Ok here's today's brain teaser..99A merc motor, fully rebuilt 5000 miles ago,ported,twin 94s, runs and drives great, bur have always had an inkling it needed more timing, just a little flat feeling in midrange. Runs a stock crab distributor .I marked the crank pulley when I built it, and ***uming my marks are accurate ( they are) it idles and runs best at 12 degrees btdc and then advances nicely up to about 28 at a bit over 3000. The weird thing is, the more until timing I give it, the better it goes..and absolutely no pinging. I am running stock heads shaved to give 40 though piston clearance, 30 thou over pistons..im guessing comp ratio is around 7 to 1 .is my compression just too low to get to a point where pinging will occur?? I have enlarged the timing slot to get more advance, but now the contact spring misses the breaker plate because its so far from stock advance ( which is 4 degrees)! Maybe I should look at modifying the distributor to provide more advance by enlarging the slots in the advance plate? Do aftermarket flathead dists. Built for performance have more than 16 degrees advance built in? Maybe that's the way forward.. And a set of Weiand cheaters.. Any sensible input appreciated!
You might want to PM GMC Bubba about a distributor solution. But I know the Weiand cheater heads will also give you a little more compression. Your flathead looks clean and speedy!
Good questions and post.... If you are running 12 btdc and then the crab should give you another 22 degrees ( most crabs are 11A units) making a total of 34 degrees at lets say 3000 rpm ....cant imagine you needing more advance.... Beware of making the slot longer as rotor phase gets out of shape when the slot is lengthened !!! Incorrect rotor phase uses way too much energy to jump the gap and often jumps the cap to case ( vertically). Our experience with our Bonneville truck is that when the advance gets a little over 28 degrees we loose horsepower on the dyno. Some of the aftermarket units ( MSD Mallory etc ) come with what i call the "chevy curve" at 36 to 38 degrees. I usually redo one of these a week where the owner doesnt like the performace of 36 degrees . We usually adjust them back with a limit of 24-28 degrees and that fixes the problem. Be more than happy to help ya have what ever you wish and can custom make a crab do what ever numbers you would like, however they are some limitations to the system...
A couple other issues , are you still running the vacuum piston and spring ( you need to run it to stablize the spark). The steering is on the wrong side of the car causing problems as well i would guess/???
Thanks Bubba, yup still running the vacuum piston and spring with a light amount of drag on it. from my estimation, and reading the interwebs, the distributor I have has 16 degrees advance in it from idle..which means 12 at idle equals 28 full advance at 3k plus.from memory (not next to the car now) it is a 21A crab distributor, if there were differing types, maybe I need to get one with more advance built in..22 as you mention, would be perfect i think,I could then back off the idle timing and still get up around thirty full advance. I have four or five of them stashed somewhere, will go through and check the parts numbers on the advance plates, see if i can find an 11A. Actually the steering actually DOES cause part of the problem! the steering shaft runs too close to the head to clear aftermarket heads with long studs on the bottom row..which is why I really need to find a set of cheaters, i think the low compression is the biggest hurdle this motor has, and 8.5 to 9.0:1 would really wake it up..its revs out beautifully, and just the mucking around this morning with timing and points gap made a noticable difference..more squeeze would be awesome!
Crab should have a max of 22 crank in the centrifugasl to be added to the static 4, just as a baseline, and with brake most of the way off I'd think it should be getting that. The 11 should be stamped on the base of the main shaft at the drive tabs and I think should also be on the brake disc, about the only part easy to swap with something else. There are also odd advances from 6 cyl crabs that could be mixed in, but all V8 crabs should be 11A advance. More was typically only needed for stuff like fuel and extra was cranked in via an advance plate (search dmarv, supplier of offy advance plates) BUT whyinhell does it want so much advance??? Could the advance be called for by or be masking something wrong?? Weak mixture, poor cylinder filling, water injection happening somehow, even accidental EGR via rot between crossover and plenum?? I'd worry about this some, do some plug readings under different cir***stances (all 8), see if pulling on choke or creating a vac leak does anything odd/unexpected, and do it all without the aircleaners, too, though it looks like you have taken care of the usual inadequacy of the little 2 inchers. Another possible reason for wanting more advance wouyld be poor turbulence because piston domes are too far from the quench area in head...that has to be tight.
I like to get full advance in just over 2000 rpm. 28 deg is more than enough. Any more and you will proboably lose power as mentioned.
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