71 LS6 breaking up at 3500+ rpm

[Mike E.]

[Dave S.]

Re: 71 LS6 breaking up at 3500+ rpm

Mike,
Maybe the fuel bowls are full of schellac and the engine is starving for fuel. That is what happened to my Holley on the LT-1 from long term storage. Have you had the carburetor apart yet.???

[Mark E.]

Re: 71 LS6 breaking up at 3500+ rpm

It hasn't been running for 41 years?

I would draw up a carefully thought out checklist for all systems of the car before starting it. Engine only systems to inspect/service/replace include the entire ignition (including centrifugal and vacuum advance, rotor, cap), entire fuel system (including lines, pick up, pump, carb rebuild), entire cooling system (including the pump, radiator, all hoses and belts), exhaust system (including heat riser), AIR system, entire vacuum system, PCV/evap control system, all fluids, all filters, all electrical connections (clean and tight, no mouse damage).

Then see if it still misses. Obviously not all of these would cause a miss but all affect reliability and safety.

[Mike E.]

Re: 71 LS6 breaking up at 3500+ rpm

Mark,
You don't really think I would start an LS6 without going through all that? I'd be a fool not to have. I appreciate your thoughts, but it isn't exactly my first rodeo. My first with TI with a miss, however. That's why I am asking.

[Terry M.]

Re: 71 LS6 breaking up at 3500+ rpm

Mark,
You don't really think I would start an LS6 without going through all that? I'd be a fool not to have. I appreciate your thoughts, but it isn't exactly my first rodeo. My first with TI with a miss, however. That's why I am asking.

Any chance you can find someone with an ignition analyzer.. Sun made the best back in the day, but there are a number of brands. Most shops have disposed of these machines because they are useless on modern cars. Here is the most recent example:

https://www.google.com/search?q=Sun+Ignition+analyzer&tbm=isch&source=iu&ictx=1&fir=Mi49w5iGJJuy8M%253A%252CJXeEBlW2gc90CM%252C_&vet=1&usg=AI4_-kQ46wjhzPSiO4CqKKjqmbfuisX7_g&sa=X&ved=2ahUKEwizq9ymyo3jAhVbVs0KHRMRAX8Q9QEwAHoECAQQBA#imgrc=Mi49w5iGJJuy8M:

Then your next challenge will be to find someone old enough to know how to operate it. Using one of those machines (by someone knowledgeable) will separate the ignition from the fuel system.

[Mark E.]

Re: 71 LS6 breaking up at 3500+ rpm

No hard feelings. I don't know you or your background. So I suggested the basics.

Enjoy your LS6.

[Patrick H.]

[James G.]

Re: 71 LS6 breaking up at 3500+ rpm

I agree with Patrick, sounds like the coil.
HOWEVER
If the cap and rotor button have not been changed I would change them out prior to swapping the coil, if they were changed out with parts house pieces I will go back with the prior parts to see if the problem stays or goes...

IF the replacement wires are LL repros I would check them with an OHM meter to ensure there are no issues there...

[Mike E.]

Re: 71 LS6 breaking up at 3500+ rpm

No shellac in carb. Tried a new repro 263 coil. Plug wires are braided NOS GM. Rotor is GM NOS (new). No change whatsoever. Talked to my neighbor 4 doors down, who builds engines for circle track race cars. He hypothesized valve spring fatigue, since they sat in the same place for 40+. What do you think about that? Possible not probable? I didn't think to do/check anything with those.

[Joe L.]

Re: 71 LS6 breaking up at 3500+ rpm

No shellac in carb. Tried a new repro 263 coil. Plug wires are braided NOS GM. Rotor is GM NOS (new). No change whatsoever. Talked to my neighbor 4 doors down, who builds engines for circle track race cars. He hypothesized valve spring fatigue, since they sat in the same place for 40+. What do you think about that? Possible not probable? I didn't think to do/check anything with those.

Mike----


I highly doubt that the problem is due to valve spring fatigue. As far as the springs go, though, I would check them for a possible broken spring (maybe that's why someone parked it and walked away in 1978). All you need to do is to remove the valve covers and attempt to rotate the valve springs by hand. You don't really need to loosen the rocker nuts since a broken valve spring is usually identifiable regardless of the position of the valve. If you can rotate it easily by hand, that's the broken spring. However, these valve springs are not really noted for failure, unlike the 1965-69 springs.

My guess would be that something is not right with the TI system even though it may have been recently refurbished in some way.

[James G.]

Re: 71 LS6 breaking up at 3500+ rpm

PERSONALLY I would check the wires, ALTHOUGH anecdotal another local member was having issues from his 19k mile 71 LS5 which had NOS wires installed last year... he put the original wires back on and they were actually better than the NOS ones (not a joke) - now if the humidity were 90% they may put on a wicked purple lights show...
I am serious that unless the wires are solid cores there is a distinct possibility they could be the culprit -- IF they are not bad - and you are headed to Greenville pull the distributor and if you don't have someone with a SUN distributor machine... bring it to Greenville and we can run out and put it on my SUN 504...


Re: adjusting the valves... were any of them MUCH looser than any of the others?

[Chris H.]

Re: 71 LS6 breaking up at 3500+ rpm

Mike, you probably already checked the fuel filters in the bowls but you didn’t mention it. I had similar symptoms to yours once on a 396 and it turned out to be the coil. And I’ve had repro TI coils that would produce your symptoms.

[Chris H.]

Re: 71 LS6 breaking up at 3500+ rpm

One other long shot is the fuel pump. Congratulations on the LS6. I drove one once, an automatic, and it was shockingly fast, chirping gears.

[Dick W.]

Re: 71 LS6 breaking up at 3500+ rpm

Any chance you can find someone with an ignition analyzer.. Sun made the best back in the day, but there are a number of brands. Most shops have disposed of these machines because they are useless on modern cars. Here is the most recent example:

https://www.google.com/search?q=Sun+Ignition+analyzer&tbm=isch&source=iu&ictx=1&fir=Mi49w5iGJJuy8M%253A%252CJXeEBlW2gc90CM%252C_&vet=1&usg=AI4_-kQ46wjhzPSiO4CqKKjqmbfuisX7_g&sa=X&ved=2ahUKEwizq9ymyo3jAhVbVs0KHRMRAX8Q9QEwAHoECAQQBA#imgrc=Mi49w5iGJJuy8M:

Then your next challenge will be to find someone old enough to know how to operate it. Using one of those machines (by someone knowledgeable) will separate the ignition from the fuel system.

Fly me there and I will diagnose using the Sun. Or bring the car to my shop. I have many hours on different models of those. Actually purchased five new ones over the years. I also have an early vintage manual that Sun gave techs when they went through training.

[Gary J.]

Re: 71 LS6 breaking up at 3500+ rpm

Now that sounds like a plan, now work the plan...