I thought it may be a vacuum issue, as there is no oil filler cap, where does the crankcase breathe from, my understanding is it draws air from the filler cap and exits through the PCV valve.
Someone told me mixture idle screws only adjust at idle rpm, with the cam minimum this car will idle is 900 to 1000, I thought that it would be too fast to adjust the mixture screws.
Thanks
Woz
So - a few things to bear in mind:
1] The PCV (Positive Crankcase Ventilation) is designed to suck fumes from inside the engine valve covers (where those fumes come up from the sump after blowing past the rings) to clean up the operation of the engine. The PCV valve opens more at low-vacuum, large throttle openings and uses the remaining manifold vacuum to suck blow-by (which have blown past the rings) into the intake manifold and the engine re-burns the fumes - it's an emissions feature. It should suck a little at idle, and open more when you're WFO and thrashing the engine producing lots of fumes which need to be sucked up by the PCV. It does not 'exit' into the engine.
2] If you have a oil filler cap, with a vent on it, then in theory those fumes travel up a hose to the inside of the air filter. This feature does far call, as there is no valve and little vacuum inside the air cleaner. It's a gimmick as much as useful.
3] The idle screws will work at small openings of the primary throttle blades. If they aren't making a difference, then a couple of things come to mind. There is either an air leak, a fuel leak, or the primary blades are open too far at idle for the idle circuits to work. If it's the last one, then the secondary throttle blades have an adjustment under the carb base plate (on a Holley, anyway) which you can use to open the secondary blades to increase the idle speed. You can then close the primaries using the idle adjustment screw to get both the speed and the mixture correct, because with the secondaries opened slightly, the primaries can be closed enough to make the air bleed circuits work, and the mixture screw settings can be corrected for optimum operation.
4] The comment about your 289 carb being too small for the 351 is both correct and incorrect. At full throttle, the 289 carb is likely to be too small to allow the engine to breathe and produce maximum power. At low engine speeds for troubleshooting purposes, it won't really matter - the purpose of the carb is to get the correct fuel mixture at a given air speed. Assume the carbs are perfectly set up and their AFR (Air Fuel Ratio) is perfect all the way through the range, then the following would be true:
289 carb - max CFM = 500 (for example)
351 carb - max CFM = 650 (for example)
351 Engine speed CFM AFR Percentage capacity used
289 carb 351 carb
1,000 rpm 86 13.0 17% 13%
3,000 rpm 259 13.0 52% 40%
5,000 rpm 431 13.0 86% 66%
5,800 rpm 501 13.0 100% 77% - the 351 would not be able to rev higher than 5,800rpm, with the smaller carb.
7,500 rpm 648 13.0 N/A 100% - with the bigger carb, the engine could run harder, longer, assuming everything
else was capable of supporting those revs.
You can see that the 351 would run ok with a smaller carb, but would run out of the steam 2,000rpm earlier than the 650cfm carb - all examples and probably hard to replicate in the real world.
Simples. Not really, but you know how it goes...
