Strange Engine Dance

[angry planet]

I have a 68 Vette with a weird problem. When I drive it for a long time (more than an hour or so), it periodically develops a weird engine dance. It rhythmically bogs down and loses power about once a second and returns to normal.

One time, whenever it tried to start its rhythm, I was able to back off the accelerator for a several seconds and it went away. Another time, it increased in amplitude to the point where the engine stalled. After about thirty minutes of letting it sit, it was able to restart and drive just fine, as if it had been flooded.

I took it to a mechanic, but obviously they couldn't get the problem to show itself and couldn't give me any suggestions.

I'm afraid to take it onto the street because I don't want to get stranded somewhere.

Any ideas / suggestions / moral support?

Thanks!

[bill81vette]

there only 2 possibilities,fuel or spark...
i wonder if you vette is vapor locking....
there is a lot of heat in the engine compartment....

[navy2kcoupe]

Maybe, just to have a baseline to go by, change points, condenser, rotor, cap, and wires,
if your car still has them and hasn't been upgraded to a newer system.
That REALLY sounds like a condenser going out on you, but it could also be that the coil
is starting to act up when it gets hot as well. Good luck with it! I know how hard it is to
find these problems when they occur only sporadically.
Andy

[lars]

You have a fuel filter that's starting to plug up. Replace the filter.

Lars

[Tom Bryant]

I would try the fuel filter first too.

Tom

[angry planet]

Thanks for the advice everyone... I'll give the fuel filter a change.

[canomjm1a1]

Cheap stuff, the dumb stuff!

[angry planet]

I hope so... haven't had a chance to deal with it yet, but I hope so

[TimAT]

Right after the fuel filter, take a look a the fuel tank vent and the hose from the frame to the tank and the one from the frame to the fuel pump..

[GerryLP]

Angry planet,

Your car is either vapor locking, or is boiling over the fuel in the carb.

Now, let me suggest the quicky verification without getting into a long explanation. Next time you drive the vette, loosen the gas tank cap, and see if it does it again. If it runs for ever without simptoms, then there may be a venting problem in your gas tank.

And if the above does not fix the problem, check the carb fuel [front] tank's level (I presume it is a Holley carb... ). If you have plugs on the tanks, back off the screw. If you have the transparent acrylic plugs, then carefully push on the Vette from side to side, to make the fuel splash against the acrylic plugs. This should give you an idea if the float is too high or too low. If you can see the fuel level just at the plug's hole bottom edge, then you are GTG. If it is way lower, then your float is set too low. The acrylic plugs are harder to tell how low the fuel level is. So if you have to get the Vette rocking pretty hard to see the fuel splash against acrylic plug, then that means that the fuel level is too low, and so the float is set too low.

There is a slight possibility that when you open the tank plug, you'll get the excess fuel running out (which it would mean that the float is set too high). If so, then hard turns and sudden stops will also make your vette run like you have been explaining or even make the engine stall.

There could be other reasons, but these are quick ways to check for a mis-adjusted float or tank fume venting problems.

GerryLP

[lars]

1. A '68 Vette has a Q-Jet Carb.
2. If it's been modified with a Holley, a misadjusted float level won't cause the intermittent problem described.
3. A venting problem, or any other fuel system delivery problem, could, fact, cause the problems. If the fuel filter has been changed, or verified to be good, might want to check to see if an aftermarket pump has been installled, eliminating the fuel return feature.

Lars