Zenith single carb problems

[Ofcalipka]

So I've been resurrecting this 1976 MG Midget that has sat at least since 2001. The fuel of course was bad so I siphoned it all out as much as possible refueled her with 92 octane and then proceeded to change the cap, rotor, wires, plugs, and all fluids. So I start her up and she runs a bit rough and I hear a strange noise from the valves. I take off the valve cover and find a push rod on cylinders 2 and 4 off. I honestly don't know how the poor thing started at all. Anyways I order new pushrods from Moss, as one bent, and a carb rebuild kit. I put the new push rods in and start her and she runs okay. It won't idle very well at all when cold and not much better when hot. I drove it around and it was ok but definitely not working at its best. So I go ahead and clean the carb, which was all gooped up with varnish, and install the rebuild kit. I put it back together and now the car barely runs at all. I think it is way too rich. The prior owner had thrown away the heat shield and the auto choke and rigged up a choke out of what is labeled the starter box in the Moss carb parts list. Now here is my question. How does one go about adjusting the mixture on this and do I need the auto choke or is the current rig okay, I live in a tropical climate, never really gets cold here. Also do I need to make a new heat shield out of sheet metal or is this okay without? Now before anyone states put a better performance carb instead there are 2 reasons I don't want to do that. 1 I want to keep the MG as original as possible and 2 I can't afford a new carb. Your help in this matter is greatly appreciated.

 

[DrEntropy]

The Zed S should work fine. No need for replacing it with something to add more issues anyway.

Adjustment is done by either getting the "special tool" shown in the Moss catalog ~or~ a long (3" or more) 3mm Allen wrench. If you had the carb down to pieces, you should have replaced a VERY small O-ring around the needle assembly. They dry out and decompose. To use the "freehand" Allen wrench, the airway of the carb must be accessible. With one hand PIN the piston from rotating (putting stress on the diaphragm) as you insert the wrench down the damper hole into the needle assembly and rotate it counter-clockwise to lean it. Can't do this running (obvious reasons) so make SMALL adjustments. 1/8 turn is a LOT on the needle travel. Try the running engine and judge from there. A thing called a "ColorTune" is VERY helpful when doing this, too.

 

[Ofcalipka]

Thanks I ordered the tool as she is definetly way to rich. I pulled off one of the vacuum hoses and it ran better plus when I throttle up with the air filter off I can see a poof of fuel/air mix blow slightly out the carb then get sucked mostly back in. I think I might invest in the colour tune I heard about it a while ago from one of my motorcycle groups and could definetly put it to use on my Royal Enfield and Ural as well. Also do I need the heat shield or will I be fine without one?

 

[DrEntropy]

If the car has a converter the shield will help. If not it's just a "nice to have".

Some like the ColorTune tool, others don't. I have two (useful with twin carb setups), I've set up anything from MGBs and Spridgets to V-12 Jags with 'em and think they're great. Only thing to beat 'em is a three gas analyzer.

 

[Ofcalipka]

No converter so I should be okay. I did not know about the o-ring in there so I pulled it apart again today and changed the o-ring and set the adjustment as lean as it will go but it is still way to rich. The diaphram on the bypass valve was the only thing that wasn't included with the rebuild kit. It was definitly old and could use replacement but still seemed servicable. I ordered a replacement one of these too now just in case. The thing that worries me now is how she seems to be running. I'm worried that I may be losing compression in one or more cylinders as she seems to not really be firing right on all 4 cylinders and I have a little more blow by than I feel to be normal. Hopefully I'm just worrying too much but I'll buy a compression tester tomorrow to check for sure. Does anyone know the proper compression for the 1500 motor?

Oh and I got a chance to check the wiring to the fuel gauge and found the gauge and wiring works but the sender seems shot. When I can get some of the fuel out of the tank I'll drop it and pull the sender out. Maybe the bad fuel has it stuck or something otherwise more parts to order from Moss.

 

[DrEntropy]

Float defect is most likely. It'll fill with fuel and becomes a "bottom feeder".

 

[Ofcalipka]

Hmm, the float looked pretty good when I had it open. No bubbles under water. After the new diaphram for the bypass valve comes in I'll pull it off again and adjust it a little more. New valve in there from the rebuild kit so I dont think that is a problem. I'm going to do the compression test this afternoon anyone know the proper compression for the 1500 motor? I still haven't gotten my repair manual yet. I think it should be 150-160 PSI if my web searches were acurate. Does that sound like an acurate number to everyone.

 

[DrEntropy]

Yup.

A word in yer ear: Water won't propagate into holes where gasoline can.

This may sound goofy but: Stuff it into a glass of milk and see if it takes on THAT fluid. Water forms a meniscus. Milk will more closely approximate gasoline.

:wink:

 

[Ofcalipka]

Well I was just looking for rising air bubbles not an actual filling of the float with fluid but I'll try anything to get this car to run right. I checked the valve clearances and none were even close to correct but now they are. I did the compression test and found the following; cylinder 1=155psi, 2=152psi, 3=148psi, and 4=163psi. I then added the proper oil to the dashpot after reading the manual, which came today, and learning that I was supposed to do that. So I started up the car and she ran great for a few seconds then the engine began to accelerate and kept going all the way up to 3000 rpms and held there. I put the air filter back on and tried to turn the idle down and got it to hold at about 900 rpms just long enough to set the timing to 2*ATDC at which point it held the idle for a moment then accelerated back up held at 3000 RPMs then flooded out and died after about a minute of trying to get the idle back down. I adjusted every screw there and nothing would bring it down. I know its flooded as when I tried to start it again it would not start and I could smell a strong gasoline oder from the exhaust as it cranked. If the diaphram on the bypass valve gave up would it cause this or do you think it is still the float level.

