The Stovebolt Site Forums The Shop Area Engine Shop Missfiring when accelerating from slow.

1969 Chevy Stepside 250 inline 6.
1st, engine running perfect except I could hear tic, tic, like an exhaust leak. Could not find any leak. I decided it was time for new points and condenser and rotar. Plugs were pulled first, and they were burning picture perfect, even though points were not that good.

Here the problem started. Immediately after changing points, rotar, and condenser, the engine would idle fine, but "miss" extremely when accelerating. As soon as I get up to speed it runs smoothly except when accelerating. I pulled the plugs and they again are burning picture perfect. I put the old points and rotar back in, but no success. I put a new fuel filter inline, I put the new points and rotar back in, I tried changing fuel/air setting on the carb to various ratios without success.

I'm beginning to suspect a fuel issue. Anybody have any thoughts on this?

Classic symptoms of a bad condenser.

Reinstall your old condenser, then don't touch it when you replace points in the future. Most new condensers are made off-shore with questionable Q.C. Often fail fresh out of the box, or shortly afterwards.

Lonnie

Classic symptoms of bad plug wires, especially if they're the radio frequency suppression type. Check the resistance of the "wires"- - -they should not have more than 1,000 ohms per inch. 12" wire- - - -12,000 ohms max, etc. Those wires are notorious for getting open-circuited spots that "grow" as the spark fires inside the insulation. I consider 30-50K miles to be the maximum lifespan for RF resistance wires.
Jerry

Since the problem began after you changed the points. Check the plate in the dist. that the points screw down to. Check the vacum advance to make sure it is functioning properly. You will find the answer to the problem when you go back and check what you did, something is not right. The advance connects to the plate the points are on. Sometimes if you have to exert a lot of force to loosen the screws holding the points or condenser, there is not a lot holding the plate and the advance together, they may have come loose.

Thanks, I will check and try these things and let you know.

Okay, plug wires...resistance for 10" wire is about 470 ohms, 12" about 500 ohms, etc.
Truck only used about 500 miles per year.

I put the old condenser back in and will test that tomorrow and vacuum advance.

Had your exact same problem twice. I agree with above and I had a bad condenser once. It killed the engine period. First time...start up fine and such but missed only on bottom end. Found the rubber vacuum hose to the vacuum canister was split. Second time....darn exact thing. Well, I had added a ballast resistor due to someone chopping the resistance wire to ignition. Must have missed a lug crimp somehow (it was also soldered) on the downstream side of the resistor. Missed then quit and there it was, no connection. Wire hanging in air. Easy to spot though.

Could be a simple loose wire at the points, when you accelerate, the vacuum advance backs off moving the plate, when you get up to speed, the plate returns and wire is back in good contact. At idle the plate is in the same position as when at speed.

Okay, to update...putting old condenser in didn't solve it. New vacuum hose, no change. Point wires are good. Has to be something with the advance. What do I look at to see that it's working? should it move down by the vacuum where it goes into the distributor when I accelerate the engine?

Also, what about the spring loaded toggle down by the manifold? My spring there has been broken for years, but never was a problem. And where is that tick, tick, tick coming from, like a manifold leak, yet cannot locate any leak. Have to be going about 20 mph at least to hear it.

The whole distributor housing moves with vacuum, not the breaker plate inside the distributor like most others. Check to see if the wire from the distributor to the coil has any broken strands, especially at the connectors. If it's got breaks inside the insulation it can cause an intermittent open circuit.

Also shake the distributor cam side to side. If there's a visible change in the point gap you can get rough running and/or an intermittent miss.
Jerry

Jerry, this is a 250 in engine.

The spring on the manifold heat plate is help warm up the carb and manifold in cold to cool weather. Once the engine is warm, it's relaxed, it's only working when the engine is cold. It's should be fixed for better start ups, but for now, most likely not the problem.

The advance pulls the rod and plate towards the vacuum pot, you should be able to suck on the hose and see it move. The hose could be direct manifold vacuum or ported vacuum. Direct manifold vacuum will pull the advance in as soon as the engine starts, ported vacuum will pull the advance once the throttle blade is opened. Look to see where the hose hooks to, above the throttle blade or below.

OOPS! I missed that little tidbit of info! He could try disconnecting and plugging the vacuum line and do a test drive to see if the miss/stumble goes away. That would eliminate movement of the breaker plate. Checking for wobble in the shaft and rotor would still apply.
Jerry

On this distributor the vacuum advance moves the plate inside the distributor. I found an old distributor with a vacuum advance and hooked it to the hose, and it works, moves the linkage. Hook the current va up and no movement when accelerating. So, I would say the current va has failed. Apparently to replace the va you have to take the dist housing out to replace the va. I wonder, can you buy just a va new for this dist.? Or, I could just use the old va I found. The linkage appears the same, but the dist is from an older truck.

