Better Beaters - Building Cars and Daily Drivers

 I found the nuts, but lost my carb parts. 

This is after I sprayed down the moving parts.

I had ordered the part to replace the broken needle/jet seat as well as some rebuild gaskets. Again, I put them somewhere that I was certain that I'd never forget, or I'd recall that clever connection.  NOT!

I'm still looking, but I kept on with my carb inspection process. The initial problem had been that the throttle shaft wouldn't move when I stepped on the pedal. I found that the linkage was bound up, frozen solid actually. I couldn't tell if it was the throttle slides stuck to the bore or if it was the butterfly plate or what. 

I decided to remove the carb assembly so that I could work on it on the bench.

That had been the idea, but it got pushed back further and further. At least I put the carb assembly back in the car.

I didn't want to work on the carbs on my wooden table since I didn't want to be spraying carb cleaner close to the laundry area.

Carts come in very handy.

Instead I brought in a metal roller cart so that I could work right next to the open garage door. I fixed up a a large shallow box that I made sure had a sold bottom panel which I installed and a metal roasting pan that I could contain the carb spray run off. 

Yes, this is only a cardboard box. I sealed up the side holes.
 I don't want to lose any more parts.

The Jag uses two large SU constant velocity carbs, at least that was what Honda called them on the CB450 back in 1965. I'd read a lot about British carburation in the motorcycle mags of the day. Variable venturi, which means that the size of the carb bore changes as a vacuum cylinder slides up and down. It not only regulated the air velocity it also regulates the fuel metering as a needle attached to the slide moves up and down within a jet. 

photo source; flicker.com
Note the dome shaped structure on the top.

The throttle plate only controls the amount of air entering the venturi, the slide rises and falls based on on the amount of vacuum present in the manifold. Therefore the  driver cannot force too much fuel into the motor by opening the throttle more than is needed at that moment. The carb maintains the setting that produces the greatest amount of vacuum. Savvy drivers used to mount a vacuum gauge inside the car, so that they could monitor the vacuum level and keep it high. They would not over throttle the motor. These were frequently refereed to as economy gauges as maintaining high vacuum provided the best power and fuel efficiency. These carbs do not employ an accelerator pump.

The carbs, unlike most American carbs also do not employ a choke plate to enrich the fuel mixture on cold start up. This also allows a greater and freer flow of air through the venturi. Honda used an actual manual choke plate like their other motorcycles. 

The heart of the starting carburetor is this electro magnet and disc valve.
I tested it and it works.

Some type of enrichment system has to be employed, often there is a built in system that most drivers referred to as the "choke," regardless. Jaguar chose a "starting carburetor" which provides the enrichment. Most American cars up until the 1950's employed a driver controlled choke. This was more reliable and effective, especially with the variety of weather that this country sees. 

Many drivers had trouble operating the choke. If left engaged for too long, the motor would chug along on it's over rich mixture. Trailing behind a cloud of black smoke and eventual fouled plugs. Then the motor would stall.

If the choke was released too soon,  the cold motor would have trouble gaining revs and would often die when pulling away from a stop. Again, a stall. 

Back in the Model T days, the driver was also responsible for retarding the spark to ease starting as well as applying the choke. Then they would advance the spark timing smoothing out the running of the motor. But they couldn't forget about the choke. There was also a hand operated throttle lever on the steering wheel. Factor in the three foot pedals and the Model T pilot was kept pretty busy!

Detroit discovered a method that relieved the driver of any responsibility of ignition timing. The automatic advance distributor uses an internal spring and cam which advances the point cam. That was later combined with a vacuum advance assembly. This system lasted well into the 1970's until ignition points were replaced by magnetic pick ups.  As the 1950's dawned manufactures found a system to automate the operation of the choke, at least for more expensive makes. Step on the accelerator once, then crank the motor. When adjusted properly the system worked pretty well. 

Jaguar designed an automatic system that activated the starting carb when cold. Upon reaching a specified minimum operating temp the system would dis engage.

Most other cars also had carbs with" accelerator pumps." These would shoot a dollop of fuel into the manifold to help transitions from low to high sudden rpm.  This shot of fuel could also help start a cold motor in conjunction with the choke. Or it could hopelessly flood the motor. 

The SU carbs do not feature an accelerator pump, so all fuel has to come from the starting carb. If it malfunctions, and from what I've read this was not uncommon, the motor would be a devil to start. Jaguars and other SU equipped cars gained a reputation as being very hard to start in the winter. Combine a weak ignition with a slow turning six volt starter and the process could become quite maddening. No wonder the cars had such poor resale value. Lucky for me, my car is 12 volts.

Jumper cables, cans of ether starting fluid, and heating the oil in the sump, (only seven quarts!) were often tried as antidotes.

The adoption of fuel injection has had far reaching improvements to the starting and drivability of our cars.

This post has been a lot of elaboration over the process of exchanging a few damaged parts. I'm just going to reorder the parts I've misplaced. I need to get moving again. I need to fire up this engine up.