Fuel

[Knot at Work]

My owner manual says 91 Octane. I am running the 350 4bbl with a rochester quadrajet.

I put 87 octane in last fill up and have about 1/2 tank left and I notice the motor wants to hestitate and trys to backfire.

Truck sat for the last 4 years in a garage.

I am going to change the fuel filter and hope that helps.

Do you guys run 91 Octane in your trucks?

[ChevLoRay]

If you are writing about a '67-72....then it doesn't apply. What used to be called 91 octane, had lead in it. When gasoline was reformulated without lead, the octane ratings were changed. The rating system that came to be at that time found that pumps were labeled 87 RON octane, which was the equivalent of the 91 octane that we had gotten before.

You may have to clean the gas tank. If it sat for 4 years, there's gonna be some varnish in the tank and in the lines. Hesitation could be caused by any one thing, or a combination of several. Fuel filter is just one of many.

[Knot at Work]

Thanks.

So can I add lead substitute and expect a change of performancce after I swap filters?

[Stocker]

I don't think so. Lead (and lead substitute) lubricates the softer valve seats in older engines. Newer engines have hardened valve seats and don't require a lead-type additive.

If your engine pings (spark knock, detonation, pre-ignition) and your ignition timing is correct -- and it isn't suffering from carbon buildup or burned valves (and probably other factors) -- then higher octane fuel may be called for.

To oversimplify, premium fuel won't increase performance from an engine designed to run on regular. It will allow performance engines to run properly.

[johny1i]

If it has sat that long, I would say some routine maintenance/checks are probably in order. Your accelerator pump in the carb could be worn out, or your timing could be way off. I'm assuming you have the original motor by how few miles the truck has on it, so I would start with the relatively cheap stuff--new points, adjust dwell/timing, rebuild carb, new fuel filter, check plugs/gaps, make sure vacuum advance is working, etc. She may just need a little TLC...

Not dodging your question, but my engine is a fairly new crate motor, so it's irrelevant that I run 87.