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what's wrong with my sbf?
I'm running a 351W with a .030 overbore, Edelbrock Performer manifold, camshaft, arburetor and double row timing chain. I use Autolite 45 plugs, fired by a MSD 6A box, Blaster 2 coil, and a Duraspark distributor. I replaced the pickup in the distributor a couple years ago. I don't know how to test it or the MSD box.
This weekend, after a spirited run through the gears, the car began running horribly, missing and backfiring at idle. I pulled the valve covers and found all the rockers and valve springs to look fine. I compression checked all the cylinders and found them all above 180#. The MSD cap is almost new and looks good. The plugs are fairly new and all look good, with a light tan color, tinged in pink. The gap is .042. The Taylor 8mm wires are, admittedly, very old, though. I have ordered new ones. I haven't pulled the distributor to look at the gear yet. I haven't sampled the fuel yet, either. The filter is clean. I'm wanting to find my solution quickly, as I have plans and reservations made for a weekend trip in the car. What am I overlooking, or what else should I check? |
Does the misfiring only occur at idle?
What carburetor do we have on here? And MSD ignition, every car I hear about with an issue has MSD. |
Is there hesitation when accelerating?
Could be vacuum leak if misfire is random. |
Sounds like timing...dumb question, but the dizzy hold down didn't loosen did it? Wasn't in there wrong to begin with as you indicated you ran through the gears...if you take the cap off is there crazy play? (Gear bad) are the guts all in place? Has the pickup corroded all to $hit? Sure sounds like timing, maybe a bad Msd box, haven't seen one myself but have seen the guts of a dizzy look like they were pulled from the wreck of the titanic...if it's running at all I wouldn't think coil...
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Carb is an Edelbrock Performer 600. Misfire is most noticeable at idle-harder to hear when the rpm comes up, but the engine is 'way down on power and will barely start. It went from running great to running terrible in just seconds, and I haven't found the cause yet.
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Have located the test procedures for the magnetic pickup and for the MSD box-will try those this evening, when I get home. The distributor was in its normal orientation-had not turned. I will pull it out tonight and have a look at the gear. The rotor, cap contacts, and inside of the distributor look to be in nearly new condition.
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Probably the wires....
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Start by checking the coil since it started suddenly.
Wires don't go that bad that fast in most cases. Bad fuel is also an option, especially if you made a fill up on the way. Check the torque on the intake in case it has worked loose. It can cause sudden vacuum leaks real fast. |
mSD
Hi,
Replace the msd box, I've had to do it at the side of the road, not fun. Perry |
How long has the gas been in the tank? Purchase gas recently? It does sound like stale gas.
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I had a similar problem. Replaced the ignition box and the magnetic pickup in the distributer, both failed at the same time. Do not forget to reset total timing when reinstalling the distributer...
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Thanks for the responses, everyone. This is a bit of a brain teaser. The gear on the distributor looks like new-no problem there. The valve train seems completely sound. I guess I'll start by replacing the plugs, next the wires, next the coil, next the magnetic pickup, and work my way back to the MSD box. This must be some sort of intermittent problem, because my spark tester is showing ignition, whenever I try it.
I turned the engine over and captured about a pint of fuel from the disconnected line. It looks perfect, with no water seen. The fuel in the tank is not over 2 weeks old. Hopefully, the new wires will arrive tomorrow. I'll put the valve covers back on, install the new wires on the new plugs, and see how it acts. Replacing the pickup is a bit of a pain. I hope to avoid that. Replacing the MSD isn't too hard, but more money, for sure. |
MSD boxes have gone down the tube! Think about Pertronix.
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MSD 6A box or Blaster 2 coil. My bet. I know there are a lot of MSD fans, but I'll pass. I'm sure MSD has its place, but I find if the voltage is low or the ground is not large enough gauge, and direct to the battery, problems surface. I have switched to Pertronix, less susceptible to the issues above, simple, and never had a problem. I hope you post your fix when you have it cured. Very Interested.
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Start it up and run it in the dark. Darker the better. If the plug wires are arcing or bleeding, you see them light up or glowing. It sounds a little odd but that's how I found a similar problem years ago on my 240Z.
Larry |
A pretty easy check
When i thought my MSD box had gone, we pulled the dizzy, reconnected all its control wires, connected the coil wire and one plug wire, inserted a plug, grounded the plug and spun the dizzy noting the spark happened at about the same rotation every time...Kind of checks the whole thing...
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With an Edelbrock, you have two bowls that feed a primary and secondary each side, unlike a Holley that has a primary and secondary bowl.
So you can lose one half of the engine, if you are running a dual plane intake. Possibly one bowl is flooding. I had this happen to friends car one night. After deciding it had a creeping needle and seat, I drove it home for him, just struggled to run at low speed, where the engine was clearly rich from flooding. Gary |
Hope to get time to finish putting it back together tomorrow after work. Late last night, I think I stumbled across the clue to all of this. I'll report back when I'm sure.
Thanks for all the input. Clearly, you're putting some thought into this. I think I may be turning the corner on it. I'll turn the key tomorrow evening. Hopefully, I'll have something to clear this mystery up by then. |
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