differnt shaped taper allowing the mixture to be richer (as the needle is thinner) in parts of the rev range.
Most needles have a similar profile around the area where tick-over will occur, but the differnt shaped taper can allow more fuel in other rev ranges which may provide better performance or economy.
As FRM suggests above, you have to be careful not to hae the mixture to weak as you could run the engine to hot and burn a hole in the piston.
From recollection the needles fitted to MGA's with a 0.90 jet were.
1500 GS standard needle
1600 No6 needle
1600 rich R0 needle
1600 weak AD needle
The minty link below is quite useful if you want to play
And I'm sure someone else will have a better explanation
|As John said, but be aware that the carb piston spring is a basic tuning component; changing it affects the entire range, with identical needles fitted. Generally, slightly modified engines with OE size carbs take the same springs as originally supplied. More highly modded engines with same carbs may require stiffer springs (or heavier pistons) and consequently completely different needles.|
A well tuned engine running at a correct slow idle, will always be running at about station 2 on the needle, progressing to 3 as idle speed increases. If you look these up, you will see that most needles are very nearly identical at stations 1 & 2, diverging at 3 and up (well that'd actually be down, yes?)
Free flow filters cause the top end to go lean, so require needles a bit richer starting mid range, with the greatest difference at the top end.
|I have always wondered why running lean causes an engine to get hotter. In theory, the exact stoichiometric mix of fuel and air gives the hottest flame. Any excess air or fuel only serves to cool the combustion. Anyone got an explanation? Diesels run very lean and don't get over hot.|
|Good point Art.|
I had the same question years ago, and it seems that the temperature drop from lean running is less than the temp increase from not evaporating the extra fuel. This can be and often is carried the other way, such that somewhat rich mixtures cool the chamber enough to get more mix in, resulting in more power but less heat, and a consequent increase in wasted fuel and thermal energy = less economy. So you gain power (volumetric efficiency) and engine life (cooling) at the expense of fuel and thermal efficiency.
Additionally, if there are carbon deposits or oil in the chamber, the excess oxygen causes them to burn, in a manner that results in more heat but usually less power, as oil especially is of very low octane and causes possibly fatal detonation.
All this is why max power mixture is usually in the 12-13:1 range, but max economy might be at 15-16:1, while best overall would be stoich, if you could run the engine there. You can only do that with a constant speed/ constant load engine though.
I guess you could always bolt the vacuum adapters between the carbs and the filter canisters?
|I don't think that works Steve - you normally need a few pipe diameter before and aft of the tapping point to get consistent readings - the experts will surely advise.|
|For what it's worth I will throw in my twopenneth and wait for it to be shot down and me educated yet again!|
My point is that I have never found twin carb air intake synchronising to be particularly accurate by any of the technical methods nor that difficult to achieve very simply. My usual method is to carefully inspect and adjust the throttle butterflies to sit perfectly in the bores so they both close perfectly. Then if both are in the fully closed position and the throttle linkage carefully tightened up, they should open accurately together over the full opening range. In many years of fiddling with SU's this method seems to have resulted in at least as good a running engine as other methods. Certainly as good as sticking a piece of pipe in an ear and trying to hear a vague similarity of hiss! I doubt other methods would be much better. The mixture control and its synchronisation is something else, however.
Any comment anyone?
"I guess you could always bolt the vacuum adapters between the carbs and the filter canisters?"
Only works if you have significant restriction in the filters. In some applications this is called a "filter service indicator", but it only reads when the filter is clogged. Also, if two filters (possible variable restriction) are involved, and two (variable) airflows, then you can deduce exactly nothing!
The UniSyn etc work by introducing a similar but much greater restriction, that is constant when you switch to the "other" carb, so giving a useful measure of the only remaining variable = flow.
In this case, you are measuring static vacuum, unless you locate the pick-off point exactly in a fast moving airstream, a pretty well nonexistent place at idle. So it really does not matter. The UniSyn actually creates a high velocity stream as a consequence of the restriction, and then monitors the active depression caused by flow - it is an "active" flow measurement, as oppose to the "static" depression of the other method.
Your method is theoretically correct but difficult in practice. The listen method is easier for some, and avoids some of the mechanical variables, but sometimes I've found carbs that just sound "different" in an unquantifiable way. Measuring the height of the piston is overall most accurate, assuming the vac chambers are correctly matched, but UniSyn amplifies the reading so it is easier to see.
