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Those are nice to haves but not necessary. The simple Uni Syn achieves just as good a job.
I have them for my Bentley, and they work on the Jag, same carbs _ that is what I use.
I have one of those Uni Syn devices, and have set the carbs up with it, and then checked with the dial indicators, I can still tweak it more with the dial indicators.
Originally Posted by Jagboi64
Isn't that dial indicators? They would measure jet or piston height, but not necessarily flow through each carb.
Yes, they measure piston height, and that is effected by air flow based upon how far the butterfly valves are open, or where the the idle jet is turned _ in the case of the HD6's
I suppose if the throttle shafts were worn, the dial indicators may not be as accurate, but the HD6 carbs have sealed shafts.
The dial indicators are nice because the plenum chamber does not have to be removed, and even better on my Bentley, as it's a Royal PITA to use the Uni Sync on it.
My '70s-era Crypton Syncro Check is working but the rubber cone is fairly hard. When it is time to synchronize the carbs I believe it will do the job. Here is the link to Fuelmiser SU-750NS on Ebay from Australia. I emailed extramileautoparts (the seller) to request expedited shipping.
Whoops! Forgot the link for the SU-750NS: https://www.ebay.com/itm/14301168230...53.m1438.l2649 . I found two pancake filters from AH Spares. I emailed them to make sure they will fit my HD-6s. One has a fitting for the breather and one does not. I am going to have to be patient while waiting for the parts. This is the first time since I got the car in December that the wait for parts has been longer than a week, except for the rear seat cushion. On close examination of the starting carb, there is evidence of overflow. I am betting that the lowered fuel level and replacing the float needle and seat will fix it. If I can find good pancakes for the HD-6s I am going to put them on. The motor is beautiful indeed without the stock air cleaner covering it and access to the carbs. I have a nice storage box for the stock bits.
What an education this series of posts has been. Thanks to All!
Just as a reminder, when adjusting the float valve level, do not push on the fork to adjust the gap to make it smaller if need be.
Pushing on the fork will transfer the force onto the Viton cones and distort them.
I have them for my Bentley, and they work on the Jag, same carbs _ that is what I use.
I have one of those Uni Syn devices, and have set the carbs up with it, and then checked with the dial indicators, I can still tweak it more with the dial indicators.
.
But which is right?? Total airflow is total airflow.
The Uni Syn is only as accurate as the little red plastic float as can be read by the lines in the window tube.
It can be hard to see it if it's slightly above, or below the line or straight on.
When reading it, I have to use one of those bright pen flash lights to take care of any shadows under the hood next to the fender well.
Even after that, I can use the scale on the dial gauges to get it a little closer, and then checking again with the Uni Syn still shows they are balanced off course or as close as one can see the red float.
The dial gauges are pretty much over kill, but they are very convenient, and just to be clear, I don't use the Uni Syn anymore.
To answer your question, they are both correct, one is just a little more accurate then the other.
I don't think the engine cares.
The down side of the dial gauges is the engine has to be in pretty healthy condition to get an accurate reading, any appreciable difference in valve clearance, compression, or even a sooty plug not igniting the fuel properly will cause slight fluctuations in the gauges movement, you have to average the fluctuations out.
Over at the RR forum guys try to, and claim they can get their engine to idle at 400 rpm, I have yet to come across any RR straight six that will idle down to a smooth 400rpm.
It even becomes more difficult on a low compression engine with a 115psi compression to get it to idle smooth at that speed.
They get so obsessed that they seem to gravitate towards the use of the dial gauge.
A Jaguar engine would probably be easier to get it to sit there and not have it move at idle with no needle fluctuation.
"The Uni Syn is only as accurate as the little red plastic float as can be read by the lines in the window tube."
Which is more than adequate as I said. I have never had any problem. Balance further up the rev range is far more important than idle.
