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MG MGB Technical - Smelly Zenith Carb

I am looking for input from all you cleaver guys out there. Here is my perplexing problem:-

I have a 77 MGB with the original Zenith carb set up. The car starts and runs beautifully but after parking in the garage for 20-30mins after driving (when the engine is really nice and hot), there is a really strong smell of gas / petrol from the carb.

I have recently done an overhaul of the carb to try and fix this problem with no luck.

I did find the auto choke assembly loose when I first removed it and, given that the mounting holes are drilled and tapped right through into the float chamber (what a dumb idea that was!), I was pretty sure that was the problem. I installed new screws, a new gasket and silicone sealant in the threads as a backup. I also checked the float setting and needle valve and, apart from the fact that a previous owner had attempted a repair of the float bracket with an epoxy material, all seemed well. A new float chamber gasket + silicone was installed for good measure.All of this failed to solve the problem.

I have conducted a couple of experiments to try and home in on the root cause.

The first was to just pressurize the fuel system by turning on the ignition and letting the pump prime the system and then turning the ignition off and waiting 20-30mins. Result = no gas smell.

The second is to install a switch to turn off the fuel pump before I reach home and to let it idle until it stalls out. Result = no smell.

So, it would appear that it is has to do with heat possibly expanding the gas in the float chamber (the heat shield is fitted by the way).

At this point I am stumped for ideas and am considering fitting a Webber 45 DOCE that I have from another car. But it seems a shame given that the Zenith otherwise seems to run so well.

Any suggestions anyone?

Thanks,
Mark
Mark

It's quite normal and your tests confirm it is expansion from heat soak. The same thing happens with my V8 in hot weather, I can see vapour 'pouring' out of the bottom of the overflow pipe. Starting it a couple of hours after that results in lots of chattering as the float chambers refill, whereas under cooler conditions there are only a few clicks. The same thing i.e. lots of chattering happens if the car is not used for some time, even after switching off in cooler conditions, as the fuel evaporates from the float chambers anyway, but over a longer period so not so noticeable.

Check the expansion isn't causing any *fluid* loss by removing the pipe from the float chamber vent and either with a loop of clear pipe or something similar on the port see if any liquid fuel is coming out. If it is, then the float height could be too high which will make the effect worse.

Paul Hunt 2

Mark, The silicone will not hold up to gasoline. Permatex makes a sealent for gaskets subjected to fuel. Ray

RAY

Oops! Forgot to mention it's name: Hylomar. Ray
RAY

Paul & Ray,

Thanks for your helpful comments. At least I can tell my wife that the smell is "perfectly normal" although I have a sneaking suspicion that is not going to cut it for me.

I did use an automotive quality silicone but, when I checked after reading your comments, you are absolutely correct that it is "...not suitable for parts immersed in petrol / gasoline..". Lesson learned.

At this point I am going to go with the fuel cut off switch until I have the time to try a carb swap.

For some reason California DMV don't send me bi-annual smog requests so I don't have that to worry about.

Thanks again.
Mark Sheward

I don't know why I didn't think of it before but if you still have the 'evaporative loss' kit fitted i.e. charcoal canister and plumbing then you probably *shouldn't* get a fuel smell, as the kit was designed to prevent fuel vapours escaping to atmosphere! However from 1973 the anti-runon valve was added, which resulted in a pipe from the inlet manifold bypassing the canister, so fumes in the manifold could again potentially escape to atmosphere. This was only addressed in 1978 when a second canister was added after the anti-runon valve.

If you have a car without the ignition relay then its quite possible that one of the emissions hoses is faulty and could be leaking fumes. For the 77 model year the ignition relay was added, but had a design fault that meant that it was only the anti-runon valve that stopped the engine, not the ignition switch. In this case if pretty-well any of the emissions plumbing were leaking or blocked it would negate the anti-runon system and the engine would continue to run normally for some minutes.

And unless the Zenith is actually faulty and leaking, a carb swap won't alter the situation in any event.
Paul Hunt 2

Mark,

Are you running an OE fuel pump? Some of the aftermarket ones require a pressure regulator. ' Something to check on.

Beyond that, call John Twist between 1 and 2 PM EST, for his call-in tech help. 1-616-682-0800. No, I don't have any interest, but he feels strongly that the Z-S has gotten a bad rap; he's convinced that if set up right, they're great.

' Think he could help you.

FWIW,
Allen
Allen Bachelder

Excess pressure overwhelming the float valve makes it leak all the time and so stink all the time.
Paul Hunt 2

Paul / Allen,
Thanks both for the suggestions and comments.

Regarding fuel pressure, I think that I have ruled this out by pressurizing the system without starting the engine and waiting 20-30 mins with no smell resulting (as Paul correctly stated).

The car is fitted with an anti-run on valve and single carbon evaporative cannister as Paul describes. I did think that this might be the problem and replaced the carbon inside the cannister with carbon pellets used in fish tanks. This did not solve the problem either.

I don't believe that there is a problem with the emissions hoses but, given that there are so many of the damn things, I will certainly take another look.

I will certainly give John Twist a call to see if he has any other suggestions.

Thanks again for the help guys.

Mark
Mark

This thread was discussed between 20/01/2008 and 01/02/2008

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