Swapped my stock 305 for a built 406 with a pretty big cam. The thing is the cam doesn't creat a whole lot of vacuum and the brakes will be fine sometimes, and others it will be really firm and hard to press in. I do have the booster hooked up to the back of my carb but I know you guys have found a work around for this, so please let me in!

Also the motor doesn't want to stay running with a holly duble pumper carb and a smaller edelbrock. I know the compression is about 11:1 so maybe its the octane? I ran unleaded for a long while in the 305 and ran it almost dry before putting premium in. Maybe I should add some octane booster and fill some more premium?

Any suggestions will be appreciated, thanks.

 

FIrst off get it tuned correctly. You will find you will have more vacuum then.

Start there.

What octane are you running? What engine specs? Regardless of what octane you are running at idle, it should be fine under no load. Get the timing right and the carb right.

 

This is off topic but 11:1 seems High for pump gas. When you get your vacuum issue sorted put, listen for any detonation.

 

not necessarily. depends on heads, cam, etc.

 

Exactly, everyone keeps talling me it won't run on pump gas. I drove it last night and it did fine. The heads and cam do allow this. It is close but yes it will run on pump.

 

As with everything else in life, TIMING is everything. What's your timing set at? Should be at least 10*BTDC, probably more. You can obtain a tad bit more vacuum at a slightly higher timing, then back the carb idle down some to compensate.

You want total timing to be at 32 - 36 degrees. To get that, you have to adjust your base timing higher. BUT - then you have to check for detonation at WOT. So, if you set your initial timing to 18 in order to get the 34 total, but you get pinging/knocking at WOT, then you need to back timing down 2 degrees to 16 and test again. You want to be 2 degrees lower than the last initial timing setting that made detonation at WOT.

Most distributors out of the box won't allow you to get 34 total timing without changing springs and/or weights inside. You need lighter springs and/or heavier weights so the centrifugal force with get the mechanical advance to "advance the timing faster" and thus get it to the 34 from an initial timing low enough to prevent knocking/pinging.

Make sense?

As an example - my car. I have initial timing set at about 14. I can get a total timing of about 28 by the time I hit 3000 rpms. If I set my initial up higher, I get spark knock/pinging/detonation at WOT, so I can't go higher on the initial. I need new springs/weights in my distributor so that the centrifugal force advances the timing farther and faster, so I can get closer to 34 total timing quicker (before detonation starts occurring).

If you can't get the necessary vacuum after getting the timing right, then you'll need to look at adding a vacuum canister with a check valve - these "store" vacuum for you, so that you have extra when needed. Available at Jegs, Summit, etc. from cheap plastic to expensive fancy chrome.