Check when your changed the points if there was a separate spring that clips on when I did mine I thought because there was a copper spring already attached and a metal spring in the box that it was for several applications. So I installed the points and had the truck running fine at idle and when I accelerated the truck would backfire and cough. Only when I put the spring steel on did it run proper as the copper piece was not putting enough pressure on the lobe of the distributer shafts to open and close properly

Good information. I know the spring has been broken since I've had the truck (6 or 7 years), and the truck starts really easy. I'm only here in the summer and it's always garaged, so it's always warm...but I understand what you mean about a cold engine. At least this, then, has nothing to do with the problem.

I have determined that the vacuum advance is not working, so, somehow I've got to replace it.

Thanks

The advance has two screws holding it in place. Pull them and wiggle the linkage out from under the plate. You should be able to change it in place.

There are different advance units, each is marked with a number 8, 10, 16, 14, etc... The stamped number is the amount of degrees it will pull in under full vacuum. If you with a couple of numbers, it should work.
http://www.corvette-restoration.com/resources/technical_papers/Vac_Adv_Spec.pdf Here is more then you ever wanted to know about advance units.

Thanks Joe;
I got it off, cleaned it, took pictures. Couldn't see a number. Now to find one. Try auto parts stores, I guess.
The other VA on an older dist can't be made to fit.

Found the VA at O'Reilly's. Be here Tuesday. I'll let you all know for sure that it was the solution.

I got and installed the new vacuum advance. It's much much better, but still have intermittent miss when accelerating. I re-gapped the points to .019 as they recommend for new points. I' will be testing that this afternoon.

I had a crack in my dizzy cap that caused similar type problem..

I believe someone mentioned this already. Check the wire that runs through the distributor body to the coil, it goes through a grommet on the side of the distributor. It way be broken, as it moves with the advance plate that the points are on. It has stranded wire, some of them may be broken.

A little better, but still get a low rpm stutter when accelerating. Thinking about the dwell, see if I can adjust that.

watch engine at dark and see if you can see any wires leaking..

I think I remember a trick, was to spray the wires with a squirt bottle while running to find a bad ign wire. Jeffrey

what would be in the squirt bottle

Water with a small amount of G.M. Opti-kleen was what the old timer did when I started in 1980. Great trick was to richen the mix of washer fluid on someones' squirt bottle & watch the show. Older & wiser me thinks flammable mist not such a good idea.

I like to take a insulated long screwdriver, attach a jumper lead to it & a good engine ground & move around the wires to find secondary voltage leaks.

I can't tonight, my cat is asleep on the hood.

Water.

Wish I still had a bottle of opti-kleen to try that stuff. Jeffrey

Okay, lights out, hood up, start up, and sure enough there was a spark down by the 2nd plug from the cowl. Found out it's a cracked plug. I replaced it and runs smooth now.

So in the end it was 2 problems, broken plug and non functioning vacuum advance.

Thanks everyone for your help and in keeping me persistent.

I bet that if you undo the vacuum advance repair there would be no misfire. Jeffrey

Yes better you found and repaired the vacuum advance.
Jeffrey

Interesting. Wouldn't there be misfire without the vacuum advance due to no dwell time? I'll try it sometime, just to see what happens. If that's the case, it could be that the old VA has not been working for some time. I know that it has never idled as smooth as it does now.

When the throttle is opened vacuum drops in the manifold so the source of vacuum to operate the advance unit is gone and so the timing is retarded to base timing.

The car accelerates faster with base timing. Base timing/retarded timing yields power.

Advance timing fuel economy low emissions.



Dwell is the amount of time the points are closed.

Vacuum advance changes when the points open in relationship to where the piston is reaching the end of the compression stroke.

As timing is retarded the spark happens with the piston closer to the end of the compression stroke/TDC.

As the timing is advance the spark takes place with the piston farther away from reaching the end of the compression stroke/TDC.

Whether there is spark advance or no spark advance the points are still going to open and fire the plugs.

When a vacuum advance unit does not work it is usually due to the vacuum advance unit having a vacuum leak. Engine idle quality will always be effected by a vacuum leak. Jeffrey

Very educational, appreciate the information Jeffrey.

and thank you for getting back, a lot of folks never do