The Stovebolt Site Forums The Shop Area Engine Shop Intermittent miss

Hi Guys, I have a 55 2nd series 3100 with a '67 Pontiac 400 in it. Engine starts, runs, drives fine. At about 2500rpm a slight, intermittent, miss starts. I pulled the plugs and they look good. The engine has a slight lope at idle, and then smoothes out till it gets to 2500. Ignition is stock Pontiac with the distributor with the little door. I have a new tuneup kit waiting to install, cap, rotor, points and new wire set. 2500 is about 50 mph in my truck, that's about as fast as I ever drive it, so the miss in not apparent when on the road. It has been doing this ever since I got it back in August, has not changed or abated one lick! Any ideas? Distributor shaft wobble?

Check the top distributor shaft bushing for wear. if you can push the cam side to side and change the point gap, replace the distributor. Every 1 degree change in point dwell from a gap change causes a 2 degree difference in spark timing at the crankshaft.
Jerry

Just to eliminate wires and switches, hot wire the coil right off the battery. I had a friend chase a miss in his race car for two years, turned out to be a faulty ignition switch.

Besides the top bushing, check the drive gear on the distributor if you are willing to pull it out, Pontiac's tend to wear the gear teeth if a non-stock oil pump has been added. I have seen them with 2/3 of each tooth wore off when running the high pressure oil pumps.

Start with the basic's, points, rotor, and cap. You can measure the resistance in the wire if you choose not to replace them, but if they look bad, just swap them as well. You might want to swap a different brand of spark plug to even if they do look good.

Joe

Well, you know the old saying "I'd rather be lucky than good" When I said I had checked the plugs, I had only pulled a couple and they looked OK.....Went out last eve and pulled all of them, everything OK till the last one in the back drivers side, hardest one to get at. Sure as heck it had a slight whisker, cleaned it off, and miss is gone! All plugs had a nice light brown center electrode, but outer ring lots of carbon and black. Guess the big poncho isn't too happy with my farmer cruising speeds! Need to blow the carbon out once and a while! Hard to believe that the whisker would last that long. Thanks guys for your help, got lucky on this one.

What is a slight whisker? (He asked politely.)

Sometimes a little piece of hard carbon can get stuck between the electrodes of a sparkplug. Dirt entering the combustion chamber can cause one to form also. Fairly common on yard equipment, chain saws, and motorcycles used in very dusty, dirty conditions. Especially when operated without air cleaners. Not so common in automobiles, but never say never..... I think my babying the big 400 probably caused it. Mixture might be a little too rich as well.

I had a lady customer who drove a big Pontiac Bonneville with a 389 engine, two blocks to work, and back home, once a day. It needed a tuneup every 6 months, and she hadn't driven it over a thousand miles each time. I finally started taking it 10 miles up the road at wide open throttle every time it started running bad- - - -60 MPH top end outbound, and 120 on the way back! She wouldn't drive it at highway speed, but she was perfectly happy to let me blow the cobwebs out every few months.
Jerry

When I worked for vw the v6 passats had insane carbon buildup. We would road test them and drive like we stole them. They would barely run it got so bad. I take my moms car and run it ragged every other month also.

Couple questions fellas. If the whisker is causing the miss, and the whisker is permanent, why is the miss intermittent? Also, I've heard these stories of carbon buildup. What exactly causes it? Internal combustion engines emit CO, so if run extensively at low rpm's is there not enough exhaust velocity to expel the heavier carbon molecules, so they build up in the heads/exhaust manifold, and even the cylinder based on the whisker? Has this been alleviated in modern vehicles?

Best I can say, is that the little piece of carbon impairs the spark somehow. It did not seem to stop the spark from occuring altogether, just the spark is not correct. Maybe the carbon moves around, why it stays there I can't say. I have had experience with whiskered plugs in desert racing motorcycles, and RC airplanes in the past. We generally do not use air cleaners on our models.

Carbon build up is mainly from heat, or lack of it. By running the engine hard, you get lots more heat in the cylinders, which in turn burns the carbon deposits into ash allowing them to exit the cylinder. If this engine was leaned out a bit, the build up would probably go away since lean-burn engines run hot. A hotter range plug would also help, not so much on how it runs, but just keeping the plugs cleaner. Even a hotter thermostat can help if the engine never really get hot putting around town.

The miss can come from carbon heating up and acting like a glow plug, lighting off the mixture before its suppose to fire. Ping or detonation is a common problem with carbon build up.

Joe

Another side effect of carbon accumulation is increased compression pressure. If an engine is running close to the pressure that would require a higher grade of fuel, say 9.5:1 or so for a regular-gas engine, a couple of tablespoons of carbon accumulation in the combustion chamber will boost the compression enough to get some detonation. A few cylinders trying to fire too soon and drive the piston backwards will definitely affect performance.
Jerry

A bad plug wire or one begiiningbto fail can cause plug fouling as well. I have gone to ahotter plug and better set of wires to keep from fouling plugs in my 235.going strong for 30, 000miles! Notice you said you had wires to possibly but in,should go ahead and do that.

what kind of ratio in the rear end, 50 mph at 2,500 seems odd
for a four speed........just curious.......I had a 2:73 in my hot rod and at 2,500 rpm I would have been doing probably 70 plus with the 350 engine

I have the stock 3.90 rear end in my pickup, and the 4 speed is a Muncie with 1.0 ratio in fourth. I put 29in dia. tires in back to give a little more speed versis RPMs. Still winds up pretty good.....