Also since I now have the manual I found out that the prior owner did some modifications of his own to the intake. The EGR it connected directly to the intake manifold with a peice of rubber vaccum line and the vent on the valve cover goes strait to the carb. No T connector or lines running elswhere and the place where a line is supposed to run to the primary adsorption canister, which is missing from the car, there is nothing just an open fitting. It seems to me that this carb was only meant to run one way and now I've found some of these items missing so can it function correctly in it's current configuration or do I have to start tracking down some obscure parts?

 

[DrEntropy]

Has it got the proper throttle return spring?

You can make a "blanking" gasket to disable that "bypass valve" thingie out of thin cardboard to start the process of elimination. As for the evap canister and plumbing, I'd plug all the vacuum ports I could find first, then begin connections with the advance/retard to the dizzy. Someplace you have a vacuum leak or the return spring isn't doing its job. The carb can run the engine without all the "extra" plumbing.

Compression seems to be a "non-issue". Good enough to run the thing.

A thought occurs: What fuel pump is on it? There should be NO MORE than four PSI pressure at the carb float needle.

 

[Ofcalipka]

Original return spring and all the vac lines are now plugged for sure. I opened up the bypass valve and sure enough its torn through. The new one won't be here till Monday so I will wait I guess. What exactly does this part do the manual is kinda vauge. I also adjusted the float per the manual and it was almost dead on. So I tried starting it again and the air filter filled with gas and it wouldn't run at all. There is an after market electric 2-3 psi fuel pump mounted in the car.

 

[DrEntropy]

That kind of fuel overflow would only result from a couple things. Float/needle not closing due to excess pressure or a bad float are the 'common' ones. That float & needle are a pain to access with carb on that car, too. Have you gone around the carb-to-intake joints with a spray can of WD-40 or somesuch? Even a spray bottle with water will work, BTW. If RPM deviates when spraying, youfound a leak...

I'm going by memory here so others may have a better grasp of that particular carb beastie.

<span style="font-style: italic">EDIT: Try making a blanking gasket for the by-pass valve, too.</span>

 

[mk2sprite]

Hi
You can also use a piece of duct tape to cover the holes in the valve body. Almost certain that is your high idle problem but now you should go back and set all the idle screws, fast idle screws to the "starting point" which should be in the manual. A good close start point for the metering needle on the air piston is the shank of the needle flush with the bottom of the piston.
If the float needle/seat is a Gross jet, throw it away and install an original.
Your settings are probably off because you tried to adjust down the high idle due to the faulty Deceleration bypass valve You can keep the air filter off until you get all adjusted. That carb should run fine with the original mechanical fuel pump. See if you can test fuel pressure also as I have seen the aftermarket electric ones go high out of range.
Mike

 

[DrEntropy]

If the float needle/seat is a Gross jet, throw it away and install an original.

Amen!

 

[Ofcalipka]

The original jet is the one installed as far as I can tell. Also I think what caused the massive flooding was the fitting that would feed to the adsorption canister. I pulled the plug I put on that and she ran pretty good but still flooding slightly. So I put the fuel lines back onto the mechanical OEM fuel pump and disconected the electric one and it ran fine for a while then flooded out and died. I still need to either plug up the bypass valve or fix it proper. Since the part to fix it correct is coming I think I'll just wait and fix it right then move on from there. I'll tell you all what though, when she does run now she sounds way better since I adjusted the valves.

 

[Ofcalipka]

Okay well it turns out I'm not as patient a man as I thought I could be. I made a blanking plate out of gasket material and closed up the bypass valve on the carb. I then cleaned the plugs as they were black with soot from the rich condition of the engine. Since the prior owner put the non electronic distributor onto the motor I reset the timing to 10* BTDC. She runs but I can't get the idle below 1300 RPM's and she still is way too rich as the plugs fouled up with thick black soot again and wouldn't restart again once it was shut down until I clean the plugs again.

 

[DrEntropy]

Gah. Something is allowin' air past the throttle plate then. You *should* be able to shut it right down with the idle screw with that bypass thing blanked off.

 

[mk2sprite]

In your original post you said the original auto choke was removed. Is the housing still attached? Is it a water choke model? could be the choke guts are still intact and stuck in the full choke position?
Are all the emissions hooked up? Not sure if the '76 has a front (behind grille) charcoal canister but if so it can get saturated with fuel and cause problems. Should have a hose coming off a port on the RH side of carb going to the front canister. Let that hose go to atmosphere while testing and if it makes a difference you will have to clean out the front canister.
I would also get the heat sheild. One other thing on that setup if you remove the carb again check the manifold nuts for tightness as they have these funky football shaped washers that hold the manifold in place, they can get loose and your manifold will be loose.
Hope this helps
Mike

 

[Ofcalipka]

No, no housing just the starter box assembly rigged to a pull cable to use as a choke. I'm sure that when the car was built it had the cannister but there is nothing now. I just have that line on the carb open to the atmosphere. I've been thinking more and more about this and even though I have the float set according to the repair manual I think I might set it lower to see if that helps lean out the motor at all. Since I'll have the carb off I might just fabricate a heat shield out of some scrap sheet metal I have lying around. It can't hurt anyways.

 

[RonMacPherson]

Ofcalipka,

Where are you located. We have a mobile mechanic that does very good work on lbc's. He lives in Kaneohe, but travels the entire island of Oahu. Lemme know if you are on Oahu and would like his phone number to contact him..