'67 BB gasoline

[David K.]

I have a '67 427/435 car and I am wanting opinions on want fuel to use. The enigine was rebuilt in the early 90s but I do not know any of the details of that rebuild. The car just does not seem to run right or better said consistant. No knocks or pings but just not right. When started most of the time the fast idle is correct at 1600-2000 rpms and then when warm it kicks down to about 900. After driving it for a short while, at a stop the idle goes down to around 500 rpm and seem like it may stall. On one occasion it did stall while braking and turning. I have put some octane boost in the gasoline, raising the octane from a very sad 93 to maybe 95 or so. It does seem to do much better when I do this but not where I would expect it to perform.

Any suggestions on gasoline or additives? Should I get some lead substitute? Will a lead additive hurt an engine if I do not know what they did on the rebuild? Is that max lead 2000 that I see advertised in the Driveline worth anything? There is a racetrack nearby me that I may be able to get some high octane fuel but it would be a pain.

Any suggestions would be appreciated.

[Douglas L.]

Re: '67 BB gasoline

If you live in a state where smog checks are done, I would have one done. A smog analysis can tell you how lean or rich your engine is running. Sounds like you might have a carb adjustment or timing problem. As far as the fuel goes, octane additives help or avaition gas, if you can get it.

[John H.]

Re: '67 BB gasoline

Doubt if the symptoms you describe have anything to do with the fuel you use; sounds more like carburetion and ignition timing/advance system issues. Basic tune-up and carb check and mapping the advance system will probably bring out the root cause. "Octane boosters" and "lead substitutes" are highly over-rated.

[Peter M.]

Re: '67 BB gasoline

I run 50% Sunoco leaded 110 octane race gas in my stock L72 with the 11:1 compression. You might want to check your compression ratio to see what the previous engine builder did. That, combined with the previous post re timing and carb adjustments will give you a better idea of what you need to do for fuel.

[Duke W.]

Re: '67 BB gasoline

I don't think it's a fuel issue. "Octane booster" points are usually 0.1 not 1.0 so they are of little value.

As John said, checking your ignition map versus OE specs and going through the idle speed mixture adjustment is a good place to start.

Also, your engine has a "ported" vacuum advance for CA emissions (All L-72/71 were set up to meet CA emissions because it was a low volume option and Chevrolet chose to go with just one configuration.)

I have always recommended that owners of L-72 and L-71 convert to full time vacuum advance, however, for L-71 you need to retrofit a L-72 vacuum can, and the most commonly available substitute is the NAPA/Echlin VC1765, which provdes 16*@12" The OE L-71 can was about 15@15.5" and the engine won't generate enough idle vacuum to keep it pulled to the limit with full time vacuum advance and this can lead to idle instability like what you have.

The easiest way to implement full time vacuum advance is to port the vacuum signal line to the choke vacuum break.

Before you start changing anything find out the current baseline. Does the ignition map meet OE spec including ported vacuum advance and an OE equivalent vacuum can.

Maybe it's already been modified in a poor way that is causing the problem.

Duke

[Richard S.]

Re: '67 BB gasoline

David,
I have he same 67 L-71 engine that you do. I can attest to the value of Dukes post. He helped me reconfigure my vacuum lines and I changed vacuum cans to his specified Napa can. Together with recurving the distributor, it made a world of difference in performance. The car also runs cooler as well so if yours heats up a bit too much....Dukes advice may be just what you need to bring back the power.

[George R.]

Re: '67 BB gasoline

Have you tuned the car up lately? I found my L71 doing the same thing when I replaced all 3 carb gaskets. The fwd carb gasket was catching the throttle plate and not closing. If you need to know how to test both F/A carbs operations let me know I'll hlep you.

[Tim G.]

Re: '67 BB gasoline

I've never found a lead substitute that works well in an L71. To get the best out of these engines you need a mixture of race and pump fuel. A 50/50 mixture will work well and for how seldom I drive my car, it is well worth it to have the fuel it likes in the tank.