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MG MGA - Engine rattle, power loss, backfire

My '58 MGA has been running great until suddenly going up a slight grade at 60mph the engine rattled, like loud valves, lost power and had small backfires through the intake. Compression is good, adjusted the valves and timing is correct. Car idles and runs fine until you put a load on it, like a hill. Then I'm down shifting to second just to make it up the hill.

I don't know the history of the motor. But I'm baffled. I just finished a frame off restoration and didn't take apart the engine because it ran great. Any suggestions? Thanks, Dave
DAR Dave Rosato

"small backfires through the intake"Fuel starvation?
Lindsay Sampford

Dave ,I had a simular problem with an engine that had been standing. Good compression etc., etc. spittig back through carbs under load. Problem was sticky inlet valves, a pressure can of the stuff Subaru use for their boxer engines fixed it within two minutes. Interestingly the tappets didn't rattle, had me beat for a day or two ! It was a twin cam, twin SU, RIley engine. Try it Sean
S Sherry

Thanks for the advice. The fuel pump seems to be OK and the fuel line between the two carbs has fuel in it. I took the carbs apart and they looked fine. I thought of sticky valves. A web search showed people putting Marvel Mystery oil in the oil and running for a while. Another sprayed Amsoil Power Foam into the carbs. Think I'll try the later first.
DAR Dave Rosato

Hi Dave. Could be a bad plug, bad condensor, bad rotor, bad distributor cap, bad plug wire, sticking valve, plugged fuel filter and lots of other things. I would do a tune up before trying too much else. Look for anything obvious, that could account for the problem. Cheers, GLenn
Glenn Hedrich

Hi again Dave. The fact that the engine idles and runs fine without load suggests to me that the engine is probably OK, and that the problem lies somewhere in the fuel or ignition system. A bad valve would likely cause the engine to run poorly on a constant basis. Glenn
Glenn Hedrich

put the MMO in the fuel, not the oil.

These symptoms are typical for a failing condnsor, particularly if you habe touched nothing before the car started playing up.

If you have a fuel problem, putting MMO in the oil will have no effect. Also clean the filters at the carbs (remove the banjo at the carb, and you will find a small thimble-like filter behind it). Is the fuel pump still delivering at full pressure?
dominic clancy

I think Glenn is on the right track, check everything. For what it is worth in my vintage motor cycling days i had a bad plug do just what your symptions are. Ditto a bad Mag. will give every symption known to man. In car ign terms , a condenser or a coil. Subtitution is the only sure fire way. Particularly condesers ( an electrolitic capacator ) Sean
S Sherry

Dave, has your car been laid up for a while? Is it old fuel in the tank? Unleaded has a limited "shelf life". Old stale petrol, sorry gasolene, loses its ability to deliver power, or its volatility. Engines will tickover but ask for power and they will protest. Probably an unlikely cause, but not one to be ruled out.
Lindsay Sampford

Shortage of fuel flow can cause loss of power under higher load and higher fuel demand conditions, possibly even once or twice backfiring through the carbs just as it goes lean. Check fuel delivery at the supply hose first, should be at least one pint per minute free flow (or more). However, running short of fuel should only cause loss of power, not any strange rattling noise.

Loss of power, rattling noise, and consistent backfire through the carburetor could be a sticky intake valve. This might happen after long term storage. It is more likely to happen with a freshly rebuilt engine when valve guides don't have enough clearance to accommodate thermal expansion. It will commonly happen under high heat conditions, fully warmed up engine pulling strong with heavy throttle, either uphill or while accelerating.
Barney Gaylord

Thanks again for all the suggestions. The week before I took a 600 mile trip in the car with no problems. The last tank was hi octane, after the problem started. Fuel screens were clean when I removed the carbs. I'll replace the condenser and plugs. The easy things first. I had a broken electrode in a plug on my Harley that slide up and down causing intermittent problems. Cap, rotor, wires and points are all new but I'll check them. I agree, sticky valves should cause problems even at idle. I haven't checked fuel pressure but it seems to get up to pressure quickly after turning on, ie the clicking stops.
DAR Dave Rosato

But if the engine is shuddering about violently with a mis-fire all sorts of external things can rattle. The exhaust pipe can hit the chassis if it's too close, loose carb overflow pipes can rattle, cracked carb heat shield, temp gauge pipe, etc.etc...so It might not be as bad as it sounds!
Lindsay Sampford

This thread was discussed between 30/12/2009 and 31/12/2009

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