As the temperature here is bouncing around 100, my thoughts return to vapor lock. I’ve got a RetroAir AC, which I’m sure exacerbates the OEM heat management challenges, with an aluminum radiotor, high volume electric fan, and vented wheel well triangles to counteract. Although the car doesn’t run excessively hot, the carbs read around 145 F after a few miles run, which is enough to boil the fuel.
Out of curiousity, I measured the air filter housing temperature, and got in excess of 200 F at the horn on the drivers side. Yikes! The plenum feeding the carbs is cooler, but still high. It seems like the air intake system is transferring heat from the exhaust side of the engine. This can’t be good for performance.
I’m wondering if replacing the original filter and heavy plenum assemply with pancake air filters would result in cooler intake and fuel bowl temperatures?
I can not understand the logic in Jaguar placing the inlet to the carbs right over the manifolds , its asking for trouble , I fitted a silver foil tube (reflects heat) to the carb intake , across the engine down the side of the engine bay , and attached it in front of the front sub frame , with a air filter attached , so it sucks up cool air !
I too have a alloy rad and electric fan , I replaced the Jaguar fan on the water pump , was surprised how much the temperature has dropped !
I agree - the design virtually guarantees a problem
Clever solution running a cold air intake. With an AC compressor and alternator on the drivers side of my engine bay, there may not be room to run ductwork. Nonetheless, worth considering.
I’m considering fabricating an intake “Log” like the Mark X/420g had, and pulling fresh air in from the front of the car. Not just to reduce intake air temperature, but also because the engine bay looks much better with the head free and visible.
there was a variety of intake filtering and silencing arrangements. i run a big flat coopers cleaner now on a standard 3.8. the car is 1960 and came with an oil bath filter in front of left fender. the filter sucked air from just in front of the rad. there was a silencer on top of engine much like a later filer. it was all connected with horrid fragile paper tube through a 3 inch tube fitting through inner fender just behind the fuse block. it would be easy to make something that looked stock with more modern material. when i first had the car i did encounter carb ice problems in moist & cold weather but we probably all avoid that now?
One of the problems with the SU carbs is that they vent the float bowls to
the atmosphere rather than to the carb throats. Because of that the mixture
changes depending on the pressure drop across the air filter. To
compensate, they have made various different needle profiles that change the
mixture depending on the air flow, which is why they had different needles
depending on which air cleaner was used. That also made the engine run
richer as the air cleaner collected dirt and plugged up. It would be
trivial to connect the overflow tubes to the drain tubes at the bottom of
the air horn, but then I would need to figure out what needle to use. If I
had a dynamometer and an A/F ratio meter I could derive the correct needle
profile, but since I don’t I have just left the original intake filter
alone.
Mike Eck
New Jersey, USA www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2
Right, but it represents the pressure that is applied to the carb throat, which is the relevant pressure for the float chamber (I think).
BTW, I sorta followed your idea of fitting an intake “log” on my XJ6, actually using a Mark X airbox and feeding it with fresh air from the grille area. It has drain tubes that could be plumbed to the HD8 overflows, but I wonder what would happen if a float valve stuck and petrol wound up being sucked into the intake?
The engine would just run correspondingly rich (i.e. like a rain soaked Sasquatch). You would notice.
I don’t think the fire risk would be significantly greater with a pool of fuel in the (sparkless) intake plenum.
A vacuum in the top of the floatchamber would presumably lower your fuel level at the jet, at least under some conditions.
And if for some reason the mixture got lean enough that the engine started spitting back then you’d probably start having problems with the pressure pulse forcing fuel past the needle, or am I missing something?
That’s the point. If there is the same pressure (vacuum) on the top of the
fuel in the float bowl as there is in the plenum then the vacuum in the
plenum (pressure drop across the air filter) will have no effect on the A/F
ratio. Think of it this way: If you were to put your hand over the intake
plenum and plug it up there would be a great vacuum inside and the fuel
would be sucked violently through the jet because of the difference between
that vacuum on the fuel in the jets and the atmospheric pressure on the top
of the fuel in the float bowls. The fuel would be pulled through the jets
because of that vacuum, rather than because of the air flow over the bridge.
The engine would run very rich. Now, if the top of the fuel in the float
bowls were connected to the plenum vacuum instead of the atmosphere there
would be the same vacuum on both sides of the fuel and it would cancel out.
Putting your hand over the plenum (or having a plugged air filter) would not
make the engine run richer, it would simply slow it down. That’s the way
most carburetors are designed, including the Strombergs. If you look down
the throat of any downdraft carburetor you will see a brass tube coming out
of the float bowl, venting it to the clean side of the air cleaner.
