If someone advanced the cam timing by changing the timing gears, that only affects the valve opening and closing times. It does not affect the ignition timing.
The ignition timing should be set to factory specs, usually about 10 degrees BTDC with the vacuum advance disconnected.

 

Don't set the ignition timing based solely on initial setting. Run the RPM up and make sure you don't have more than about 34 or 35 with the vac advance plugged. If maximum advance is at 35 or so and initial is at 10 then 10 is okay.

These engines are sometimes a little picky about the advance curve. You may have to play with the timing a little to get it running decent.

All that said, don't get too carried away perfecting the ignition timing until you know that everything else is okay. The compression check sounded good and the vacuum sounds reasonable. How do your plugs look beyond the oil on the ones you mentioned? Do they look rich or lean?

 

well just replaced them but the ones i just pulled out the rear 4 cylinders were black as where the 4 front cylinders were brown so it seems like it was running a little rich but since than i readjusted the mixture and turned the idle down a bit i think part of the problem may have been a high set idle so it may have been getting too much gas in the mornings i dunno how accurate i am on this

 

Sorry to change the subject but does anyone know how to take those compression readings and figure his compression ratio? I have a fresh built 400 and i was going to do a compression test to find my exact ratio.

never mind

http://en.wikipedia.org/wiki/Compression_ratio

 

The compression ratio cannot be determined via a compression test. The compression test is only used to determine cylinder sealing condition.

 

whata re your specs?

 

question. since static compression is simply a comparison of space and dynamic (operating) compression should reflect the actual pressure in the cylinder...thus is newer engine there should be way to determine if calculated dynamic compression correlates with the psi results as seen during a compression test. makes sense as long as the engine isn't worn much. AND the cam is accounted for. It would be a good way of determine ceilings etc of different builds. I'd bet that is a positive correlation and that things like octane could be, in part, gauged by a compression test....of course quench and stuff could also be compared.

Back to the prob at hand. If you remove spark from each plug individually while the engine is running you will notice a 2-300 rpm drop that goes away once the spark is returned...ie the engine runs noticeably worse...makes sense. I wonder if you were to do that if you wouldn't find that there is no real difference when you pull the spark on #4. Mine is doing the same thing...my vacuum is a bit low and i'm wondering if i might be leaking at the intake at #4- as pondered above. I'd also be curious about valve train geometry and rocker/lifter noise.

 

There are just too many variables in play to determine CR via a compression test with any accuracy at all.

 

sorry didnt want to chime in here, but a quick question. at 6000 ft what is the NORMAL reading for a vacume gauge reading? Also what should the timing be set for a 78 400?

 

sorry didnt want to chime in here, but i have a quick question. what is the (normal) vacume reading at 6000 ft? Also on a 78 400 what should the timing be set at?
thanks

 

Mine is around 15 but I'm at approx. 5300 ft in the Denver area.