Oh Dear , What Can the Matter Be ?

[JohnLiles]

My 103,00 mile 356C with Zenith carbs was serviced a few weeks ago , trip back from the service shop was fine , about a 1 hour run , engine running sweetly on highway and in town traffic crawl , steady tickover , no issues .

I go to start it up yesterday after a few weeks standing , almost fired up first time . Was then probably over enthusiastic with pumping the gas pedal after which it would not fire up . Left on battery charger for couple hours ,tried again, no luck.

Left it again , but when came back could smell fuel , looked under engine lid and saw RH carb dripping fuel , actually running off the throttle spindle end but on examination fuel seemed to be coming from higher up , just under air filter and level with fuel inlet pipe. The fuel inlet banjo joint itself was not leaking . Turned off fuel tap in cabin , left it overnight to ponder.

Came back this morning , all looked dry , turned fuel tap on , left it for 10 minutes , still dry on RH carb . Tried firing up , started first time , felt like it would stall so kept blipping throttle to get fuel flowing and things warmed up , after a minute or so was still having to give it some throttle to keep it going but running very lumpy and felt like not all 4 cylinders firing. Got out of car to have a look under engine lid , tickover then dropped and seemed on verge of stopping , and RH carb was leaking fuel again . Switched off , no tools and no time to do anything today.

?? Assuming that fuel is being pumped into RH carb and not getting out again then this could explain
A it would overflow
B the cylinders on that side would not be working

I am hoping that I can fix this myself but not sure where to start ! Any advice most welcome !

[David Jones]

There is a strong likelihood that you have a sticking float valve which is not unusual, I have had it happen a few times. Unfortunately it is a fiddly job to fix it in the car. Easiest is to remove the carb and replace the float valve. Do you have a workshop manual? Be careful when you remove the top of the carb with the accelerator pump linkage and be very careful as you remove the top not to damage the top gasket. It may be a good idea to take pictures as you go to enable re-assembly. There are some small parts that are easily lost. On re-asembly be very careful to get the top retaining fasteners in correctly. It is easy to damage the threads on the carb base, do not over tighten as the metal is weak and the threads are easily stripped.

[WilliamVaughan]

sticking or held open by rust particles

[Doug McDonnell]

John If you have a sticking float valve gas will have gone into your crankcase. Check your oil level And smell the oil. High level and oil smell like gas? Change oil before trying to star your car again.

[Ned Gorski]

John
be sure to check your oil a to see if it smells like fuel or if the level is elevated . a stuck float or a bad needle valve can fill your engine with fuel. bearings don't take so well to thinned out oil. its a good habit to always close your fuel cock. i learned my lesson years ago, i always close the fuel cock now its also a good theft deterrent.

Regards Ned

[JohnLiles]

Thanks guys ! I think that I may have introduced some swarf into the feed to the RH carb when I had to hacksaw thru the metal fuel line a few weeks ago -to remove the crimped-on rubber coupling which had failed . I was very careful (!) but that pipe is hard to get at being behind the fan housing and a) I was too lazy to disconnect it at the banjo joints to get better access ; b) did not have to hand replacement red fibre gaskets for the banjos ( Murphys Law predicts the old gaskets will leak on re-assembly )

Will check for fuel in sump oil . Oil level looked OK , I checked it and it was up to the top line but had only just been serviced so expected that . I will go back and have a good sniff though . The elapse time between creating the overflow and shutting off the fuel tap was around 60 minutes , after that the carb continued to drip for a while say another hour . Before yesterdays failed start up there was no evidence of a leak or smell of fuel so I am convincing myself the oil may have had minimal contamination .

My understanding is that the carbs were rebuilt and new jets fitted by one of the top German specialists under the car's previous ownership and have only done around 15,000 miles since so hopefully a bit of maintenance will sort it out , and yes have a workshop manual and also the tech articles on this website to hand.

[Richard Shilling]

To test for gas in the crankcase, smelling is good, tasting is better. Put a drop of oil from the dipstick on a finger. Touch your tongue to the drop. Oil tastes oily, like cooking oil. Gasoline has a sharp taste. Wipe off tongue and rinse mouth. Works every time!

[Larry Brooks]

I also would be checking the big brass plug that is swedged into place next to the fuel inlet (right around the corner).

[Vic Skirmants]

That only "leaks" when cornering. Won't have any effect in this case.

[JohnLiles]

Update . I managed to get the leaking carb off , had to borrow a 12mm ring spanner with short handle from a friendly local garage to remove the 'front' two nuts holding the carb to the inlet manifold.
During this process I had a senior moment and was trying very hard to shift the nuts in a direction that would only tighten them up .

I took the carb home and carefully took the top cover off . I have ordered up a carb kit with gaskets and bits like float valve included , probably overkill but was nervous about damaging gaskets.
All looks OK , the twin floats float and move freely , don't have a sensitive weight balance but suspended the assembly on a straight edge and it appeared to balance halfway to the naked eye suggesting both floats equal weight and unlikely to have fuel in ( except for an equal amount of fuel in each !)

The float valve assembly did not look like it would break down to remove the valve, tried blowing in the open end and then holding the movable valve stem in , appeared to work ie it sealed . However obviously it is suspect so will replace it when the carb kit arrives.

On rechecking oil level , it was about 1 cm too high and dipstick definitely smelt like fuel . Tried tasting it as advised but my sense of taste failed . Could (and should) have compared it with straight oil tasted from the can but will accept that have to drop the oil and change it. Will be another week or so before have chance to do it

[David Jones]

John, if there were any debris in the fuel line that got into the float valve it may well have been flushed out by now through your ministrations and you may never know the cause of your leak. When you re-assemle the carb be sure to use the same number of fiber washers under the new float valve and you may be close to the correct float level. Do a search on google and read about setting the float level on Zenith carbs. It is important to be as close as you can to the correct level.

[Thomas Sottile]

A little sugar helps.

[Thomas Sottile]

A little sugar helps getting down.

[Wes Bender]

You can buy a 12 mm ring spanner and heat it with a propane torch and bend it like the one below and it will be much easier to remove those four mounting nuts on each carburetor. Be careful not to bend the idle mixture screws or the accelerator rods when tightening or loosening the nuts.

To use the tool, merely insert a screwdriver shaft or socket extension in the open end. Don't overtorque the nuts.

[David Jones]

It also helps to shave some material off the box end to make it easier to get on the nut and be able to turn it. I use a stubby wrench which allows access to turn the wrench and prevents over torquing.
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