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MG MGB Technical - SU Carb explosion

Every time my car backfires, not very often, the front carb loses it's top. It's a 73B and has the HIF carbs with plastic dmapers. I've changed dampers and it still does it. Anyone got any ideas?
J Arthurs

First, fix the backfiring problem. A properly tuned engine should not backfire, even occasionally. Does your gulp valve work correctly? Is your engine running too lean? Are the springs on the popoff valves in the butterflys worn out? (this last fault would make it hard to set the mixture accurately at idle)

Check to see if the threads on the dashpot where the damper inserts are in good condition. If they're worn out, you might have to replace the dashpot/piston assembly. Perhaps there is a junked pair out there that someone is willing to sell parts off.

Mark B.

Or maybe just the piston, its top being plastic as opposed to the metal of the dashpot cover. The Metro Turbo with single SU had a screw-on clamp arrangement holding the plastic damper cap in place.
Paul Hunt

Agreed that the real problem to fix is the backfiring. As noted, check for lean mixture, which includes vacuum leaks. A check of ignition dwell and timing wouldn't hurt. And as long as you're at it, check your valve lash. Ahem.

As to the damper cap loosening, I had the same problem but it was not due to backfire. It would just work loose from time to time.

Figured it was the plastic threads worn out, bought a new damper. Same problem. I imagine that after 30 years, the aluminum threads in the housing must be worn. The good news is that a few drops of loctite holds it in place.

I now check the damper oil by: Remove air cleaner. Place clean finger under piston. Raise piston. If resistance is felt after 1/8" or so of lift, enough oil remains in the damper cylinder. I feel pretty confident with this, as my carbs have never consumed oil.

Matt Kulka

To check the level of the damper oil just lift the piston with the lifting pin - no need to touch the air filters.
Paul Hunt

The backfiring isn't a real problem. The car sits, sometimes for several weeks, and sometimes when I start it, it has a small hick-up. You can't even hear it, but I know there is back pressure as this is how I originally found out the damper would blow off. I never really know untill I wonder why it is running sooooo bad and then after I finally open the hood, presto. I've switched plastic damper tops and it still does it. I'll try the dashpot covers, that never occured to me. Thanks to all for the suggestions.
J Arthurs

Paul,

Re: check oil level with lifting pin.

Good advice. I can only save face by saying the only time I check the damper oil is at tune-up, when I have the air cleaners off anyway.

J,

Even if I let my car sit while on vacation it doesn't backfire when starting afterward. I'd still check your state of tune, especially your ignition timing. Sounds a little too advanced if it pops back when starting. Barring that, an intake valve which isn't seating when really cold, but let's not even consider that. It would be pretty unusual.

Matt Kulka

Matt - the other way I use is to remove and refit the damper piston which also indicates how much 'leeway' is left - with the piston lifting method it could be on the verge of not enough, but it does contribute to thread wear, of course. It should not be necessary to do this much more frequently than the service intervals though.
Paul Hunt

J-
The most common cause of backfiring is simplest to fix: a leak in the exhaust system. Unburnt fuel condenses in the exhaust system and when hot exhaust gases hit it- Bang! To find out if this is the origin of your particular problem, mix up a thick solution of water and dishwashing detergent. Not the kind you put in the dishwasher, the other kind. You know, the liquid stuff that your wife uses as a degreaser. With the system cold, squirt it on the joints of the exhaust system (don't forget the joint at the bottom of the header), then fire up the engine and look for bubbles. If you see bubbles, you've found the leak.
Steve S.

This thread was discussed between 17/10/2002 and 19/10/2002

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