Carburetor issues

[Yngbld14]

New to the site as well as to British cars. I have owned and worked my older cars (71 GTO and 56 Chrysler Windsor) and decided to try something different, so I got an MG Midget. I have lots of questions. I am not sure of the Stromberg carb setup on this car. I have several starting and idling issues.

I just got the car last week so I am trying to figure it out. The car can only be started after its warm by either pulling the manual choke out to the fast idle or depressing the gas pedal 1/2 way. The car also burns real rich and often idles irratically after warmed up. I did figure out that after the car has warmed up and I remove the damper on top of the carb and depress the air valve til it bottoms out, the car starts with no problem. Does it need to be rebuilt? Not sure of all the adjustments.

 

[R6MGS]

I am not all that familiar with Midgets but I assume you've got a single zenith stromberg carb?? The good thing is that you've got a manual choke and not one of those sorry excuses for an auto choke my 78's have.
Often these carbs tend to get gummed up and sticky and that can cause them to run poorly so thats usually the first thing I check before starting to rebuild anything. Try shooting some carb cleaner into the throat, if you can remove the dashpot on the top you can also try shooting some carb cleaner int othe carb from there to help remove the gumminess....Also the weight of the oil in the damper can affect the way it runs...A light machine oil is usually best, but regular 3 in 1 oil can work ok too if it's all you've got handy....I am sure someone will chime in with the proper weight required.
Try those too things first....if those don't help you may need a rebuild...or at least a proper tuning

 

[mk2sprite]

Doing what the previous poster says is good ,cleaning it should help. Sounds like you may Not have any oil in your damper. I always use regular 20/50 engine oil but there are many different opinions on what weight to use. Have also seen ATF used. Put in enough oil so when you put the damper cap back on and pusg down with slight pressure it leaves aproc. a 1/2" gap between the bottom of the damper cap threads and the top of the carb. Then push it home and screw it on. See how it goes and let us know what happens.
Mike

 

[Yngbld14]

I put in 3 and 1 oil didn't seem to make a difference. It seems like the air valve should move as I run it, but I am not sure. I start the car and let it warm up to normal temp. I rev the engine a few times and shut the car down. I notice after I rev it the air valve goes up and does not return downwards when I shut it down. Is it suppose to? The car will not start right up unless I push the air valve down. Not sure what to look for. Seems like the damper is long enough.

 

[bthompson]

Welcome to the site! I have a '76 Midget that <span style="font-style: italic">used </span>to have the Zenith. That carb's known for being a bit troublesome. It works well when it's in tune, but it's very sensitive to vacuum leaks of any sort--and <span style="font-style: italic">any </span>leak will drive it dramatically rich. The first thing I'd do is remove the dashpot cover (the 4 screws holding the dome on) and see if the cylinder is gummed up. It sounds like it's not falling freely. The manifold vacuum sucks it up; with the engine off it should fall like a stone. Check the rubber diaphragm and cylinder orientation when you put it back together--it previous owner may have put it together wrong. Oh, and don't squirt carb cleaner down the dashpot hole--it'll eat the rubber.

Pop over to the Spridget forum and ask there, too--there's a lot of us there with Zenith experience. Good luck!

 

[Yngbld14]

THANK YOU ALL!! Your input was very helpful. Come to find out the previous owner tried to take the air vent apart and put a burr on the end of the sleeve. Filed it off and bingo the air vent drops like a brick when the car shuts off. Thanx guys. I still have the issue of it burning rich. Maybe the egr isn't working properly?

Dave

 

[bthompson]

The EGR is a long shot. Remember most carburetter problems are electrical. :wink: "Too much gas" could actually be "too little spark." Make sure you have a good fat spark at the plugs first! Beyond that, there are several reasons for a Zenith to richen up...

--Vacuum leaks. With the engine warmed up and running, squirt some of that carb cleaner around anywhere air might be sneakin' in. Manifold gaskets, vacuum hose connections, choke body, air bleed valve, throttle butterfly rod, etc. If you hit the spot where the air leak is, the idle speed will suddenly change.

--Worn needle or jet. The needle is spring-loaded to run against the side of the jet, and as it wears, it'll let more gas by.

--Broken needle seat. The needle is held in its seat against its spring by a teeny roll pin. If the pin breaks, the needle will pop upwards into the seat by a few mm, enlarging the jet orifice and letting more gas by.

--Needle adjustment. There's an Allen head screw at the bottom of the dashpot hole that'll screw the needle down by a few turns to lean it out. (There's a nifty special tool that you can use: its outer sleeve holds the piston from turning while you turn the screw in. You don't need it, though, if you take the dashpot dome off and hold the piston with your fingers while you tighten the screw with an Allen wrench.)

--Float bowl level. If it's set too high, it'll be too easy for the venturi to pull the gas up.

--Float bowl needle valve. If it's worn (or sticking!) the fuel pump will just keep pumpin' gas out the jet.

--Plugged air filter. Since the carb balances vacuum both sides of the venturi, a restricted filter on the 'in' side will increase the vacuum relative to the other side; like a vacuum leak in reverse, but causing the same effect. (Zeniths are <span style="font-style: italic">very </span>sensitive to pressure on the air filter side.)

I'm sure there are others, but those are the major causes I can think of off the top of my head. Honestly, the Zenith isn't a <span style="font-style: italic">bad </span>carb...Just needs some looking after. Of course, if NY doesn't require smog tests on older cars, you have other options than just rebuilding the ol' Z-S. The 'home market' cars used dual SU's, and you can run that setup if you can find it. I <span style="font-style: italic">think </span>the big SU HS6 (1 3/4" throat) bolts right up to your existing manifold, so that's an option too. (The SU and Zenith do the same thing the same way, but the SU does it better.) And the Weber DGV is a popular (Italian) replacement, but requires a little creative engineering to get the manifold to fit sometimes.

p.s. A lot different from your '56 Chrysler, isn't it? :laugh: I had a '56 Dodge not too long ago. As nice as the Super Red Ram, Forward Look fins, and pushbutton Torquflite are...I still like my Midget better.