dead miss at 4500

[Allen Mulcahy]

I have a 351w,mild cam, holley 770, msd distr, with vaccum, I have a dead miss at about 4500 rpm. Engine with 1800 miles, 15* initial timing ,21* mechanical timing. Timing light @ 4500 rpm said 50* Msd distr has 21* bushing in it. All parts are new. I can disconnect vaccum hose, plug carb no miss what gives? Do I run with no vaccum or do I have a problem with distr? Plan on changing carb to Quick Fuel and adding NOS in future. How much timing does vaccum add to total timing? With vaccum plugged off 36* to 6000 rpm runs good......

[wtm442]

50 degrees total? WOW

1) Do you have light and turn dial to see 50? Not recommended. Put light on zero and use timing tape/marks on damper.

2) Where is vacuum line? Attached to intake manifold? Wrong place. Should be attached to port at base of carb.


How much timing does vaccum add to total timing?

None. At WOT there is little or NO vacuum at the port at base of carb. The vacuum advance is only operational at steady RPM's and improves gas milage.

[Excaliber]

Where the vacuum line should be attached is highly debatable. But for SURE, 50 degrees advance sounds to high!

[Allen Mulcahy]

Vaccum line on carb on pass side high on carb, light on O using marks on damper. Have two ports in front of carb one small one large. Both low in front.

[Tommy]

Vacuum advance does indeed advance the timing when the vacuum is high (i.e., throttle not fully open). 4500 RPM with no load would likely produce a lot of vacuum and 50* total advance doesn't sound unexpected to me. That's why timing is always set with the vacuum advance disconnected and the outlet plugged.

How does the engine run under load (i.e., on the road with full throttle)? Does it miss at 4500 RPM then too? If I were looking for a problem, I'd start with a weak ignition from bad plug wires or a weak coil.

[wtm442]

Distributor vacuum advance line should be attached to small port, usually located just beneath the primary fuel bowl on the passenger side.

Timing at 36 º at 2500 RPM and above with vacuum line disconnect at distributor.

I am not familiar with MSD. Could the 21 º bushing be "distributor degrees" which would be 42 º damper degrees of mechanical advance?

If you are having 50 degrees advance with vacuum line disconnected, I would switch bushing to a 10 degree unit (10 degrees distributor). That would be 20 degrees crank mechanical advance. With the 15 º initial timing, total would then be 35 º total.

Of course, check everything with timing light.

Good luck. Your 50 º+ could hurt the motor. Make that will hurt the motor if you add laughing gas.

Of course, if you are having 50 º with vacuum attached at a steady RPM, that is OK, and your miss is being caused by something else ... weak coil, bad plug wire(s), time for new spark plugs, etc.