Hi All,

This is my first time asking a question on the C-3 fourm.  I purshased a 1970 convertible about 2 years ago.  The engine had already been rebuilt to stock specs (By a well known racer in the Atlanta area).  If the car sits for a while it starts and runs like a champ.  After about it warms up all the way the problem starts.  If I'm running down the road she runs fine.  If I stop at a light,  slow down under 5 mph, or have to stop quick, the following happens.

This problem first started shortly after I changed the stock ignition to an electronic ignition with the ACCEL conversion kit.  Everything was fine for about 200 miles, then the engine started acting up.  It acted like it was loading up on gas.  Since, then I've had the distributor rebuilt through I beleive Mid-America (Using the same ACCEL kit).  The brass dist shaft bushing was getting egg shaped, and the shaft did not turn freely at all (I had to use pliers tio turn it).  They did put the original vaccum advance back on.  I checed it and it starts to move at 8 lbs of vaccum and is wide open at 16 lbs.

Here's what happens now.  As I start to accelerate the cars acts like it's loading up on gas.  Sometimes it will almost die.  Other times it will slowly die, then actually run backwarsds for a few seconds.  I know that a bad thing, so I try to time the lights, etc. to keep it in a moving situation.  I purchased a spread bore replacement through one the the Corvette parts houses.  It's doing the same thing with that one too.  Sometimes as I get up to about 3 mph, the engines seems to miss (Almost like a plug wire is pulled off), then a few seconds later, runs fine.  I don't think it's electrical since it's not acting up all the time.  Is there something in the TCM system that could be causing this?  It's currently disconnected.  I am close to a straight jacket at this point. 

Where is your timing set, and what is it at just off idle?  It sounds like too much advance, even if all is working correctly.  Do you have a ported vacuum to the dist, or manifold vacuum?  

What about the ignition coil?  What brand/type?  Which plug wires and plugs? 

It sounds like a ignition problem in the fact that it tends to turn backwards.  A performance coil may overpower ignition systems and could cause unwanted flashover, sending spark to the wrong place.

A few more details please.

Hi Ken,

Thanks for the reply.  The timing is set at 8 degrees BTDC with the vaccum line disconnected and plugged, and the engine idleing at about 800 rpm.  When the vaccum hose is put back onto the advance on the dist. it brings the timing to approx 32 degrees.

When I changed the distributor over to the electronic kit from Accel they reccomedned a new 12 volt coil that did not require the balast resistor.  I bought the Accel model they suggested for that particuler electronic ignition kit application.  The plugs (AC Delco R45) and plug wires are the original style.

The vaccum advance hose is running to the carb, to a port that has full vaccum.

Dan

That coil can overpower the original type wires.  The resulting arcing can cause a lot of problems.  The moisture in the AM can contribute to that problem.  You may want to try some high performance wires.

And 32 degrees at idle is way too much.  The vacuum line to the distrubutor should be a ported vacuum.  That's to say there should be no vacuum on the line until the throttle is opened up a bit.  You do not want full manifold vacuum at idle. 

Try finding a correct vacuum source first.  Then you may need to go to the wires as well.

Hi Ken,

I appreciate the information.  I'll try the vaccum first.  I'm thinking I did upgrade the wires on the 70 when I did the conversion.  I have three C-3's that keep me on my toes.  I'll let you know how things turn out. 

Thanks again,

Dan

Hi Ken,

I have a follow up for you.  I changed the vacumn advance hose to a port that wasn't "Full vaccum" all the time.  Then, I adjusted the timing.  The problem is all but gone now.  I get a little pinging on ocassion, but the motor stopped acting up.  I think I'm a few minor adjustments away from being good to go.  I appreciate your knowledge and help through the process.  Maybe we'll meet up in Pigeon Forge.

Thanks again,

Dan

I dont' know if I will make it to Pigeon Forge.  I have my severe doubts but I haven't given up on it yet.  Perhaps I will if all goes well.

The pinging is a result of too much heat in the cylinder when it fires.  Over 2500 degress will make the ping.

This is a result of several things.  Too much timing advance.   Engine too hot.  Too low of octane fuel.  Carbon in the cylinder.

The EGR valve is designed to stop pinging.  Exhaust gas has burned, it cannot burn again, so as far as the engine is concerned it's inert gas.  No oxygen, no fuel.   By adding EGR gas, the mixture in the cylinder is diluted without adding air or fuel, so the mixture percentage does not change, but does get diluted.  The result is a cooler burn, and no ping.

So in addition to the above items, check the EGR operation as well.

Hi Ken,

Thanks for the follow up.  It's great information.  I've been adding an octane booster to the fuel.  I didn't on the last couple of tanks,  That may be it.  I also used one of those heat guns to check the temps at the thermostat, radiator (In and out), along with spots on the intake manifold.  That seems to be good. 

I'll double check the things you mentioned.  Thanks again for everything. 

Dan