The Stovebolt Site Forums The Shop Area Engine Shop troubleshooting ..misfire and so on.

Hey everybody..

Well, my old truck is having a little trouble. It's a '65 C-10 with a stock 350 GM crate motor that's probably 10 years old or so at least. It's tight and does fine.

It has a quadra-jet,headers,point and condenser distributer and a Moroso air cleaner on it. Well, I'm telling all this to give you an idea of my engine setup.
Ok, now to the problem. The truck starts fine cold with a little pumping on the gas pedal when cold and idles smoothly and starts right up when warm. Lately it has gotten to the point that when I accelerate and wind the engine around 3000 and even less now, the engine feels like it's misfiring some and kind of stutters with just a little pedal into it and it gets worse as I wind it out some more,but if I stay out of it and drive kind of normal it does ok if I don't wind it out much. It used to be better ,but it has slowly gotten worse. If I punch it and open the secondaries it sort of blubbers along with a lot of stumbling and misfires on what sounds like all cylinders. I hope I described that in a way that you can get an idea of what I'm dealing with. And with most q-jets..it's gets pretty good gas mileage if I keep the revs low and my foot out of it.

I pulled a spark plug and it's burning with a nice tan color with a normal gap and little to no wear on the electrode and so on. The fuel filter is fine..the gas cap is vented well. The plug wires look ok physically and they are a nicer set of yellow wires with good boots and clean connections. I started it up with the hood up on a very dark night and revved it up..saw no arcing or halos.

The points look ok,but I haven't crawled up in the engine bay to sit and took a real close look at them, but they look ok with a lot of contact area.
The distributer cap doesn't appear to have any cracks and the contacts are pretty clean. The q-jet has two vacuum hoses that have been blocked off , but I think it was fine like that for quite a while. I snugged the carb down on the intake manifold and it's still the same.

So, I'm thinking..maybe a weak ignition coil? ..

I think the fuel pump is ok..

Does my bumbling,misfiring-stumbling when the revs start to go up a bit sound like a weak coil?

Tell me what you think.

I sure appreciate the read and any suggestions.

Marc

sounds like weak spark. I would start with replacing points and condensor as they are cheep. check timming. the critical thing is to not assume something is ok cause if you skip over somthing simple you will wind up chasing your tail

Agreed. You might check ALL the plugs.
Carb cleaner sprayed around gasket areas will reveal vacuum leaks. I dunno...I still remember guys calling them "quadra-junks"...I've never had one. But if your ignition is up to snuff, I'd suspect a carb issue.
Good luck.

cm

Make sure the points are properly set. If the points are either too close or too wide, you can have ignition mis-fires as you increase the engine speed, but nothing will show up on the spark plugs or points.

Also, check the ignition current if you have an ammeter. You should get about 4 amps of current with the key on, points closed, engine stopped. If you don't have an ammeter, measure the voltage at the coil positive terminal to ground, key on, points closed, engine stopped. You should find about 6 volts there. A little variation, a few tenths of a volt, above or below is not critical. Low voltage at that point indicates low current or high resistance in the primary ignition circuit.

Also, make sure you don't have a coil with an internal resistor connected to the ballast resistor. Measure the resistance between the coil primary terminals. You should find about 1.5 ohms, a little above or below is not critical, but if you measure something around 3 ohms, you likely have a resistor coil, which you probably shouldn't have. I believe your truck has a ballast resistor wire layed in the wire harness. If your truck has the harness resistor, one of the wires connected to the coil positive terminal will be stiffer than the other, and originally it had a white fiber glass shroud around black insulation on the wire. In most cases the fiber glass shroud is missing for an inch or so, so you may only find the black insulation and the stiff wire, which is the nichrome resistor wire.

You could have carburetor trouble, as well, but one thing at a time at this point.

Harvester

After you check out the ign. system, you can check out the flow rate on the pump, (disconnect the ign. system, unplug the coil, etc.) put a fuel hose on the line coming from the pump to the carb, stick it in a quart jar. See how much comes out when you crank it over. Also, you said you think the fuel filter is ok, do you have 1 in the carburator, as well as before it ? (Just incase , I know many people don't know of the 1 in the inlet on those carbs...)

>>The points look ok,<<

Get a dwell meter. There is an allen screw inside the window on the cap. Turn that until the meter reads 30 degrees.
Then set the timing and for that you need a timing light.

Worry about any of that other stuff if needed, after you know the dwell and timing are correct.

One word... CONDENSER. I have seen bad ones on countless occasions causing the exact symptoms on many different older engines.

Install a "unipoint" set that has a condenser built in, and set the dwell to 32 degrees. If you're going to do a lot of high-rpm running, swipe the spring off an old point set, and use it to double-spring the new points. We routinely ran point-ignition engines to 6,000 RPM or so without spark scatter back in the dark ages before electronic ignition by doubling up on the point spring. The rubbing block will wear a little quicker, so check and adjust the dwell frequently.
Jerry