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MG MGB Technical - Idles great/misfires under load

Hi: 74B/stock desmogged 18V HIF4s. Idles great at 900rpm,hot, revs up fine with no load on it. Spits and sputters and little power under load. Will cruise along fine under light load.. once you get it there... New BP6ES/points,cond/cap/rotor/fresh fuel/timing at 20. Any suggestionswould be greatly appreciated...Thanks Anthony
Anthony Henderson

Sounds like it may be running lean. Have you tried the lift the damper check?
.

I've tried this-they seem to respond properly speed rises slightly when piston lifted. With HIFs do you turn the mixture screw in or out to richen. HSs are out to richen/Are HIFs the same? Thanks for the idea though. Anthony
Anthony Henderson

The symptoms you describe sound more like point gap/ plug gap/ timing than carburation. Was the engine running well before you made any changes? Sometime new parts are not good, so try changing back to the old parts, one at a time, starting with the condenser/capacitor. You might also try varying the timing, bearing in mind that the timing at various RPMs differs between non-load and load.

In is leaner on both carb models.
George B.

For HIF4s, turn the screw out to lean and in to richen.
Gary Cash

Anthony:

I think I'd try new plug/coil wires and a new coil.

What you describe could be a classic case of ignition breakdown.

When the engine's idling, the "pressure" within the cylinders is low, relatively speaking. Thus, it's relatively easy for the voltage which fires the plug to jump the spark-plug gap. At this point, the engine has basically only to push the crankshaft around to keep running. When you drive the car, especially when accelerating or going up a hill, the pressure within the cylinders is much greater. After all, the engine now has to not only push the crankshaft around, it also has to push your car and you around. This increased pressure within the cylinders makes it far more difficult for the spark to jump the plug gap. In fact, there is so much pressure at this point, the voltage to fire the plugs will take the path of least resistance and if your plug/coil wires are bad, or if the coil itself is weak, you'll get a misfire and the symptoms you describe.

This may not be THE reason for reduced power, but again, it's the one I'd try next.

Best regards,

Mike Brown,
Seven Shop British
Mike Brown

Check your dwell. A very large gap caused misfiring under load for me. If you can get the missfire by revving the engine at a standstill then I would check for HT breakdown in the dark.
Paul Hunt


Hi,
ive had this, if its not the point gap, then try a new coil. this has happend to me Twice. Its easy to try if you have a spare
Ray


Had this problem on two occasions, with the problem in secondary igition both times. Weak spark plug wire, and a loose nut holding the condenser wire.
Phil Collura

HI Again! Problem solved I think. Replaced brand new Lucas points, and low tension wire inside distributor to points and the original coil. Life is normal again. Anybody have any favourite brands of points these days? I have used Lucas only for years as I had poor luck with blue streak and some other generics. This is the third set of new defective Lucas points in the last year. Maybe I should break down and go pertronix. Thanks to all for your suggestions and help. Anthony
Anthony Henderson

By Low Tension wire inside distributor, is that the braided wire, with cloth insulation? Where did you find a replacement? Can another type of wire be used? Mine is beginning to fray. Bart
BartTS

Bart: You should not use plastic insulated wire here as it is to stiff to allow proper movement within the distributor. You can buy these from Christian Hollum Imports in Montreal at 514-639-4639 or Obsolete Automotive www.obsoleteautomotive.com. I think they are about $5.00. Anthony
Anthony Henderson

This thread was discussed between 31/03/2002 and 02/04/2002

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