Front Carburetor puts gas into vacuum line to distributor at idle and at engine shutdown

When I remove the vacuum line from the front carburetor after engine shutdown, about a quarter of a thimble of gas drains out of the line. This happens both if I have been running the car at idle and if I remove the line immediately after a fast drive.

I presume this is fuel that the needle position allowed to be sucked up out of the float bowl when the engine was running and it condensed at the butterfly valve when the engine was shut down and then is draining into the vacuum line port, but I cannot believe this is “normal.” The carburetor is not leaking anywhere. The reason I cannot believe this is “normal” is because fuel draining into the vacuum retard will soon deteriorate the diaphragm in the vacuum retard. I think this has been going on in this car for years because I can never get the vacuum retard diaphragm to last very long. I just assumed it was poor quality units. Now I am thinking that it has been gas discharging at engine shutdown into the diaphragm. Anyway, I have put a “T” in the line so that there is a length of tube into which the gas is deposited until it evaporates or is sucked back into the manifold when the engine is again started. off the horizontal leg of the “T” I run the vacuum line with a steep rise in the line before it drops down to the distributor vacuum retard to keep gas from getting to the vacuum diaphragm.

Anyone ever had this problem or heard of this problem?d

Thank you


There sure is something odd here, Gordon…

Ported vacuum is tapped into the airflow past the throttle butterfly into the engine - the vacuum line to the distributor is blocked by the diaphragm. There is no air flow through this connection - and while ported vacuum is low with throttle closed, it is still a vacuum applied to the dist hose, and
no fuel/ fuel mixture should enter up this hose…

Have you measured vacuum at the spigot used - and are there other spigots at the carb?

The reason for asking is that some carb models were fitted with an anti run-on system. As the engine is turned off, vacuum is briefly applied to the carb bowls - stopping fuel being sucked into the airflow. However, it was not meant to suck fuel into the attached vacuum hose, only bow air - but it could happen…?

xj6 85 Sov Europe (UK/NZ)


I think I have confused what is happening. Immediately after engine shutdown if I remove the vacuum line at the port on the front carburetor right behind the throttle plate, about a 1/4 thimble of fuel runs out of the port. I put a “T” in the vacuum line at a point below the port. The one port of the “T” is connected by vacuum line to the port. The horizontal leg of the “T” is connected to the distributor vacuum retard and to the third leg of the “T” (directly opposite the leg of the “T” connected to the port) I attached a vacuum line with a cap on the end. After I shut down the engine, if I remove the cap at the end of the line that is now directly below the port, about a 1/4 of a thimble of gas drains out the line and the removed line cap has a little fuel in it. I have measured the vacuum at this port and at idle I have about 17 inches of vacuum. Of course when I increase the engine speed the vacuum drops as the butterfly opening increases releasing the distributor retard.

When I said that fuel comes out of the vacuum line with the engine running, I meant that if I restart the engine very shortly after shutdown, I presume any fuel in the vacuum line is sucked back up into the engine. There is no fuel originating from the vacuum line, just whatever fuel has drained through the carburetor port at shutdown. Just seems odd to me that there should be any fuel to drain out of the port. But if it is not unusual for a little fuel to drain out of the port immediately after engine shutdown, then it seems that it is important to make sure that the vacuum line from the port to the vacuum retard on the distributor travel upward part of the way to the vacuum retard before traveling down to the retard so that this after shutdown draining gas does not end up in the vacuum retard.


Is this the hideous twin Stromberg set up in a series 1 from the states?

Good point. Gordon says “vacuum retard.” I can’t remember how that was configured, but it’s best to get rid of the whole thing and just leave the dizzy mechanical only (although they can be modified to provide advance, as some S2 E-type owners do).

To sum up if its a series 2 etype with strombergs or the same in a series 1 xj6, its an abortion.
Gas and electric spark doesn’t mix…
p.s. I JUST REREAD YOUR THREAD, The vacuum line UNDERNEATH! the manifold was a STUPID idea.
If I’m reading you right you have the abortion double butterfly set up.
The unburnt gas is getting trapped between the two butterfly plates then leak down into the line…
Real smart
Its not the diaphrams, its the horrid set up.