Mike Eck
New Jersey, USA www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2
EDIT: Wait a second… There’s a slight vacuum in the plenum, isn’t there?
Sometimes (I’ve changed to pancakes on non Jags and ended up having to change to a different needle).
By the way, if you want to change to pancakes, beware the cheapo ones… They’re made of sharp edges and foam bits with gaps. Fine for keeping mice and small children out of the intake manifold, but that’s about it.
That would still be a giant improvement over the ones provided by the
factory on the XK120, which will keep out “birds and larger stones”. You
could pass a popsicle stick through the mesh without distorting it.
Mike Eck
New Jersey, USA www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2
Fine for keeping mice and small children out of the intake manifold, but
that’s about it.
My car came with the pancake cooper air cleaner. Years ago, a mechanic rebuilt the carbs. Several years later I got a Pertonix electronic ignition installed. Last summer, after running the car with the Bosch coil which the PO had installed, I replaced it with a gold colored Lucas coil. Over the past few weeks I stripped the several coats of paint off the pancake down to bare metal, then painted it with Hammerite silver gray paint. I also cleaned and polished the visible metal parts on the engine: valve covers, metal hardware leading to the carbs, and thermostat housing. To complete the picture, I installed underbonnet insulation from SNG. I guess I’ve just been lucky: since the carb rebuild & electronic ignition installation, I haven’t experienced any problems with vapor lock . Maybe that’s because in 1998 I had the radiator replaced with what was described to me as a ‘recirculating’ radiator. Last weekend we took the car to a club event 110 miles away, 220 miles round trip. Temperatures were warm & humid, in the high 80’s low 90’s. My temp guage always remained below 70 C, and , having an all synchro trans with overdrive, managed a respectable 22 MPG fuel economy. I think the pancake filer looks best, especially now that I repainted it. I wouldn’t change to anything else.
By “Pancake filters” I meant the individual ones which fit directly on the carb intakes:
70 is a bit low for 90 ambient. Have you checked with an IR thermometer? I’ve a really cheap one, (30$) but it is within 5 degrees at that range (calibrated by pointing it at the temp sensor of a new OBD equipped engine and reading the OBD value).
I wouldn’t say the Mark 2 is prone to vapour lock. (Not compared to a TR3, or a big Healey with the heatshields missing or damaged).
I’m not so sure about the Mk2 not being inclined towards vapor lock. I’ve heard and read too many references to the issue to believe there isn’t truth to it. I have a TR4 and have had a TR3, and operated both in southern climates, with neither ever exhibiting vapor lock.
Maybe I’ve got some really atypical situation beyond having an A/C unit, but my car routinely exhibits the symptoms on days in the mid-80s and above in spite of having most of the standard heat mitigation features installed. The engine temp will go a bit over 70C on a hot day with the AC running, but never close to the top of the gauge, so I don’t believe the issue is overheating. The 145+ temperature readings off the carburetor bodies suggest to me that fuel is boiling in the bowls. The horns on the air cleaner read even higher, with the drivers side being 165+
I’m starting to think that an exhaust manifold shield is the next step.
I’ve seen more problems with overheating than with vapor lock on the XK engine. Perhaps it’s the alpine conditions (In Switzerland, if you’re not going downhill, you’re going uphill).
Have you tried changing the heat insulators at the carbs? Carbs are normally more or less self cooling - pulling cool liquid from the tank and then atomizing it takes a lot of energy out of the mechanism, but bolting them to a heat source negates that.
As the exhaust is on the opposite side of the engine to the carbs it certainly seems like your problem is the ambient air temp under the bonnet - that being the case I’m not sure a heat shield will make much difference. Angling the air cleaner cover to point as close to the battery as possible will mean that you’re pulling air from the cooler area of the exhaust side.
Another really, really long shot: Does your car have the landing seal at the rear of the bonnet? I can sort of imagine that leaving this off might change the way the enginebay airflow works. On lots of older cars omitting a single seal, shield, or cowl can push cooling over the tipping point. A couple of years ago I had a Healey which had terrible problems with boiling fuel… someone had got a bit overenthusiastic with the power washer while spring cleaning and blown most of the asbestos off the carb heat shield. I twiddled and tweaked with engine tune etc, but it wasn’t until I stuck a bit of water soaked cardboard where the Asbestos should have been that I twigged to what was going on.
I moved the air cleaner around so the intake is far forward on the drivers side, but it didn’t make much difference.
Good point about the self-cooling aspect of a carb. I’m not sure that is enough to counteract the very hot intake air and cool the fuel bowls as well.
The rear bonnet seal is missing on my car, and I bought a set of new carb insulators, but haven’t installed them yet. Thanks for giving me two more things to try.