If you can get satisfactory running with your throttle synch method, then cold start mixture control should be no problem, as it is much less sensitive. Main mixture setting should be no problem, as it can be set quite accurately by measurement, again assuming that all else is correct. If "all else" is not correct, then any/all of these settings become make-do compromises.
One tip for carb setup: All screws which are adjustable controls should have the ends rounded to effective hemispherical points, else you get varying effects of equal fractions of a turn. Screws are commonly rough sheared, which amounts to a random cam.
|I still haven't tried them, but I'll let you know how they come out after this weekend. I doubt FRM is correct in saying it's a waste of effort...but we'll see to what extent it is any better or worse than the other methods, all of which I have tried.|
I have found corroborating evidence for this method online since I started this, and have been encouraged by what I have read.
I will let you know!
I am still having trouble getting my mind around airflow and pressure in the carb inlet to the common manifold. I agree that once the airflow from both carbs reach the common manifold the pressures will equalise; but how far back up the carb inlets does this apply? Let us assume that the carbs are way out of balance and that perhaps twice the flow is going through one carb. By the laws of physics there will be a significant difference of pressure on both carb walls. Where does this equalise? Immediately after the butterfly valves or when the air actually enters and mixes in the common gallery?
I am certainly interested in hearing about Jim's results.
1 atmosphere = 30" Hg = 15PSI = 30 Feet water
1"Hg = 13.4" water.
"H2O are used for small measurements, "Hg for larger ones, and PSI for big stuff.
Pressure (vacuum) differentials only exist across a restriction in a flowing system. In a long pipe with flow, the restriction is distributed along the pipe, measured as unit pressure/unit length ("Hg/foot,etc.). In a carb, the major differential is across the greatest restriction and is local, at the throttle plate, when it is not wide open. Other sections of interest are across the air filter, across the venturi, and across the throttle plate when open. All of these except closed throttle are low at low flow rates. The genius of the SU "constant depression/variable choke" is that the venturi (choke) differentials are maintained at near constant levels over a wide flow range, unlike the "constant choke/variable depression" of most carbs.
For NA engines, the max inlet differential (ambient air vs cylinder pressure) is 1 atmosphere or 15psi or 30"Hg. Usual achieved idle vacuum is around 18"Hg. This is the measured difference between ambient and the downstream side of the carb butterfly.
The rating restriction for a clean air filter is 1.5"H2O
A really filthy one is around 20"H2O.
The rating restriction for a carb at WOT/full flow is 1.5"Hg
The "Constant Depression" design of an SU tells you that the reading on the upstream side of the throttle butterfly is always around 1.5"Hg, if the carb is optimally matched to the engine.
The CD vacuum can be measured on late carbs with breathers in the body; early ones you would need to drill in the same area.
I keep meaning to do this as a matter of information, but it is not very pressing. I expect the real numbers are somewhat less than the 1.5"Hg used for WOT flow ratings - let's say 1"Hg for discussion.
So, we have a maximum depression across the throttle plates of about 18 (manifold) minus 1 (CD)"Hg = 17"Hg
The entire manifold will see this, if all is in balance. If all the flow is from one carb, then the balance tube restrictor in the manifold becomes the only source of differential between manifold legs. Any pressure differential between manifold legs will be the result of flow from one carb through this restriction to feed the "other" side. If the carbs are balanced, flow will be zero; if one is closed and one is open, the flow will be at max. As the throttle openings get closer to equal, flow and restriction decreases. Since the balance restrictor is a large hole relative to the throttle open area, the flow signal or it's surrogate two vacuum readings will be very weak, ie, in the "H20 vs "Hg range. AJ might be better off to measure the balance with a water manometer connected to both vac adaptors than with comparing two mercury or gauge readings.
Big problem with these low & sensitive measurements is that cylinder pulses, standing waves etc. get bigger than the main signal.
I too am awaiting AJ's results, and also his "corroborating evidence".
|hoping to get time for this tomorrow morning...parts are still sitting on my desk, but have been traveling for work this week.|
|I agree with FRM's suggestion that for balancing the best arrangement for using these two ports would be to connect the two downstream ports using a water manometer..a big word for a plastic tube with about 12in of water in a downward loop . |
one of these ports would also be good for measuring the 'before and after,beneficial affect of insering the 'Steve duet ' stub stacks in the filter etc.
|"plastic tube with about 12in of water in a downward loop . "|
Better make it taller than that! 12" H20 is less than 1"Hg, per previous. If one carb were sealed and the balance pipe blocked, the max vacuum seen would be manifold vac, 18"Hg or 20 FEET H2O. Additionally, you want the liquid to be such that you can compare height in the parallel vertical tubes, not getting involved with the loop at the bottom.
