MG Car Repair: vac on the carb., poor vacuum, vacuum gauge

Question
QUESTION: Rebuild my 1976 midget carb. running good but a little rough, do I need to set the vacuum , I have a friend  with a vacuum check, is there a certain # I need to get it at ? How is the best way to check it ? Thanks

Steve

ANSWER: Hi Steve,

Over the years I have seen people try to set adjustments by reading a vacuum gauge.

After studying engines for years I do not follow that method for many reasons. Some cams will make an engine produce very poor vacuum at low RPM like you would expect that same engine with a different cam.

I found that there are too many variables involved. Racing motorcycles professionally and building my own engines I arrived at a procedure that I felt worked. Also as an instructor in two commercial mechanics schools teaching engine theory and engine rebuild, the same was true in auto gasoline car engines.

Ignition timing is strictly controlled by compression ratio, temp of the engine and the O/S air and the quality of the fuel.

Fuel mixture depends on what you want to do with the engine. A smooth idle and mild street driving requires a stoichiometric or a hair leaner mixture where the max horse power is from 15% to as high as 40% richer then stoichiometric.

The difference in manifold vacuum in all of these cases is wide spread.

On your 76 Midget you have only one constant velocity carburetor (Stromberg) and all you need to do is change an air cleaner and you will see a different manifold vacuum.

First run a compression test (throttle open) and IF it is ok then start tuning.
Unless you have modified the engine in any way you should set timing to factory specs and leave it there. Then adjust the carburetor mixture with the 3mm Allen wrench (if it is not factory plugged) Then when you are happy with the running check the vacuum and log it as a vacuum that you want to aim at (first checking what the barometric pressure and temp is for that day and location).



---------- FOLLOW-UP ----------

QUESTION: Thanks Howard

This has been a big help, I didn't want to mess with the vac anyway.

Shod the compression # be the same on all  and what would be a good # for this old car ? and [Throttle open } pleast explain,,,,,,   where on the carb, do I use the 3mm allen wrench to adjust ?

Thanks, you have been a big help

Steve

ANSWER: I like to see from 135 to 150 PSI and only a little difference between cylinders (5 PSI) If it is lower down close to 125, it may still be ok but lower then that is an indication of a problem. If it is higher then 150, that's fine but could be an indication of excess carbon build up in the combustion chambers. Still not a problem but then requires a good high octane fuel.

The reason for the (open throttle) is "No air in = nothing to compress" and a false compression test.

The 3mm Allen wrench is inserted into the top pot after you remove the large thumb screw with the  dampener on on it. Clockwise to go rich and counter clockwise to lean. Only turn a half turn at a  time and rev the engine to see the change. BLM plugged some cars but I don't remember exactly which models. You will know because you won't find a Allen screw in the bottom to insert the wrench into.

Without CO equipment you can go rich until you see a light puff of black smoke from the tail pipe on a rev up, then turn counter clockwise a half turn and put oil in the top (if it is not already there), put the cap back on an give a few quick revs of the engine while someone watches the tail pipe. You will probably see the puffs of black smoke return so turn lean another half turn so that you just barely detect any black in the smoke. The adjustment will be close to correct then.

Always do the compression test first and if that is not correct it is a waste of time looking at ignition or fuel. If compression is ok always do ignition next and fuel last.

Howard

---------- FOLLOW-UP ----------

QUESTION: Thanks Howard for all your help.

I am still a little confussed on the 3mm allen wrench and where to adjust it,,, are you talking about unscrewing the top of the pot or carb. where I put oil into the carb, and the allen screw is the one that holds the metering needle that I need the Stromberg tool to get to  it ????

Answer
Steve, there is only one big plastic cap on top of the carburetor. remove it and it will have a long pin attached with a piston on the end which sits down in oil. lift it out and put the long end of a 3mm Allen down into the oil and see if there is a socket for the Allen wrench to fit into. That is the adjusting screw. As you turn it clockwise you lift the needle which makes it rich and counter clockwise forces the needle down which leans the mixture.

There is a Stromberg tool that holds the piston in the carburetor so as not to put a strain on the diaphragm when turning the adjusting screw as some are tight. You can get the tool if you like but you can just reach into the front of the carb and hand hold the piston if the screw is tight.

Howard