Frank, I did not know much about how SU carbs work. In one youtube video of Adam from “living with a classic” he explained with a different SU carb than the one I have, but stated a flooding carb can also cause the engine to stall. I checked the overflow tube of my front carbs a few time. It is wet, although not dripping or leaking badly like yesterday. The back carb’s overflow tube has been dry when I inspected it. So the “starvation” problem I have for now may be a “flooding” problem. I will try to observe the front overflow tube again and see if it leaks during the rough running of engine. Just get one question though, wont flooding cause black smoke? just like a rich mixture? I have not seen any black smoke when the engine was running rough.
The fuel flow to the carbs can be simply checked by disconnecting the supply pipe immediately before the float chambers, directing the end into a container and switching on the ignition to run the pumps. This may also clear and remaining debris in the pipes. Thereafter I think the problem must be with the carbs themselves - either in the float chambers or piston/suction chamber assemblies. I know you are reluctant to open up the carbs but I would suggest you look at these next. I had two similar problems with the HS8s on my S1 which turned out to be:- low/incorrect dashpot oil (a very easy check) and a jet needle which had fallen out of the bottom of one of the pistons (required limited dismantling but a very easy fix.)
Frankie
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It usually does, Fan Yi, but not necessarily visibly so. It was just a thought referring to the ‘hot’ problems; the xk is fairly tolerant to excessive fuel - but there is a limit, particularly when hot…
Frank
xj6 85 Sov Europe (UK/NZ)
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Fan,
Frank’s hint is pertinent - now that we know your fuel filter is as intended, do you have the regular twin pump setup? One for the right tank, one for the left tank? If you undo the five screws you can remove the shields behind the spare wheel well and check. Then you can see them, hear them tick … and maybe see them spill or leak fuel.
Assuming that your problem lies somewhere inside the front carb, you might not touch the main setup, as syncing and tuning twin carbs isn’t exactly a first-timer’s job. That being said you can’t do much wrong opening the float chamber and checking why the carb floods. Maybe the float doesn’t float anymore, maybe some debris has made it to the float chamber, maybe (one of) the fuel pump(s) is wrong: the float chambers will take only like 0.3 bar of pump pressure - about one tenth of what a pump built for FI delivers. If someone installed a wrong pump it may have been ok for some time, but go beyond what your carbs can take over a certain period of running time.
BTW, there is a wonderful writeup on the functioning of HD8 carbs on the Burlen website, I think.
Good luck
Jochen
75 XJ6L 4.2 auto (UK spec)
Gents,
Thank you very much for all your help and suggestions.
I watched more videos from Moss Motors on youtube, surprisingly found out my carbs are HS model rather than the HD I believed for so long. There is an easy enough to follow, step by step, videos on their channel. I will run the engine again today, with both tanks, after putting some more fuel in them. I will check the model/brand of the pumps as well. They are SU pumps, but with some black sticky tapes around. So I am not sure about the models yet.
About the dash port oil, I use only engine oils in them. I read all the previous discussions on the forum, different types of oils like ATF, engine oil and others were used by owners. One even compared the possible difference of them. I stick for engine oil as they may end up in the engine anyway.
I remember that when the carb leaking happened last time, the flowing stream out of the front carb was quite a lot, almost like a baby peeing. I will observe carefully, not turn then engine on when there is a leak. Maybe a needle and float renew is a good idea.
Good afternoon gents,
I did more investigation today when the sun was shiny. First I open the boot again to check on the two fuel pumps. They are identical SU pumps and look like this：
I tried to apply the flow bowl tapping technique to see if the fuel over flow problem will go away, but with no success. After the pump clicks for about 3~5 seconds, without stopping and still clicks once every few seconds, there is a strong leak at the front carb overflow tube again. This is what I get for the 3~5 seconds with ignition on. Maybe 10~20ml in the cup. I feel I definitely need to do something to the needle valve and floater in the front float bowl.
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Indicating that the pump stops as pressure is reached, Fan Yi - then restarts as fuel bleeds off. Which is exactly what is expected from the pump - but they should really both do the same. If they differ in behavior, it is likely no great matter - but you could check both pumps’ pressure, for a possible explanation. Carb pumps are usually rated to 15 psi while some 5 psi is usually adequate - high pump pressure tend to raise carb fuel level even with float levels set to spec…
Obviously, the ‘leaking’ carb is flooding - certainly causing a fueling issue, and should be attended to as Jochen says.
While using engine oil in dampers may work satisfactorily; the specified oil for carbs is SAE 20 - thicker oil may interfere negatively with piston movement.
Frank
xj6 85 Sov Europe (UK/NZ)
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Thanks Frank,
I put 10W30 engine oil in the dampers. This is the left over engine oil for my Subaru. I will attend to the float bowl in midweek and see what surprise I can find in there.
