That was my initial thought but the same argument could be applied to the air venturi (greater depression = greater flow); and of course the needle and piston rise will always be in the same ratio. Acceleration requires a richer mixture so logically you must be right.
Frankie
As I understand it, the purpose of the damper is not so much to change the fuel flow as to slow down the air flow. The normal reason for an accelerator pump is that when the throttle is opened the air responds immediately while the fuel takes a bit longer, thereby resulting in a brief lean period. An accelerator pump isnāt to richen but rather to avoid the lean. In SUās and ZSās, the damper slows the air so there is no lean period. Thicker oil in the damper slows the air more. Most users, if they fiddle with it, conclude that thinner oil results in better throttle response, perhaps because any oil at all slows the air enough to avoid a lean misfire on accel.
The difference between ambient air pressure in the car bowls and vacuum at the jet forces fuel into the air stream. The higher the speed of the air past the jet, the higher the vacuum, and restrictions in the passages increases air speed - as of course does engine air demands. The same applies to the vacuum port(s) serving the piston, but lifting the piston also alters the jet orifice, controlled by the precisely tapered needle - increasing fueling as the piston rises. The combined effect; you are of course right; and the āacceleration pumpā effect is vital to prevent āleanā engine stumbling. Essential function on all carbs, and indeed on EFI, where changes in pedal position is used by the ECUā¦
As an aside; wear in the damper, piston/bore will affect the damping effect - as will oil viscosityā¦
Frank
xj6 85 Sov Europe (UK/NZ)
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Guys, I am back. This afternoon I had one hour to look into the leaking float bowl. I watched Joeās youtube video about different SU flow bowl covers last night, and felt I should find out what part I have in there, before ordering some wrong parts online. This is how the flow bowl looks like. These bolts and old gaskets achieved a mysterious balance to seal the fuel. I know that once I open it, I wont be able to re-seal then unless new parts are used.
Many swear by Burlen Fuel Systems. If in any doubt give them a call - they are friendly and helpful. I used this kit on my HS8s. HS StayUp Float Kit - HS6 & HS8 Includes .096" Needle Valve - SU Carburetters
Frankie
Great work, Fanā¦
it is a float valve issue. Found. A huge step forward.
Now rthe fix. and yoiu are on track there. Seals!!!
Carl
Great, Fan!
Big chances you got it! As much as us kids of modern technology hate to admit, but, if left unfettered, it is not unlikely that a connection made 50 years ago is still tight. So Iām not that surprised by the fact that your fuel chamber was still well-sealed ā¦ btw while gasket materials age there is absolutely no reason for the high quality screws used to do the same in a fairly comfy environment.
Getting it sealed again is a much harder job and, as Frankie wrote, Burlen is a fairly safe harbour when it comes to OEM or close material.
Good luck
Jochen
75 XJ6L 4.2 (UK spec)
Fan, whilst youāre in there perhaps some new fuel line? Your one is the braided type, with a cap to cover the strand ends. Given itās all previously crushed it might be difficult to get a seal. If you really want to run it before Jagdaim delivery could always cut a rough gasket with gasket paper. I have done same before. Repco or similar should stock. Happy for you that itās coming along. Paul.
To clarify my earlier post re sealing washers, your cabs have a different top cover to the one I was referring to, so my comments are irrelevant in this case
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According to my manuals, Fan Yi; the HS8 bowl is to be inverted for checking float level - and there is no adjustment; needle/float to be replaced? And manuals specifies that the alignment of the lid and float chamber be marked for exact position when matedā¦?
Frank
xj6 85 Sov Europe (UK/NZ)
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You are right about the fuel line Paul.
I can see petrol leaking from where the fuel line joining the bowl cover. Some gasket/sealant need to be used to seal it.
I think this is correct, Frank. On my S1 (HS8s) there was a brass washer under the needle/seat housing. I remember reducing this thickness on glass/wet and dry paper to come to spec. My kits were complete including throttle shafts and needle/seat valves. Now it purrs like the big cat it is. Paul.
Thanks Frank, I checked carefully with the manual after watching Joeās video about setting the gap of floater. The mechanic used by PO used a shim to adjust the height of plastic float. But strangely, the gap was about 10mm, much bigger than the preferred value. So I am thinking the fuel level in front carb could have been too low for most of the time and run lean. That is what I noticed when measuring the engine bay temperature. The front header was abour 40 degrees hotter than the rear one when running (340c vs 300~310c). I followed the following page to set the gap.
Thanks a lot for all reply and info guys. I will post again after I have the parts.
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A reasonable inference, Fan Yi - lean does generally runs hotterā¦
The gap of course reflects the float level position when the needle valve closes; the larger the gap the lower the bowl fuel level - but it is equally important that the needle valves do close at the set level! In my experience; the usual fault with incorrect fuel levels were failed needle valves - given intact floats properly adjustedā¦
Frank
xj6 85 Sov Europe (UK/NZ)
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Aye, most if not all carb rebuild kits contain fresh needles and seats. Brass to brassor at time a Viton tiped needle to a brass seat.
Never did t that I recall, but a possible test for a leaking seat. Gengly hold the float in the hod position/Gently blow in to the fuel inlet. Resistance or leakā¦
In a rebuild most just relace as a proactive measure.
Guys I am back.
The Jag has been sitting in the garage for more than 1 month. I had this fear that I may end up with a rusty car if I donāt get a chance to work on it sooner, just like my professorās ā2 rusty Jagsā joke.
So I finally collected all the required parts to replace needle, seat and floater in the front carb, also some 7mm fuel lines to feed it. This 7mm fuel lines are not carried by my local repco auto and took me awhile to get it from a shop working with vintage German cars.
However, after I replaced everything, there came more leaks from the fuel line connecting to the T metal tube before carbs. It seems all these old fuel hoses had a mysterious balance, which worked perfectly fine until I touched them. I ended up buying more 8mm fuel lines to replace that section. I would eliminate any noticeable fuel leak before attempting to start the car.
In this afternoon, we got stranded at home because of a short covid lockdown in Brisbane and it was raining outside. I made sure there was no fuel leaks after priming the pump then started cranking. It took almost 20 times turning the ignition key until the engine was back to life, partially may due to a weak battery. It puffed out a cloud of white smoke/steam until the temperature gauge started to move. I then warmed it up for about 15 mins. The loss of power problem did not occur during the time. Hope I can test it on road tomorrow.
Frank, I get more confused now. After setting the new floater level at correct value, the front exhaust branch still measured 40 degrees hotter (350 vs 310). Looks like there are more things I need to work on.
That sounds like the carbs are not balanced for fuel delivery, have you tried lifting the pins situated below the piston to check if the revs rise and then fall back?
The lifting pin method is well tried and tested but does not give sufficiently clear results for me, but maybe thatās just me. I prefer to whip the plugs out so I can see the evidence of a weak/rich mixture.
Frankie