If you hook this up and there actually is any significant difference between the two sides, it will instantly suck all the liquid into the engine. Usual procedure is to measure with good gauges before hooking up manometers, unless you are certain the range of the instrument is not exceeded.
I don't think any vacuum measurement will show any effect of the stub stacks, as it is mostly a transient phenomenon, and local to the inlet and jet area of the carb. As I said a long time ago, I think the only way to measure is on a dyno that can measure quick response acceleration figures under controlled throttle openings. Peter Burgess' new toy may be able to do so.
|FRM..being a rational practical engineer I presume an element of common and a realistic situation. The only practical approach is to get the carbs balanced approx., drill out the obstacle blocking the two halves of the manifold inserted by the local car terrorist.....and then try the fine balance with the manometer. You then don't need a python of a plastic tube going twice around the garage.and down the local |
also I do believe you would see a stubs stack affect. I did intend to run this test myself but now in Judson territory ..but do I recollect you are not a true believer?
|Thanks FRM. I got the gist of all that. Some of the scientific detail flowed over my head leaving a small vacuum between the ears! I get the impression that you are saying the pressure may still vary between the carbs at the spacer measuring points but may not be sufficient to measure on a typical auto vacuum gauge?|
"not a true believer?"
What, you are questioning my religion? Which belief are we after now?
Was just pointing out that especially with a water manometer, things can be surprising. Ask me how do I know this...
Do you know what size the restriction is in any given manifold? Especially the one that sat in the garden for 23 years? I personally have seen them blocked deliberately and otherwise, modified, or missing altogether.
I do think there is an effect of the stub stacks, but I do not think you can measure it there, and possibly not anywhere without very special high speed instrumentation - this is millisecond range stuff. Once I wanted to measure some things in this range, and found out that normal instruments and computers cannot do it. IF dyno sampling and response rates are fast enough, you could get a damped real world average as acceleration. Seems to me I was suggesting that you and everybody else measure all sorts of stuff before, during and after making changes, but no, nobody listens!
Steve- Not just "may not be sufficient to measure on a typical auto vacuum gauge?", but not recoverable from various "noise" resulting from pressure waves and etc and so forth, which are characteristic of piston engines.
But, I love measurements so long as they are done and communicated in a way that might mean summat, so have at it all!
|FRM....I regret the 'true believer' comment. I thought I recollected that you were somewhat cynical about the potential for stub stack benefits..but reading back I was wrong and it was another soul. I also never comment on religion (incl my belief in natural selection ) except on atheist sites.|
trust only the measurements need sensitivity......
You said: "...but not recoverable from various "noise" resulting from pressure waves and etc and so forth". Surely these are constants that affect both carbs through the common gallery so cancel one another out. i.e. they will show on both gauges (assuming a pair is used). Any difference in values shown on the gauges is therefore down to mass airflow and therefore valid.
|Well Neil, I am a guy who radiuses the ends of pipes inside plumbing connectors! And I think I can claim credit as a leading proponent of stub stacks, and some input into the design of the ones under discussion.|
As for God, if She shows up, I am ready to share a beer and discuss Darwin, or go motorbike racing, or whatever - guest's choice.
Steve- Sounds good, but it only works (maybe) if everything is absolutely symmetrical in the manifolds. If one throttle plate is open more than the other, the only situation giving overall unidirectional flow through the restrictor, then the situation clearly is not symmetrical. I expect that the open throttle induces some effective damping in the passage, while the balance flow induces transverse pulses into what should be longitudinal flow through the manifold. All this bounces around in complex ways which would give time dependent shifting pressure reflections, nodes and such.
