Back story... Cadillac engine swap into '55 chevy pickup. I was having a number of issues to sort out including tuning tripower carbs, fuel pressure, and et cetera. At the same time, I had a battery discharge issue. This was not as important to me as getting it running reliably, I thought I'd figure that out after. The problem... Misfire under load when driving/accelerating. Engine idles well and does not stumble when not under load. Timing light does not flash or flashes intermittently, two lights were tried. The tries to fix. Three different coils. Two different distributors (one dual points fresh rebuilt by Bubba, one known good pertronix stock distributor). Three sets of plugs (first set ruined by really rich conditions when tuning the carbs, third set just to be sure, lol). Two sets of new plug wires, one for points, one for the pertronix. After and during all this, there was still a stumble/misfire under load. I was talking to my dad, an electrical engineer, yesterday. I mentioned my problems and he suggested taking care of the battery discharge before trying anything else. I had my generator chromed. Turns out it wasn't polarized and was putting out zero volts with the engine running. So I figured that out. I polarized the generator and not have approximately 14 volts at idle. The battery is low from the before condition of the generator but is on a charger to fully recharge before continuing. With all that said, could the misfire be caused by the low battery and/or not being recharged by the generator? My theory is that when the generator was not charging, and the battery was lower than normal, that it provided enough spark to idle but not enough under load causing a misfire and intermittent timing light function. Am I on the right track????? I will know tomorrow for sure when the battery is fully charged and doesn't discharge with the generator now working if the low voltage was the cause of the misfire/timing light not flashing. With the battery fully charged and the generator working correctly, the misfire will either be gone as that was the fix, or it won't and there will be a short in the wiring somewhere (don't think this is the case as the harness is less than two years old and ran the old engine great and all connections were checked for proper routing). Just wondering your thoughts on my theory and/or if any of you guys have experienced something similar. Thanks in advance!!! And thanks for the HAMB!!!!!
The PerTronix distributor, with the correct coil, will spark fine down to around 5 or 6 volts. The points one probably around the same. If there is an issue with oiling, or fouling, or timing problems, where a hotter spark is needed then maybe it could be the problem
I should also add that before talking to my dad, I was going to try running the engine on a single four barrel carburetor to see if I could make sure it wasn't carb related. The carbs (3 Stromberg WW's) were rebuilt some time before I got them and not run. I pulled the tops of to adjust the floats and gaskets and acclerator pumps were new. They are running a straight linkage and seem to run well after turning down the fuel pressure and adjusting the idle mixture by vacuum. My dad talked me into diagnosing and fixing the battery discharge issue before making any other changes. So I did. That said, I guess the carbs can still be suspect as I didn't rebuild them myself. But the whole timing light not flashing leads me to believe the misfire is electrical/ignition related.
Timing has been set at stock 5*btdc (stock recommendation for 4bbl.) and 10*btdc (stock recommendation for tripower). I had also tried further advance and retardation. The misfire happened at all timing settings. No issue with oiling of plugs. There was a rich condition when first setting up the carbs but that has been pretty much eliminated.
I'd have to agree with your dad in that you need to fix the known issues (low battery) before proceeding. Eliminate that issue then move on to the next.
one word yes. got one doing it right now.or atleast till i get off the computer and go out to the shop.
Remember that running voltage is a couple volts ABOVE 12 with generator working, a couple volts below after running a while with dead generator. And if you are running the ignition off of the stock resistance circuit you can knock off MORE. What it would affect would be the coil's ability to fully charge for each spark. You are trouble shooting an untested package, new ignition, new timing specs, new carburetion package all at once...so to avoid going mad, for sure get charging and ignition under decent control before you even think abou carbs.
Also check your voltage to the coil. If your resistance going to the coil is to high it will miss fire past idle. So check your out put on your ballist resistor or resister wire. I think the positive side of the coil should not have more than 9 volts. This is checked with the key on and not running. If you have more than 9 volts change your ballist or wire to a say 1.3 ohms or what ever the manufacturer suggests for you ignition set up.
I had a '62 Chevy II at the track. Since it was a track only car, the pulleys were underdriven. By the time I got to the line, the battery was dead. I got a jump, but with the low battery, I was "shooting ducks" all the way down the track. The next time, I charged the battery before going to the track and the car ran perfect.
