Solex Carburetor Vacuum Ports

Mark Thompson · Post by **Mark Thompson** » Thu Sep 07, 2006 3:55 pm

Last night, Russell Natali and I were discussing a problem he is having with his 4ADT. His car runs fine on level ground, but stalls whenever he attempts to go up a hill. He says it seems to be starved for fuel. He is working on an alternate fuel pump as a possible solution but would welcome any suggestions to solve his problem. As part of the discussion we got off on a tangent about vacuum ports on the Solex carburetor. He was asking if the vacuum line going to the distributor is supposed to be connected to both carburetors. His is connected only to the rear carburetor. I told him that I thought that was the correct arrangement, but I wasn't sure. My working 4AD has a single carburetor. I am restoring a 4ADT and the vacuum line on it is connected only to the rear carburetor, however I have not yet had the car running and so I am not sure if that is correct. Can anyone advise?
Russell then raised the question about whether the vacuum port on the front carburetor should be plugged if it is not being used. I checked the port on my single carburetor 4AD and found that the port is not plugged and doesn't appear to be threaded to accept either a vacuum line or a plug. My car runs without any difficulty whatsoever. So, now we are wondering, should the ports be plugged, or does it matter?

Post by **mikeyr** » Thu Sep 07, 2006 5:59 pm

first make sure you are talking about the port on the carburator and not the ones on the manifold below the carburator. Some people will connect the vacuum line to the distributor on the manifold, its supposed to be on the carburator, front or rear does not matter, rear is easier to feed the line back to the distributor. Mine is plugged on the front carburator.

Since the car appears to run fine at speed and only die out on hills, I would assume that the fuel pump is ok, the car needs the same amount of fuel to go uphill as it does at high speed, a car dying out at speed is usualy the fuel pump. On any other car, failing up hills and running fine otherwise I would look at the carb float levels and the floats themselves. On the 4AD there is no adjustment for the float levels but that needle valve above them could be bad or the float could have slight leak and one float heavier than the other.

Last item would be the fuel system itself, could the gas tank be dirty and have something that rolls around to plug the fuel inlet on a hill and back on level ground it moves away (for example a broken baffle in the tank).

On a non-fuel note, how is the timing of the car and does the centrifugal advance really advance ? You mention his car runs fine on flats does it really ? by that I mean how is the power ? could it be that his centrifugal advance is not working correctly and he has some power, enough for freeway speeds but not enough advance to get power up a hill ?

Peter McKercher · Post by **Peter McKercher** » Fri Sep 08, 2006 1:06 am

Mark/Russ
Mike makes a good point. Is it really running ok on level ground. I just resolved a similar symptom with mine. It seemed to run fine at idle and even in cruise mode, but crapped out as if fuel starved under real load. I thought sure it was gas, but it wasn't. It's easy to jump to conclusions.

With respect to fuel, dirt is a possibility. Does the car have a fuel filter. Is it clogged.

Timing might also be an issue as Mike suggests. You might also want to check the high tension and plug leads for arcing. Do this in a pitch black environment. You might be surprised to see the fireworks under the hood.

My problem was two fold, The low tension lead from distributor to points was hanging on by 3 strands - too much reistance for the current. With the darkness test I also found both terminals on the coil were lighting up. I assume there was a hairline crack allowing the current to take a shortcut. The higher the output the worse the performance.

Post by **mikeyr** » Sat Sep 09, 2006 4:45 pm

Ok, come up other people am I wrong on this ?

I spoke with Russell yesterday and after he plugged the front carb vacuum port, the car started flooding, it had gas overflowing the top of the front carb. I would say that is from the needle valve not shutting off the gas but overflowing is not something I would have expected from plugging that vacuum leak, so come on anyone else with a suggestion ?