Good morning members of the Corvette Forum. I just joined the Forum as I recently acquired a 1969 L71. I am looking for advice. The drive train in the car is original, including the motor. The former owner recommended I use high octane gas plus an octane boost given the motor is set up for leaded gas and the 11:1 compession ratio. I tried some stuff off the shelf but the car has a tendency to run on once the key is turned off. The car runs strong otherwise.

I reviewed some older postings recommending Torco Accelerator or race fuel. Any recommendations would be appreciated.

Thanks
Jeff H.

 

Check your idle speed when the engine is hot....if the idle speed is a little high, that will cause a run on problem. Also verify your timing is set correctly. If the engine really is a true 11-1 compression and still has iron heads, it will potentially ping and run on when running 93 oct. Thats really just the way it is....

Your options are to mix some race fuel with 93, really detune the engine (pull a bunch of timing out/add some jet and basically kill the HP and responsiveness), or mess with your additives and hope they are working...(I dont trust any additives). The camshaft has a lot to do with an engine's ability to run on 93 oct at a given compression ratio.

You can limp it along but ultimately...if its pinging, you are doing damage to it...eventually it will hurt it.


My engine is a true 10-1 compression(210 psi cranking pressure), iron heads, has a pretty tight quench and a relatively small cam and it is very sensitive to timing advance. I only run 31* total timing so I don't get any pinging...every now and then I will still get a couple hints of run on when shutting it off but its really due more to my idle speed being around 1100 rpms in PARK.. I have blown a head gasket on it once from running 38* of timing on 93 oct and beating on it hard...

You're best bet is to mix a little fuel for it if you really are at 11-1 and iron heads.... But if you are just quoting GM's original compression ratio off 11-1, its likely really only slightly above 10 ish -1... That you can work with and keep it on 93 oct and keep it responsive..

Factory compression ratios were always a little optimistic....especially now after the engine has some miles on it.

 

Thanks for response. The motor is a real 11-1 compression with iron heads. The motor runs very stong with no pinging. it idles around 1100 rpm while in neutral. It just shuts down rough and wants to run on. Not sure of the timing. Any recommendations on octance boosters and race fuels?

 

You might have carbon buildup in the combustion chambers/ piston tops that glow after the ignition spark stops, causing the run-on. Try cleaning that out by using a petroleum distillate available over the parts counter.

 

Try backing the idle speed off a bit first and see what it does... Try setting it at 1000 rpms at hot idle in neutral and see if it helps...Maybe even 900 rpms if it will still idle smooth there... Is it a stick or auto? (Im guessing auto if you have it idling at 1100 in neutral to probably get a 900 idle speed in gear)...

What cam is in it? We really need the entire engine combo to even remotely guess at what the timing/jetting needs to be.

As for fuel, I would mix 5 gals of VP110(107 oct/Purple) to the rest of the tank of 93 oct(12 gals)... That will get you about 97 oct average and that should cover you.... Just depends on how much you drive the car...It will get expensive but the engine will run better and be safe. VP110 is about the cheapest race gas out there...

 

Manual transmission. Don't have specs on the cam but should be stock.

 

Well... if its a manual then try lowering the idle rpm to 900... I bet that cures the run on. I would still check the timing and make sure its where it needs to be.

If its basically a stock/rebuilt L71, you should be fine running it on pump gas as long as the timing is conservative....I doubt its near a true 11-1 comp unless they decked the block/milled the heads etc or went to a taller dome piston... But even at say 10.5-1 comp, its on the ragged edge with iron heads and 93 oct... The timing curve will be critical. If the L71 is the solid lifter motor, that cam should have enough over lap to help with cylinder pressure..(lower DCR)...

(I don't recall all of the data on the L71, I thought it was the solid lift cam)

 

First, welcome aboard.

Second, congrats on the "new" L71. Had a manual L71 a few years back myself.

Third, ajrothm has already given you some good advice here. FWIW, the L71 had a solid cam # 3863143; .520/.520 lift, 242*/242* @ .050, 114* LSA.

IIRC the target idle speed for a manual L71 was 800 RPM (warm, thru air filter), so 1100 would definitely be high. In addition to lowering the idle, I'd graph your timing curve out and get back to us. Could be your initial and/or advance are a bit much for 93 octane, and you might well have some wiggle room there. There's a sticky on timing that recommends bringing mech advance in ASAP, but before you find it IMOE many a BB won't tolerate doing so without higher octane. You might find some tuning solves, but I'd keep an eye on the plugs even if you can't hear anything bad going on.

As for octane boosters themselves, there's nothing you can buy over the regular auto parts counter that does very much, if anything other than lighten your wallet (claims of a point typically = 0.1), but the Torco is for real. That said, blending in some VP110 might still be the better route.

Before messing with the carb, be certain to get the ignition system absolutely dialed in first. Also, in case you're ever tempted, don't try to improve on the factory linkage system; you can't. At the risk of over-simplifying BBC Tri-power basics: 1) disconnect the secondary linkages and tune the center carb as you would any Holley primary; 2) hook up the secondaries so they're just closed at idle by the linkage; 3) install the softest secondary vaccum springs you can which don't cause bog at WOT under load, and you should be good to go carb wise.

edit - Another thought, IIRC the L71 cam has 6* advance gound in, so retarding it 2-4* might well lower DCR enough to take care of this. Not the same build, but FWIW I'm installing the cam (custom grind) straight up during my high-perf 427 makeover (yep, have a 427 BBC in my '78).

 

Thanks for the advice.