Mr Fufu

Hi Guys,

Any quick tips on where to start to fix a misfiring condition in my 300hp 327?

Being winter, I hadn't taken my '67 coupe for a drive in about 6 weeks. This past weekend the weather was great, so I took her out for a drive on both Saturday and Sunday. But, she's not running so good. The engine started up fine, but a 'roughness' was immediately noticeable at idle. This roughness and accompanying lack its usual eagerness to run was apparent at all engine speeds. The misfire is there when the engine is cold, and remains once it's warmed up. All other systems seem okay - it doesn't run any hotter than normal, and oil pressure is normal. It just feels like I'm running on 7 cylinders, as the engine lacks it's usual vim. The last time I drove the car six weeks ago it performed just fine.

The car was tuned up about a year ago, and I've put less than 2,000 miles on it since then.

Before I start taking things apart, can you suggest likely causes, and where I should start my investigations?

Thanks!

Avispa

3 things for each cylinder to fire. Ignition, compression and fuel. Check ignition first. Pull one plug wire at a time out of the distributor cap with the motor running to figure out is any cylinder isn't firing. A dead cylinder will show no RPM change and the roughness won't change. If you isolate a bad cylinder, pull the plug to see if it's fouled so bad as to need cleaning or replacement. You should also put the plug end of that same plug wire near a ground, maybe using a screwdriver put inside the plug boot. Crank the motor and check for spark to figure out if the wire is good.

If that doesn't work, do a compression test to find whether one or more cylinders has bad compression. Could be a leaky valve, which you can identify with a compression test.

Last, check for intake manifold leaks, using engine starting fluid, spray around where the manifold bolts to the heads. You may want to check vacuum hoses for cracks or breaks at the same time.

That should get you through the obvious and easy stuff.

AZDoug

Or, just pull all the plugs and compare them, and look for an oddball. Easier to pull cold plugs than run the car to find bad one and wait to cool or work on a hot motor.
I woudl suspect a carb issue, maybe even just the gas has finally gone bad. When did you last put gas in it?
Doug

GUSTO14

You say the weather was great, what was the temperature and humidity like. Someone mentioned the carburetor, something I always think of when temps drop to near freezing and humidity is high is carburetor icing. It tends to be more of a problem with piston driven airplanes with carburetors, but it can happen to automobile engines as well.

Here's a chart that indicates the relative potential of carb icing forming based on outside air temp and relative humidity.
There is a discussion of what it is and how it forms here. While it is directed at aircraft, the principle of carb icing is the same.

Good luck... GUSTO

Frankie the Fink

Never hurts to pop the distributor cap and make sure the points lead wire at the breaker plate is attached solidly - look for acrcing inside the cap. Swab it out well as condensation can cause ignition issues. If the contacts are dark on the cap or rotor it may be time for replacement...
Make sure the distributor hold-down bolt is secure.

You can pull plug wires off one at a time with a rag to prevent shocks - if each cylinder is firing you should get a 75-100 RPM drop in idle...

Pour some fresh gas down the carb throats and see if the miss temporarily clears up..

GTOguy

As a tune -up technician for decades, I would do what Avispa said. He is right on track. one cylinder dropping out is an ignition problem 99% of the time with a carbureted engine. Probably a fouled plug or an open ignition wire. If that isn't it, it's time to connect a vacuum gauge and check the condition of the valves. Good luck.

MikeM

Here's quick tip. I'd pull the air cleaner off and observe if the choke is pulling off after the engine starts like it's supposed to.

Frankie the Fink

That -- and make sure the passenger side exhaust manifold heat riser valve opens when the car is warmed up...

Two threads in a row where I've agreed with Mike - maybe a trend ?

Rusler John

Dennisscars

This.. Seems this new weenie gas doesn't hold like it did back in the day. Last weekend I took carburetored motorcycle out and it ran poorly and wouldn't run with a good amount of (manual) choke at all times even at temp. Would run off choke underway but would immediately fall dead once you slowed down. Drained petrol and refilled with fresh. Cleared up pretty quickly thereafter. Never have had over winter fuel act like it was too lean. Fuel did not smell of days gone by bad fuel, actually didn't smell much at all.

I'd try to manually close the choke some to see if that helps. If it clears up after you enriched it, it's probably stale fuel.

dcamick

Check your coil, also.......