Fan
This is indeed the officially recommended grade of oil but it caused me great problems. A thin mono grade oil looses viscosity when hot which in turn allows the piston and jet to rise more quickly and adversely affects the mixture. My S1 (HS8s) had a significant lack of power when hot. Changing to a multi-grade 20/50 improved performance no end. Back in my youth (longer than I care to remember now) I experimented with thicker grades, ending up with an EP90 in the SUs on my Triumph Spitfire - and concluded that 20/50 or thereabouts resulted in optimum performance.
Frankie
May I suggest looking at the sparking plugs? Run the car round the block until the engine reaches temperature; park up and allow to cool a bit; remove all six plugs and keep them in a numbered row in order. Take a photo of all together and post here. The colour may give us a clue or allow us to definitely rule out a possible cause. When replacing the plugs be sure not to over-tighten.16ft/lbs will be enough.
Frankie
Frankie,
Thanks for the info. I run 20W60 for my S1. Will try to put some in the damper and see if I can tell the difference. But first thing, I will sort out the overflow problem. With a carb leaking like this, I feel like sitting on a gas bomb. Strangely, I never noticed this sort of leaking before. If it has been leaking that badly for a long time, it will be difficult to miss the petrol smell.
I replaced the spark plugs not long ago. Maybe 200 miles are done since the spark plugs changed. Now all look like this. Mixture is rich I think.
Regards
Fan
I would be suspecting a sunken float causing the overflow.
Fan:
Needle and seat . A simple device/ used in many applications. But, yet tricky. Worn or corroded seat? Same for the needle. Misalignment of the pivot. A hole in the float… it no longer floats. Teh elvel of the float has to be just right. too high and it overflows. too low and the engine starves for fuel.
I am not familiar with yoru car’s carbs. But, i do know that none that I ever woked on, and there ahve been more than a few, could be set in situ… The carb must come off…
Carl . .
Fan,
it is a lot easier to miss a fuel leak in the engine compartment than in the trunk. As long as you’re sitting comfortably in the cabin and leaking fuel is condensating quickly in the warm engine compartment you won’t notice until you open the hood and lean over the carbs from the right wing.
Follow this line and check the float chamber.
Good luck
Jochen
75 XJ6L 4.2 auto (UK spec)
Hi, when one of my carbs overflowed due to the valve in the float chamber having some foreign body in it, my car would not tick over, it would run normally if the accelerator was depressed but it refused to tick over. The petrol was running out of the overflow pipe and onto the ground.
Easy fix though,
Bob
You don’t need to remove the carb to set the float level, it is set in the top cover of the float chamber. Remove, one bolt on top and fuel line in, invert and try blowing through the inlet. The weight of the lever should be sufficient to close the valve. Also check that the float floats, not sitting in the bottom of the fuel bowl. Also, there is a tang on the back of the lever that prevents it dropping too far down, which could allow the needle valve to drop down and jam. If all there is well, check the float level measurement as described in workshop manuals. The line into the carb is a real pain to seal ( the banjo bolt connection) and any weeping there will also allow the fuel pump to keep clicking. Some washers are fibre, some aluminium. If aluminium they sometimes get a lip on them, which you can rectify by sanding on a flat surface with wet and dry sandpaper, ditto to the float chamber lid. It’s a simple system so attention to detail will sort it.
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Indeed, Frankie - but I would have thought Jaguar had thought of that. Besides, the carbs should not get very hot - they are cooled both by fuel and air flow.
Problems encountered may have been due to other factors than damper oil viscosity - but if thicker oil solved a problem, so be it…
Frank
xj6 85 Sov Europe (UK/NZ)
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I was taking my wife’s nephew sightseeing the winter olympics area in Oslo in my Simca/Talbot, Fan Yi. Going back towards home I got an occasional wee whiff of petrol, nothing actually alarming. However, as I drove on; I noticed that the petrol gauge was noticeably dropping gradually. Now, my idea with car trouble is that if the car is indicating trouble, but actually moving in the right direction - I prefer to get home to tackle the problem…
So I got home in half an hour, backed into the garage and opened the bonnet. Oh boy; a blind bung on the carb had popped out and fuel was actually spraying out at (carb) pump pressure - it was a bit ‘disconcerting’! 20/20 hindsight is good - but does not beat dumb luck…
Frank
xj6 85 Sov Europe (UK/NZ)
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It has been a puzzle for me for some time and I can’t quite get my head round it. If the damper oil is thicker and the piston therefore rises more slowly does that give a richer or leaner mixture?
Frankie
Slower piston rise would be causing a higher depression over the jet causing more fuel to be drawn out of the jet?