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MG Midget and Sprite Technical - Zenith-Strongberg 150 CD4 CARB - DRIVING ME CRAZY

I am putting together my 76 midget 1500 after it was in storage for 10 years. All going pretty good till I ran up against my flooding carb. I have the stock ZS 150CD4 carb. I thoroughly took the carb apart and cleaned everything first. I have a Triumph TR3 so I am familiar with the SU carbs. (or so I thought!!). When I first tried to start up the rebuilt engine I noticed that I was flooding badly with gas shooting up through the jet. Thus, I ordered a new float needle & seat from Moss and put it in and adjusted the float level. Same thing - gas gushing up through the jet. I then took the carb off the car and did my testing with 5/16 tubing filled with gas. If I press the float (softly)up against the shut off I cannot blow air through the incoming gas line. If I put a soft spring on the bottom of the bowl against the float it will not let gas flow (as it should). If I take the carb and tip it up and down I can see and hear the float move up and down. I have changed the float level from .625 to .68. No matter what I do gasoline still flows up out of the needle jet when I fill a tube with gas and hold it up. I am baffled. For testing, I took off the piston, needle and cover. What am I doing wrong??? Help!! Help!!
AL MILLER

AL. I know it sounds strange but I ran into one that had an overflow pipe. If by chance you got the inlet and overflow mixed up this fuel problem could be solved.
SANDY
SANDY SANDERS

You don't have a leaky float do you? A crack or split in it would certainly explain your problem.
Tim Michnay

Thanks for your quick response. I just ran out and made sure I was connected to the gas line, not the overflow. I was OK, the overflow is above the in fuel line. The float is OK ....it floats in a bucket of gas and no gas is inside the float.
AL MILLER

Do you have the original-spec needle style float valve, or is your carb fitted with a Grose Jet? The only time I tried the latter, I got bad overflowing - and heard a similar story from a guy who had a TR6. Returning to the old style valve cured the issue completely.

Are you running an aftermarket electric pump? The factory mechanical pump is designed to deliver the correct pressure. With anything else, you might need a regulator.

One last thought, having been there and done that - have you had the choke unit off and back on? If you get the screws mixed up, you run the risk of getting the long one in the wrong hole, where its tip will foul the float and keep it from lifting all the way. Again, I found that one out the hard way...

Good luck,

-:G:-
Gryf Ketcherside

Yes, I have the original float valve, not the Grose Jet. I have a new one (366-705) from Moss. I am not using the fuel pump at all, just a 5/16 hose with gas in it - gravity feed. I had to stop all the fuel from going into my intake manifold so I took the carb off the motor. I did have the choke off to clean it and I will check in the morning on the screws. Thanks ......AL
AL MILLER

Keep us posted then, Al... I've dealt with flooding in my ZS on a couple of occasions, and once sorted, it's run perfectly for years. Hopefully I can brain out what might be happening with yours.

Have you actually run the engine under these conditions? Rampant flooding like that can cause fuel to blow past the rings and dilute the oil in the sump. Been there, too... if so, be sure to change your oil once you've cured the flooding. I remember changing mine three times one summer before I finally figured out what was causing the flooding. (The first was the Grose Jet, the second was something else, and the third was the choke screw fouling a new float.)

And mentioning the new float reminds me - on the original float, there was a little indentation molded in to clear the choke mounting screw tip. The new float didn't have that indentation, and that contributed to the problem. I'd bought the new float because I suspected the old one might have gone bad, but in the end I reused the old one (which was fine) and stuck the new one in my parts drawer when I found that it was hitting that screw.

Cheers,

-:G:-
Gryf Ketcherside

Gryf,
No, I did not run the engine with the carb flooding. As soon as I noticed it I pulled it off the car because I did not want all the gas to go into the cylinders. What I did do, just to test the engine was to cut off the fuel pump line and the inject a small amount of gas into the carb throat and play with the throttle till it started. It ran great, but only for 8 seconds or so till the gas ran out! Guess that is why they invented the carb! Attached is a pic that I took of the carb today just in case someone sees something that is not right. I did see the choke screw that you are referring to and it does not come through the carb side. I have a guy coming over this Sat with a spare carb for me to look at and compare to mine and possibly swap some parts. Until that I will keep myself busy in putting all the lights on the car. Lots of lights for such a little car! Thanks again ....AL


AL MILLER

Hm... nothing appears to be amiss. As long as the float height is set correctly, you should be okay there in the float chamber.

You said you'd cleaned the choke - did you disassemble it fully and check the little O-ring around the needle? Those can disintegrate over time and allow fuel to flow even with the choke closed. What it's supposed to do is to seal off a little passage between the float chamber and the carb throat, just downstream of the throttle plate. If the O-ring is shot (which mine was, BTW), it will allow enrichment regardless, but it won't make anything overflow, as you've been seeing. Just wanted to mention that... and the choke O-ring should be included in a carb rebuild kit, if you have it.

Also, this link will take you to the definitive guide to rebuilding the ZS choke:

http://www.sterlingbritishmotoringsociety.org/files/choke.pdf

That's a good one to print and stick in your shop manual. I did... it applies to both the auto and manual choke. Mine's got a manual conversion kit on it, thanks to the PO, but they work the same regardless of actuation.

