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MG MGB Technical - Engine rough at 1500 to 2000 rpm

1973 B Roadster
Original engine with pollution control removed years ago.
pertronix ignition
Spark plug color is a deep rusty red

At the end of last season I had serious intermittent engine faltering. I would be driving along and all of a sudden the engine would sputter and stop. When I pulled over the engine would start up and run fine until it happened again. This started right after a fill up so I figured water in the gas.

I did two things. First I drained the gas tank and second I pulled the carbs and dismantled them to see if anything was going on there (I had rebuilt the carbs the previous year). However, before I could do a test drive it snowed and I had to park the car for the winter.

This spring I tuned the carbs and filled the tank with fresh gas. The car now runs great with one exception.

When going up a gentle incline in 4th gear, with the engine running at about 1500 to 2000 rpm, I'll get some hesitation and engine sputtering. However if I step on the gas (still in 4th gear) it will take off under full power just fine and accelerate up the hill.

I've tried a slightly richer and leaner mixture with no change in symptoms. The vacuum advance tube between the manifold and distributor vacuum unit is connected both ends (although it seems that if there were a vacuum advance issue I'd have different symptoms). I haven't checked timing yet but every time I've checked it over the past few years it has been right on. I adjusted the valves at the beginning of last season (a year ago) but since I only put a 1000 or so miles on the car each summer I haven't tried readjusting those yet.

I do not have the hesitation problem in 2nd gear going very slow on a flat road at 1500 to 2000 rpm so it seems to me this is some sort of problem occurring at that engine speed while under a heavier load.

Any thoughts?

Thanks,
BH
BH Davis

Well..............it WAS the timing. When I thought about it that seemed to make the most sense. It it was firing too soon under load at low rpm then it seemed I would get the results I was getting.

The DCO (dumb current owner........doh!) adjusted the timing last fall at idle instead of at 1500 rpm. That made it fire about 4 to 5 degrees too soon at 1500 rpm.

Just ran the car up a mile long hill in 4th at 1800 rpm and it ran smooth as can be.

BH
BH Davis

Nice
James

BHDAvis: I ran into the same problem. When I read your initial post about the Pertronix Ignition and other engine mods, it was clear to me. The other thing you may want to check, is make sure the main coil lead is well seated in the coil and dizzy cap. That was an issue for me last year. Engine started to misfire under load after 2000 RPM.

Setting the timing in my 79 is still a trial. I started out at factory spec as noted in the engine compartment, 13 BTDC @ 1500 RPM. From there I had your symtoms until I worked the timing up to my current 17 BTDC @ 1500 RPM. I am still going to set it up another 1-2 degrees and check results.

You are headed in the right direction.

Cheers

Gary
79MGB
gnhansen

This thread was discussed between 03/06/2009 and 04/06/2009

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