The dial gauges are nice to haves but far from essential when Jaguar only recommends the listening test. The pull a plug wire & equal rev drop is probably the most accurate as it is what the engine is "seeing" at various RPM. Carbs wear over time and don't feed cylinders equally. Manifolds are not perfect unless flow benched, porting is not perfect. etc. You want each cylinder to pull equal weight.
Last edited by Glyn M Ruck; 05-31-2022 at 07:33 AM.
I have also had good luck with the "rubber hose method" detailed in the manual of listening to the intensity of the hiss. I balanced the carbs by listening and then put the flow meter on and the carbs were sucking equally.
The important thing with the hose to do it exactly the same on each carb, and make sure the hose is positioned identically in relation to the bore on each carb. Repeatability between each carb is more important than the exact technique.
That's what I used to do. Of course my ears worked better in those days. I think that the noise is that created by the turbulence in the flow around the piston blocking the venturi. Since the noise power increases very rapidly with velocity, it could potentially be the most sensitive method, better than all the gadgets. That's assuming the rubber hose is in the same place and the listener has good ears.
"The Uni Syn is only as accurate as the little red plastic float as can be read by the lines in the window tube."
Which is more than adequate as I said. I have never had any problem. Balance further up the rev range is far more important than idle.
The dial gauges are nice to haves but far from essential when Jaguar only recommends the listening test. The pull a plug wire & equal rev drop is probably the most accurate as it is what the engine is "seeing" at various RPM. Carbs wear over time and don't feed cylinders equally. Manifolds are not perfect unless flow benched, porting is not perfect. etc. You want each cylinder to pull equal weight.
Yes, I'm not arguing anything here, but they HD6's can't be balanced further up the rev range, so what is the point of that statement ?
One could balance a set of carbs from my Bentley that way because there is no idle jet adjustment, but the engine would be puking out all sorts of exhaust out
Because pulling a plug wire at 3000 RPM as an example & measuring rev drop would, as I say, make sure that each cylinder is pulling equal weight. The rest is irrelevant.
Last edited by Glyn M Ruck; 05-31-2022 at 05:06 PM.
My properly balanced S Type engine. Crankshaft, flywheel & clutch. Equal weight rods & pistons. Idles beautifully. You can put a glass of water on the engine & only observe a slight disturbance of the surface at 600 RPM. Friend's Aston's (3 twin side draught Webers ~ Vantage engines) & V12 Ferrari's (with 6 Webers) will only let me touch their carbs because they like the outcome. All done pulling plug wires. Obviously engines in good nick.
Last edited by Glyn M Ruck; 06-01-2022 at 07:08 AM.
The following is how I’ve always understood the synchronisation of HD6 carbs.
The objective is to make the two carbs supply close to equal amounts of air at idle. This is to achieve a smooth idle (similar power from all cylinders) and also to facilitate adjustment of the mixture strength (which obviously requires that neither carb is dominating the supply of mixture). The butterflies have been set to act together and are closed so the air flow through each carb is (almost) entirely determined by the slow running screw. As its name indicates, the slow running screw influences slow running only. That’s the adjustment we’ve been talking about and can be made with the help of various gadgets or pulling plug wires. At higher revs and load, the butterflies are open and have control of the air flow and have largely taken over from the flow past the slow running screw. The butterflies were synchronised at the beginning of the process by setting the linkages such they both fully close together.
Once the air flow at idle is equal for the two carbs, the mixture can be adjusted. After that, the air flow might be re-checked and the perfectionist can re-cycle the process.
With everything in good order, the idle of the 3.4 litre XK is remarkably smooth, better than a fuel injected V12.
Plug pulling seems a good idea to me especially for one carb choke per cylinder arrangements. Since Glyn uses it on his car, it works for the less isolated cylinders with twin SUs. Doing it at 3,000 rev/min presumably gives an extra check on the synchronisation of the butterflies.
Absolutely correct Peter. The slow idle circuit is irrelevant once higher power & revs are demanded from the engine & one wants all cylinders pulling equal weight & taking into account less than perfect manifolds & breathing. As you say these engines idle extremely smoothly ~ all things being correct.