Yes, it is the hideous Series I set up from the States and you have described it perfectly. I think you are telling me that other than having this car (for now almost 50 years as my wife bought it new in 1970, a year before we were married, there is little I can do about this condition.

I cannot just eliminate the entire vacuum retard system because I have the distributor retarded as far as it will go and without the vacuum retard it spark knocks terribly even on premium gas. It still knocks a tinny bit but a lot less than without the vacuum retard. I have tried rotating the plug wires one spot counterclockwise but then even without the vacuum retard connected, I cannot rotate the distributor clockwise enough to have the proper timing advance.

I think I have executed the best solution by putting the “T” in the line so that the little bit of gas that drains through the port has a place to go until it evaporates back up through the tube into the carburetor. I have also tilted the “T” up so that the gas has to drain to the lower tube and not along the horizontal leg of the “T.” Also by having a high rise in the line from the horizontal leg of the “T” to the distributor keeps the gas and likely any fumes from reaching the vacuum retard.

Thank you all so much for your time. it did not make sense that the design would allow gas to exit the tube and give it a path to the distributor, but meeting US pollution control standards at the time made for strange results in more than one car situation.


Sorry to hear about your issues. Can the vac line be relocated, perhaps by Joe Curto?

If you need more retard, send the distributor out to advanced distributors. The guy will recurve mechanical advance in the Lucas distributor.

You can also have the vac advance changed around by

Good luck! -John

Gordon, I’ve owned two S1s. Although I’ve fitted SUs to the current one, I remember disconnecting the retard on both cars very soon after I bought them. Check the E-type archives, but I’m pretty sure that the retard is only supposed to work when the engine is cold, or perhaps only when it is idling. It would make no sense to have it operational under load. The dizzy should work fine, regarding setting advance, with no retard. I think some E-type models were provided that way–no vacuum canister at all. Perhaps your mechanical advance is stuck?

Anyway, I think there’s something wrong. IMHO.

17" Hg means that you are tapping into manifold vacuum dropping with increasing pedal input - not ‘ported’ vacuum, Gordon…

The ‘T’ connection you describe connects the ‘drain’ spigot in the manifold to the dist vacuum. And the ‘drain’ has manifold vacuum, overriding the proper ported vacuum connection - and drains pure fuel from the manifold into the bargain…

No matter what; you have messed up the vacuum set-up - possibly due to the assumption that you have a ‘retard’ dist. All of which explains the problems with the ign timing/engine…

Remove the ‘T’, connect the dist vacuum directly to the carb spigot - and verify that the carb spigot shows normal ‘ported’ vacuum; around 0 in idle varying with throttle settings. Whether you have advance or retard; the single spigot on the carb is the one to use…

Check ignition advance at 800 rpms with the dist vacuum disconnected - set advance to 17 deg BTDC, as a starting point. Tweaking can then be done from this - but we need a standard set-up to clarify matters. Do not(!) proceed with the present ‘T’ set-up - it is plain wrong…

Questions arising; the dist can be turned 360 degress, but we usually ensure that the vacuum capsule is aligned ‘for and aft’ with the ign timing correctly set. Any reason why you cannot rotate the dist as desired?

What initial advance have you set with vacuum disconnected. If manifold vacuum is applied to an ‘advance’ dist, it will increase advance by some 10 - 12 deg when vacuum is connected. If the ign is set to the standard used for ‘ported’ vacuum - ign will be far too advanced. Have you checked what happens to the advance when vacuum is connected/disconnected with your present set-up - which will reveal if you have the advance or retard…?

