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Unread 05-09-2015, 10:43 PM   #1
burley
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Cylinder #3 Misfire

I'm getting a rough idle which keeps stalling when cold, until I rev it up to running temp. It dies/lugs when I open the throttle slightly, does better when I rev it up.

Pulled #3 plug and changed it because it was fouled. The rest of the plugs are fine. When idling, I can pull plug wire #3 and the idle doesn't change. I'm guessing dist cap/rotor, but cap is only a year old. Rotor is unknown. Should I be checking anything else? Just reset timing to 8*. Engine loved driving around the block at 16*, but thats way too high.

Background: I recently rejetted my new-to-me MC2100 from 50 to 47, and I'm at about 650'. The carb ran/idled GREAT with 50's, lots of power, no hesitation, but after 20 min of driving or idling would I would foul a plug, and lug around on 5 cylinders. Plenty of power in 3rd (top) gear to get up to 70MPH on the highway.

I have a Holley FPR installed, and set to 3psi. No fuel return line from the filter, but I've already ordered the parts to reinstall it. I could use a new fuel filter, but FPR reads plenty of pressure. Air filter is clean. I've been running rich for about 6 months, did I clog my new cat already?

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Unread 05-10-2015, 05:01 AM   #2
gmakra
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I think you hit the nail on the head if you arent getting a spark on 3 time to replace cap, rotor and wire.
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Unread 05-11-2015, 06:57 AM   #3
burley
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I replaced the the wire on plug #3, and the rotor. I had to order a cap, so that will have to wait. #3 ran great to the carwash yesterday to get the mud off, and misfired on the way back. Dont know if it was a coincidence or maybe it was the moisture. I pulled the new plug and it was a little black, no oil/gas smell. Hopefully a new cap fixes this.
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Unread 05-11-2015, 07:13 AM   #4
Matt1981CJ7
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If #3 continues to foul with a new plug, wire, rotor and cap, the next step would be to do a leak-down test.

I had a similar situation. I did all of the above and nothing changed. I also tried re-orienting the distributer, no change. The leak-down test didn't show anything abnormal. When I took the valve cover off, below is what I found.

Matt
rockerfailure.jpg

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Unread 05-11-2015, 07:49 AM   #5
burley
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Quote:
Originally Posted by Matt1981CJ7
If #3 continues to foul with a new plug, wire, rotor and cap, the next step would be to do a leak-down test. I had a similar situation. I did all of the above and nothing changed. I also tried re-orienting the distributer, no change. The leak-down test didn't show anything abnormal. When I took the valve cover off, below is what I found. Matt
So what exactly would be causing the cylinder not to fire if I fail the leak down?
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Unread 05-11-2015, 08:05 AM   #6
Matt1981CJ7
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In my case the rocker failed and the pushrod poked right thru it, causing the valve not to open. This condition didn't show up on a leak down test.

Google "cylinder leakage test" and you will get more info than you ever wanted.

Matt
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Unread 05-11-2015, 10:36 AM   #7
Partsmke
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Quote:
Originally Posted by Matt1981CJ7 View Post
In my case the rocker failed and the pushrod poked right thru it, causing the valve not to open. This condition didn't show up on a leak down test.

Google "cylinder leakage test" and you will get more info than you ever wanted.

Matt
The push rod through the rocker arm would show up if you do a compression test. If the valves are not opening a leak down test won't show a problem if the rings are good.
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Unread 05-11-2015, 11:09 AM   #8
mastercraftka
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Rule out intake mani leak! I had problems with #2 went through it all and finally found I was getting air around the intake at 2 tightened and straightened the gasket and all was better.
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Unread 05-11-2015, 11:42 AM   #9
burley
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Quote:
Originally Posted by mastercraftka
Rule out intake mani leak! I had problems with #2 went through it all and finally found I was getting air around the intake at 2 tightened and straightened the gasket and all was better.
This feels like a stupid question. I followed the directions included when I put on the small 2 bbl to large 2 bbl adapter (the expensive one from Gronk Performance in Colorado) for my MC2100. It included two gaskets for between the carb and adaptor, but none for intake to adaptor. It specifically said no gasket needed, if I remember correctly.

I tightened the bolts down nice and tight. Might this be the issue? I'll spray come carb cleaner at the intake manifold tonight.
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Unread 05-11-2015, 12:00 PM   #10
mastercraftka
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I don't think you would have this specific issue to #3 if that was the case. I have a 2100 on mine as well can't remember if i had another gasket will have to look.

Did this problem start right after you swapped out the carbs?
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Unread 05-11-2015, 12:07 PM   #11
burley
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Quote:
Originally Posted by mastercraftka
I don't think you would have this specific issue to #3 if that was the case. I have a 2100 on mine as well can't remember if i had another gasket will have to look. Did this problem start right after you swapped out the carbs?
It's been an ongoing issue. It seems to be pretty random. I'll drive to work and it'll have plenty of power, turn off, wait 15, and it's running on 5 cylinders again. When I went wheeling it was more pronounced after idling for 3-5 min, then would go away after driving on the highway after a few.

I chalked it up to running rich, which is why I got rid of my Carter bbd. Thought this time it was the large #50 jets causing the issue. I jetted down to #47 and the damn thing would hardly idle. I rechecked timing (was at 3.5*), set to 8* and it's better. Seems to run well, then just lug randomly.
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Unread 05-11-2015, 12:23 PM   #12
mastercraftka
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Well mine at high speeds or rpm was hardly noticeable but at idle was bad and was consistent. Wont hurt to spray the carb cleaner or wd40 on the each intake and see. Also around the bottom of the carb where you might suspect a leak.
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Unread 05-12-2015, 03:23 PM   #13
burley
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New cap and dielectric grease on the cap seems to have helped. The power curve feels like it has much less grunt on the low end, but at least all the cylinders are firing. I wonder if the shift in power has to do with the #50 to #47 jet swap and shifting timing from 3.5* to 8*.

I couldn't get a good vacuum reading on the PCV hose with a T. I was only pulling about 2.5. When I pulled manifold vacuum straight from PCV nipple on carb (no T) I get a solid 21-22 with a steady needle.
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Unread 05-12-2015, 03:38 PM   #14
gmakra
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Quote:
I get a solid 21-22 with a steady needle
Good sounds like you hit the nail on the head, run it for a few days and pull the plugs and see what they look like.
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Unread 05-12-2015, 04:03 PM   #15
burley
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Quote:
Originally Posted by gmakra
Good sounds like you hit the nail on the head, run it for a few days and pull the plugs and see what they look like.
I'm getting 21-22 by going straight from PVC nipple on the back straight to the vacuum gauge. Everything I've read says to T it off and also connect to the PVC on valve cover for a proper reading, but when I do that I only get 2.5 or so.
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