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MG Midget and Sprite Technical - 1500 CARB ADJUSTING

I have a 76 midget with the stock Zenith-Stromberg 150 Carb. The car runs well but my idle seems a little rough at 800 - 1000 RPM's. When I lift up on the carb piston my idle speed increases and seems to run smoother. When I do this I "assume" I am making the mixture richer? Thus, I thought that I should "adjust" the needle with the special tool to raise the needle and make the mix richer. When I use the tool it is very difficult to turn and does not seem to move the needle any. Any suggestions? I do not have the air pump connected and all the other air recirculating stuff. Attached is a pic of my carb. My choke is a manual one but I do not use it (Miami temps). Thanks ....AL



The first thing I can think of is that somebody might have fitted a conventional metering needle retainer set screw instead of the original-spec item that has a spring-loaded tip that rides in the slot in the needle carrier without actually pinning it in place.

The purpose of that set screw is to prevent the needle from turning with the adjuster screw when you crank on it with the special tool. The adjuster screw head is fixed in place with a little star-washer retainer down at the bottom of the air piston, meaning that when you turn it, the needle should ride up or down the threaded part of the adjuster. With a solid setscrew, the needle will be unable to move up and down.

That's just a theory, but it's easy enough to check by removing the top of the carb, lifting it away, and then lifting out the air piston. Remove the little set screw in the side of the piston, and see if the needle then moves in or out when you turn the adjuster. Also check the set screw to ensure that it has a springy tip. If not, you've found the problem. If so, I dunno...

There's also the chance that the needle has reached the limits of its travel, but I wouldn't expect the engine to run with the adjustment at either extreme.

Cheers, and keep us posted,

Gryf Ketcherside

Al, the set screw in the side of the piston may be too tight preventing the needle from turning. You might need to remove the piston from the carburetor to access it. Sorry I can't be more specific on that screw but it's been years since I worked on the needle.

Raising the needle allows more fuel to the intake manifold.
Clive Reddin

Thanks for your input. I removed the piston and it has a set screw in it with the springy tip. Removed the set screw and needle still does not want to turn with the adjusting tool....only turns about 1/8 turn either way and very hard. I don't want to destroy anything so that is why I am asking? In the past I have found that forcing things is not a good way to go! My needle is movable side to side and I can push it up about 1/4 inch or so. Seems to be spring loaded? Should I put WD40 in the well and let it soak? Thanks ......AL

Oops ...I mean I can pull the needle down, not push it up...AL

An special reason why you are sticking with the single Stromberg?
Daniel Thirteen-Twelve

Only reason I have the single Stromberg is because that is what the car came with as standard equipment. I "think" that the UK version had duel carbs, but in order to meet the USA smog limits they had to go to a single carb? Actually, I would not mind going to duels if I could find them plus the manifold at a good price.

It's a common modification to convert any Stromberg 1500 Midget to either have an HS6/HIF44 single carb or fit a whole UK spec twin HS4 carb setup.

Not sure what you'd call a good price but the exchange rate is in your favour at the moment.
Daniel Thirteen-Twelve

Clive wrote:

>>> Al, the set screw in the side of the piston may be too tight preventing the needle from turning. <<<

I think he and I cross-posted... as I mentioned in my previous note, if the correct set screw is in there (and Al says that it is) you actually CAN'T overtighten it in such a way that the needle carrier jams. The threaded part of the screw bottoms out, but the springy tip of the screw will still ride lightly in the slot without locking it up. The effect is that needle holder won't rotate with the adjuster screw... it will just slide in or out, and that's what you want. (I learned this the funny way... left it loose to prevent the needle locking up, only for the needle to fall out of the piston later when I was out on the road. I got to reassemble stuff on the shoulder of the highway. Only later did I notice the spring tip on the setscrew...)

>>> My needle is movable side to side and I can push it up about 1/4 inch or so. Seems to be spring loaded? <<<

Al, the movement you're seeing in the needle is normal. It's mounted in its holder using a little spring that biases it to one side, causing it to ride lightly along the inside of the jet instead of rattling around and causing variations in the mixture.

When you take out the set screw, the needle PLUS the cylindrical holder should slide out of the piston, but ONLY after you've turned the adjuster screw counterclockwise far enough to disengage it from the needle carrier.

