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Weird "rough" idle problem after ignition changes.

Robert560

Senior Member
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Weird "rough" idle problem after ignition changes.

I'm a bit confused by this one. Hopefully someone out there can offer some advice. When I say "rough" idle, I mean shaky... increased engine shake, but not poor running. Here's the story...

Before:
- Stock coil
- NGK BP6ES plugs with .025" gap
- Bumblebee wires
- Premium rotor
- Stock style distributor cap

Ran OK, idled OK, no major issues. Had a problem with the wire boots breaking during a plug change, plus I wanted better shielding so my garage door opener would work, so I upgraded the system.

After:
- Lucas Sports Coil
- New NGK BP6ES plugs, increased gap to .032", figuring a better mate to the higher coil output
- Cobalt Blue ignition wires
- Replaced rotor again, since I was there
- Updated the distributor cap, since I had push-on plug wires now
- While I was doing all of this, I also installed a new Lockheed style brake servo and new check valve.

After firing the car back up, the idle speed had dropped significantly. I had to adjust the idle screws nearly a full turn, if I remember right, to get the idle back up to where it was (adjusted the fast idle to match).

Now here's the weird part. The car runs fantastic, but at idle it has noticeably more shake than it had before. It really doesn't sound any different though... it just shakes more - like it's idling rougher but not sounding or acting like it's idling rougher. The air-fuel mixture seems fine. I'm not getting any smoke or anything funny. And as I said, it runs beautifully. Off idle I have perfect throttle response.

Another weird thing is that the drop in idle speed from start-to-warm (just after you come off the choke) to fully warmed up is much greater than it was before. There was always some, of course, but now there's noticeably more. And it's even more noticeable when the temperature outside is warmer. It's WAY more than before, and as the idle drops, the engine shake/roughness increases. But none of this seems to have any effect on performance - just idle. I know these things don't idle very smoothly anyway... but the difference is significant from what it was before to what it is now... even though it seems to run better now in all other ways.

I've checked the timing, checked for vacuum leaks (including plugging off the new servo just in case the problem was there somewhere), and recently adjusted the valve lash. The carbs were never touched during this process, nor was the Pertronix ignition module, which I've had in the car for over a year now, long before doing these ignition changes.

Does anyone have any ideas on what could be going on here? I'm at a loss as to what to try next. I'm afraid that when summer gets here, the idle drop will be so extreme that I have a hard time keeping the car running at idle. Was it something I did? Or has something happened elsewhere and it was just coincidence the problem turned up now? Any ideas would be appreciated.

Thanks!

~Bob
 

RAC68

Darth Vader
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Re: Weird "rough" idle problem after ignition changes.

Hi Bob,

A true example for the saying: “If the car runs good don’t lift the hood”.

Assuming none of the components are defective and you have checked your timing to correct advance, my first thought focuses on the added plug gap. Although added gap does provide for a better overall burn when using a hotter coil and CD ignition, I have also found that this addition does create a rougher low RPM run (idle). I was able to adjust most of the roughness out but, since I am not after ultimate performance, decided to return back to standard gap.

I would suggest rechecking your timing and return the gap back to 0.25 to observe the result. If all seems as before the modification, then it’s your choice whether to keep the greater gap or return to standard. If all is not as before, you have another problem and I would start exchanging replaced parts to find the problem component.

Good Luck,
Ray (64BJ8P1)
 
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Re: Weird "rough" idle problem after ignition changes.

Sure sounds like a vacuum leak. How did you check for leaks?

IMO, you had the better setup to start.
 

Keoke

Great Pumpkin
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Re: Weird "rough" idle problem after ignition changes.

Bob_Spidell said:
Sure sounds like a vacuum leak. How did you check for leaks?

:iagree:

Check the one way valve to the brake booster. Keep the point gap between 0.026 and 0.035"
 
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Re: Weird "rough" idle problem after ignition changes.

Check spark plugs for color. You may have missed on the HT wires when you screwed them into the dizzy cap and get no spark. Just a guess but it often happens.
 
D

Deleted member 8987

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Re: Weird "rough" idle problem after ignition changes.

In my experience, opening up plugs is not a good idea.
You just "eyeballed" an additional .007" with what study as a reference?
Do you have the old plugs?
If so, just put the old plugs in and see what happens.
You are doing a "system restore" on the plugs top a known good point, and see what happens.

It's free. No parts to buy.
 
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Re: Weird "rough" idle problem after ignition changes.

A couple things come to mind: a vacuum leak usually raises the idle speed, as more air is going into the engine.

Changing the points, thus affecting the timing, could easily drop the idle RPM.

The rough running; any chance you <<excessively>> oiled the advance mechanism?

I'll admit that I didn't read your entire post (ADD...) so you may have already addressed what I observed.
 
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Robert560

Robert560

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Re: Weird "rough" idle problem after ignition changes.

Thanks all for the replies!

I was having a really hard time picturing how plug gap could affect idle speed... so it's nice to hear second opinions that it could be a factor.