I (we) at the shop should've thought of that. You threw us off by saying the gen doesnt charge at idle, So I let it leave my mind. The "hard" sounding miss (hard to describe) sounds to me like timing too advanced and fighting the compression stroke, but I definitely learn things every day. It may just be how it sounds w/short pipes.
if the voltage is critically low, the spark will be weak. under load, with the carbs dumping larger amounts of fuel, the spark may be getting quenched by the fuel. I will be surprised if a fully charged battery and properly functioning generator doesn't solve the problem
Here's what happened today. I went back this afternoon, the battery was at full charge, 12.8 volts. I started the engine and checked voltage output at the generator (14.6 volts) and then at the battery (14.5 volts). So, now the charging problem is definitely fixed and the battery has a full charge. So I put the timing light on it to set the timing accurately since I'd previously been advancing and retarding it by ear to eliminate the stumble. The timing light at first does the same thing as before. Then it starts working and I set it a 10* btdc (which was approximately where it was the last time I jiggered with it). Now I'm ready to test. I put it in drive with my foot on the brake and roll on the gas. It felt like it had much more power and the tires squealed. More inportantly, no misfire. At this point I'm feeling good. I take it down the block and the first time I roll on the throttle its the same old stutter as before. With everything ignition related replaced at least twice with new or pro rebuilt, its hard to believe that two sets of plugs, two distributors, and three coils are bad. Two different ignition resistors as well, the stock chevy one which was known good and a new one Bubba sent to use with the Mallory. I'm at the point now of thinking the timing light may be an anomaly and the stutter or misfire is carburetor related. I'm going to swap a stock intake with a known good 4 barrel and see what happens. Let me know what I've overlooked and what I didn't check!!! As always, thanks!!
I'll check incoming to the coil tomorrow. I have used two different resistors. First the stock chevy that worked before the swap and then the new one GMC Bubba sent to use with the Mallory. Same stutter or misfire throughout.
Before I polarized the generator, it was putting out zero volts. Same deal with the misfire before and after.
Terd, Reading this reminded me that another friend of mine was telling me that when he changed to a pertronx in his corvair he had to remove the resistance wire from the coil curcit because it did not need/ call for the resitance. I will give him a call tomorrow to verify, if the coil is not getting the required voltage that would kill the spark. HJ
A generator charges less at idle than an alternator, which is why headlights dim at the stop light. I assumed (wrongly) that we were cranking it a lot without a lot of driving to recharge. You're right about how it sounds being hard to describe. You heard it and felt it. It doesn't fall on its nose and slow down like a bog. And it doesn't pop and backfire. The engine doesn't feel or sound like its going to die. Its like a stutter, sounds kind of like a motorcycle stutter box. Any speed, any gear when you roll past half throttle. It doesn't clear until you back off. And only under load.
Yeah, and then I'm back on it. I'm going to swap the intake for a four barrel and see if I can eliminate or pinpoint fuel issues. That's what I thought could be happening, but it turned out to be wrong. A fully charged battery and functioning generator did not solve the problem. I'm sure this is going to turn out to be one of those "why didn't I think of that" moments when I finally figure it out.
You've covered the electronics/battery/charging issue.That should be a "go". Just for kicks,possibly a valve(s) a "lil" tight? You've done the induction with the same results,so weed that out.You've covered about all else.....Just trying to cover all aspects.... I know you'll get it,but doesn't make diagnosing fun.
Listen to 1:29 in the 1st vid, and :08 on the second vid to hear the hard to describe stumble. 1st vid is newer http://www.youtube.com/watch?v=Gm_WMu73bYM&feature=related http://www.youtube.com/watch?v=obr8ds0b6hQ oh, and don't forget to check out my Mustang II (the pinto) toy doin a burnout in related videos.
Had your same set of circumstances a few times w my 51 Plymouth. A discharged battery situation caused a high speed miss in mine. Fixed generator and charged battery....works good. Also check to see if carbs is getting fuel. Good luck
Its been all worked out. The generator was not polarized after chroming. There were other ignition issues along with the wrong jetting in the carbs. The ultimate issue was the carb jetting. A good four barrel fixed everything.
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