I'm having trouble starting my 1973 S3 V12 after about 8 years without running. I've fitted a new fuel tank and pump, and confirmed that I have petrol (gas) pumping to the engine - it was even leaking from the air inlet side of the carbs before I replenished the dashpots.

I have decent sparks on the new plugs, and have confirmed that the starter is turning the engine.

If I spray "Quick Start" into the carbs, the engine will fire and burn the Quick Start, but won't continue to run on its own.

To me this suggests that I'm not getting petrol from the carbs into the cylinders, but I can't work out why

Does anyone have any suggestions on what the cause could be and what I could do about it (other than removing and overhauling all 4 carbs)?

 

How old is the fuel? On a friend's Norton we putzed around for two weeks until we decided to drain the fuel (6 months old) and replace it with fresh gas... she started up on the second kick. Make sure the carbs have fresh fuel in their float bowls too. Newer gas with ethanol absorbs water from the air, it messed up my boat motor this summer as well.

 

Thanks Alfred. In doing all the other things, and putting fresh fuel into the tank, I completely forgot about the old fuel that would still be in the lines and the float bowl. I'll get that all drained down and try again.

 

Hello Stuart,
Stopping the leaking fuel from the inlet side of the carburetor by replenishing the dashpots was a coincidence, as the two are not linked.

Its probable that the fuel was leaking from the fuel reservoir vent. This is a hole located at the top left area of the inlet face. You will see that this is a brass tube that extends through to the fuel reservoir of the carburetor. Fuel will leak from this hole if the needle and seat valve that controls the flow of fuel into the carburetor reservoir does not close. Conversely, if this valve becomes jammed closed, either within itself, or via the fuel level float, then fuel won't flow into the fuel reservoir. Also, the fuel metering jet can become blocked if rubbish is sucked into it from the bottom of the reservoir cap.

It would be unlucky for all carburetors to be effected in this way, but it is possible. A V12 engine will start if only one carburetor is working, and will start and sound as if running correctly when only two carburetors are working, particularly if the two working carburetors are on the one cylinder bank, if you're not familiar with how it should sound with all carburetors working.

If after changing the fuel the engine still will not continue to run, I would definitely remove the carburetors and at least clean and fit a new minor service kit (not expensive), particularly when the car has been sitting for 8 years. Make sure that the Temperature Compensation valve is not stuck open. This is the unit on the R/H side of the carburetor, looking at the inlet face of the carburetor, that has the white plastic cover.

The carburetors are more easily removed if removed complete with the manifolds, and are much more easily reinstalled when already attached to the manifolds. However, you need to replace the gaskets between the Inlet Manifolds and the Cylinder Heads if the manifolds are removed with the carburetors.

Regards,

Brent