Miss at idle 72 350 SLC
 

I have a miss at idle on my 350 SLC. I replaced most of the smaller vacuum hoses, but not the plastic lines. I also used carb and choke cleaner to spray around all the vacuum lines. No affect. I did not find a leak. I plugged the main vacuum T at the back of the intake. No affect.

I just bought a new fuel filter, but have not replaced it yet. Could this be a bad injector. Is there a way to test each of the injectors, or should I just remove the spark plugs one by one and read them. I'm sure if the miss is just on one cylinder, the offending cylinder would show this with a plug that stands out.

This is the D-jet fuel system I believe. Any suggestions?

Steve

miss at idle
 

If you have not found problem yet, here are some suggestions

plug wires
plugs, can leak after awhile, or develop a hair line break, check engine in the dark and see if any signs of spark around plugs or wires and cap, I have found Bosch plugs does this more that others
hair line break in dist cap
plug wires not seated completely
fuel problems would tend to be a lop in idle, not a miss

I love my old 72 350 SLC, have gone through a lot of changes, issues, and accomplishments with her, I would be glad to share my developments with you

Tim
Woodstock, GA
99,000
Zender production 72 model

Sounds like the problem is in the ignition system. Start with rotor cap and work your way to each of the plugs.

A single (or multiple) cylinder misfire should first have the cylinder/s identified. This is done with a cylinder balance test. If you were a well equipted shop you would use your engine analyser to do it for you. It would kill one cylinder at a time by shorting the primary. It would give a reading of rpm drop per event and if a real well equipted machine, it will also give the hydro carbon increase per cylinder by matching up the exhaust gas readings to the cyl shut down one at a time. A misfire will be the cylinder that drops the least when turned off.

The hydrocarbon readout evaluates the balance of fuel flow through the injectors. A cylinder with a clogged or malfuntioning injector will have no hydrocarbon increase when the ignition is stopped. A good cylinder will cause a large amount of hydrocarbon (gas) to exit if the spark is shut off (it goes into the exhaust because it wasn't burned).

If you are out in the woods, one must inovate. The same basic test can be done by pulling spark plug wires one at a time, this can also curl your toenails. It helps to pull the wire at the dist end rather than the plug end, but if you find an open wire (that is one not properly shorted by the sparkplug attached to the other end) then you will be the best ground for 25-30kv.

The D-jet cars can be done by pulling the injector leads and I have done it that way in a pinch, but the harnesses are often so fragile by this time that I would advise caution.

Later cars with idle control require real smart machines that can repetively stop only a precise number of ignition strokes to evaluate rpm differences. This is where your hip comes in. Place your hip against the fender and evaluate the misfire as each cylinder is shut down. The speed won't change but the misfire vibration level can be felt if the difference is one cylinder at a time.

If you have a ton of miles on that sled, a worn distributor shaft can give you a rough idle and excellent performance otherwise. I've seen it.

Peter