67 L71, so cal nightmare

[Dave C.]

I trailered my 67 roadster down to Southern California from Victoria BC, in hopes of having it judged at the chapter meet in Chatworth on the 8th of April. And if any of you have been following my threads on the corvette forum, you will know what I have been going through to try and get it to run properly, after having to rebuild the motor in a week before I left for my holiday.
Duke and a few other people have been helping me out daily, trying to diagnose my overheating and spark knock issues.
just thought I would bring it over here even though it is not a real restoration question.
Heres the story: completely rebuilt L71 , all stock, heads cam etc. New recored rad with high capacity core. Car runs great when it is cold but was overheating like crazy when I got here. Duke and the other guys walked me though the changeover to full manifold vacuum advance, huge improvement, they also helped me to set the timing properly. I installed an electric fan, I checked the thermostat on the stove. It cools better now but eventually it starts to heat up to say 210 and then it starts to knock, then it won't idle anymore and just dies when you are at a light. I put in a replacement coil this morning thought I had it fixed, but a few
miles down the road same thing all over again. Duke has suggested getting some high octane fuel, which I plan to do in the morning, to see if that helps. Just looking for anything I can try, as there is none here in Hemet that has a clue how to tune these cars properly. Or maybe one of you know someone good that is reasonably close to here that can help me out.

thanks,

Dave

[Leif A.]

Re: 67 L71, so cal nightmare

Dave,
I have been following your trevails' over on the CF. Thought you had her fixed. I know you have a new engine but could it be possible that you may have a bad fan clutch?? Just throwing another idea out there. I was having similar issues with my L79 (everything new) and mine turned out to be a faulty fan clutch that wasn't fully engaging as the car heated up causing an overheating issue...new fan clutch from K&B and problem solved.

[Dave C.]

Re: 67 L71, so cal nightmare

Thanks Leif, I did replace the clutch when I installed the motor this time, and I have since installed a 16" electric fan to assist the mechanical one.

[Michael J.]

Re: 67 L71, so cal nightmare

Duke is the best there is about '67 L71 overheating issues. He solved mine but it takes many things as there are so many moving parts and the L71 was so poorly engineered for street use. The knocking and failure to idle are new things I have not seen in mine. Something different than just cooling/timing issues there. Running hot (210+) is normal in summer traffic situations, but you should always check the idle vacuum and check advance at idle. I used to always tune mine at idle by sound and feel but using the vacuum gauge. Max idle RPM and vacuum without pre-detonantion when revved when I set the distributor advance. Good luck, I was planning on being at Chatsworth for judging '63-'64s, or ''67s, but Texas plans derailed that, but many in our SoCal chapter to help you with the car. Hope you have a good experience.

[Bill W.]

Re: 67 L71, so cal nightmare

Dave,
Sounds like the timing may be retarded, as in the rubber ring on the crank damper may have softened over time to the point that the outer ring may have slipped. I know time is short you but see if you can rig a positive stop on the number one piston. Remove ALL plugs, turn engine over by hand (DO NOT USE STARTER!) make chalk mark on damper when the piston contacts your stop. Reverse rotate crank till piston again contacts stop. Make chalk mark. If the TDC line on your damper is an equal distance on either side of your chalk marks, your damper has not moved.
Instead of all the above you may also try just advancing initial timing. This may also tell you if your timing marks have slipped.
Good luck and let us know what you find. Bill

[Dave C.]

Re: 67 L71, so cal nightmare

Thanks for your reply Michael, too bad you won't be at the Chatworth meet!

[Dave C.]

Re: 67 L71, so cal nightmare

Thanks Bill, my mechanic friend in Victoria, did the exact same test you described when he initially tuned the car, and the balancer was right on , but it may have slipped since then. I will check

[Mark E.]

Re: 67 L71, so cal nightmare

Well, the good news is you're in Southern California which has a lot of talent that can help. Duke has a solid reputation; if he isn't available maybe he or someone from the local chapter can refer you to a good mechanic.

