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MG MGB Technical - backfiring during deceleration

This is driving me crazy. There are absolutely no exhaust leaks anywhere in the system yet when I come off the gas I get the popping through the exhaust. Running a Zenith Carb 175 CD with manual choke. Adjusted mix leaner and richer with no improvement either way. If I move the manual choke 1/4 turn clockwise on the carb it improves the popping. Not sure where to go from here, any suggestions would be appreciated. Does anyone have a manual choke on a Zenith Carb 175 CD that might have a picture so that I can see how it should be positioned. Timing also has no effect I believe the problem has to be related to the choke.
JCH Hibbard

This is usually caused by a faulty gulp valve, which should put more air into the cylinders on the overrun to counteract that particular effect of the exhaust air-injection system.

Improves the popping meaning reduces it? I'm assuming that the bimetal spring on the water choke tightens up whe heated, it is wound anti-clockwise when looking at how if fits up against the carb body, which means it will move the peg anti-clockwise as it heats up, which should weaken the mixture. Therefore if turning the manual choke assembly that replaces the water jacket, heat mass and bimetal spring *clockwise* reduces popping, that indicates a richer mixture *is* having an effect, and you should get the same effect by running with the choke out a certain amount (not recommending it as a 'cure' ...). However unless you go for a gross readjustment of the needle and jet the choke gives a much bigger mixture change, which may explain why simply tweaking the mixture at the needle and jet seems to have no effect.
Paul Hunt

Thanks Paul, the gulp valve was removed some time ago so the problem isn't there. "Improves the popping" does indeed mean reduces it. I failed to mention that I have also tried running with the choke ever so slightly out as you have mentioned and that does reduce the popping as well. Is it alright to run with the choke out slightly or can that have any negative effects?
JCH Hibbard

The gulp valve having been removed, if the air pump is still present, is exactly what would cause the problem. If the air pump has been removed as well then there is some other problem. My UK-spec roadster pops and bangs slightly, but only on a long downhill on the overrun at medium revs. It's audible, but mostly masked by the silencers. I suppose if you only have one silencer and/or what you have are straight-through or approaching it (either by design or by blowing the stuffing out of them), then it will certainly be more noticeable, and maybe more likely.

Once you have the correct mixture setting at the needle and jet any additional choke will cause a rich mixture, which not only increases fuel consumption and pollution, but is also likely to wash oil off the bores so not a Good Thing.

Do you still have the poppet valve on the carb butterfly? As that opens on the overrun I'm wondering if that also helps to reduce backfiring, which means soldering it shut or replacing it with a solid butterfly, as many including me do because the valves are troublesome, may also make it more likely.
Paul Hunt

Air pump has been removed. This is a 77 B and the catalytic converter was removed with a pipe in it's place running to the front pipe and then back to two glass packs. Have had this system on the car for two to three years with no popping before. However a new problem has come up in an effort to solve the popping problem. Thinking that movement of the manual choke or giving the car a little choke while it is running reduces the problem, I removed the choke for inspection and cleaning. The kick piston was sticking as well as the enrichement needle. I took everything apart cleaned all parts a put everyting back together correctly with brand new gaskets. I used a touch of oil on the enrichment needle and kick piston all parts worked and moved beautifully. The only thing I did differently was when replacing the enrichment tube I noticed when removing it for inspection that the top hole in the tube was not lined up with the hole on the back of the choke assembly. My information say this is the correct procedure and I re-installed it that way so that the holes line up. After putting the choke back on the carb and re-attaching the manual choke, the car no has a highly noticeable hesitation in third and fourth gear. The popping still remains in addition to this new problem.
Car starts fine, hesitates in third and fourth, and runs nicely at 60 MPH on up. Everything on the choke was working nicely and re-installed correcly. I'm not even sure where to begin with this problem and could use some direction. I tried adjusting the needle both richer and leaner and this has no effect whatsover. The popping still exists and now the hesitation. If anyone can help I'd appreciate it.
JCH Hibbard

Have you read http://www.sterlingbritishmotoringsociety.org/files/choke.pdf? I know it concerns the water choke but the watery bits are only a small part of the overall choke system and this article, so the rest may be of some help.
Paul Hunt

PS. On the face of it I can't really see why something affects the engine in some gears but not others, especially when 1 and 4 are different to 2 and 3. However from late 76 Transmission Controlled Spark Advance was used on American spec engines which meant that vacuum advance was only applied in 4th gear, hence with that I can understand why 4th could be different to other gears - i.e. if the timing or vacuum advance mechanism has gone awry for some reason. This *could* be a leak in the pipe from the solenoid valve to the distributor, which would only affect the engine in 4th gear. I know the problem is in 2nd and 3rd, but if the mixture was over-rich it may be that a leak from the diaphragm in 4th gear only is putting it back to about what it should be.