A possible path of investigation is to have an adjustable balance restrictor, which could be shut completely. Then AJ could set his carb balance, and finally open the restrictor again. Does seem a lot of fiddle to synch the carbs though. OTH, Vizard relates extensive research on changing the size, shape, volume of the balance passage, with interesting results of an expensive manifold.
|FRM finally hit on the point why the manometer will not work in this proposed arrangement. The vacuum noise level is much higher that the vacuum differential that is proposed to be measured. The problem is that you would be measuring (comparing) vacuum levels rather than air flow.|
With small throttle setting (idle speed) the throttle opening is a couple orders of magnitude smaller than the opening in the balance tube (cross over tube) of the intake manifold, so air flows freely from side to side through the balance tube. This almost completely evens up the vacuum level in the two throats immediately downstream from the carbs. The vacuum signal differential that you would be measuring will be extremely small (unless the carbs are grossly out of balance).
Next issue is the flow of air from left to right in the manifold balance tube. This engine has Siamese intake ports with #1 & #2 cylinders sharing one intake port, and #3 & #4 cylinders sharing the other intake port. Firing order is 1-3-4-2, but you can run the same sequence starting with 2, and you get 2-1-3-4. For one revolution of the engine you have two intake strokes through one port. For the next revolution of the engine you have two intake strokes through the other port.
Result is about half of the intake flow comes through each carb while all of the the flow goes into one port. It follows then that half of the flow goes in through one carb and passes through the balance tube to get to the opposite port. With each full revolution of the engine the flow through the balance tube changes direction. Flow through the balance tube is driven by pressure differential (or causes a pressure differential, depending on your point of view). The magnitude of this induced and reversing pressure differential (with each rotation of the engine) will be substantially higher than the vacuum differential you are trying to measure between the two carburetors. At 900 rpm idle speed flow through the balance tube reverses direction 15 times per second.
Additionally, air flow is not a constant speed, and pressure drop is likewise not constant. At low engine speed pressure drop and air flow vary dramatically from beginning to end of each intake stroke. If the engine was turned slow enough the flow and vacuum level would go from zero to maximum and back to zero with each intake stroke. At higher engine speed (idle speed) and restricted throttle opening (idle speed setting) the result is wild fluctuation and pulsation of air pressure (vacuum level) in the intake manifold. That's the noise.
When the carbs are nicely balanced for air flow the vacuum differential signal you would like to measure would be zero, while the pressure noise in the manifold would be relatively large (much larger). The manometer would be dancing (vibrating fluid) all over the place while you are trying to read it. Solution would be to place a flow restrictor (small orifice like an adjustable needle valve for instance) in the manometer to damp out fluctuations of the liquid flow. If this is properly adjusted the manometer would dance within a relatively small range, and you may be able to see a difference in height of fluid in the two tubes. But the pressure (vacuum) differential signal you read would still be very small unless the carbs are grossly out of balance.
Back to the first paragraph, when balancing the carbs you really want to be measuring air flow rather than vacuum level. Instruments (like Slosyn for instance) that monitor one carb throat at a time are measuring air flow, not vacuum. The instrument has a venturi that generates a relatively high internal vacuum signal that is proportional to air flow and is easy to read. In reality the Slosyn unit is using the generated vacuum signal to drive a small air flow through a slightly tapered vertical tube, with the moving air column supporting the visible puck.
The Slosyn unit would also work if you would connect a manometer in place of the vertical air flow tube to measure amplitude of the generated vacuum signal. You could also use two Slosyn units with a manometer connected between them. Assuming the two units are exactly matched for output signal, the manometer would then provide a reading of air flow differential rather than vacuum differential. A small imbalance of air flow would generate a relatively large signal in the manometer for good accuracy.
Bottom line is monitoring vacuum differential between the carb spacers isn't going to work when there is a balance tube in the intake manifold. If you could momentarily plug the balance tube the engine might die, because you shut off half of the intake air flow that is feeding one intake port at a time at slow speed.
|Addendum: Even with one throttle plate completely closed and the engine idling on one carb, the downstream vacuum differential would still be quite small due to presence of the balance tube in the manifold. You won't be able to measure it (through the noise) without some expensive high speed instruments.|
Water. Methanol. Injection.
|OK - tried it.|
had to use a lot of damping to get the needles to stop bouncing, but I did get a reading.
There is a large "grey area" at low idle where you can adjust the throttle plates quite a bit and get no reaction - due to the balance tube effect I am sure.
It works after a fashion, but maybe not great.
After taking the time to remove and repace the carbs, I didn't have a lot of time to play with it, and the engine wasn't even warmed up, so I will have to play more later on to see how good or bad it is really.
