TVR Vixen - Engine
This car was fitted with twin Weber 40 DCOEs when I bought the car although the rest of the engine is standard. The choke cable was only connected to the rear carburettor and I extended to this to the other by using the inside of an electrical terminal block to clamp an extension cable on without having to replace the choke cable.
During the first few months I had the car it was plagued by going off tune and refusing to idle. It was easy to fix once I worked out that the cause was an air leak at the carburettor - although it took me a long time to work this out and try it. Even though the carburettors were correctly mounted with rubber O-rings, thackeray washers, and nyloc nuts they kept coming loose as the studs would not stay tight in the manifold.
When the engine was out of the car I decided to remove all the studs and retighten and loctite them in place. I used a pair of nuts to lock on each stud so that I could turn it. By the time I had finished I had three pairs of nuts to cope with three different threads - fortunately the manifold end used only two different threads and only one was wrong. I bought a new set of manifold studs and found that these were too long as the they would not screw far enough into the manifold as the holes were not fully tapped. Rather than buy a tap I extended the threads by filing grooves into an old bolt which was quite hard enough to cut threads in the aluminium manifold. Problem solved - but why is nothing ever straightforward?
I also discovered when refitting these that I needed to use a better technique. In the past I have had no trouble with balancing SU carburettors by listening to the "hiss" with a bit of tube, but SU's are closed right down at idle and there is quite a strong airflow. These have a much larger intake area and I couldn't hear the difference. A borrowed synchroniser device solved the problem.
One of the consequences of fitting twin carburettors to this engine is that the distributor becomes almost totally inaccessible making electronic ignition very desirable. The cheapest way of doing this is to use the electronic distributor from a Ford Fiesta or Escort with the Valencia engine. The way to do it without altering the ignition advance curve and retaining the original coil and ballast resistor is to use an electronic ignition kit and this car is fitted with a Lumenition electronic ignition kit.
Fitting electronic ignition can cause difficulties with the rev counter on cars of this age
Changing the carburettors shouldn't need alteration to the advance curve so I reconnected the vacuum advance, threw away the vacuum gauge and stopped the engine misfiring. I filled the hole in the dashboard with a clock.
Those of you with an eye for detail will have noticed in the picture of the engine bay that the top of the dipstick is elegantly topped off with a bit of oily rag. This is there to stop oil blowing out of the dipstick hole as I don't know how else to do it. I have run the engine with the oil filler off and there is no question of crankcase pressurisation - air is sucked in all the way up to the red line. One suggestion has been that it is caused by the lack of a vent in the alloy rocker box but I am reluctant to start drilling this without understanding why. On the original steel rocker box the vent would have been connected to the air filter housing - and that's not there so I can't do that.
As I understand the original system it is supposed to draw air into the top of the engine and out through the crankcase breather - a valve regulates the flow to avoid affecting the fuel mixture too much, so would an extra vent at the top let more air in or out? Why when air is being sucked from the crankcase breather does oil come from the dipstick which enters the crankcase next to the breather. Can anyone help please?
I've now established that this is a common problem on fitting twin Webers because the vacuum is provided by only one cylinder and this is apparently not enough. I imagine that the problem will only reveal itself when the engine is working hard and breathing more heavily. I'm told that the solution is to forget about recirculating crankcase fumes and to fit an oil catch tank and breathers.