Hi group,
I just got an early 74 MGB (US). The previous owner had had a rebuild done about 5 or 10 years ago and then put antique tags on it, so it hasn't been driven much lately. It's got 70Kmiles on the odometer, less than 10K since the rebuild. I've got the receipts and the work was done by a reputable local shop that specializes in MGs. I'm slowly easing it back to 50 mile/week or so service, keeping an eye on lubrication and such. I've been driving Hondas for 20 years so have forgotten what little I knew about the art of auto maintenance.
My question is about ignition timing. I recall from messing with my old Moto Guzzi (years ago, the lag again due to Honda-induced laziness) that I could match timing to fuel and riding style by advancing until it pinged on acceleration and then backing off a bit. When I tried that with the MG, it seemed as though I couldn't get the timing advanced far enough. I turn the distributor clockwise as far as it will go until the vacuum advance unit hits the oil filter and it only runs better. I even pulled the plug wires off the distributor, shifted them 90 degrees, turned the distributor 90 degrees the other way, and advanced some more. I finally got it to run worse on acceleration, but still without pinging or knocking: it just hesitates if I try to gun it. I put a timing light on and saw that it runs best with the mark advanced a good 30 degrees or more at idle (as best as I can judge, since it's way past the marks). The mechanical advance shifts the timing appropriately when I gun it, although I haven't compared exact values with the tables. I pulled the plugs out and confirmed that the mark really does correspond to TDC on #1. When I time it according to the marks, the distributor position looks about like the photo in the Haynes manual, but it seems pretty sluggish and stops making power pretty early in the range. It definitely runs better at all speeds advanced significantly from this position.
I am pretty sure that the vacuum advance is not working. It makes no difference when I pull the vacuum hose off either with the engine idling or revved. I did a quick vacuum test on the advance unit and it gives some resistance but will not hold a vacuum for more than a fraction of a second. I took the distributor apart and can physically push it back and forth some, but don't know how much play it should have or how much resistance it should give. I'm holding off on replacing the vacuum advance unit ($80 from Moss) until I decide whether to replace the whole distributor, possibly with an electronic version. Still, the vacuum advance is only good for 10 degrees or so, and if it's not pulling vacuum shouldn't the timing be too advanced rather than too retarded, and ... would it make much difference anyway to this aspect of performance? I haven't messed with the carburetor or checked the point gaps or dwell yet. That's next.
My current theory is that the carb (Weber downdraft) is being stingy with the fuel, and therefore it doesn't provide enough fuel to strain the engine. Someone suggested that I might have a bad accelerator pump. I see a squirt in the inboard barrel when I gun the engine, but don't know what it's supposed to look like. The plugs are light tan and dry (not black and oily, like the ones on my old Triumph Bonneville). Maybe it's perfectly normal. I may just pay someone competent to look it over, but would like to learn a bit on my own. I'd love to drive a known good MG to get a sense of how one should behave; maybe I could arrange that at the shop.
Ideas?
TNX! Dave