So far, my earlier method of adjusting using the RPM/Dwell meter seems more accurate at low speeds.
This method does tell you what your actual vacuum is, though, so it would be interesting to set the valves at 0.17 and then at 0.15 and see if there was any difference.
I took pics and will do the test again and write down all the numbers and such and then report more comprehensively - as well as posting the link to the article I mentioned earlier.
Just not enough time this weekend!
I have to protest the "finally hit on the point..." part, since I said that in the first and most of my subsequent posts here. For everybody else, Barney has written an excellent alternative statement of what I've been trying to get across, and AJ's results are exactly what I expected.
Note that assuming a SloSyn is like a UniSyn, the part you read actually IS a manometer, albeit an air one rather than a water or mercury one. It is reading pressure in a venturi against atmospheric, and is calibrated as a flowmeter, biased by the little piston weight.
|Barney and FRM..I am persuaded by the true force of the arguments. In particular the major function of the balance manifold pipe to even flows to cylinder strokes.|
I have to say in times past I had excellent results using a suction synchronizer with air manometer I bought from Moss.
Thanks for the treatises on the subject .
|FRM, -- SloSyn is my pet name of Unisyn, because it is the slow way to synchronize the carbs (especially when you have to R&R the air cleaners). I have a Unisyn, but the only thing I ever use it for is to prove that you don't need it. It is stored way in the back of the bottom drawer along with catalog picture of Click-Adjust (because I never bought one of those).|
|Barney- Great name; might be some where near mine, which somebody gave me!|
Everybody interested, but especially you guys in England with Steve & Steve stubstacks:
On the MGB Tech board, thread "Need needle suggestions...."
I have gotten Peter Burgess engaged in a conversation about measuring throttle response on his new dyno toy. He can do it. So, get yourselfs over there and get us some numbers - and get your car set up right in the bargain. It can't be far - he is designated as "Near the centre of England"! I think you need to make an adjustable throttle stop for testing, but it will likely save you enough pounds in fuel costs, plus added smiles, to pay for the trip.
If you are desperate for a Click-Adjust you can have mine....!
Some good technical arguments have been put forward; very informative.
In conjunction with my SU balancing rods I have always used the gunson Carbalancer (photo) which I have found very awkward to use. Nowhere to clip the tube and the pipe is very stiff and forever falling off. I have been trawling the UK side of the web but have yet to find the better quality Unisyn shown in John Twist's YouTube video. Can someone point me to a (UK) supplier please.
|Following Jim's opening post and my initial response I ordered a pair of vacuum adapters from APT before all the subsequent discussions.|
First I would mention what an excellent postal service from APT. They arrived in 2 days! Faster than we often get with internal mail.
Second. The internal diameter of the adapters is ok. However, the slotted bolted holes are too small for the SU H4 carb, but I was able to drill them out using a correct gasket as a template. The adapters are also slightly thinner than the standard.
I fitted them on the car this morning and ran a test. Both off-takes read identical which I would expect as the carbs are in balance anyway. The Gunson's gauge gave a rock steady reading of 19 InsHg at idle, dipped to about 13 as I opened the throttle but immediately returned to 19 once the RPM stabilised. Looked a healthy setup to me.
When I am next bored I may deliberately 'unbalance' the carbs to note any different readings - if any!
Incidently, many years ago I found an interesting article on vacuum analysis. I have put it on my website for your interest: http://tinyurl.com/c86t5zx
|Great article - thanks.|
I will do a more thorough investigation now that I have this to reference!
|Steve, -- Maybe you already know there is en error in that article. Caption under picture "B" on page 4, "Mixture on most SU carbs fitted to classics is by screwing this nut up to richen". That should be down to richen.|
|Good spot Barney, I hadn't noticed. Incidentally, I note that the article concurs with our experts that the vacuum gauge cannot be used as a balancer on twin carbs with a balancing tube.|
|Too bad it didn't work out as originally hoped for me, but you can get lots of other good information at least! |
The best method I have used for balancing the throttles is to use my dwell/rpm meter rather than any direct measurement of vacuum or flow at the carbs, so this will be my fall-back. (If you hook up the rpm meter and split the carbs, you can adjust them very exactly so that if you adjust either screw the rpm changes - works really well, and no "grey" area.)
JIM in NH
This thread was discussed between 02/06/2012 and 08/08/2015
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