You don’t mention if you still have the original, mechanical points system. And, if so; have you verified that the contact points are unpitted and gapped to the prescribed 0.014" - 0.016"…?

xj6 85 Sov Europe (UK/NZ)


The distributor that came with the car is the one still in the car. It has a vacuum retard as evidenced by both the fact that the vacuum line connection is on the distributor side of the mechanism instead of the opposite side so the vacuum is pulling the shaft connected to the distributor towards the distributor and not away from the distributor. The fact that it is a retard system is also evidenced by the car gaining about 300 rpm when vacuum is removed. Also this distributor does not have a vernier adjuster to advance or retard the ignition within a narrow range.

The only fixture on the carburetor to which to connect the vacuum line is at the engine block side of the carburetor on the bottom of the carburetor flange where it mounts to the engine. The vacuum at idle today at 600 rpm is 18 inches as evidenced by the below picture of the vacuum gauge connected to this fixture. The vacuum reduces as the engine speed increases, which makes sense with a vacuum retard system as the timing should advance as engine speed increases. This is accomplished by reducing the amount of retard.

As you can see the carburetor set up is the two butterfly system as evidenced by the linkage shown in the picture below.

I have also shown a picture of the vacuum line set up. What I have done has no effect on the vacuum retard, but gives the draining gas a place to pool until it evaporates other than the vacuum line to the distributor. It does not affect any operation.

When I have replaced the vacuum retard, I have to be very careful to specify both the distributor model and tell them it is the retard system. Inevitably they send me the advance distributor with the vernier and I have to get them to re-send, but this retard and distributor really exist and is what came on the car. We have owned the car since 1970.


Linkage connecting butterfly in carburetor base to butterfly in manifold connecting front and rear carburetors

with both connected to intake manifold.

Vacuum retard on distributor. Vacuum line connection is on distributor side of vacuum fixture. The line at

top of the picture angling downward to the left is the line going to the carburetor fixture. The yellow on

the distributor is painters tape I have attached to the line with cylinder number on them.

T set up I installed. As you can see the orange/red lines are vertically in line with the carburetor fixture so any

gas at shutdown drains passed the black vacuum line to the distributor and sits in the lower line (which is

capped but not shown in the picture) until it evaporates or is sucked back into the carburetor at engine start

and idle.

Vacuum gauge reading at 500 rpm with gauge line connected to front carburetor fixture where vacuum

line to distributor is connected. When I increase engine speed, the level of vacuum decreases.

All of the following IMHO, Gordon.

Timing should indeed advance with engine speed, in a precise way that is determined solely by the centrifugal advance (until computers, that is). Additional advance beyond that provided by the centrifugal mechanism is detrimental under load (pinking) but does provide some increased fuel economy whilst cruising. That is the role of vacuum advance (fuel economy; no increase in performance; not at all necessary).

There is no simple relation between manifold vacuum and engine speed–only engine load, which, although minimal at idle, can vary greatly when the car is operational. This is because load depends on the inclination of the road, wind, if the driver is trying to accelerate or slow down, and other factors. Manifold vacuum can’t predict engine speed.

The Jaguar vacuum retard system on your car is designed to operate only at idle (throttle closed). Its function is related to pollution…engine runs hotter and warms up faster. Idle is smoother too, perhaps. On the earlier cars (pre 72, like yours) the retard dizzy was fed from a drilling on the front carb. The drilling was just on the manifold side of the butterfly. It picks up manifold vacuum when closed (idle), but this vacuum largely goes away when drilling is exposed to the carb throat. So retard exists only, or at least primarily, at idle. Note that this drilling is exactly opposite in function to the conventional vacuum tap located just on the carb side of the butterfly–which provides ported vacuum.

On later S1 cars, a vacuum switch sensed coolant temperature, and provided a better source of vacuum to the retard dizzy during warmup. As I mentioned before, there is a ton of info in the archives–I just refreshed my memory of a long discussion with RustFreeMike (whose cars I greatly envy).


Thank you for the detailed and comprehensive reply. You have described exactly how my car is working. The idle is lower and smoother with the retard functioning properly. When the vacuum retard diaphragm gets a hole in it, the car idles up to 300 rpm higher and idles rougher.

As soon as I add a little throttle even at idle, the vacuum drops way off and right to zero if I open the throttle much at all.

