The carb flooding question prompts me to ask one of my own. My 3.8 Mk2 has rebuilt carbs, starts and runs well. But as I drive there is a strong smell of gas or unburned fuel in the cabin. I have searched diligently in the trunk for fuel pump leaks. The tank is new & has no indication of leakage. In the engine compartment, the fuel filter bowl & fittings are tight, and the carbs themselves aren’t leaking. The auxiliary carb (hisser) is wired in thru a switch and is working properly. I have tried leaning out the idle jets, and driven with air cleaner off. No luck yet. I once had a car that sucked exhaust gas back into the trunk, but the smell in the Jag is there even at low speeds. Does anyone have any ideas? On a side note, I’ve been working on a '51 BMW motorcycle resto for 2 years now, and just got her running & ready for the road. Just in time for winter, of course. And simultaneously, I’ve been resurrecting a '97 BMW enduro, which I also just finished. It is interesting to contemplate how much technology has changed in the 50 years since the war. I must admit that the new bike is way more fun to ride than the old one. One of the things I love about the Jag is that, for all it’s old-school vibe, it is actually a very modern car.
Enough digression. Thanks, David
Keep in mind the hisser is open to the atmosphere at the top. I find (1965 3.8S) that in very hot weather (85F +) fuel vapors will “boil” out of the hisser on start up after a hot shut down and there will be a strong fuel smell present. It normally dissipates in a few miles of driving as the engine bay is ventilated by the fan. I have also heard that the ethanol diluted fuel is more prone to this issue. It can also vary as summer and winter blend fuels are phased in and out here in the USA.
I am in full agreement with the ethanol fuel problems here in the USA. I also get a strong fuel smell when driving aggressively on curves and the hisser tends to leak a bit. I have the fuel pump shut off switch for shutdowns to empty the bowls and I also use this when I smell fuel on the run and I switch off the pump to use up any “boiling fuel”. I have on long trips turned the fuel on and off numerous times for about 30 seconds at a stretch and this solves the smell problem.
Gerard
do you have old flex lines in fuel system? they perish but leaks can be hard to spot as are rust pinholes in steel line by frame rail.
All useful responses. I use non ethanol fuel, so I can rule that out. It’ll be a few weeks before I get to the jag again. If I figure it out I’ll post the solution. Thanks, David
The hisser does expose a fuel column to the atmosphere, so there is a bit of evaporation, but the smell usually isn’t evident inside the cabin. A whiff of fuel fumes when you open the bonnet is normal.
If the engine spits back the pressure will spray fuel out of the hisser.
Some of your firewall grommets may have perished with age and will allow fumes into the cabin. I have found a few like this.
Gerard
Hi David,
I have resorted to manual choke on my 340 to try to elevate petrol smells with some, but not with complete success.
As John says, the hisser is open to the air under the bonnet and is therefore going to smell of petrol. Also the overflow pipes on the carbs are open. I have gone over the engine bay trying to make sure there is no way for leaks to get into the cabin, but I don’t think the shell is designed to be completely sealed. I can’t decide whether my current problem is coming from the engine bay or the boot. In the boot I have wrapped the fuel pump temporarily in plastic sheeting, also the fuel sender area without much success. I have reduced the problem, but not eliminated it and it spoils my enjoyment of the car. I can open the boot and there is a faint smell petrol, but when I try to smell each component I still can’t find it.
I have read somewhere that some of the restorers mount the fuel pump under the car to resolve this problem. Maybe somehow the pump lets out vapour??
The exhaust manifold where the down pipe is bolted on, has a tendency to become loose; are you sure it is petrol you are smelling?
Paul
Hi Paul;
Here’s my thinking so far. The car is on a bit of a back burner right now
while I finish a bike I’m working on, but as soon as I’m done with that
I’ll pull the jag out and approach the problem methodically.
- It may be >one problem
- if the fuel pump isn’t leaking, it’s not the fuel pump
- check all fuel tank connects carefully, including filler hose
- get the car up in the air and look underneath it while it’s running,
track out the fuel lines & make sure they’re intact - check tuning/carb mixture (idle jets and needle jet position). Check for
exhaust leaks in the engine bay (manifold & header). The smell could be
unburned fuel in the exhaust.