It might not be a bad idea to remove the adjuster screw and have a look at it. It may be jamming up for a reason. Also, there's an O-ring that fits into a groove around the adjuster screw head, the purpose being to keep damper oil from being pressured out past the screw head and down the needle into the intake airstream. To remove the adjuster screw, first note the current position of the needle holder... i.e., see how far the holder is sticking out of the bottom of the piston. You'll want to aim for the same location when you reinstall it later. Turn the adjuster screw counterclockwise as mentioned above until it disengages the needle holder. Yeah, it may be stiff... but as you turn it, you should see the needle and holder gradually emerging from the bottom of the piston. Remove the set screw, and you should then be able to pull the needle and holder out of the piston. (Be sure to pull the carrier, not the needle. You can damage that little biasing spring). Once the needle's out you can remove the adjuster screw; find a skinny, pointed tool like an ice pick and bend a tiny hook in the pointed end. Carefully reach down the air piston bore - try not to scratch anything - hook the star-shaped lock washer that retains the screw, and flip it upright. Then just shake the star out and use the butt end of a drill bit (or similar) to push the adjuster screw up and out of the damper piston. Some guys will tell you to just drift the screw and star washer out with a pin punch, but in my opinion, that's risking damage to the damper bore. You should then see the O-ring around the screw head. In my case, I just took the screw to the hardware store and tried new O-rings from their stock until I found one that fit and sealed well without binding in the damper bore.

When you reassemble things later, note that the star washer needs to be tapped into place with the dished side up. The easiest way I found to get it back into place in the correct orientation was to thread it onto a skinny drill bit and feed it back into the piston. Try to slide it down the drill bit somehow until it's over the screw, and then use a suitable punch to "set" it back in position so it will hold the adjuster screw in place and not let it rise.

Also, when re-inserting the piston into the carb body not the little tab on the rubber diaphragm. It needs to engage in the corresponding space in the carb body to ensure that the air piston is positioned correctly in the intake air flow.

Hope this helps... it's all pretty straightforward. Post questions, if you have any.


Gryf Ketcherside

Gaaahhh. I wrote:

>>> Also, when re-inserting the piston into the carb body not the little tab on the rubber diaphragm. <<<

That should say, "NOTE the little tab..."

Gryf Ketcherside


Thanks for your input. Penetrating oil managed to free everything up and I was then able to turn the adjusting screw easily and as you said the needle and holder did protrude from the base of the piston and I was about to pull it out (easy). Since all seemed to work well now I did not go up and remove the adjusting screw assembly, but I will save the thought. I wanted to adjust the carb tonight and test it out, but I was told that "I have to get ready for tonight". I will adjust the first day of 2011 and let all know the results...........AL

You should need the choke even in Miami, especially lately. If not, it is too rich.
Most common cause of rich idle is the beginning of a hole in the diaphragm. When the hole gets bigger it runs awful and very rich.
Problems with the choke will also cause rich idle that you can't adjust out with the needle, except maybe temporarily. Easiest fix is just tightening the screws that hold it on - it goes rich if the screws are loose. If some of these screws are missing, they can be had from Moss or TRF; they are very special strange screws and nothing else fits, and it is imperative that you get the right ones in the right holes else it jams the float. This and other choke issues are not cured by the manual choke conversions.
Best Choke info is in the Moss MGB catalog (John Twist), or on the web - see ZS choke - Zasko

FR Millmore

>>> Most common cause of rich idle is the beginning of a hole in the diaphragm. <<<

That, and a perished O-ring on the choke needle valve, meaning it won't seal even with the knob pushed home.

Gryf Ketcherside

Adjusted the carb needle with the "special tool" and it now seems to idle pretty good. At 850 - 900 RPM's it is pretty smooth. If I bring the RPM's down any more it starts to jump around and run rough. I closely checked the diaphragm on the piston and it looked fine. I think it was replaced when the restoration started 12 years ago. All else seems to be good. I will get it out on the road more this week and see how it does. Thanks for all your input .........AL

Good news, Al!

Yes, please do keep us posted.

Gryf Ketcherside

Ran the Midget quite a bit yesterday and it ran fine, especially at idle, which was my main problem. Only problem now is that I need to hook up the choke because it was quite rough till it heated up (which it should do - and I was told that). Thus, I mush of had it running too rich. The choke is a manual one I already have it in the dash, just not attached to the carb. I will hook it up today before another trek and all should be well. Thanks again ....AL

I have a manual choke conversion on mine too. A PO installed it years ago, and it was on there when I got the car... just with no cable or knob. He had it wired partway open. Go figure. Didn't take me long to correct that issue.

Anyway, it works great. Best of luck getting yours set up,

Gryf Ketcherside

This thread was discussed between 27/12/2010 and 08/01/2011

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