View Full Version : Down on power, lumpy idle

07-31-2014, 08:46 AM
I've got a 1967 GTX with a 440 that I built up in 2007. Recently, I've noticed that, at idle, the engine simply isn't as smooth (visually) as it was, say, at the end of last year. It shakes and vibrates on the mounts. Then, once it's on the road, it just doesn't have the seat-of-the-pants feel that it used to have. I've put about 6000 trouble-free miles on it since the rebuild so I have a pretty good feel for where it should be. It feels like it's down a cylinder.

My first thought was a loose or bad plug wire so I got an IR thermometer and checked all cylinders. They all seem to come up to temp at the same rate.

Next step, I think, would be to check compression but I thought I'd post here to see if there's anything else. Timing, maybe?

Here are the details of the motor:

Original 440 block, .030 over
915 heads with 2.08 intake and 1.81 exhaust valves
Stock cast iron intake and HP exhaust manifolds.
Comp cam XE275HL grind, 231 int./237 exh. duration, .525 in lift
FBO ignition
Stock rockers, lifters and pushrods
Stock AFB carb (4326S)

07-31-2014, 07:58 PM
Check compression. The stock pushrods may be too long for that cam, so it might be a burned valve or two. Could also be something as simple as a bad plug or wire. I'm curious. Never heard of using a thermometer to check wires and plugs. How do you do that? What I've always done is to pull one wire at a time to check for rpm drop. If no drop, the problem is that cylinder, its plug or wire or cap contact, or a vacuum leak affecting it. If you're running a points type ignition, could be the points.

07-31-2014, 09:27 PM
The thermometer trick I learned at the race track. Works best on headers but cast iron manifolds, if they're cool enough, will differ in temp on whichever cylinder isn't firing. Once they're not, though, it doesn't work as well as it does with headers.

Ignition is electronic with vacuum advance, but vacuum disconnected made no difference.

Motor isn't making any strange noises which I would expect from a bent pushrod. Bent valve, though, or collapsed lifter, that's another story.

I also forgot to mention that the pistons used were "six pack" pistons with valve pockets. Clearance checked out and I don't tend to rev high enough for valve float to be a concern; it's gone 6000 miles so far, though anything is possible.

08-01-2014, 01:39 AM
I would pull the valve covers of and check for possibly a broken valve spring,rocker or worn cam lobe/lobes.

08-01-2014, 12:52 PM
I like that thermometer trick. Unfortunately, I don't have an electronic one. Sounds like that would indicate they're all coming up to temp. But I would still pull one wire at a time and check rpm drop, if any. I am encouraged that you did check clearance on assembly. But a pushrod that is slightly too long might keep valve from closing completely (even though valve clearance is good), which would damage valve over time.

John Kunkel
08-01-2014, 01:41 PM
Save yourself some heartache and find a shop with a scope, they'll pinpoint the problem and it'll probably cost you less than "shotgunning" parts.

08-01-2014, 03:07 PM
Save yourself some heartache and find a shop with a scope, they'll pinpoint the problem and it'll probably cost you less than "shotgunning" parts.

By scope do you mean those little cameras they can put down the spark plug hole (or whatever they are troubleshooting)?

08-01-2014, 10:30 PM
Well, I'm not 100% sure but I think the problem might be cured. I ran a compression check on all cylinders and the lowest was at 185 psi and the highest at 200 psi, so not to wide a gap.

In the process, I reseated the plug wires at the cap and found the #2 wire felt a bit loose, like it was held in by the rubber boot rather than really making contact, so I adjusted that. I also found the choke plate a bit stuck, though I'm not sure it was such wide open or not. I don't run the choke connected so wide open would not have caused a problem but if it was partly closed and running way rich, I could see that affecting idle and power throughout.

I just started it but didn't have time to go for a drive to get up to full temp. But it seemed to idle better; less rocking back and forth. I'll try to get a good drive in this weekend.

08-02-2014, 11:49 AM
John is referring to an oscilloscope calibrated for automotive diagnosis. Most shops used to have them. I love them. Pestering my wife to buy me one for Mother's Day sometime. A scope is terrific for ignition diagnosis and can often pinpoint things like fuel problems and compression problems. I remember a 3-day-old Datsun 260Z that wouldn't start. Flat line on the scope = ignition box. Solved, bang. You can do a dynamic compression without pulling any wires. Very nifty and there are scopes you can get for home use.

John Kunkel
08-02-2014, 05:08 PM
By scope do you mean those little cameras they can put down the spark plug hole (or whatever they are troubleshooting)?

No, an analyzer like pictured below, a skilled operator can pinpoint a multitude of problems.

08-03-2014, 12:02 PM
That is basically what that is, John, an oscilloscope. Perhaps a bit fancier than the ones I used to use, probably with a lot more problem solving built in. Also probably has built-in scanning.