Apparently when the butterfly closes and the vacuum disappears at engine shutdown, any fuel-air mixture in the manifold condenses into a liquid that is draining towards the front carburetor and drains into the port on the manifold side of the butterfly (the port to which the distributor retard vacuum line connects. Intermittently, there is gas on the engine side of the float chamber that shows up a few minutes after shutting the engine down after actually driving the car, but no such gas if the car is just idling and then shutdown. I do not have a similar circumstance with the back carburetor. I have run the fuel pump for an hour with the engine shutdown and no fuel in the front carburetor throat or around the float bowl. Same for the rear carburetor.

This gas in the out the front carburetor vacuum line fixture and the gas on the engine side of the float bowl are most probably related. The gas coming out of the vacuum line fixture is definitely coming from inside the tube not outside but there is no gas wick from the carburetor vacuum fixture to the back of the carburetor bowl. I have checked with paper towel. Now that I know that my vacuum line to the vacuum retard is properly connected, I am wondering if the condensed fuel at the butterfly valve is weeping along the bottom of the throttle shaft in the carburetor at ending shutdown then to the backside of the carburetor. There is also an over pressure bypass valve on the front side of the carburetor so maybe gas at shutdown is leaking from the bypass valve cover interface with the carburetor. I will check these the next time I drive the car, but this seems like a lot of reaching for a reason.

Thank you again Robert for your diligence and help.


When you disconnect vacuum line; air is entering, a vacuum leak, the engine and it revs up, Gordon - this increases idle, and is not in itself a proof of ‘advance’ or ‘retard’ set-up. There should be minimal idle reaction to changes in ign advance when properly set…

Irrespective of this; initial timing must be set properly in either case; apart from engine reaction you have given no numbers of advance in degrees. Have you checked it with an ignition timing light?

You have two issues; ignition timing improperly set, which is the cause of you running problems - ign timing must be set as described for the relevant set-up. And your concern about petrol leaking - which is an irrelevant issue in this context, and your remedial actions does(!) interfere adversely with ign timing…

As Bob explains the ‘retard’ set-up, manifold vacuum only applied while idling - with your set-up, manifold vacuum is applied throughout by the ‘T’ connection to the manifold.

Petrol cannot enter the vacuum capsule through the vacuum lines - both with ‘retard’ and ‘advance’ vacuum is sucked away from the capsule by the engine. While the observed petrol accumulation at shut-down is interesting, it does not warrant your remedial actions - it should be pursued in other ways. The constant break-down of the vacuum capsule has other causes - and should be pursued as a separate issue…

To get the engine to behave; get rid of the makeshift ‘T’ and cap the manifold spigot - and connect the vacuum line from the dist directly to the carb spigot. Verify that you then have manifold vacuum in idle, disappearing as throttle is opened - as Bob describes for the ‘retard’ set-up.

The use an ignition timing light to verify, and set, the prescribed ign advance. If you do not have an ignition timing light; either get one - or with a points distributor, prepare to set timing ‘static’. To be explained if required…

Assuming a ‘retard’ system; there is no way to make the ign timing working correctly with your present set-up - get rid of it. You may certainly disconnect, and plug, the vacuum line at the dist, as Bob implies - running the engine with centrifugal advance only. But you still need to ensure that the initial advance is set correctly - it’s an absolute must…

xj6 85 Sov Europe (UK/NZ)

Is it possible to obtain a set of used SU HD8 or HS8, which were fitted in Oz, and probably UK, and fit them?

With the diaphragm and piston removed I found two things of interest. First, the threaded holes for the long center screws that hold the carburetor bowl on are drilled all the way into the carburetor air inlet (threaded screw drilling on the air inlet side of the piston and the threaded screw drilling for the center screw on the block side of the engine is drilled on the engine block side of the needle just beyond the air piston. So gas puddling behind the piston drains along the screw threads making the bottom of the float chamber wet and it would not occur until the car sat after shut-off for a few minutes. I found gas puddled behind the piston. I suspect this same thing is occurring behind the butterfly causing gas to drain into the vacuum line. Question is “why the pooling gas?” Given that I found the O ring on the needle adjusting fixture to be almost completely dissolved (my guess caused by ethanol in US gas), I am guessing that this is causing a rich mixture out of the front carburetor which I may be compensating for with the mixture setting on the back carburetor. Anyway, I will report back with results in a day or so after I get everything back on the car.


