My Stromberg engine has been spitting back at times over the last year or two, and just not running quite right. Nothing I could put my finger on, although it got better when I cleaned and aligned the carb pistons.
After changing the rear crankshaft oil seal a few days ago I routinely measured the engine compression and got 130 psi on all cylinders. That was disappointing, as the engine was not burning any oil and I had done the valves 2 years ago. At that time when I set the valve timing the marks seemed to be in the right place at TDC, with the exhaust sprocket mark slightly down, and the inlet sprocket half a tooth up compared with the head.
So I looked at past posts and valve timing was mentioned as a possible cause. I checked again and moved the inlet cam one tooth backward to advance it. Both the inlet and exhaust timing marks were now slightly down with respect to the head. I suppose that could be better then before, but it wasn't obvious.
Rechecking the compression and it's 160 on all cylinders, with no spitting back and the engine idling a lot slower than before. It pulls well and I'm a happy bunny!
So if you get spitting back and so-so performance on a Stromberg (and maybe a Weber) engine, check the compressions and possibly the inlet vacuum with a gauge. If you find the compressions are down by 30 psi or so but even, and vacuum is low, then it could well be the inlet valve timing at fault
You could of course time the valves accurately with a timing degree disc, but this post is intended for those who just use the timing marks.
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