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Old 01-14-2013, 11:35 AM
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Possible electrical problem. Need help bad.

Hello guys I have developed some sort of ignition or electrical related issue here and would like some advice on what else to check. Engine is a sbc 350 with a hei distributor and a holley 600 carburetor. About a month ago I developed a miss fire that was somewhat minor and was not noticible at higher rpms cruising.

I could hear it like my plugs was not firing all the time and it was missfiring on both sides by listening to my exhaust. I thought maybe my choke was on a ilttle rich so I leaned it out some and I checked my plugs and 6 out of 8 was fuel fouled and the other two still had gas on them.

So I looked into my plugs and it turned out they where a colder plug so I got a hotter plug ngk heat range 4 which is for street driving type vehicles and here in ohio even though its winter we had a nice 60 degree weekend so I had time to change my plugs.

Well the problem was still there and I could'nt understand what was causing my plugs missfire and its at random. There is no particular one misfiring as it happens random. I checked the idle mixture screws on the carb and they where fine and turning them in would kill engine so no power valve issues. I looked into the barrels and there is no fuel dripping from the boosters and the float level is fine and carburetor was redone just a few months ago so I have ruled it out.

After changing plugs I took my truck out for a drive and it would run ok except the random misfire and at cruising you could not really notice it but it did happen at times but was not a reall bad misfire. Came back home and let it sit for a while. I then start to think am I getting enough spark to I got one of those hei spark plug testers and I grounded it and had my wife turn my truck over and no spark now. So I thought either the coil was going bad or the control module was bad. MY Dad had a pair brand new so I just replaced both the coil and the module and my truck fired right up with no issue. I checked the power source to my distributor and its a good 12 volts and with engine running about 14 volts so its getting power to it.

I then proceed to take my truck out for a test drive and all of a sudden my sun tachometer which worked ok before changing out my coil and module worked just fine and while taking it out the take worked fine at idle but if I stomp on it and the tach gets up close to 3000 rpm the needle will bounce all the way to 7000 rpm plus and back and it will bounce back and forth and I never had this problem before. And I still had the misfiring at idle but yet it still seems to run decent under load without really noticing anything wrong but at cruising you can hear it occassionaly misfire.

So I thought maybe something with coil so I changed out the whole cap with a different coil and still have the same problem. So at this time I got my multi meter out and started to check things and the batter was good and steady with 14.5 volts charging and the same thing at the alternator and was good and steady at 13.75 volts at my electric choke and was around 14 volts at my ignition wire going to my distributor.

I had my multi meter sitting on top of my breather while testing a few other 12 volt source wires and I don't know if that had anything to play with this or not but I grouned my meter on the battery and while checking some wires at times even my 12 source wire for the distributor it would read 14 volts then the meter went haywire and it spiked up to more voltage and then would go way down to a negative number and back up and be all over the place.

There was one time that I was not touching anything with the terminals and it was doing the same thing it was going all over the place which was weird considering I was not touching anything with the probes. I then rechecked the voltage at batter and alternator etc with my meter thinking it might be bad but it read ok.

So I have replaced all major components in distributor and plug wires are not even two years old and are good qauality ones not chepos and new plugs put in with only a .045 gap which has always worked fine and now developed issues with my tachometer not working right during acceleration which tell me there is something going on somewhere ignitoin wise but I don't know what else to check as I have been all around the engine.

Battery is also new and all connections where checked and are nice and tight along with the other ground straps. Even my volt meter guage inside my truck was reading 14 volts so it matched up with my volt meter.

All plugs are in correct firing order and on nice and tight. Any clues and has anyone had this problem before? Truck was running fine all summer long and even good as up to two to three weeks ago when this problem started and has not gotten any better. Thanks guys.

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Old 01-14-2013, 12:25 PM
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Are there redundant grounds between the engine, battery, chassis, firewall? Is the distributor body grounded through the clamp, w/no paint between the metal of the clamp, the hold down bolt and distributor body?