Gordon, I just had to jump back in, back in the day, many CUTOUT the secondary butterflies.
Could that also be why a puddling effect?
Make sure they are both there and the shaft holes are plugged, sounds crazy but…
I already explain my distain for the set up, but they are right.
Timing is the key for spark and burn, but either the butterflies are off and not closing together or you have leaks.
Leaks from removed butterflies were common as well.
If your keeping the car for the long haul, get triples.

There is no way to compensate for a fat ‘front’ by leaning out the ‘rear’ carb, Gordon…

If the carb has been ‘modified’; it is defective and a replacement is called for - though perished seals should of course be replaced. But you should indeed check the other carb - to verify if it shows the same as the front one. Which may mean that the carbs are as they should be - but which carb type do you actually have…?

To me the pooling of fuel is of secondary interest compared to the misbehaviour of the engine at speed. The possible interference of petrol with the vacuum capsule membrane is easily remedied by just disconnecting the vacuum hose at the dist and run the engine on centrifugal only.

This will work perfectly OK when the timing is set correctly - leaving a simple problem of carburation to sort out. If the carbs are indeed out of sync symptoms will be distinctive - and open for proper remedial actions.

Then, with the engine running properly, the ‘pooling’ can be pursued if still relevant. As the vacuum lines are under vacuum as long as the engine is turning - it’s difficult to see petrol backing into the vacuum capsule with the engine running or stationary. The petrol source must be higher than the capsule…

xj6 85 Sov Europe (UK/NZ)

Distributor Timing:

Frank, in response to your message from last week, yes I have checked the timing. At 1,000 rpm the timing using number 6 cylinder should be 10 degrees and I am showing close to 12 degrees advance.

Later this week, I am going to remove the distributor and see if I can figure out why it will not rotate more counterclockwise (retard) than it does. Visually there are no non-distributor engine parts or accessories that would interfere with more counterclockwise rotation. At one time I rotated the pinch bolt bracket more counterclockwise by raising it up over the centering nipple on the block which caused the pinch bolt to be at a slight angle to the distributor shaft body. This gave me more spare retard but resulted in cracking a piece out of the lip on the distributor where the pinch bolt bracket goes around the distributor. I looked at this when I replaced the vacuum retard fixture, but maybe the pinch bolt assembly is distorted in a manner that it limits the counterclockwise rotation of the distributor body. More to come end of week

Front Carburetor Gas Pooling:

J Limongelli,

All four butterflies are still installed in the carburetor.


I noted the day before yesterday that before I started the car (and with the air cleaner off) that the bottom of the front carburetor piston was wet with gas. I removed the piston and found a small puddle of gas adjacent to the threaded drilling in the carburetor throat located just beyond the block side of the piston into which the carburetor bowl long fastening screw on the block side of the carburetor bowl is installed, which explained why I thought that gas was draining around this bolt ending up on the bottom of the carburetor. The piston was also wet on the bottom and actually all the way up to and on the diaphragm. I wondered if after engine shutdown gas was wicking up the needle and condensing on the piston. I poked a small hole in a piece of white paper towel and inserted it in the carburetor throat and inserted the piston with the needle going through the hole in the paper towel. Over night there was a small very light gas stain around the needle hole and a very light stain about the diameter of the piston. The piston bottom was not touching the piston and the piston was not wet with gas. After replacing the O ring on the needle adjusting fixture yesterday and running the fuel pump (engine off) for about 30 minutes, I had not gas in the carburetor throat nor on the bottom of the piston. However when I drove the car around the block (45 mph highest speed) and returned to the hangar, shortly after engine shutdown I had gas on the bottom of the carburetor bowl that originated from the long center block side bowl fastening screw (the threaded drilling for which is right into the carburetor throat).

