Hi

I just build a 1973 400m motor. 9/1 pistons, cleveland 2V heads, pocket ported, edelbrock performer cam, edelbrock intake and 1406 600cfm edelbrock carb all new off course. also have a hei distributor.
After brake in motor run ok for 1 week than just died. Obviously it was to much gas from the carb. I put carb apart and find out that one brass float was full of gasoline. I repaired this by soldering the float (factory mistake). Carb works ok now, but whe I start the engine it stumbels alot, like it has a huge cam, rpm are going up and down like crazy and this happens just of the idle, like when it's cold and choke is on. Vacuum advance is connected to ported vaccum port. If I disconnect vacuum advance engine runs smooth. Also when engine is warm it runs ok. Choke is working ok.
I have 14 initial, 32 combined and 48 with the vacuum advance.
Should I just disconect the vacuum advance or is there some other solution?

Thank you

 

It really sounds like the vacuum advance is connected to manifold vacuum. Or, the throttle stop is adjusted up so much that the ported vacuum port has vacuum at all throttle positions. Check to see if you have vacuum at idle.

 

It's connected to ported vacuum. If engine is hot it runs ok, at idle there is no vacuum, but when I started cold, rpm are higher and there is vacuum also on ported side because of the choke engaged. I had total advance with vacuum 53 and I put it down to 48, it's better but still not ok.

 

What you are seeing is due to there being vacuum at cold "idle". And that is happening because your fast idle setting is too high, thereby opening the throttle plates enough to expose the port to vacuum. Reduce the fast idle setting on the carb.

 

I run 10 initial, and I have a stop-bushing (18 degrees) for a total of 29; My vac advance allows up to 10 degrees at partial throttle.

10-15 initial is not all that un-reasonable, and an 18 degree stop isn't too bad...so your settings sound OK.

What are your idle mixture screws set at?

Mine are at 2.0 turns out (Spec is 1.5 - 2.0 turns out)

 

.
. X 2... sounds like the HEI doesn't know what to make of the fast idle... a fresh engine shouldn't need to idle very fast when cold to keep running... also, maybe some fuel-fouled sparkplugs... did you clean or change them after fixing the carb. float?

 

Wouldn't 53 deg. of ignition advance blow the bottom end clear out of the engine? I honestly don't know it just seems most people are running 28 to 34.

 

. When cruising at steady speed/part throttle, initial advance + mechanical advance + vacuum advance can run the total up high like that for better MPG... it backs off as soon as you give it more pedal...

 

It isn't unusual to have 14 on initial, 18 for mechanical, and up to 28 for vacuum, which says you could have a total of 60. But that would be unusual in the real world as the mechanical will not be fully in until something like 3000 RPM, and you'd have to have a light foot to get that much vacuum. So it would be at high RPM in lower gears or with maybe 4.56 finals.

 

. Yes, sadly, stock 1970's engines needed almost WOT to merely cruise at 60 in a pickup... but a powerful rebuild with good compression ratio and good engine quench effect can cruise 60 with very little throttle opening...

 

So if you hook up a vacuum pump to the distributor, you can actually pull the advance past the mechanical limit?

I thought the mechanical advance WAS the limit.

I always thought the vacuum advance was just to supplement the mechanical advance in high vacuum, low RPM situations.

 

They are two separate things. The mechanical advances the central cam, and the vacuum advances the base plate to which the pick is attached.

The mechanical advance is to compensate for the fact that each explosion takes the same time so as the RPM's increase the spark needs happen earlier and earlier. Vacuum advance is purely for economy and is partially because a really lean mix doesn't burn as fast as a 14.7:1 mix does.