6)Block off any openings between engine bay and cab, same for drive shaft
tunnel. - check trunk weather stripping/tail light enclosures in case exhaust
fumes are getting pulled back into the trunk and cabin by low pressure
behind the car as it rolls.
If it still stinks after all this I’ll be ready to spit the dunny (you get
bonus points if you know what that means).
About 25 years ago I had an almost constant all be it not too strong fuel smell in the cabin of my 3.4 MK2 which was slightly worse in warm weather.
I have to admit it turned out to be self inflicted! A year or so before I noticed the smell I had an issue with vacuum being crated in the fuel tank which was noticeable when removing the filler cap to refuel, surprised it never caused fuel starvation. Anyway a quick fix was to drill a small hole of about 1 to 2mm in the filler cap. it was only when much later I decided to investigate the smell that I twigged that the previous issue with the fuel tank vacuum could be linked, actually with hindsight it was a complete shot in the foot!
Looking into the fuel tank vent arrangement it became obvious that it was blocked solid hence the vacuum as the fuel was consumed. The plastic pipe that runs from the filler neck stub across the top of the tank before dropping down to vent to atmosphere had, since new I assume, been pinched between the top of the tank & the boot floor & had eventually closed up completely! Drilling the cap cured that but obviously allowed vented warm fuel fumes into the filler access box which were then drawn into the lower pressure saloon when the NDV’s are open in the summer. Restoring the tank vent & a blob of epoxy in the filler cap hole & normal service resumed!!
Great tip from Chris. you might also wrap the ends of the carb bowl overflow pipes with tissue when you take it n a test run to determine if fuel is overflowing when running.
Not sure of what you have in terms of set up but I now install a fuel pressure regulator (filer king filter/regulator) on al my cars as the modern replacement SU fuel pumps push out too much pressure.
Best regards
Phil D
I will definitely look into fuel tank vent as a possible source, also into
the condition of the filler cap. Thanks Chris. And thanks, Phil, for the
idea about the tissue. I will report back, since I have a little time to
play with the Jag while the machine shop does magic on my motorbike
cylinder heads. David
To check my vent overflow pipe, could I use a high pressure air line and shoot air into the vent hole near the filler neck and clearly hear air being vented out of the other end of the hose over top of the tank? Years ago I R&R the tank and I may have made a mistake. Taking the entire tank out could prove to be a days work with shipwrights luck.
Sounds reasonable to me. D
Also check the underside of the auxiliary carb for leakage, they have a tendency to leak where the aux carb fits on the connector to the front carb.
And if all else fails get yourself a TPI 725L pocket combustible gas detector - pinpointed the source of a gas(petrol) smell for me - turned out to be porous fuel hose; would never have found it otherwise.
Frankie
thats a really good idea, as fuel hose gets porous when it touches something else and weeps
make sure any vents and grommets in firewall are functional
the ASC has an open fuel well into engine bay, so all engine bay fumes must be excluded from cabin
I would have thought that a pretty good idea, anything that avoids having to drop the exhaust tail pipes unnecessarily has got to be worth a go. Be careful not to pressurise the tank itself, it would only take a few psi to do some serious damage! Might be worth locating the atmospheric end of the vent pipe & try to blow through it with the filler cap off & get an assistant to confirm air is actually being blown up the tank vent. My vent pipe, when I dropped the tank for renewal owing to corrosion, was so hard & brittle that it just snapped if you tried to bend it as well as being crushed for many years no doubt.
The ads seem to indicate it is more directed towards methane and natural gas. You say that it detects raw gasoline easily? Does it also detect carbon monoxide leaks?
Gerard
Hi Gerard.
Carbon monoxide is not combustible so I doubt very much it will detect that gas, but I haven’t tried it on that. The package states:- “Detects combustible gases including:- Methane, Butane, Propane, Gasoline, Diesel, Kerosene.” The sensitivity rate is stated as 40 PPM Methane - how that translates into gasoline, I have no idea.
You may need to hold the sensor head within half an inch or so of the gasoline leak for it to register but a weak smell in the atmosphere is too dilute for it to sense. It responds to a combustible concentration of a gas - not just a background smell. It detected porous fuel hose in my trunk, vapour spilling from the end of the tank vent and a poorly sealing petrol cap for me.
Hope this helps.
Frankie