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MG Midget and Sprite Technical - New Engine--Won't Start. Need diagnosis help

77 MG 1500 with a single ZS carb. Won't start.

The only (intentional) changes I made are a 4-2-1 header (open right now), and a longflow air cleaner. Carb, distributor, etc were working when they came off, so I just bolted them back on. I did replace the water choke with a manual choke. Seem to work, but I don't really know.

It has spark, and I think I have the distributor on right but from what I understand I should get something even if I'm 180 off. I am not sure it has fuel in the cylinders. Fuel is coming out of the pump and from the line to the carb, but I don't know if the carb is using it. When I look into the air intake, I see the carb piston covering about 80% of the opening. After cranking, I pull out a spark plug and it is dry.

I'd like some ideas on what I should do next to diagnose the problem.

Thanks.

--john
jf Falconer

John -

Makes me wonder if your float valve is stuck shut. You'd still be seeing fuel at the carb, but it just wouldn't be entering the float chamber.

It's normal for the carb piston to rest at the bottom of its travel when the engine's off, and to hover near the bottom at idle. If the piston doesn't rise when you crank the engine, that would point to a bad diaphragm, easily replaced.

Keep us posted... this shouldn't be too difficult to figure out!

-:G:-
Gryf Ketcherside


Tip a thimble full of gas through the the top of the carb, if it fires, then it's gas supply problems, if not, the problem is most likely to be no spark. Rotate the dizzy through 180 and trying that.

If you do all the above and you still have no luck, put the dizzy back to where it was when you started and grab a Haynes/Bentley book and start trouble shooting!

Andy
a borris

If you can't wet the plugs it's either not firing or not getting any petrol.
Daniel Thirteen-Twelve

In reverse order:

Firing: It is. I held a plug to the block while charming assistant turned the key. Lots of happy little sparks.

Fuel: There is no top to this carb (intake comes in from the side). Can I squirt a little gas in there, and hope it drains the right direction? But...

Float valve: Perhaps sitting dry for a year did this. Does it mean taking apart the carb? I can pry the piston up, but then it returns to about 80% closed. Too much/too little oil in the float chamber?

Firing order: If I am 180 degrees off, shouldn't I get a some kind of fire/backfire?

--john
jf Falconer

You shouldn't be able to get the dizzy in the wrong way round if you simply removed it and put it back
The drive dog is offset for this purpose.
Yes if you add a squirt of fuel you ought to get a short scond or so of "Pseudo-running" until it dries up again
Be aware this is not a good idea, uncontrolled fuel can cause lots of grief if there is a blow back.
If you do this and it does ignite outside the carb the best way I ever found to deal with it was to keep the engine cranking and put the "fire" where it belongs, back inside the engine and down the exhaust pipe
I suspect a stuck needle valve blocking the supply to the float chamber and the jet, gumming up is not rare in such circumstances
Keep an extinguisher ready
If the float has stuck it may respond to the tradtional smack with a firm object to jar it loose
Wooden end of a hammer stale would suit.
Bill

>>> I can pry the piston up, but then it returns to about 80% closed. <<<

Hm... you shouldn't have to pry it! Normally, you should be able to lift the piston with a fingertip. It should move smoothly, but with some resistance from the damper; and then it should drop freely back to the bottom with a clunk. This is with the engine OFF, note.

If it feels like there's excessive resistance or interference, you have other problems. The piston and damper assembly is easily removable via the four screws on the carb's top cover. It can't hurt to remove things for a peek. Just be sure that when you reassemble it, the little tab on the edge of the diaphragm engages the slot on the carb body. This ensures that the holes in the bottom of the piston are correctly oriented to the incoming air flow.

Keep us posted!

-:G:-
Gryf Ketcherside

Took the top off the carb yesterday. The diaphragm had no obvious tears. The piston moves smoothly up and down, but takes a bit of pressure.

My Haynes diagram left me with some confusion, so forgive the novice question. At the bottom of the piston is a needle...I take it this is the oft-mentioned needle valve. It has a little travel, but not much. Perhaps 1/4 inch. Is this right?

