Rough Idle--327/300

**Duke W.** · July 29, 2004, 01:37 PM

Re: Rough Idle--327/300

"I know the car is running a little rich because the idle speed increases when I disconnect a vacuum line."

If you disconnect the signal like to the vacuum can idle speed should increase, but become rough due to what is essentially a vacuum leak. If you PLUG the signal line, speed should drop below idle speed and the engine will also run rough.

So which is it?

Also what is the idle speed/vacuum you are measuring?

I would first suspect some kind of ignition problem. A check of all the secondary components is called for. Pull all wires including the coil wire and look for signs of arcing at the terminals. Thoroughly inspect the inside of the cap and rotor. A small crack in the cap can cause these symptoms. Also run end to end resistance checks on all the high voltage wires. If you are using the OE type RFI wires I believe they should measure about 5K ohms per foot, so the front four cylinders will be heigher than the rear four.

Duke

**Bruce Boatner** · July 29, 2004, 02:26 PM

Re: Rough Idle--327/300

Duke,

I'm a little confused. I always thought that introducing a vacuum leak anywhere would typically cause the engine to idle rougher AND decrease idle speed, so I may be in need of some fundamental education here. Also, I thought that disconnecting the vacuum line from the can should always result in decreased idle speed, whether or not the line was plugged or open, due to the absence of additional advance from the can.

In my case, the idle increases if I remove the vacuum line going to the heater control valve (i.e., not the vacuum can line). If memory serves correctly, the idle DECREASES when I remove the vacuum can line, and decreases even more if I plug the disconnected vacuum can line.

I adjusted the idle screws to get the most vacuum at about 500 rpms.

I have a new distributor cap and rotor, but I will check all of the wires as you suggested. I will check the cap and rotor too--I guess "new" does not alway mean "good".

Thanks for your help.

**William V.** · July 29, 2004, 04:28 PM

Re: Rough Idle--327/300

Bruce

When I have a problem like this, while at idle, I removing the plug wires at the distributor one at a time (be careful or you may get shocked). If there is no change when a wire is removed, (engine should run rougher) then that cylinder should be suspect as having a trouble. By using this method, many times I have found bad or cossed plug wires.

Another quick check is to be sure all plug wires are secure at the spark plug and cap. A wire off a spark plug is a very common problem that is often overlooked.

Good luck

Bill

**Brian Monticello** · July 29, 2004, 06:56 PM

Re: Rough Idle--327/300

Bruce,
My 327/300 had a similar miss and I found a few worn cam lobes when I tore it down. Hopefully your issue is something less involved. How many miles are on this engine?

Brian

**Chas Henderson #28127** · July 29, 2004, 08:39 PM

Re: Rough Idle--327/300

Bruce,
I had a similar problem with a "miss in the engine. I had a friend put a scope on the car and he foung the #5 and #7 and #8 plug wires were not up to snuff. I tried the pull the wire trick, but I really couldn't notice the problem. It seemed to run fine at speed, but that was just masking the problem. It was noticable at low idle. So the gas usage was up but I thought it was a ill adjusted carb.

Chasman

**Bruce Boatner** · July 30, 2004, 08:49 AM

Re: Rough Idle--327/300

Brian,

I'm not sure how many miles are on the engine. I've been told that the entire top end had been rebuilt less than 10,000 miles ago, but who knows? I imagine if the cam lobes were worn, the miss would be constant instead of intermittent. Was the miss constant in your case? In my case, the miss may go away completely at one stop light, and at the next stoplight it misses every 2 or 3 seconds.

**Brian Monticello** · July 30, 2004, 08:59 AM

Re: Rough Idle--327/300

Bruce,
I'm not even sure if my mystery motor had the 300 or 250 HP cam but It had a terrible idle at anything below 700. With a 250 or 300 hp motor it should be smooth and easy. It never missed under load and would occassinally miss when I free revved the engine at a steady RPM up to about 3000 and then it was nice and smooth.

Brian

**Bruce Boatner** · July 30, 2004, 08:59 AM

Re: Rough Idle--327/300--Addendum

I replaced all of the plug wires recently. Of course, I had two wires crossed and the engine ran rough as hell, so I know what to look for there. I will recheck all of the wires to make sure everything is snug. It seems like the miss would be CONSTANT if a wire was the problem, but I've given up trying to figure out anything to do with electrical problems.

**Roy B.** · July 30, 2004, 01:00 PM

Re: Rough Idle--327/300

This may sound to simple , many think it's nuts , many will never try it, most don't believe it? I've mentioned this before, try running your engine to 3000 while spraying a mist of water down the Carb for a minute. . Even tho you have every thing perfect CARBON still builds up on your valves and piston causing leakage around the valves at idol and when you increased RPM the leakage is less noticeable and smoothes out.
( If any one notices carbon build up on their spark plug, put them back in and do this , then remove them and see that they look new again just using water). That should show you that your also removing piston and valve CARBON.
I'll stop mentioning this any more in this forum but only on a one to one person because I realize many people are weary to try it

**Bruce Boatner** · July 30, 2004, 01:13 PM

Re: Rough Idle--327/300

Roy,

You just found someone crazy enough to try this. Even if it does damage the engine (which I think is highly unlikely), it will just give me an excuse for a rebuild which is something I have thinking about doing anyway.

What is your recommended water delivery method? How much water are we talking about here? Would a pump-up sprayer work?

Thanks!

Bruce

**Roy B.** · July 30, 2004, 01:39 PM

Re: Rough Idle--327/300

Hay !you sound like my find of guy. Just increase the water from a hose ( or what ever) enough where the engine just start to stumble and as I said for about one minute . Then stop the water flow while keep running the engine for 20 seconds and that's it. Just don't drawn the engine causing it to stop.

It's the cool water hitting the hot engine CARBON that knocks it off the valves and any where els ( EVEN THE OIL RINGS)
Ask some OLD fart that you may know that was mechanics about it.

**Steve Pettit** · July 30, 2004, 02:14 PM

Re: Rough Idle--327/300

Bruce,

I took Roy's advice the day before yesterday, use a garden hose, you will be amazed at the amount of water that it takes at 3000 rpm to make the engine stumble, spark plugs bright and shiny and no mosquitoes in the whole neighborhood.

Thanks Roy,
Good luck Bruce
Steve

**Bruce Boatner** · July 30, 2004, 02:56 PM

Re: Rough Idle--327/300

Steve,

Did you have a sprayer on the end of the hose or just dump that bad boy full on into the carb? In addition to the mosquito fatalities (no shortage of those here in Houston BTW), did you notice a smoother idle afterwards?

Bruce

**Steve Pettit** · July 30, 2004, 03:20 PM

Re: Rough Idle--327/300

Bruce,

No sprayer on the end of the hose, just kinked it and had it running at a pretty good clip. Ran smoother I think, though it was pretty smooth anyhow, it even seemed to run a little cooler this AM on the way to work, although that may just be my imagination. I was a little sceptical, but it worked just fine.

Steve

Rough Idle--327/300

Rough Idle--327/300

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