Ignition Problem
Thread Starter
Intermediate
Joined: Nov 2008
Posts: 29
Likes: 0
From: Lawrence Kansas
Ignition Problem
Hey everyone, I have a strange problem I can't manage to figure out. While I was out driving today my engine started idling rough and it was even worse when accelerating (it's a 71 350 base motor). I had burnt a spark plug wire in the past and though it might be the same thing, but when I looked at them nothing was wrong. However, when I pulled the passenger rear cylinder wire from the distributor the rpm didn't change (even though the cap was arcing to the plug wire), so I figured I found the problem. When I pulled the plug it was somewhat wet, but not too bad. I put in a spare plug I had and no luck. I tried a spare wire and still no difference. I put a new cap on the distributor and still no luck. I pulled the plug and held it against ground and it fires, but when I use an inline checker when the plug is in the cylinder it's not firing. Does anyone have any idea why the plugs don't want to work while in the cylinder or where my problem might reside? Thanks in advance for any help you guys can give me.
Le Mans Master
Joined: Jan 2000
Posts: 8,918
Likes: 27
From: New Hill NC
You should check the compression on that cylinder.
Also that is the 5/8 runner. Vacuum for accessories is pulled off that runner. A vacuum leak is common and will cause a misfire at idle, but not at speed.
You should also check #5 plug.
Also that is the 5/8 runner. Vacuum for accessories is pulled off that runner. A vacuum leak is common and will cause a misfire at idle, but not at speed.
You should also check #5 plug.
Thread Starter
Intermediate
Joined: Nov 2008
Posts: 29
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From: Lawrence Kansas
I can double check, but even when I replaced the one wire that I knew wasn't working it didn't make any difference. Also, they check out fine when the plug isn't in the cylinder, it's only when everything is hooked up that they fail. I put the wire on a multimeter and didn't see any problems with it, but I'll probably double check all of them soon.
Le Mans Master
Joined: Jan 2000
Posts: 8,918
Likes: 27
From: New Hill NC
If the compression or the fuel mix is not right on that cylinder or the corresponding cylinder on that runner, you might not get anything out of that cylinder. The plug is firing, the explosion dont happen.
Thread Starter
Intermediate
Joined: Nov 2008
Posts: 29
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From: Lawrence Kansas
The strange part is that is was running just fine earlier in the day. I took the car out for a cruise since the weather was nice (it's winter around here right now) and about 30 minutes in or so is when it dropped the plug. It's been running fine for a long time before this, just some weird gremlin right now. Also, it does not run any better at speed, it's still dropping spark (to some degree at least), though it is much more noticeable under acceleration.
Thread Starter
Intermediate
Joined: Nov 2008
Posts: 29
Likes: 0
From: Lawrence Kansas
The plug actually isn't firing when it is in the cylinder. I've put an induction checker on it and other cylinders and it works on other cylinders but not the #8. It's just weird that the plug works when it's not in the cylinder...
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That fits your symptoms!
Thread Starter
Intermediate
Joined: Nov 2008
Posts: 29
Likes: 0
From: Lawrence Kansas
Just pulled the valve cover today and didn't see anything wrong. Cranked it over a few times and nothing seemed out of place. I can move the rockers okay with my fingers, so I know they're not too tight and they look like they go down as much as any of the others. Going to see if I can do some kind of compression test on it sometime soon, though it's pretty hard with my headers in the way. I just can't figure out why it would have gone out so suddenly without something physically breaking. The car makes no strange noises that I can pick up, which I almost wish it did at this point so I can pinpoint the problem better...
Instructor
Joined: Jul 2007
Posts: 167
Likes: 0
From: Sheboygan Wisconsin
Check the rockers and valve springs on the other side. I had this happen many years ago. A spring broke on the other side, causing a cylinder to die out on the opposite side. It might even be the inner spring that's broke. Compression test may not show this.
Good Luck
Good Luck
Thread Starter
Intermediate
Joined: Nov 2008
Posts: 29
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From: Lawrence Kansas
Well, looks like I might have tracked it down. I did a low tech compression test and just held my finger over the plug hole and cranked the engine. It only held compression for one stroke, then nothing, so I backed off the rockers a little and it seems to have fixed the problem. I'll know better tomorrow when I take the car out, but thanks for all the help and advice!
Le Mans Master
Joined: Jan 2000
Posts: 8,918
Likes: 27
From: New Hill NC
Well, looks like I might have tracked it down. I did a low tech compression test and just held my finger over the plug hole and cranked the engine. It only held compression for one stroke, then nothing, so I backed off the rockers a little and it seems to have fixed the problem. I'll know better tomorrow when I take the car out, but thanks for all the help and advice!
Thread Starter
Intermediate
Joined: Nov 2008
Posts: 29
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From: Lawrence Kansas
I didn't see anything when I was looking at them. I think what's happening is as soon as the lifters see oil pressure they're sticking pumped up and not letting the valves close. Backing off the rocker adjustment seemed to fix the problem since it's now compressing properly on each stroke.
Le Mans Master
Joined: Jan 2000
Posts: 8,918
Likes: 27
From: New Hill NC
I didn't see anything when I was looking at them. I think what's happening is as soon as the lifters see oil pressure they're sticking pumped up and not letting the valves close. Backing off the rocker adjustment seemed to fix the problem since it's now compressing properly on each stroke.