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Twin Plug, dual MSD question?? Interesting Misfire?

OK 1982 3.0, twin webers 46IDA3C, dual distributor twin plug, two MSD 6al's

Pulled the bottom plugs when I recently adjusted the valves (less than 300 miles)
they were clean but I gave them a fresh clean and reinstalled.
everything back together, she was running absolutely awesome!
roughly 200 miles later developed a misfire, seemed like misfire at low RPM's
but consistently went away when the tach hit 3000 rpm's this lasted for 40-50 miles around town so I parked the car went on vacation and said "I'll have to take a look at her when I get home." Few weeks later, get home, take her out and no miss. I mean absolutely running terrific again! I'm quite perplexed?? I mean I consider myself a pretty good mechanic but that is ridiculous! lol

Any thoughts? Not that I'm looking for problems. But I just don't want to get several miles away like to a c&c and it comes back.

Do you think that I'm looking in the correct direction? it really sounded like ignition because of how it completely went away at higher RPM's or is it possible I have imbalanced Webers? I do not have a lot of experience with Weber Carbs. I do have to mention that all the engine work was done before I purchased the car and I do not have any history. I know that is a no no. Sorry

I pretty sure the car/engine has some track dayz behind her. Check back in 3-5 years then I can tell you everything about my girl!!

Thanks in advance for all your help you guys are awesome!!



Old 08-31-2015, 07:30 PM
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Maybe double check that the plug wires are fully seated on the plugs.
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Old 08-31-2015, 07:54 PM
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Make sure your idle jets are clean as this will cause a light throttle misfire until the engine revs are over 3K-4K.
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Old 08-31-2015, 08:29 PM
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How are you set up?

How many coils? How are the boxes connected related to the dizzy or is one Box switchable to be inactive unless you have a failure? Etc.!!!!!!!!
Old 08-31-2015, 09:13 PM
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There are 2 coils, not sure how they are wired. Any suggestions how to figure that out. From what I can tell each MSD is connected to its own coil. They are definately synced together because the second distributor has nothing inside except a rotor. The timing is only controlled by the main dizzy. When you say "switchable" would that be something that is controlled by a driver or is that something automatic. I guess I should have mentioned I have no experience with twin plug set ups either.

Thanks for you input!!

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Originally Posted by TCracingCA View Post
How many coils? How are the boxes connected related to the dizzy or is one Box switchable to be inactive unless you have a failure? Etc.!!!!!!!!
Old 09-05-2015, 04:51 AM
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As the esteemed Mr. Weiner mentioned, I would be looking at the idle circuit and webers before ignition.

Double check everything is clean and that the carbs are well balanced.
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Old 09-05-2015, 06:32 AM
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+1 on idle jets. Happens to me quite often. You're in main jets >3k. Take them out, remove jets from holders and run compressed air through them. Bet it's like night and day.

I'm going to get a higher quality inline fuel filter.

To check firing, put a timing light pickup (the clip thingy) on each of the 12 plug wires. Sounds like you're running a dual distributor setup rather than a 12 plug distributor. No experience with that, I have a 12 plug JB unit that has a pig tail, 2-into-1 connection from both MSD boxes.

Todd
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Last edited by toddu; 09-05-2015 at 10:22 AM..
Old 09-05-2015, 10:11 AM
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I run a main filter coming into the engine bay and then two smaller filters at each fuel inlet, one per carb. This significantly cut down on having to clean the idle jets. Basically, I don't ever clean them.
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Old 09-05-2015, 10:15 AM
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Do you have any engine bay photos? I'm interested how this set up looks.

Quote:
Originally Posted by jpnovak View Post
I run a main filter coming into the engine bay and then two smaller filters at each fuel inlet, one per carb. This significantly cut down on having to clean the idle jets. Basically, I don't ever clean them.
Old 09-25-2015, 02:46 AM
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I wanted to thank everyone for their input. It was the idle circuit of the Weber carbs that were dirty. I've been working on cars for many years but I could of swore with how exact the misfire sounded that it was an electrical issue. Anyhow thanks again!!
Old 09-25-2015, 02:50 AM
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Quote:
Originally Posted by Steve@Rennsport View Post
Make sure your idle jets are clean as this will cause a light throttle misfire until the engine revs are over 3K-4K.
this is what I was going to say, I'm betting something was in there and came loose. Common problem.
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Old 09-25-2015, 03:37 AM
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