On 1 choke per cylinder set ups plug pulling works a dream but also works well on the twin SU engines feeding 3 cylinders each & 3 SU engines feeding 2 cylinders each.
Last edited by Glyn M Ruck; 06-01-2022 at 09:34 AM.
I managed to put this original post in the wrong place so thisis a repeat
With all this discussion I am referring back to the original problem of carby fuel smell when hot,
Fuel will expand when heated. In fact this is a well known factor in aviation. The specific gravity for aviation gasoline varies between .68 in the tropics to .72 in colder climates at typical temperatures as an example..
Stopping a hot XK engine means that the carby can become "heat soaked" and fuel already in the float bowl will expand.
This could be compounded by a poor or almost non existent venting of a float bowl due to a blocked "overflow" or vent pipe.
This could also lead to flooding as the float needle and seat can no longer function as intended.
This could also lead to the "reluctance to start".
I have just been through this problem myself with my MK2 as the "bug stopper" mesh I installed over the vent pipe outlet of one of the carbies fell off and our friendly mud dobbing critters made a home in it.
Symptoms were as per Blue 340s original post.
__________________
Bill Mac
MK1
MK2
S3XJ6
X300
15 previous Jags Mk5 to X308
Absolutely Bill ~ as hard starting hot can also be due to fuel vapourisation. The fuel companies make Winter & Summer grade gasoline that most are not even aware of because we make the change over unannounced. Winter grade has more light ends to aid cold starting (lower density or specific gravity) & Summer grade less light ends to prevent vapourisation.
e.g. In a low use car you can still have Winter grade gas in the tank/tanks. This can lead to vapourisation in summer and hard starting. Then endless efforts to start the car can lead to flooding.
Listen to Bill ~ he's spot on. It's why F1 cars put the coldest fuel permissible into their tanks as they are fuel mass restricted by the Regs. i.e. 110 kilograms of fuel per race. They want highest density possible through the injectors. Density is related to water which is deemed to have a density of 1.00.
Wasps etc. love building nests in low use older car vent pipes. Gauze capping the fuel drains is a very good idea. Depending on where you live.
All of these situations can lead to fuel smell. The Thermo SU's always emit slight fuel smell from the starter carb. But it is slight.
Our old cars smell different to the modern car where everything is captive. Even refueling site nozzles on the pump are captive of fumes today.
Fume captive nozzle. Outer concertina shroud recovers fumes back to the underground tank.
Last edited by Glyn M Ruck; 06-03-2022 at 11:43 AM.
I checked the clearance between the inlet flange on the front HD-6 and the fender. Neither my Crypton Syncro Check nor a pancake will fit, so, back to stock even if it hides the beautiful valve covers. The air intake bolts that came out were 3/4 inch and 1 inch long. Perhaps shorter ones will mount up easier. I am waiting for new floats and a 375-324 synchronizer from Moss. I needed a synchronizer fifty years ago and assuredly need one now. The Fuelmiser SU-750NS float needle and seat are on the way from Australia (30 days away). the attached photo shows the fire extinguisher, screw jack, and tire tool mounted to a board under the front and back seats. The 2 x 4 to put on top of the jack is behind the fire extinguisher next to the rear seat. The new floats should be here next week. I am anxious to see what is lurking in the float chamber... After cleaning the float chambers the needle and seats from BritishSUperior will go in unless the floats are sinkers. Regards, Morgan
I know the stock air intake is big and bulky and not the most pleasing of things to look at when you open the bonnet but what I did as I had a spare one lying around was to paint the air box the same as the colour of the car, Old English white. Stands out even more now but for all the good reasons. The original metallic silver does nothing for the engine bay.
I got this idea from Jose who had painted his air box red the same colour as his car. If he has one I am sure Jose will post a picture of his.
05-30-2022, 02:23 PM
