BBCorv70

Stock 70 LS5.

Thought I had this problem solved. Car starts up fine, idles great, all is well when cold. If I drive only on back roads, speeds less than 50, the car will run fine for hours with only a hint of a tendency to stall at a light or stumble taking off. This doesn't happen when it's cold or partially warmed up. However, if I drive on the highway for more than 1/2 hour, the idle will deteriorate while waiting at a light on the off ramp, sometimes stalling.

I noticed the temps tend to rise a bit while highway driving, need to install the air dam and seal on top of the radiator support. This may help but I suspect it won't solve the problem entirely.

Pretty sure the correct gasket/spacer is installed between carb and manifold.

Seems like a heat related problem. Modern fuel causing the carb to percolate? Could there be an issue with the fuel pump? Doesn't seem to fit... I have two carbs, seems to happen with either.

Anyone else with a BB have similar issues?

This is a fairly stock LS5 with a mild cam. Stock exhaust with heat riser. Exhaust pass through intake manifold not blocked.

fishslayer143

What kind of distributor?

BBCorv70

Stock with points and condenser.

BBCorv70

ttt
Nobody has had this problem or similar?
Wondering if this may be related to modern fuel, related to vapor lock? The car can be restarted though sometimes difficult, may need to be nursed, rev the engine until it clears.

Maybe I need more insulation under the carb or possibly a heat shield?

Wrap the fuel lines where they pass near the exhaust manifolds?

I may consider installing pointless ignition, one which preserves the stock appearance. Not sure that would change anything.

Running out of ideas.

OMF

How hot is the coil after a hiway run compared to a local run at slow speed? Maybe the coil is at the end of it's useful life.....

clwi

My BB runs flawless, i have a MSD distributor and MSD 6AL ignition.
Idle drops a bit when really hot but no issues besides that.
Qjet and 468cui with a Straub cam.

fishslayer143

could be vapor lock or percolation . when it won't start, remover Air Cleaner cover and see if fuel is boiling into bores.. or if not, see if the accelerator pump squirts fuel.. I assume your fuel lines and filter are in good condition?

Jeffs82c3

Maybe a plugged fuel tank vent...

rickimvette

+1 for coil and gas cap

suprspooky

I'm not sure if the 70 uses a vented cap or not but I'm giving a +2 for a venting problem or fuel starving (fuel level can affect idle before it's noticeable while driving), when was your last fuel filter change?

BBCorv70

Good to know. Does your gas have ethanol added? It's 10% ethanol where I'm located. Possible this is an ignition problem though seems a bit unlikely, running points and condenser.

I may try an experiment next weekend, see if I can get the symptoms to show again, peer into the carb when it's happening, see if I can spot fuel dribbling.


Coil was replaced a few years ago, same thought, didn't change anything.


I've owned this car for over 30 years. It was a semi daily driver in the 80's, don't recall having this problem. Did a frame off in the 90's, car wasn't operation for many years. Parts which were replaced may provide a clue. The gas tank, fuel lines, fuel pump, and carb were replaced. It's possible there's a problem with the tank though I'm running a 'vented' fuel cap. Thinking about driving with the cap loose once I burn off a bit of fuel, full tank right now.

When it begins to misbehave, I'm not sure whether it's running too rich or too lean. Too rich may suggest percolation? Too lean may imply restriction somewhere or vapor lock? I have to feather the gas pedal when taking off from a light when it's hot, else risk a stall. Clue? Last time I had it out, when exiting the highway I had to wait at a light. It nearly stalled when I tried to take off, had to keep working the gas pedal to try to clear, took a while to settle out. This is becoming very annoying...

I'm open to the possibility of an ignition problem as well though it doesn't appear to be a problem with the coil. Thought about breaker-less ignition for convenience if not improvement in performance.

lvmyvt76

did you put the molded "s" fuel line hose from fuel line to fuel pump on the car, or just use a piece of fuel line??? the fuel line will get warm and kink, slowing down fuel to pump & carb. ask me how I know.

cv67

Its the gas, did the coil, all hard lines same deal as yours
even left the gas cap off

thinking of one of two solutions:

- install a cool can no other way to get around it short of EFI and thats one thing Idont want in an older car.

-install a solenoid,that when it happens will bump the idle up some. A/c cars use it I believe.

If I use some VP or the like it never happens. 190 deg and the rides over far as Im concerned. Anything under and it will idle all day long with a cam that should be according the internet loading up constantly.

boseone

Did you checked heat riser on exhaust manifold? I had the same symptoms on my 59´oldsmobile, heat riser was rusted in closed postition...

MelWff

did the stock 70 LS5 have a fuel return line either from the gas filter or the pump and if so is it still there?
have you attempted to adjust the idle mixture screws when fully warmed up, if an automatic transmission in Drive or Reverse with the wheels chocked?

ajrothm

We had this exact problem on my dad's freshly restored 66' 327/350hp vert... Chased my tail for months... turned out that the reproduction coil for the TI system was the issue. It was a good quality one from Lectric Limited, however, after 30 mins of driving it would want to die, miss, sputter etc...

They sent us another one, and it did the exact same thing.

I rolled the dice and bought one from another vendor, it instantly worked great and no more issues..

So definitely try swapping the coil with a KNOWN GOOD ONE....

Brcmpbl

My 70 L-46 behaved like this for a time, and I did lots of things trying to sort it out. In the end, I'm nearly certain that what I had was a combination of not enough initial timing and a vacuum leak. I had the timing set at 4* advanced initial, and that wasn't nearly enough (I have 10* initial now without vacuum), and I had a series of small vacuum leaks from an improperly torqued intake manifold and a couple of older vacuum hoses. The problem was always worse after the car got very warm, such as after coming off the highway.

Does this help you? I don't know, but this was the cause of my problems.

Iceaxe

Because the problem only happens when the engine is hot I'm betting it's a gas issue. You are probably boiling the fuel in the metal fuel line between the carburetor and the fuel pump. This is a pretty common problem with classic cars. The gas we have these days is nothing like the gas we had 40 years ago.

OMF

This is an interesting problem....I just reread the entire thread.
When you rev the engine to clear it, is it rich and there black smoke out of the exhaust, or is it starving for fuel and just missing due to being lean? Answer this and it will give us a better direction for advise.

7T1vette

If you are using common fuel with ethanol in it, I suggest that you look for some fuel source without it. Ethanol has a lower boiling point than gasoline and has caused these types of problems.

Putting a front air dam on the car will push a LOT more cool air through your radiator and may also solve or help to solve your problem.

If you have 'no ethanol' fuel and have installed the air dam, you might want to install a metal heat shield under the carburetor (along with the gaskets). That can help the carb stay cooler.