What would be the reasoning in changing the primary jets on a 650 DP from 71 to 67's?Thanks

Todd,

Part of tuning a Holley carb is making sure you have proper jetting in both the primarys and secondarys. In general, jetting up will richen the air fuel mixture and jetting down will lean the air fuel mixture.
For general driving you are using the primarys or front half of the carb so moving the jet size down will lean the air fuel mixture. If you have black smoke comming from the tail pipes and or your spark plugs look powdery black you are running too rich and should jet down. However be careful you do not want a too lean condition as this can cause serious problems such as burnt valves and such. If you are not sure always error on the side of too rich.
When jetting down do it in small increments, drive the car and then check spark plugs. Look for tan, or medium to dark gray color on the ends of the plugs. If you see white or very light gray you may be getting too lean.

Timming, idle adjustments and type of fuel will affect these readings and how the car performs. Too rich or lean may prevent the car from performing at it's best.

Note: New cars burn very lean and you will see white on plugs.

Would there be any benefit to putting it back to stock?I'm trying to get my timing and carb issues squared away.

As a GENERAL rule, if you are having tuning problems, yes, go back to stock settings and then make one change at a time to solve your problem.

However, not knowing exactly what's goin on in your case, I'd put a fresh set of plugs in and then make a WOT run on them. Don't cruise around on them. After the WOT (if your situation allows you to even try this), pull a plug and check it. If you are already lean as it is, I wouldn't lean it any farther out. If it looks a bit rich or so, then I'd drop back to your stock jetting and it should be safe to go from there.

What exactly is the combo and problem?

Well I have some what of a rich condition I can't solve.Only one side of my air/fuel mixture screws work.When i drive it for a while and park it and then go back to start it,it acts like its out of time.I'v changed the power valve per the vaccum reading.I'm going to do a complete tune up today and start over and see what happens.

Ok,I think I got the timing close and the carb close,the idle mixture screws actually do something now.My question is should my engine shake excessivly at idle?Thanks

Todd,

No, your engine should not be shaking. Get the timming set where it needs to be not just close. Once the timing is set then you can dial in the carb. If you still have excessive shaking you may have a miss.