MP 528 cam vacuum

[moper]

ok. 10% over the 67s it came with is 70s. I'd be starting there in the primaries and 10% larger than the secondary 73s are 78s. So your primaries are lean right off the bat, the secondaries are at a starting point, and you have a ton if initial timing. Find out how much total you have.

 

[Marcelo Dart BR]

The total is 32*. As I said, the timing doesn't change from 1800 to 3000, I don't think it would advance after this. It's working way better with that much of initial.

Ok, I have these jets here.

 

[moper]

It's using timing to cover the other issues. Have it your way, but there's a reason car makers have timing curves rather than huge amounts of initial. My theory is it's lean, and the high ignition timing is sending it into a ping situation which once started, will not go away until the throttle is closed again.
Plus, the fuel's burning very fast. With that much lead, the burn and maximum cylinder pressure is coming too early. Part of the available power is wasted on pushing against a piston while it's in the wrong position to make use if it. Get the tming curve set up properly, and then tune the carb.

 

[Marcelo Dart BR]

What would be a proper timing curve? I'll get a new distributor from procomp, only mec. advance. What's a good starting point?

Any thoughts why the plugs are dark brown and the vac. gauge pointer was shaking at idle?

Thanks

 

[Marcelo Dart BR]

Just to let you know, I rebuilt the carb and rejeted it with 70-77 and the problem at high rpm is gone. I pulled it to 7k easily.

Thanks!

 

[moper]

Cool. In terms of timing, a lot has to do with trial and error with your car, with your fuel, and in your hometown. The basic is somewhere around 15-18° initial, another 22-36° of centrifical depending on a few other things, and then vacuum if you have it, which you dont. The last part is how fast the curve is brought in. If you have street gears (numberically lower than 3.55) then you don't want to bring it on too fast. Racier gearing you usually can. A typical street curve would be having the centrifical start coming in around 1800rpm and being "all in" by 26-2800. Once the timing curve is set, go back and tweak the carb in terms of idle quality and response. It's not a fast process, but you'll get there.