I have a friend who recently got himself an '85 Vette with a 350 TPI in it.

For some time now, it has gotten the bad habit of backfiring in higher rpms in the 3500-5500 range, anyone who has got a clue as to what the problem might be?

We have checked most things that one usually would suspect..

* TPS Sensor
* Pickup coil in distributor
* Fuel pressure (43 PSI)
* NO vacuum leaks found

And it has got new spark plus, wires, distrubutor cap & rotor and coil...

We haven't changed the MAF yet, since it's probably a different model on the '85:S I've been told - Is that so?

My friend is considering upgrading the ecm to a 1227730, maybe now would be a good time if it should show that the MAF is the problem?

 

Did you check your timing

 

Timing set at 6 degrees base timing, and it runs good at lower rpm (0-3500), and we have also replaced the ignition module and tried with another MAF, with the same results

 

First we need to define "backfire". Are we talking a "popping" back through the intake? If so does it have a definitive "rythym" or regularity? If this is the case then you need to look at the valve train. High milage engines are notorious for losing a lobe on the camshaft (flat tappet cams, rollers are not as prone to this condition). When this happens to an exhaust valve lobe, the cylinder is not properly scavenged and it starts to "relieve" itself when the intake valve opens to refill the cylinder. It repeats this cycle ...fills normally on one stroke, fires, doesn't exhaust on the normal exhaust stroke, holds the cylinder pressure until the intake valve opens then exhaust through the intake then runs a "weak" or "dead" cycle because the cylinder did not fill properly on that cycle, then fills normally on the next and repeats the process. When this condition is in the early stages of lobe decay, the only time it is immediately apparent is in the higher RPM range. As the lobe continues to decay it becomes noticeable lower and lower in the RPM range.
Also note that a standard "compression test" will not show any irregularities when this condition is in all but the most extream state.
I don't know if this will help, you may know as much about this as I do but I have seen many cams come and go....give it a look.

 

You hit it right on the spot....

We removed the valvecovers yesterday on the Vette to have a closer look, and two of the exhaust valves barely moves when the engine is running!!!

So I think my friend is looking forward to at minimum a cam swap, but since he has to tear down the engine anyway I think he'll go for a full rebuild probably....

 

Sorry for your friend that I was right, but glad to help. Good luck on the rebuild.