Originally Posted by stewartfn1
I've got a 86 S10 w/ a 350 throttlebodied and have had timing issues from the second I got the engine running. It also has a mild cam. I keep putting everything on top dead center and try to get the timing down, but it's never run quite right. So I need to know how to get RIGHT. Thanks!
Most likely the problem is the chip. It controls the fuel and ignition curves, but the cam changes everything. Manifold Absolute Pressure (MAP) managed EFI systems, of which TBI is one, are very sensitive to the relationship of manifold pressure (think vacuum to keep the approach simple) to throttle opening angle and RPM. A hotter cam changes these relationships which causes the computer to look in the wrong places of the fuel and ignition schedules. It doesn't take much of a cam or rocker arm ratio change a displacement or compression ratio change, a change to the CFM capacity of the throttle body, headers and duals, removal of the cats, etc. to start getting into trouble. Keep in mind that changes are additive to the problem, more changes, bigger the error of the PROM trying to match inputs against schedules to determine outputs.
This requires a custom PROM be made that redefines where on the schedules the computer looks for fuel and ignition values for the given references of manifold vacuum to throttle opening to RPMs to transmission gearing, etc . Also, if the overall final drive ratio changes, such as the effects of gear and tire size changes, the computer also needs to be reprogrammed for this. Just google "custom chips" and tons of shops will show up.
Note, there are people who will tell you the computer has learning capability, it does but it is very small. Block Point Learn as it's called is only intended to compensate for production line component variations from part to part, it does not have the capacity to deal with things like camshaft, header, compression ratio, gearing, etc. type of changes and it only works if you have a functioning O2 sensor and the computer is able to enter Closed Loop mode.