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MG MGB Technical - won't run over 2500rpm

Hello To All, Having fits, am desperate. Quick summary... 73 B starts easily, idles smoothly, race engine to 1500, 2000 rpm runs smoothly, hit 2500 to 300 rpm engine gets rough, sputters, backfires through rear carb, valves get rattling, ease off throtle and returns to idle and smooths out in a few seconds. Swapped carbs, dizzy, coil, condensors, fuel pumps, checked timing, fuel floats, dwell, carb adjusments,valve clearances, all on the mark. Ran fine, now this. canged fuel filters too. Help, it is spring.
Dennis P.

Sounds like it might be ignition HT problem. This is usually evident under load. Replace plug wires.

Mike
Mike Phillips

Dennis,

How about ignition timing. Has car been sitting for a while before starting up? Advance weights/plate could be sticking.

Cheers, Doug
Doug Keene

Swapped wires and dizzy, checked weights and vacuum advances, all good. This is all happening while in driveway, not on road, not under load. Timing dead on, as well as dwell.
Dennis P

Sounds like it may be the fuel filter or a restriction in the fuel line.
J W Hopkins

Try-CAREFULLY- slowly dumping a half treaspoon of raw gas into the carbs while its doing this. See quickly if its a starvation problem (doesn't really seem like it). Is the choke stuck (are the jets retracting back into the carbs when the choke is released)? What do the plugs and exhaust smoke look like? Black and sooty=rich.

Mike
Mike Phillips

How many miles on the engine? Cam timing jumped a cog due to timing chain wear?
Blake

Try pulling the choke out when hitting the 3000RPM mark, if things get better, then it is a fuel problem. Ignition and advance will also cause the same problem. If only the rear carb sputters, then things are not syncronized. Check that the carbs are opening together, and that the pistons are moving freely IE do they move up and down freely. Also the HIF carbs can have trouble with the temp compensator. Does the probelm change with engine temp?

HTH

Ron
Ron Smith

If you have changed leads, have you got the order right - the symptoms you describe are what you get with 2 & 3 reversed.
Chris Betson

Hi, Yes, swapped wires, balanced and tuned HS4s, wires not crossed...Fuel fresh, from a can, choke not help and added fuel while happening and no change. Plugs clean, no oil burn or foul and compression 165 to 175.
Dennis P.

This is a far shot but- have you replaced your spark plugs with new ones? Sometimes the porcelin will crack allowing the plug to short out. Spark plugs do strange things when compression is put on them. It is possible that your compression at 3K is enough to cause the plugs to short through a crack. Try swapping your rear plugs with the front plugs and see if the problem changes. Just a thought.
Robert

If you can reproduce it on the driveway then clip on a 12v powered, inductive pickup timing light and watch the flashes. Any erraticality (?) in the flashes when the misfiring starts confirms it is ignition, if there is none it says fuel or something else. If the tach flickers as well that says it is ignition LT. I say "12v powered, inductive pickup" timing light because these won't flash if there is no HT from the coil *or* the plug is breaking down. The in-line self-powered type won't flash either if there is no voltage from the coil but they *will* if the plug is breaking down. Check the king lead and each plug lead, I have had a distributor cap/rotor arm fail such that I got flashes on the former but not the latter, but this was only under load. Another thought - as the revs increase centrifugal advance comes into play. If for any reason the points phasing (physical relationship between rotor and cap contacts when the points open) is bad the rotor could be moving too far from the cap contact.
Paul Hunt

Is erraticicity a real word do you think? :o)

Only thing no-one has suggested is checking your plug gaps... but you'd have seen one that was closed up when you checked them for colour.

Good luck,
--
Olly
Oliver Stephenson

Hmmmm.....sounds like maybe the distributor shaft bushings may be worn letting it dance above idle.
Hugh

Have seen similar symptoms when an exhaust lobe on the camshaft is wiped.

Wayne
Wayne Pearson

Thanks you, all. Changed the plugs and the car revs better abd did not sputter and break up until 4000rpm Back to idle and idle was erratic, richened the dickens out of it and problem went away.. Now I noticed that the idle varied in speed and roughness. Sucking air through throttle shaft on rear carb. Smeaed grease i on shft end and idle smoothed out. Rebush of SUs next. Thanks to all!
Dennis Pantano

Dennis, thanks for posting your solution... it's always nice to see a thread finished off properly so it's useful when it goes into the archives!

Have fun with the re-bushing (or, W. bushing if you will... :o)
--
Olly
Oliver Stephenson

You folks are sooper kewl! I love the helpful demeaner.
I am presently in search of my first MGB, and already want to modify/upgrade it. Just so pleased to know so many helpful cats are out there should I get my head stuck in a can. Great day to all. Ross
rooster

This thread was discussed between 26/04/2003 and 02/05/2003

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