|The MGA With An Attitude
VACUUM LINE ROUTING - IG-121
Common question: How do you install the vacuum line from carburetor to distributor?
When you buy a "pre-bent" vacuum line for MGA (or early MGB) it may look something look this illustration. This does not show all of the bends, correct orientation of the end fittings, where it follows around the engine, or where it connects at the ends. There should also be a small bolt and nut running horizontally through the bracket to pinch onto the bulb. Item 29 is an optional rubber connector used for the later MGB style vacuum advance unit with push-on connector.
The metal pipe should be all one piece from carburetor to distributor with a fuel separator bulb in the middle. The bulb is filled with wire wool with the metal pipe on each end to serve as a fuel separator. The bulb goes upright with bottom end connected to carb and top end connected to dizzy. The bracket holding the bulb was originally mounted on the second manifold bolt from back. However, with engine in the car you may find it easier to install the bracket on the rear manifold bolt. The P-clip goes on the rear head stud. Use a second nut there if the stud is long enough. Otherwise you have to R&R the nut to install the P-clip.
Illus# Part# Description
AUC4490 Union, automatic ignition, rear carburetor
32 1H830 Pipe, vacuum ignition control (with fuel trap)
32 1H919 Pipe (new part number) Com. (E) GB11301
34 2K6192 Nipple, carburetor end Fin. (E) GB11300
35 2K6193 Tube nut, carburetor end Fin. (E) GB11300
35 2A459 Tube nut (new part number) Com. (E) GB11301
36 6K649 Olive - Qty 1/2 - (qty increased at (E) GB11301
37 6K650 Tube nut, distributor end
Item 34 was originally a long brass nipple soldered to the pipe. After 11,200 engines (October 1956) the soldered fitting was changed to the small brass olive, and the matching compression nut changed to smaller bore. The current Moss Motors replacement pipe
has a male threaded nut and brass olive (items 27 & 25 at top of page). To use the Moss Motors pipe as received you need to remove the original adapter fitting, and screw the male threaded nut directly into the throttle body. Alternately, for concours show, you can cut the end fitting off the Moss pipe and install the original female threaded compression nut and brass olive to use with the original adapter fitting.
Photo at right below shows the fuel separator bulb and bracket correctly on the second stud from the back. However, the lower tube is turned back upward to run over the top of the heat shield before going underneath the carb. This was claimed to be an original MGA 1600 survivor car with low mileage, but it appears to have been "tampered" with in various ways. The valve cover was repainted (poor masking job on the patent plate), the firing order plate is missing (proof of prior disassembly), heater hose changed with incorrect hose clamp, apparently incorrect vented dashpot cap on the carburetor (not the later dustless carbs), and the vacuum pipe is painted (I think not originally painted). Subsequently I think re-routing of the vacuum pipe to the carburetor is incorrect.
I believe the pipe from bulb to carb originally ran down the inboard side of the the heat shield to pass underneath, then back up the outboard side to connect to the under side of the rear carburetor. If you put the bracket on the rear manifold bolt (not original) you may find it easier to run the pipe around back of heat shield then under the carb.
Connection port is on bottom of carb, left of center, underneath the edge of the throttle plate. That threaded port is to the left of center underneath, (toward front of car) opposite the float chamber. If you haven't installed the carbs yet it may be prudent to connect the vac line to the rear carb before installing the carbs on the engine.
Two pipes in the picture are the vacuum line and fuel vent/overflow pipe. These would be better off running behind the spring bracket, leaving better access for wrenching on the lower carb mounting nut.
The distributor end of the pipe originally has a female threaded nut and another compression olive. If the distributor drive gear is installed properly, the vacuum unit will point straight up. The pipe connection nut will be underneath the heater valve, requiring a 90 degree turn in the pipe just above the nut. Life will be easier if you leave 1/4" of straight pipe above the nut. Pipe between the vac unit and P-clip on rear head stud has to flex when you rotate the dizzy for spark timing.
More photos from Ron Bissland in Kenosha, WI, USA