I chose the increased plug gap based on scientifically sound voodoo magic. Seriously though, I read that a plug gap should theoretically be .001" per kv of the coil. So a 40k coil would be .040". I went less than that figuring a slightly hotter spark would help keep oil off the plugs (it's an old engine after all). Looking at my available feeler gages, I had a .032", so it sounded like a good gap to go with. See? Scientific voodoo :smile:

My vacuum leak check was pretty crude, and was really "checking vacuum for its effect on the problem". I simply plugged vacuum ports to see if it had any effect on the idle shake (which it didn't). So in reality, I haven't confirmed I have no vacuum leaks, but have confirmed that, if there is, it's having no effect on this problem. And as Randy said, a vacuum leak will usually increase idle, not decrease it.

The timing is dead on (and hasn't moved from what it was long before the ignition changes), and the advance mechanism has no more oil on it than it did before all this started. Didn't touch anything in the distributor except to swap out the rotor and cap.

Having typed it all out and gotten everyone's feedback, it seems pretty obvious now that the plug gap seems the most likely root cause. I'll change it back to .025 with a fresh set of plugs and see what happens.

~Bob
 

Brinkerhoff

Jedi Knight
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Re: Weird "rough" idle problem after ignition changes.

The other thing is you could have damaged one of the plugs when you installed it or bumped its gap closed. I'd also pull each plug wire off when the car is idling to see if its dropped a cylinder at idle meaning its too lean ( vacuum issue). Of course you could double check the firing order first ( that would be too easy for me ) ! It will be simple whatever it is. Kevin
 

big6

Jedi Trainee
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Re: Weird "rough" idle problem after ignition changes.

After you replaced your plug wires, have you rechecked the firing order at the distributor cap? What you are describing sounds to me like a cylinder misfire. Does the engine stumble when you increase the engine RPMs? I don’t believe you will see any difference in changing the plug gap from .038 to 025… Unless you are superstitious having the difference of .013…
 
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Robert560

Robert560

Senior Member
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Re: Weird "rough" idle problem after ignition changes.

big6, that's what makes this weird. As I mentioned above, the car is otherwise running flawlessly. There is zero hesitation coming off idle, and throttle responsiveness and acceleration feel better than ever. So the upgrades to the system have shown noticeable improvement... except for increased shake/vibration at idle accompanied by increased rpm drop as either the car temp or ambient temp increase. It's definitely odd.

At any rate, I'll rule out the plug gap this weekend. Hopefully that solves it. If not, I'll be really baffled, and will probably have to start swapping parts until I hit on something.

~Bob
 

red57

Jedi Warrior
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Re: Weird "rough" idle problem after ignition changes.

How about this?
As Randy said, vacuum leaks usually raise idle speeds because more air is getting in.
You said you changed the brake booster and check valve at the same time as your tune-up.
What if the old booster or check valve were leaking? Could it be that in the past, without knowing you had a vacuum leak, you had set the butterflys to give a good idle speed then chaged the servo and, again without knowing, solved your previous vacuum leak and now the idle is too slow because no excessive air is coming in?
I know, it sounds like a stretch to me too, but......
I think the plug gap at .032 with a Sport coil should be fine, least it has for me for years.

Dave
 
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Robert560

Robert560

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Re: Weird "rough" idle problem after ignition changes.

Dave, great observation! I was starting to wonder that same thing. It doesn't sound like a huge stretch to me... and it could certainly explain a lot. If I had set up the carbs with a vacuum leak I didn't know I had, and then fixed the vacuum leak, my carb settings could be completely out now, or my carbs way out of sync, since the brake servo comes in on the back of the manifold. I'm thinking I need to baseline the carbs again and re-set everything. Make sure all that's good, then see what happens, then if I still have the idle problem I'll start messing with parts. But the more I think about your idea, the more sense it makes.

~Bob
 

Johnny

Darth Vader
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Re: Weird "rough" idle problem after ignition changes.

Hi Bob, you might check that the plug wires are seated tight on the plugs and the new distributor. I hear that the new caps on the dist. have larger holes where the wires go in and when you go to tighten them they don't pierce the wires properly. Worth a check...
 

Keoke

Great Pumpkin
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Re: Weird "rough" idle problem after ignition changes.

Cobalt Blue ignition wires-

w BOB!!!what are these wires stranded copper or String impregnated with carbon?????
 
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Re: Weird "rough" idle problem after ignition changes.

They requires a top entry dizzy cap. Did you change the cap also? Pull each wire off its plug in turn and hold near the plug or engine to see if you are getting a good spark. As I said earlier you may not have a good contact with the cap. I had this problem decades ago with my Jag. Ran fine at speed but not at idle.
 
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Robert560

Robert560

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Re: Weird "rough" idle problem after ignition changes.

Yup, I changed the cap and rotor when I did the wires, along with the new plugs and new coil. That's interesting though that you had a similar problem. I will certainly check for good contact on all the plugs. Thanks for the tip!

~Bob
 
D

Deleted member 8987

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Re: Weird "rough" idle problem after ignition changes.

Any extra gap in wires to cap, or cap to rotor, is exacerbated by wider plug gap.
 
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