Cheers,

-:G:-
Gryf Ketcherside

In the UK you can still find 175 CD Strombergs for about the cost of a couple of beers. I bought some new Landrover ones for £5 each. The older UK spec ones are all manual choke and get used for aero VW conversions. The reason is that CDs are one of the few carbs to still work at 45 degrees to the vertical. Some have the jet adjustment at the top and some at the bottom (if you have the brass wheel underneath)

When I rebuilt the old one I also found perished seals including the ones on the brass jet 'carrier' that seals the jet assembly from the alloy carb body.

The second link shows a handy shot. Somewhere between items 21 and 23?




http://www.gowerlee.dircon.co.uk/Stromberg.html

http://remyluc.ibelgique.com/document/Stromberg%20150CD.pdf

Hope this may be of some use.




rob multi-sheds thomas

Al,

I had a similar problem when I first got my 75 1500. All of a sudden one day gas was just pouring out.

You didn't mention if you had an autochoke or manual choke on your carb. For starters if you have an autochoke I would take it off and see if the carb runs without pouring out gas, bet it does. I went through all the steps you took only to find out that stupid choke was my problem and to be honest I was never able to figure out why.

Eventually I switched to dual SU carbs. Something to eventually consider but you could just switch it over to a manual choke mechanism instead.

Good luck


Thor Patterson-Ritz

My Triumph runs twin 150's, i had the same flooding problem, ended up it was just float height adjustment after rebuild, but it took a long time to get it just right.
Brad (Sprite IV 1380)

>>> I went through all the steps you took only to find out that stupid choke was my problem and to be honest I was never able to figure out why. <<<

Most likely a perished O-ring on the choke needle:

http://www.sterlingbritishmotoringsociety.org/files/choke.pdf

-:G:-
Gryf Ketcherside

Thanks again all for your help. I have the manual choke. I will play with it again tomorrow. The wife gets kind of upset when the house smells like gasoline on T Day........AL
AL MILLER

Well, back to the trials today. I am getting kind of used to the taste of petrol now. Tests 31, 32, and 33 proved fruitless (adjusting the tab on the float up, down, 90 degrees, etc). I am still pouring gas out of the needle jet. I adjusted the float so low that it contacted the bottom of the bowl, which finally stopped the flow of petrol, but also stopped any petrol from entering the bowl. This and placing a spring under the float to keep it it the up position is the only way I have found to stop the flow of gas. I "think" that the needle valve is not seating properly in the seat. When extra pressure is applied it will seal it, but the floating of the float alone is not sufficient to seal it. I know the float floats, and it will move up and down when I tip the carb up and down so I know it is not hanging up on the inside of the bowl. Tomorrow I will put the carb in a bucket of gas without the bowl connected to see it the floating float will shut off the gas supply. If it does not, then I have to believe that it is a defective needle and seat and I will contact Moss for a replacement. Fortunately I have many other things to do with my Midget so it is not holding me up too much, just frustrating and contributing to a potential alcohol abuse problem ..........AL
AL MILLER

What sort of washer do you have under the float valve seat? The ones I have here are aluminium and crush down nicely when the float valve brass seat is torqued down. The Manual I have suggests that thicker washers can be used to adjust the effective float height if you want to.

When refitting he jet assembly, there are rubber seals on the jet adjusting screw (small o-ring) and one on the main jet body to float lid area (large ring).

From this manual...."correct sequence: jet orifice surrounded by spring, brass washer, bushing with "O" ring, aluminium washer"

In the UK it is possible to specify different size bores for the float chamber valve and seat to allow greater flow for engines that don't use pumps but rather rely on gravity feed. The aero engines with gravity feed require a huge hole. 2.5mm rings a bell but it was a long time ago. Could you have different sized seat and valve?
rob multi-sheds thomas

Rob,

Yes, I have the aluminum washer and all seems to fit properly. They sent me 2 different size washers with the new needle and seat - I tested both. I did not touch the jet assembly yet. I will make a test today as I mentioned in my last note to see what happens. I am thinking that I just got a bad needle and seat from Moss. Will fine out. Thanks for your input .....AL
AL MILLER

PROBLEM SOLVED ......FINALLY!!! IT WAS THE %$#&(&^%#$*&^%^#$&^*^%&* OVERFLOW PIPE THAT I "BLOCKED". As soon as I pulled the carb off the car I put a short hose on the overflow outlet and plugged off the end because I did not want gas (petrol) to come out of it. Never removed it till today. It must have prevented air from escaping from the carb and sealed the chamber, forcing gas out the jet pipe because I was putting pressure on the bowl with the incoming fuel. The first time I pulled it off all was OK. Hope I do not run into too many more "issues" like this again. Thanks for all your help. Now I can proceed with my assembly........AL
AL MILLER

Bingo!

That explains it precisely. I ran into this same issue a few years ago when I tried a Grose Jet, as mentioned above. Enough fuel overflowed the float chamber that it drained through the overflow port into the front charcoal canister; when enough fuel had run into the canister, that blocked airflow through the vapor recovery lines, and fuel started squirting up through the jet, to be ingested by the engine.

At least in your case you didn't have the carb on the engine, so you shouldn't have to worry about petrol in your oil, as discussed earlier.

Great to hear you sorted it out! Once I got my Zenith set up properly, the engine ran great. I haven't had to do much with the carb for the last few years, short of topping up the damper oil occasionally.

Cheers,

-:G:-
Gryf Ketcherside

This thread was discussed between 24/11/2009 and 28/11/2009

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