How much does vacuum matter in starting? My lines are not quite what they were pre-rebuild, because I switched the exhaust manifold and thus dropped one branch of the line.


--jf
jf Falconer

>>> At the bottom of the piston is a needle...I take it this is the oft-mentioned needle valve. It has a little travel, but not much. Perhaps 1/4 inch. Is this right? <<<

Nope, that's the metering needle. It's mounted in the piston bottom with a little spring that causes it to ride gently along the bore of the jet, minimizing erratic fuel flow due to vibration. Note that the needle emerges from a cylindrical brass "holder" in the piston bottom; when properly adjusted, that holder should just barely extend beyond the piston bottom.

As for the piston itself, if you unscrew the yellowish knob at the very top of the carb and withdraw the damper rod/valve, the piston should slide up and down smoothly and freely. There shouldn't be any binding at all. And again, with the damper in place and proper filled with oil (i.e., 1/4" inch from the top of the hollow rod), it should cause stiff resistance to upward movement, but none at all as the piston falls.

The needle valve, or more properly, "Float Valve" we're talking about can only be accessed by removing the carb from the manifold, inverting it, and removing the float chamber. Undo the six screws, and it will lift right off. You'll see the plastic float, with a little brass tang that presses against the bottom of the float valve. Unscrew the valve and lift it out... the idea here is that as the fuel pump fills the float chamber, the float will lift until it contacts the float valve, thus shutting off fuel flow and maintaining the proper depth of fuel in the chamber. This is important, as there needs to be a specific level of fuel in the chamber and jet to ensure the proper "atomizer" effect. You can check the (removed) float valve by blowing into the top and pressing up on the pin; it should shut off air flow. These things can get fouled by gummy old fuel, so clean it well in carb cleaner and make sure it works.

This is also a good time to check the bottom of the float chamber itself to see if it has any sediment, etc. Clean it well, and I'd recommend fitting an inline fuel filter there at the carb to minimize any further crap getting into it.

When you reassemble the float valve, you'll want to check the float height. Invert the carb again, and with the valve installed, measure from the chamber gasket face (with the gasket removed) to the uppermost part of the float. You'll need to check your manual, but I think the proper distance is 5/8". I'm at work, or I'd check. If it's not accurate, carefully bend the brass tang until it's in spec.

That's basically it. Note that a properly set up Zenith carb can work very dependably! Once I studied up on mine and rebuilt it, and then tweaked the settings, it's been practically trouble-free, as long as I keep oil in the damper.

Keep us posted,

-:G:-
Gryf Ketcherside

Thanks very much, Gryf. With your guidance I will explore the theory that sitting idle for a year may have led to a stuck needle.

I'll check the valve and clean as you suggest. I'll post an update as soon as I go thru the process.

(glad I'm not the only one who pursues his avocation while at his vocation.)

--jf
jf Falconer

Good luck!

When working on the float chamber, valve, etc., just think of it as a miniature toilet tank filled with gasoline. The theory's exactly the same, in that incoming water (or fuel) raises a float until it hits a valve, thus shutting off said fluid at a specific level, until the level drops again and reopens the valve.

If your carb's float valve is stuck shut, it's sort of like when you flush the biffy and the bowl doesn't refill. Similarly, if the valve isn't shutting properly, that's like doing the ol' flush only to have the water continue to run after the bowl's filled. The bad part is that if the carb float valve doesn't seal, the chamber will fill to the top, flow out of the chamber vent into your charcoal canister (if fitted), and eventually cause your engine to ingest raw fuel, which can be blown past the rings into your oil pan. Fuel-diluted oil is a Very Bad Thing. Been there three times, until I finally figured out the problem.

-:G:-
Gryf Ketcherside

Gryf--

Pulled the carb off, pulled the float chamber, and pulled the float valve. It had some sediment (seemed metalic, which is odd) which I cleaned out. I tested the valve as you said, and it flows when open and block when pushed shut.