Today before even starting the engine I removed the front carburetor piston and it was damp with gas on the bottom of the piston, but not wet. As a side note, oil was still in the air valve guide rod.

I noticed earlier in the week that the temperature controlled vacuum shutoff controlling the air cleaner diversion valve that allows engine exhaust manifold heated air into the air cleaner was not properly working. No matter the temperature of the exhaust air, the diversion valve opened to be the 100% provided air to the air cleaner. I fixed the fixture in the top of the air cleaner between the carburetors so that the fixture would reduce and ultimately eliminate vacuum to the air cleaner diversion valve so that once the engine warms up outside air rather than exhaust manifold heated air would be the primary air source to the air cleaner and consequently to the carburetors. I did not have it quite right for yesterdays around the block run, but made another adjustment last night.

So, today I drove the car about 20 miles including on the expressway at 70 mph. When I got back to the hangar, I did not immediately shut down the engine, but instead removed the carburetor piston cover screws. As soon as I shut down the engine I turned the ignition key back on so the pump would continue to pressurize. I immediately removed the front carburetor piston. There was light layer of gas on the bottom of the piston, but no gas on the side of the piston or the diaphragm. Also no pool of gas although there was a small bubble of gas in the threaded bowl screw hole (screw in the hole).

I put the piston back in its place and set the cover on top. Every 15 minutes (fuel pump still running) I checked for gas on the bottom of the piston. There was a very slight amount the first time, but by the third time, no gas at all. The needle had no gas on it the first time or any of the other times. I let the fuel pump run for 30 minutes and no pool of gas, but a small amount on top of the needle receptacle.

So, could the problem be that the quite warm air to the air cleaner and thus to the carburetor throat and piston at engine shutdown caused the cooler fuel fumes from the bowl into the throat to condense on the bottom of the piston to the point that after a few minutes fuel dripped off the piston and created a pool of fuel in the carburetor throat? Maybe my fixing the air cleaner diversion valve last night reduced this condensation?

I checked the diversion valve just before I got on the expressway and the valve was still allowing only exhaust manifold heated air to the air cleaner. After the expressway run and the 20 minute or so drive back the the hangar from the expressway, the diversion valve was fully closed so outside are was the air source to the carburetors.

I apologize for the long message, but complicated problem.


Distributor Timing

Turns out there were two problems with the distributor. The biggest was the radiator side of the distributor cap clip that clips the cap to the distributor. It is designed so that the clip can only move far enough from the cap to allow the cap to clear. However, somewhere in its life, the radiator side cap clip caught on the lower radiator hose and bent the clip so that the “L” shaped piece limiting clip travel was no longer tight under the clip mounting point on the distributor. (See picture below.) This clip protrusion caught the lower radiator hose limiting the counterclockwise rotation. Also the forging on the distributor body to which the clip is attached touches the lower radiator hose. Fix was to cut off the “L” shaped piece of the clip and file the forging so that it hits the lower radiator hose after farther counterclockwise rotation of the distributor.

Second problem was the fixture that is tightened or loosened to rotate the distributor. This bracket was a little deformed and did not allow the distributor to rotate counterclockwise as far as the now fixed distributor cap radiator side clip. Repaired the fixture so now can retard the distributor to a point where there is no spark knock under full load in high gear.

Note “L” shaped piece on the bottom clip. There is a similar piece on the top shown clip, but it is properly bent under the clip mounting points. The idea is that the clip could only move far enough from the distributor to allow the cap to be removed. Makes it easy to find the clip when the cap is put back on the distributor.

Front Carburetor Gas Puddle:

On both drives today (earlier at high speed and this evening to set timing where engine does not spark knock), no gas leaking or puddling. What remains to be checked is that with the gas tanks about half full, the front carburetor did not leak before. I will fill with gas tonight and see what shows up tomorrow.

Thank you all for your patience and your perseverance in helping me solve these problems.