OK for a correct and accurate compression test you will need to follow this instruction I wrote a while back for Old School..
If you just jam a compression tester in the hole and wind it over then write down the compression then you are doing yourself a dis-service..
My technique for compression test is this.
1 - Get engine up to operating temp
2 - Remove HT leads and crack off spark plugs a quarter turn each, reattach HT leads in the correct order.
3 - Run engine for 30 seconds at around 2-3000rpm (this gets rid/burns off any carbon that may have been in the spark plugs threads and eliminates carbon chunks getting jammed under the valve seat when you are performing compression test)
4 - Remove the spark plugs
5 - Disconnect the distributor to eliminate the spark
6 - Jam the throttle wide open (best to have someone sit in the car with their foot held down) make sure the throttle stays WOT until the entire test is complete (opening and closing the throttle between compression checks causes bore wash and can alter the compression from first to last)
7 - Start on number 1 and thread the compression tester in till it is firmly seated
8 - Crank Engine over so the engine turns over 20 times (10 compression strokes)
9 - Write down the result from the gauge (generally between 100-180psi) for each cylinder
10 - Back to stage 7 but move to next cylinder and continue
11 - Compressions shouldn't alter more than 10% between worst and best (up to 20% at most)
Once you have compressions report back and we can take a look at whats going on.
As a theory for right now - you say number 3 and 4 sparkplugs are your problem but as they are running on different carbs as such then I doubt carb balance is your issue. If the first 3 cylinders or last 3 cylinders were running rich then yes it could point to carb balance being a problem.
My best assumption is that because the carb balance tube runs between cylinders 3 and 4 (I am assuming you are running a 240/280Z factory twin carb manifold) that the mixture is richer at that point in the manifold and the firing order is 153624 so the engine turns an entire 360 degrees between cylinders 3 and 4 firing.
As I said before I fitted an MSD and my L26 just stopped fouling plugs. I was changing plugs every 500-1000Km or so before I fitted the MSD and after I fitted it I didn't change any and the guy who owns it hasn't had to change the plugs ever.
I'm not saying its going to fix YOUR problem but it did fix mine..
Another thing are you running Round Tops or the Flat Tops carbs?
Hitachi Flat Tops
Hitachi Round Tops
