Postby Hasbeen » 07 May 2014 16:23
Hi LUC, & welcome to the forum. That firing order is 1,3,4,2.
I have had backfiring through the carbs with 2 7s that had sat for a while. One had sat because the PO had adjusted his own tappets, & got his math wrong.
He had one inlet & one exhaust valves riding. The thing would start & sort of run, backfiring through the carb often. He had put it away discouraged, which meant it was a cheap buy a couple of years later.
Once the valve clearance was corrected the rest was easy.
The other one was harder. It has sat for 3 or 4 years. It would run a bit, shaking like a freezing dog. When I checked the valve clearances no 2 inlet was 46 thou, [should be 8] & some others were only 2 or 3 thou.
I shimmed them to 8 inlet 18 exhaust, the thing was running a bit better, still backfiring through the front carb. The compression test was quite low, so I added some STP oil treatment to the oil, as this frees up sticking piston rings, & cleans carbon out of cylinders & off valves.
I have a private track on my property, & I gave it a bit of thrashing around it, which improved it somewhat.
I still could not get it to idle, & it was still backfiring. I checked the valve clearances again, & found No2 inlet riding.
I ultimately worked out the No2 inlet valve had been actually sticking open, at least at low revs, due either to rust or carbon on the valve stem. My shimming had held it off the seat.
My running had cleaned this off, & once shimmed correctly, the thing started to run quite well. With the STP & some running the compression came up from less than 100 PSI to around 150 PSI.
I would suggest you look at valve clearances, valve & ignition timing, add some STP to the oil, & hope someone with injection experience comes in with suggestions.
After more remedial work, that engine performed very well for about another 70,000 kilometer. So don't despair, with a bit of luck, yours will too, once sorted.
Hasbeen