As Michael said, your symptoms can have any number of causes, and to make it challenging, since you rebuilt/replaced/changed so many things, more than one issue may be contributing to the problem.

Cheer up; this can be fixed. A good mechanic familiar with old analog V8s almost has a six sense, especially after spending time with the car using his other five senses and some basic diagnostic tools.

We on the Forum can't experience the engine in person, so our ability to help is limited. But if you decide to tackle this yourself, one approach is to go through a methodical process that addresses possible causes in logical sequence until the problem(s) is/are found. Several shop manuals (but not most vintage Chevy FSMs) include a troubleshooting flowchart for analog V8s that may help guide your diagnostics.

With all that said, you described the symptom as:
"...it eventually starts to heat up to say 210 and then it starts to knock, then it won't idle anymore and just dies when you are at a light..."

Can you provide more specifics? When does it heat up to 210 (while idling, cruising above or below a certain speed, under load, all the time)? How long does it take to act up from a cold start? What is the knocking like (continuous at idle, only under load, only partial throttle,...)? Does the knocking sound like pre-ignition, or something more mechanical? Understanding the frequency of the knock is helpful. For example, semi-random pinging is often pre-ignition; in contrast a faithful tick at half engine speed is usually valve train related.

Do you have a sense why it dies (running rich and loading up, running lean and fuel starved,...)?

[Mark E.]

Re: 67 L71, so cal nightmare

After thinking about the situation (rebuilt engine, restored car with cooling system thoroughly checked), I would be inclined to begin with the ignition timing, then valve adjustment (whoever adjusted your solid lifters knew what they were doing, right?), then valve timing.

Understanding when the engine overheats provides a clue where to poke around. Only overheating at idle? Check initial timing. Only overheating at cruise speed? Check advance curve and VAC. Overheating all the time and ignition timing is ok? Check valve timing.

[Dave C.]

Re: 67 L71, so cal nightmare

Mark , thanks for your interest in my problem, and my thanks to all the other members that have tried to help me out. I will try to give you some more details, I have set the mechanical advance to 34 * , I have changed out the vac to a b20? ( as per dukes advise, 16* at 12") I have 14" at idle. Idle set at 900. Set the mixture with a vacuum gauge, but I am no expert at this and maybe I have it too lean @ 1 full turn counterclockwise from seated. I have 24* at idle with the vacuum in. At this point if someone knows a great tuneup guy anywhere in the greater LA area, I will load this thing up in my trailer, and haul it there with my motor home, and buy them beer and giv them cash until it is fixed.

[Dave C.]

Re: 67 L71, so cal nightmare

Oh I forgot to mention, Duke and I had a great conversation the other night , and now he has registered for the Chatworth meet, so he can judge my car personally! This whole thing is very exiting for me. I hope I can get my car running properly soon.

[Loren L.]

Re: 67 L71, so cal nightmare

Find Mike Scott, formerly of Corvette Corrections in Anaheim.

[Michael H.]

Re: 67 L71, so cal nightmare

When you rebuilt the motor how far did you bore it ? If you went .060 or more that could be your problem.

Mike

[Michael J.]

Re: 67 L71, so cal nightmare

When you rebuilt the motor how far did you bore it ? If you went .060 or more that could be your problem.

Mike

+1, if over bored much (I set my limits at .03) and not sleeved, overheating is a common issue with all engines.

[Gene M.]

Re: 67 L71, so cal nightmare

When you rebuilt the motor how far did you bore it ? If you went .060 or more that could be your problem.

Mike

Also what other machine work done? Such as milled heads, over bore, piston dome size, index of camshaft, piston to cylinder wall clearance, bearing clearance, torque to rotate assembly as built. Engine should run fine at 210 degrees. Check cylinder compression pressure. Is vacuum correct and steady at idle? Does your heat riser work correctly?