Thread: Engine running trouble with Chevy 350

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Hello all. I am a new comer here in search for some assistance. I have a 67 Nova SS with a 355ci, fenderwell Hooker headers, Holley 650 double pumper, Edelbrock Performer intake, Fuelie 2.02 heads, TRW flat tops and a Vertex Magneto. The car is kept mostly in storage, sometimes for months at a time, and has seemed to develop a new problem. I noticed that the engine was running a little rough and figured that it was misfiring from a set of fouled plugs or something. So I changed the plugs, although they looked fine. Needless to say the problem still exists. I figured that maybe it might be the carb. The carb is old and beat and needs replacing regardless, but I haven't had the chance to do that yet. The more I drive it the more I think that maybe it's not the carb but the Vertex Magneto that’s causing the trouble. The car appears to be misfiring at all times and even produces some small backfiring with normal driving behavior. It's at its worst around 2500-3000 RPM where it shakes the car vigorously and almost seems to fall on its face. My tach is also going synchronously insane at this point. Once I am passed 3000 RPM it returns to its "normal" crappiness. After sitting for a day or so I'll take it out and it appears to run better and even seems to improve the more I drive it, but once it sits for a few hours after that, it REALLY runs like crap. Is it possible that something in the Magneto could be causing this or do you think that I have a completely different issue altogether? The car sat for year in a fairly damp warehouse before this problem began. Any thoughts would be appreciated. Thanks.

did you try a deferent dist like a hei? or is the carb a holley with a bad power valve and bad gas ?

Right, 1'st thing ! Is that tach wire in any way hooked up the magneto grounding post ? That has the black bakelite screw sticking out the side of the magneto. It is only to be used as a grounding wire. NO OTHER WRES MUST BE ATTACHED TO IT !!!! AS IT WILL BURN OUT THE MAGNETO.

Because what your describing is that the magneto is overheating and is beginning to break down. The grounding post wire should go through a toggle switch 10 amp in the wire and it should go straight to ground flicking the switch to close turns off your engine. For your speedo you need a seperate magnetic pickup, they are available from Magsaurus. You will probably need to get your mag checked out for damage.

Pat-
I have not tried another distributor yet. I dont have one to try at the moment. I'll see if I can get my hands on one. I want to be sure that the mag is the issue before I buy a new distributor or choose to have the mag repaired. What would you recommend for a replacement distributor?

As far as the gas goes, I filled up with a fresh tank of gas and still have the issue. The power valve could be shot at this point but these symptoms seem to be pointing towards someting else.

Southerner-
My kill switch is the only thing connected to grounding post on the mag.

If it's an older DP and the engine has backfired you can almost be sure that the power vavle is blown. www.summit.com sells an inexpensive kit to solve that problem.

So do you feel that the power valve is the issue or do you think that its more involved than that?

Did you check the wiring up to your mag, did it have 1 wire coming of that P post or several ?

i would look at putting in a no blow back check valve in the base plate like said . put a new power valve in it or two if it as one in the rear then you need two kits for this carb i would run the msd if you are looking for a new dist if you want to keep the mag any dist will work to see if it is the mag is going south make sure if it is a roller cam engine that any dist you use must have a bronze gear on it and if the power valve is bad it will empty the bowls of the carb to check this take out the sight plug and it will be empty

The only wire connected to the post on the mag is the kill switch.

The carb has clear fuel bowl sight plugs. I usually rock the front end to check the fuel level. I did this recently when I was adjusting the fuel mixture on the carb and there was definitley fuel in the bowl.

Any other ideas? Any idea what component inside the mag could be causing the issue. I know that an ignition module will start to break up at hot tempatures once they go bad, but I am not sure what to look for in the mag. Is this mag even worth fixing if it is indeed the problem? Will I get better performance from an MSD?

Thanks for your help thus far!

Okay, you have looked at the mag and verified that it has one wire coming off the p post. Next, now this may sound a bit weird, remove the grounding wire from the p post and wire a tempory wire and toggle switch onto the p post. and ground the wire downend from the p post to the body or engine. what you are doing is checking that there is no phantom wiring coming off that ground wire further up the loom. Try that and see what your results are. S

Yep mags are 100% reliable they last for years if treated right. They are a self contained generating plant that operate on AC voltatage. So those platinum points will probably outlast the life on the engine. Since the power is AC there is no metal to metal transfer on the points.They were first designed for airplanes when the engineers understood what would happen to the pilot if the old kettering ignition system should fail !!!!!

Wouldn't that be eliminated by removing the wire from the post altogether? At one point I did remove the wire from the post but I still had the issue. I just had to reconnect it when I was ready to kill the motor.

At this stage it is probably not the magneto then. Notice that I said at this stage. THe next thing to consider what type of rev counter you have is it electronic or manual, with a cable comong out the back and going down to a drive gear. If the rev counter is electronic or electric, where does it get it's signal sorce ? This will mean tracing it back through the wiring. So the easy way to do this is to:

1 check what kind of rev counter it is

2 If electronic see if the maneto has an electronic pickup mounted on the side of the magneto

3 If it does not have an electronic pickup then you will have to search through the wiring to see where the signal sorce is.

I am using a magneto signal converter. It has been hooked up this way from the day one. Here is a diagram showing exactly how I have it connected. https://www.autometer.com/download_instruction/753A.pdf

This issue is so strange. Now you got me thinking that its the carb again. I need to replace it anyway. I am thinking about installing the Edelbrock (Carter AFB type) carb, not too sure what size yet, maybe 750cfm. I might even get rid of the mag too. Maybe an HEI or MSD to replace it. I would eventually like to install the Edelbrock top end performance kit so I wanna make sure that I purchase the right carb as it is not included in the kit.

750 is too big unless you have the cam and heads to support it. Put a new power valve in the DP and a blow out protection kit along with it and it'll probably solve your problems or put a 750 Eddy on it and create more.