Following this line-of-fuel theory, I also blew into the longer down pipe that sits in the center of the chamber. I'm guessing this is the fuel up take. It let the air into the center of the carb and then out. Fine. Next to that (slightly off-center) is a shorter piple. I can't figure out from Haynes what it is or what it is supposed to do, but it won't let air pass anywhere when I blow into it. Is that a fuel uptake too?

I measured the float level and both sides were at 5/8".

Next question is what moves the fuel thru the intake and into the cylinders? I'm nervous that my vacuum line modifications have affected something. Does the fuel-air mixture get pushed, or pulled into the cylinders? If pulled, does the valve train action create the necessary vacuum?

Looking forward to responses! :)

--john
John

Hi John

The vacuum is created by the downward movement of the piston. Did your float chamber have signs of fuel in it when you dismantled it? Try placing the end of the fuel pipe in to a jar and crank the engine over to see if your fuel pump is working.

Carl
C Bintcliffe

Carl--

Yep. Fuel line shoots a nice stream when cranking. When I pulled the float chamber a little leaked out, but not much. Perhaps the sediment I found was indeed blocking the flow.

I am curious about that other down pipe... do you know what it does? Maybe it's not relevant.

--john

john

John --

Have you ever seen an atomizer? Imagine a drinking straw in a glass of Pepsi, with the level of Pepsi just below the top of the straw. If you blow across the end of the straw, you increase the velocity of air right at that point, thus decreasing the pressure to below ambient, and causing Pepsi to be drawn out of the top of the straw and sprayed all over your friends.

The carb works the same way. The tube you saw in the center of the float chamber is the main jet, and the tapered metering needle that extends downward from the carb air piston rides in that jet. As the piston rises and pulls the tapered needle out of the jet, it enlarges the annular space around the needle, and more fuel is drawn off into the intake air flow. Simple... the faster the engine turns, and the more air it draws in, the more fuel is pulled from the float chamber and mixed with the air.

FYI, the theory behind the rising/falling air piston is pretty interesting. Have you heard of the "venturi" effect? Basically, if you pass air through a tube, and the air encounters a restriction, the velocity increases and the pressure decreases. This is the same phenomenon that keeps airplanes in the air, which is a good place for them... In the carb, the "bridge" area right under the piston, coupled with the piston itself, create the venturi that accelerates air flow and decreases pressure, ensuring that fuel is drawn up and into the air flow. BUT - the rising/falling air piston create what's called a "variable venturi." Whereas a fixed restriction in the carb would cause the velocity to increase and the pressure to decrease according to engine speed, you can mix fuel much more accurately if the vacuum is constant, not variable. So in the SU and Zenith carbs, as more air is drawn through them, the piston rises, enlarging the venturi and creating a constant vacuum at all engine speeds. Thusly, all fuel metering is handled by the tapered metering needle.

BTW, that vacuum is referred to as "depression" by the British, so these carbs are known as "constant depression" carburetors for that reason. Apparently...

;-)

Now... as for the damper in the top of the carb, the idea there is that it restricts motion in the upward direction only; namely while you're accelerating. This restriction causes a temporary lag in piston lift, and therefore a higher air velocity and lower pressure at the venturi. The upshot is that more fuel is drawn into the intake air, enriching the mixture for better acceleration. The piston will eventually reach its "normal" height, or equilibrium, and the mixture will lean out to the standard "cruise" mixture.

Hope this helps,

-:G:-
Gryf Ketcherside

Yep; familiar with Venturi. And acquainted with Bernuli. Just wasn't sure where the draw was coming from.

I refitted the carb this afternoon and tried to start it, and of course it wouldn't. So I'm giving in and planning to take it to a shop. I don't want to be looking at a parked car in July.

Thanks all for your comments. If they can resolve it, I will let you know what the problem was.

--jf
John

John, if you email me your address I'll send you a booklet on how the Z-S carb works. It's as easy as anything once you understand it.

Clive Reddin 76 Midget

Try a shot of starting fluid. If it kicks in then you know it's the carb.If not then look at the ignition a lot closer.
Sandy
Sandy Sanders

This thread was discussed between 21/04/2008 and 26/04/2008

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