Other than that I'd be realigning the enrichment tube as it was before - a bit desparate I know!
Paul Hunt

I had this problem with a Triumph Spitfire a long time ago. The only thing I did to the engine was to raise the CR to nearly 10/1 & fit tubular headers to it. After that it popped on decelleration. I never did cure it & was told at the time by "experts" that the headers were to blame. I seem to remember that this symptom is caused by the engine trying to drive the car faster than the transmission will allow it.If this is the case, a stronger return spring on the accellerator might help. As I said before, I never cured it but here's a few more straws to grasp. Barrie E
Barrie Egerton

"caused by the engine trying to drive the car faster than the transmission will allow it"

Eh? You are on the overrun, it's the momentum of the car that is driving the transmission and engine. The pedal is already released, a stronger return spring will simply make it harder to push. Might save you some petrol though.
Paul Hunt

Thank you both Paul and BArrie. I would still like to see a picture of a manual choke connected to the ZS 175CD just to compare the position of the choke on the carb.
JCH Hibbard

There is a good one here http://tinyurl.com/5bsxhz

Part of setting up the water choke involves turning the heat-mass and bi-metal spring assembly to the correct position. Too far one way won't apply enough choke for a given temperature and too far the other will apply too much. There should be index marks on the heat-mass and body, these are lined up initially, then tweaked one way or the other during trial and error tests until you get good running when cold, during warm-up, and when fully hot. I'm assuming the manual replacement for the heat mass must be positioned in much the same way, although in that case it should be simpler as all you need to do is make sure that when the control knob is pushed fully home the enrichment and fast-idle is fully off. However that is controlled by where the cable is clamped in the lever as well as where the assembly is rotated to. Does your replacement for the heat-mass have an index mark? If so I'd line those up then adjust the cable so that the lever is pushed fully back just as the knob reaches the fully home position.
Paul Hunt

No mark on the manual choke Paul, I'm going to check the website you recommended and try to go from there. As always you are a wealth of information, thanks.
JCH Hibbard

If no marks the other way is to make sure the pin on the carb is in its fully un-enriched position, the lever on the manual unit is in its fully home position (if it has one), then line up the lever on the inside of the manual unit with the pin on the carb and make a mark on the manual unit yourself that lines up with that on the carb body. Alternatively if the lever on the manual unit has no obvious end-stop in the fully home position the pin on the carb must stop it going any further, so within reason it won't matter how the manual unit is orientated as it will be the carb pin that determines the fully home and fully operated positions of the manual lever.

Unfortunately none of which solves your backfiring problem :o(
Paul Hunt

After looking at the website and other information I understand that the pin in in the out position when the car is cold. The choke would hold the pin out when manually pulled and as the car warms the choke being pushed in would set the needle in the enrichment tube. After looking at the photo I the website I positioned the choke in that manner and did a test run which ironically seemed to decrase a great deal of the backing off noise.
JCH Hibbard

There's a bit of a catch in the Zenith cold-running enrichment mechanism, especially with the water choke. If you let the car cool right down from hot then the enrichment mechanism will still be in the 'hot' position. It is only when you blip the throttle prior to starting that it will flip into the 'cold' i.e. enriched position. With a manual conversion I'm not sure what happens, i.e. when you pull the choke does it go to the enriched position immediately? Or only when you blip the throttle?
Paul Hunt

Got the manual choke back to it's correct orientation. Basically the enrichment needle needed to be in the full choke open position and then the manual choke is offered up to the lever in that position, rotated anticlockwise about a 110 degrees to match up to a mark on the housing. That being accomplished I am now back to square one on the popping situation. The only thing that seems to dramatically reduce it is a richer mixture, but as I said before even adjusting the needle as far as it will go in the carb will not take it away completely. But leaving the choke open slightly will. Curious.
JCH Hibbard

The choke gives much more enrichment than the mixture screws for a given apparent movement. The mixture screws are required to give very fine control for the majority of running. The choke is a very coarse and gross mixture enrichment device, usually used for relatively short distances, the driver continually backing it off as the engine warms and needs less enrichment. I think the Zenith has a fine screw, a coarse nut, and if you need to go beyond that you can adjust the needle in the piston (which is something not available on the SU). I'm sure you could adjust the latter to give reduced popping, if the former two won't. But that, for the same reasons as leaving the choke open, would be a bad thing to do.
Paul Hunt

This thread was discussed between 05/12/2008 and 15/12/2008

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