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MG Midget and Sprite Technical - 1500 after-fire

A little over a year that I've had the 79 there have alway been random exhaust pops on upshifting, now a pretty good bang at higher RPM up shift. All stock except the water choke to manual conversion on the Zenith 150 cd carb. I understand rich/lean, timing, exhaust system leaks etc. Wondering if this is a common fault in the 1500 and looking for starting point solution experience from folks who have dealt with it on their rides.
Bob Ketcham

now a pretty good bang at higher RPM up shift.

Is this inside the engine...

Prop and the Blackhole Midget

Jeez Bob it sounds like the car has burnt to the waterline
Get a manifold and some SU's or a weber.
Maybe check the timing first. Does it run on after you turn off the ignition?
R W Bowers

Inspite of being a long-stroke engine, the 1500 is as sensitive to ignition timing as any other. Check the ignition timing by first setting and gapping the points and then dynamically time with with a strobe. You'll probably find the ignition is too far retarded so the end of the burn on the power stroke is overlapping with the exhaust valves opening. When you come off throttle (when changing gear)the flow of fuel-air is momentarily mismatched with the slower engine speed so there's a surfeit of fuel; if the carb is flooding a bit the problem will be exacerbated so you might need to service the carb too. Changing manifolds and carbs isn't necessary as the same problem can occur with all carbs and manifolds. Worn or sticking valves can also amplify the problem so start with the easier tasks and work towards the more difficult ones.
Nick Nakorn

Thanks for the input. There is no run on at shut down and aside from this exhaust pop-bang the old girl runs like new. When I found the car it had been stored in an abandoned spray booth for five years at a dealership, and it was clear the PO had been doing some good upkeep work, so a battery, alternator, fuel pump, and brake bleed were all it took to get back on the road. I'll get a strobe on it and see where we're reading at idle and how the advance looks. Changing out the water choke revealed that mechanism to be well crapped up so I'd expect more of the same if I were to move further into the carb. Not having the expertise to tear into the Zenith any gurus you folks would endorse to do the job? Since the car is a mostly a survivor and running so nice I plan to leave it pretty much stock, but the Weber's or SU's are tempting.

Bob Ketcham

I know that the ZS carbs suffer from torn diaphragms, but I'm not sure if that might be causing your problem.
Dave O'Neill 2

This thread was discussed between 01/09/2014 and 02/09/2014

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