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MG MGB Technical - Popping on Decel

In the past 7 summers I have owned my '72B, there has always been some popping in the exhaust when I let of the gas, typically when I am going downhill. This past summer it has been more pronounced than ever. The engine is still fairly original and to my knowledge (I have 30 years of records) the engine has never been rebuilt. It now has 106,000 miles on the odometer. I last adjusted the valves about 3 years ago (2000 miles) and I intend to do that again when I take it off the road this fall. I am also considering having the distributor rebuilt, because there is play in the shaft which the previous owner compensated for by replacing the points with electronic ignition. Any suggestions as to the source of this popping on deceleration? I believe it may be valve adjustment, but am not certain. Compression has been good, @ ~ 150 lbs./cylinder and power has been satisfactory for an engine with this mileage.
Joe

Check for an air leak in the exhaust.
David DuBois

I agree with David. Places to check first are head to exhaust manifold and manifold to down pipes.

Iain
I D Cameron

The usual cause is a faulty gulp valve. This counteracts the effect of the 'air leak' from the exhaust injection pump by letting even more air into the system.
Paul Hunt

I have a bad gulp valve on my '74. a new gulp valve is $144 at Moss.....is it possible to change diaphragm on the existing valve cheaply? since I dont feel like paying $144, my other option is to scrap emission equipment.
jjralston

Thanks for the advice. The gulp valve was removed long before I acquired the car, so that can't be it. I will re-tighten the exhaust manifold to the down pipe, and see if this helps. With brass nuts, it may have loosened. The rest of the exhaust is tight to the muffler in the back.
Joe

Do you still have the poppet valves on the carb butterflies? I first had mine soldered shut, then years later removed altogether (to improve flow). I noticed after they were soldered that there was no more popping like you describe. I too have no emissions equipment on my car other than the fuel vapour canister.
Erick Vesterback

I removed the carb butterflies and replaced with earlier versions without poppet valves.
Joe

Joe-
Mix up a thick solution of water and liquid dishwashing detergent. Not the kind you put in the dishwasher, the other kind that your wife uses when she washes stuff in the sink. You know, the thick liquid stuff that she uses to cut grease with. With the system cold, squirt it on the joints of the exhaust system (do not forget the joint at the bottom of the exhaust manifold), then fire up the engine and look for bubbles. If you see bubbles, then you have found the leak. If tightening up the clamps does not cure the problem, your friendly local auto parts store or muffler shop can supply you with some exhaust system putty to take up the gaps in the connections that result from poorly matched exhaust system tubing diameters. A set of SuperTrapp T-bolt style exhaust clamps will give perfect 360 degree sealing in order to eliminate the leakage completely. They also have the advantage of being manufactured from durable stainless steel so that they will not corrode and are complete with Nyloc nuts. These are available from Summit Racing in both 1 ¾ “ (Part # SUP-094-1750) and 2” (Part # SUP-094-2000) through their website at http://store.summitracing.com/ . It should also be noted that the juncture of the exhaust manifold and header pipe is the most common location of such a leak. Because the interior of the bottom of the exhaust manifold has a conical sealing surface, the exhaust manifold gasket should be installed with the conically-shaped end facing upwards into the conical recess of the exhaust manifold. The gasket is squeezed between the exhaust manifold and the exhaust pipe flange in order to achieve its seal.
Steve S.

I hate to be simplistic, but how is the oil level in the carb dampers?


Dan Robinson

Confirm ign.timing is correct AND/OR valve clearence,I found that excessive valve clearence can cause "popping" on de-accel.
t burnett

Thanks T. Burnett, I have suspected valve clearance, and quite probably worn rocker arms which makes it more difficult to accurately measure clearance, so I may be changing the rocker arm assembly out if I can see there is wear.
Joe

Another common cause is an overly rich mixture.
Jeff Schlemmer

This thread was discussed between 09/10/2008 and 18/10/2008

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