Is the module tight to the distributor? Is the coil ground strap or wire in place and tight?
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Old 01-14-2013, 12:50 PM
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Hello there is no paint between the distributor body and hold down clamp as its all bare as can be.And all ground connections are fine as everything was running just fine until a few weeks ago when this problem started.
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Old 01-14-2013, 01:33 PM
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I suspect your plug wires. Set your multimeter on the ohm scale, ( it's little symbol looks like headphones), 2000 or less, and grip each end as firmly as you can and look for crazy fluctuations, or loss of continuity altogether. (Meter goes blank, however briefly). MSD wires offer around 50 ohms PER FOOT. An 18-inch wire should read around 75 ohms, though it's not super crucial. I've seen some ordinary production wire-sets run several hundred ohms fer foot. Talk about giving your coil a workout... I had 2 separate Taylor wire-sets fail within a year. Never again. Remember, it only takes one to produce the effects you're experiencing.
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Old 01-14-2013, 02:45 PM
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I forgot about checking my plug wires I will do that. Thanks for the input.
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Old 01-14-2013, 03:50 PM
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That also could explain the 'noise' the meter was seeing.
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Old 01-19-2013, 03:14 PM
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hello guys I checked my plug wires and the resistance I got was about 5.8 ohms for short wires and 7k for longest wires. I am using auto lite wires from advance auto parts. I am assuming that these wires are still good and within range. yes I have used aftermarket wires with very low resistance but had bad radio static problems so got the auto lite wires about year and a half ago and problem solved with that issue. I don't know what else to check. I took one plug out and it was dry but still had a gas smell to it. I don't know what else to check. any ideas.
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Old 01-19-2013, 05:26 PM
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JHello guys just a update here i wanted to double check my wires and I had one that was either dead or it just would not read on multimeter cause of being that bad. I went ahead and bought a set of wires from autozone and each wire has only about 1k or resistance or less so there way better then the autolite ones so hopefully this will cure my problem. Thanks for tips.
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Old 01-30-2013, 10:36 AM
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Well guys I am posting back and I changed out all my plug wires with a brand new set and I checked thema ll before putting them on and they all have 1000 ohms of resistence or less so they are good. So I have put a new coil and cap along with a control module in the distributor which is less then two years old and I put new plug wires on. I just installed a brand new battery and all the ground wires are all new and have all been checked and I get a good 13.75 to 14 volts at the distributor when running. I also put a new set of plugs in and after all this stuff I still have the miss firing on both sides and you can hear it at the exhaust pipe as it makes a poof sound when it happens. I have checked the idle mixture screws and basic settings on the carburetor and its all fine wise there. Any suggestions on anything else to check. I am at a lost here.
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Old 01-30-2013, 11:36 AM
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Did you do a compression test ??
Did you happen to mess with the valves ??
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Old 01-30-2013, 05:33 PM
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Nothing wrong with valves as I have rolled rockers and they are locked and sound normal. As far as compression test goes it has been running fine then all of a sudden it developed the issues it has. I have checked so many things I don't know what else to do. The misfire is random and is not at just any particular cylinder and is happening on both sides.
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Old 01-30-2013, 06:07 PM
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You need to take your car to a competent tune up shop that has an ignition analyzer scope and an exhaust gas analyzer. With a scope, you can "see" the exact cause of the problem without throwing parts at it.
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Old 01-30-2013, 07:45 PM
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x2 sparkc

Bad PCV valve or other vacuum leak.

vicrod
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Old 01-31-2013, 03:58 AM
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Intake manifold leak
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Old 01-31-2013, 04:29 AM
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What does a vacuum gauge reading look like? Have you done a compression test?

What about the float level? You mentioned the plugs were gas fouled- that should not be happening. Fuel pressure excessive? Plugged air bleeds? Blown power valve? See if it'll stay running or stall out when you screw the idle mixture screws all the way in (count the turns so you can return them to where they were, or plan on readjusting the idle mixture, engine fully warmed up).

Could be too-tight valves. Might be time to bite the bullet and adjust the valves w/the engine running. That way there's no doubt where zero lash is. I'd still check for vacuum leaks, etc. first.

Another thing to look at is the rotor phasing.
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