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New MG Owner - Need Help

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BigBird

BigBird

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I'm a new MG owner, purchased a 1977 MGB for my wife. The car backfires through the exhaust upon deceleration. It also seems to idle rough when cold, at least rougher than the cars I am more accustomed to working on. I have checked the timing. I also replaced the air and air pump filters. Other items replaced includes the distributor cap, ignition wires, spark plugs, rotor, gulp valve, EGR valve, and check valve. There are no exhaust or vaccuum leaks. I am planning on replacing the distributor points, condensor, and coil next.

I suspect that the engine in running rich because of the backfire through the exhaust. The car has a single Zenith carb. I have NO experience with side draft carbs or MG's for that matter, so any help would be greatly appreciated... Suggestions on what I should be looking at to cure this issue? Thanks!
 
There is a rubber diaphram that frequently tears in the carb. Take the 4 screws out and remove the "dome" of the carb and you'll see what I'm talking about. Any tears or holes will cause poor running.
 
&, is it backfiring through the end of the tailpipe or up at the carb?
 
At times it is more of a popping, but it does "backfire" toward the end of the tailpipe at times. When this does happen, black smoke does accompany the backfire.

Additionally, I failed to mention that I did check the valve lash and it was set correctly.
 
I recall that way back when my B was driving, replacing the aging exhaust system solved these particular symptoms for me. Backfiring and popping. The engine requires backpressure and an open exhaust is a problem.
 
Having gone some relatively similar experience with my '76 recently, I'd suggest checking your emissions lines, and resetting that mixture. My car currently makes small pops out the exhaust on deceleration, but when cold and the choke is on it'll make louder pops similar to a back-fire out the exhaust.

I recently replaced my smog pump with a new one, and that had the immediate effect of creating back-fires out the exhaust every time I let off the accelorator (even when shifting). Replacing the "Gulp Valve" resolved that issue for me.

Since you mention that you've also already replaced the check-valve, gulp valve, and egr valves, I believe you're just running too rich.

If you don't have a mixture adjustment tool for your Zenith Carb, I highly suggest obtaining one. It's very handy and is well worth the cost. If you don't have one, you can adjust the mixture with a really long allen wrench, but I only recommend using the proper tool. There's a chance of you tearing the diaphram on the vacuum piston if you don't use the correct tool. To adjust the mixture, take out the damper (the plastic screw on the top of the carb), insert the tool, and turn counter-clockwise to lean, or clockwise to richen.

Pick up a Haynes MGB manual, or a Bentley MGB shop manual, or even the Haynes Weber/SU/ZS Carburetor manual. They're handy, and if you get all 3 that's even better, because sometimes one of them will better cover a topic than any of the others. The Carburetor manual will come in really handy if the mixture is always too rich no matter how lean you make it.

Best of luck, and welcome aboard!
 
Replace the coil...It can cause these exact symptoms....It's a cheap and easy replacement and eliminates one variable.
 
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