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I agree with Robin, Fan Yi - despite you careful work; the carbs are not fully synchronized…
However, there may be more subtle causes, not caught by carb settings. Like float levels correctly set, but differences in needle valves and/or floats themselves may cause differences. Admittedly, that should be compensated for by the carbs’ mixture controls, individually set in carb synchronizing - but the latter depends on perceptions rather than precise measurements.
Your stating problems is an issue in itself, but may be related to exhaust temps? The AED feeds both carbs equally, but single carb ‘choke’ variants may be involved. It’s very important that the ‘choke’, whatever fitted, is completely off and the engine fully warmed up, before synchronizing the carbs. Just for the record…
I assume that the temps are measured at similar spots on the exhausts (downpipes?) - and indeed that the higher temps are caused by leaner running on the front carb (or fatter at the rear one?). There are no specified temps given, of course - for all we know; both branches may run on ‘wrong’ temps…:). And theoretically; they are differently cooled, though one may expect the rear branch to run hotter in that case. You may measure temps during warm-up - though, likely, it may not clarify anything…
There may also be internal reasons, engine or exhaust, for different temps as well - but we are into guesswork territory, where imaginations may run riot…
All that said; if the engine is performing as it should starting and running; there is no cause for alarm or immediate actions…
Thank you guys for your response.
Today I tried to duplicate the problem that I started the post for. The loss of power only happened when engine fully warmed up, but not with the car in garage. I did notice one thing different, that I kept the engine bonnet open in garage to observe the fuel lines. This may prevent the engine bay temperature rise enough to cause the problem.
I then closed the engine bonnet and kept it running @1500rpm for about 25 mins. The power loss and stalling problem occurred. So it seems a high engine bay temperature is one of the criteria!
Would the mixture and fuel delivery by carbs get affected when temperature rises? Or it looks more like a coil problem now?
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High tamps may cause the fuel to boil, Fan Yi - causing vapor lock; fuel not reaching the carbs in sufficient quantity…
Look at your fuel hose routing; a high point in the routing may trap fuel ‘steam’ - restricting flow. Another thing; check fuel pressure; your pump may be set weak or failing increasing hose diameters will enhance the possibility of vapor lock. Certainly, hot fuel has less density which means loss of power - and if the carbs are already set on the lean side it may be a factor…
Vapor lock is not common in temperate climate, and some cars are more susceptible than others - possibly for individual reasons mentioned above.
Also; we know some ignition items, coil and EI amplifiers, if fitted, may start failing at high temps. You can check sparking, hot and cold, using a spare spark plug connected to any plug lead…
Thank you very much Frank. I watched some video about vapor lock, but still think it more like some kind of magic. I checked the fuel lines in engine bay, the higherst point will be the feeding tube to the float bowl. However, no bends are found. The posssible point for steam to get stuck I would say is the T shape metal tube connecting the main fuel line of 8mm to the 7mm fuel lines to the carbs. I may replace this with a Y shape connector to eliminate the large section of tube laying horizontally.
I found the same coil GT40R in one local auto store and will replace it in coming days.
I will report back.
How would vapour lock happen in our cars? Anything not liquid would be vented out the float bowls immediately and replaced with cooler petrol right away, as vapours wouldn’t lift the float?
Ignition sounds better as an attack point.
1 Like
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
67
The idea of vapor lock is that some fuel turns to vapor in a line somewhere, forms a bubble, and then the bubble gets jammed in a fitting and prevents flow. You’re correct, not likely to happen in the float bowl.
I see, but why wouldn’t the bubble be flushed towards the float bowl as soon as the valve opens and the pump sees the resulting pressure drop? Does the bubble create additional pressure by expanding from the heat, so the pump does not see a pressure drop large enough? If so, I understand it now.
1 Like
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
69
Carb fuel pumps are lame, only producing 3 psi or so. That’s weak, so I guess a bubble can stop the flow. Of course, we have XJ-S owners claiming they have vapor lock problems in an EFI car with a fuel pump capable of producing 100 psi.
I have not checked the pump pressure yet. Will visit the local auto store to get a pressure gauge for that job. I did switch the pumps when the problem happened earlier for the first few times. It did not help to cure the problem though. Now the right tank is quite empty. Connecting the right tank would just leave the pump clicking like crazy. so I have to use the left tank only for now. The poor thing can not make it to the nearest petrol station and back.
I am quite sure it is temperature related. At least this problem never happens during the warm up period.
Probably because if the bubble has reached the injectors it isn’t ‘vented’ to the tank, hence the fuel rail is always at the highest point. I believe it can happen, what other reason would there be for the hot start switch in the S3 4.2s, the one that kills vacuum to the pressure regulator (and I believe exists on the V12 as well)?
Now, if this S1 is particularly plagued by a hot start problem while others are not I would first look at parts that can degrade over time. That can be coils, condensers, points, diaphragms, parts like these. Shielding them from engine heat to narrow it down would be worth a try.
David and Kirbert, I had this thought that changing all fuel lines to transparent ones would help me to see the fuel flow. But I cannot find any transparent fuel lines in 7mm, only 8mm ones are available.
If the pump clicks like crazy the tank is empty, or the feed line or in tank filter is clogged, or the tank is not vented. Open the fuel filler. Add some fuel. If it still pumps like mad, remove the bottom drain plug and the pump as well. Blow through the line towards the tank and make sure air gets through. If it does, the pump needs adjustment.
Heat shielding the coil is something I would like to try. I may start with some aluminum foil for kitchen use.
The coil for S1 is attached to the right hand side not like an S3 on the block. It still feels too hot to touch after use of 30mins. Is it normal? But that is with the engine bonnet close.
I will put some fuel in the right tank and try again. It is my prime pump to use when the car was working well. I do remember it to click more when I had lower fuel reading.
The coil gets hot too so you mustn’t insulate it from the air. Cool air flow would be good. It’s on the coolest place in the engine bay anyways.
About 60 watts I think, it will get quite warm on its own. You might want to substitute it with another coil and go from there.
Another possibility is that the line to your RH pump has a small leak. This causes them to click faster as well.
Have you checked the temp
Sensor that feeds the ECU? If it’s not correct when warm it could cause issues generally that would mean a rich running engine when warm but you never know