Gavin65

Op says it feels like its running on 7 cyl, if you have ignition shielding at the distributer , remove it to see if one or more wires are arcing off the top of the shielding, there should be a plastic insert to prevent just such a condition,

GTOguy

This is another thread where the poor OP is getting pulled in two directions at once. OP: can you post a video of the engine running with a vacuum gauge attached? My personal interpretation of your post tells me that your engine is missing on one cylinder...the classic 'dead hole' syndrome. If that is the case, chasing fuel system/carb issues is not where you want to begin, generally. You start with ignition and go from there. Could turn out you have a fouled plug due to a fueling issue, but you have to replace the plug first. If your car was belching black smoke, stinging your eyes, and randomly running rough at certain speeds, I would suspect carburetion. But for a single dead cylinder, a V7 with a stutter, I would NOT look at fuel first.....I would look into ignition and go from there. I know a lot less than most, but have made a living tuning cars for the past 40 years, so this is FWIW.............

karkrafter

See hoof prints think 'horse' not 'zebra'. You have a fouled pug or plug wire 'off' …

BTW, the way you describe it you have a dead skip, not a misfire, at least not the way I define those terms.

MikeM

This is true, generally as you say but the OP asked for a quick tip. Checking choke operation is about as quick as it gets.

If he has a rich fuel condition going on and he replaces the plug(s), depending, it doesn't take long to screw up a fresh set.

So, if the choke is stuck shut as I suggested above or, the float is stuck and gas is running every where, I'll stick with my "quick tip" that he asked for.

But you're right. A stuck choke by itself will not make a dead miss until it fouls the plugs.

GTOguy

A stuck choke can lead to the cause of the cause of a single cylinder misfire (one plug more fouled to the others to the point where it fails first), but in itself, a sticking choke will load up all the plugs, not just one. 99.9% of the time, a single cylinder dead miss is not fuel system related. Not on a carbureted engine/fuel injection system with a wet manifold. Port fuel injection, or direct injection, yes. But we're dealing with a carbureted 327 here, running on 7 cylinders. So: fouled plug/bad ignition wire/bad cap/ burnt valve/flat cam/yadda yadda yadda. P-----p----putt-----P----p----putt-----P-----P-----putt.....and so on.
Agree that replacing plugs with a known choke/fuel problem is not prudent.

Gavin65

First thing he needs to do is isolate what cylinder is dead, I agree its an ignition issue, bad plug wire, cracked cap, arcing off shielding.

MikeM

We agree on that much. We also agree that big picture, the putt, putt, putt, putt, putt (no PUTT) is likely ignition.

AZDoug

A very rhythmic misfire is obviously ignition, random misfire, eh, maybe not. Hard to diagnose on a verbal description that may, or may not be accurate. An obviously dead cylinder is kind of hard to miss. Driving at low speed feels like the car hit a brick every so many feet on a dead cylinder. Suddenly appearing misfire is usually not ignition related unless a plug got fouled, for what ever reason, so bad it won't fire.

Fouled plugs from old gas causes random, non rythmic misfire, unless just one plug is so fouled it won't fire at all. Just examining the plugs cold, can tell a lot. A clean set of plugs installed can also tell a lot.
Doug

Mr Fufu

Gentlemen, an update.

First of all thank you so much for all your insight and tips into my misfire/rough-running problem!

So, last night I set out to investigate per your instructions.

I hooked up a dwell/tach meter to the coil, and a vacuum gauge to the engine, and started her up. I connected the vacuum gauge to the brass port with 3 nipples in the manifold (in front of the distributor), using the port intended for the auto trans vacuum modulator. I'm presuming this to be a proper source of direct manifold vacuum, correct?

Here's a pic of the top of my engine. I connected the vacuum gauge to the top nipple (it's hose is the smallest of the three connected here):


After a few minutes, the choke unloaded fine.

Vacuum held steady at 18" at this time.

Idle was about 800rpm with choke on, dropping to 600 upon warm up.

I then removed the top shield and pulled each wire from the distributor, one at a time. For each wire removed I observed an RPM drop, some wires caused a more noticeable drop than others, however. Nonetheless, all produced a drop in the idle.

As luck would have it, when I pulled the #8 wire the cable came out, leaving the copper connector behind in the distributor! I fished that out and used needle-nosed pliers to re-crimp it onto the wire. Not sure if that'll hold for long... I'm not looking forward to replacing the ignition wires if they're bad as this looks to be a crappy job with all the shielding to re & re, the rubber grommets at the top shield, and routing the wires behind the exhaust manifolds to the plugs...

I then went for a drive, hoping maybe a loose wire was the cause of the rough running. Nope, the car felt the same - rough idling and running. Strange that pulling the spark plug wires didn't detect the offending cylinder...

When I returned from the test drive I again fiddled with the spark plug wires and hooked up the vacuum gauge again. This time the vacuum reading wasn't stable - instead the needle fluctuated rapidly between 15 and 18" when the engine was idling. Speeding up the engine smoothed out the vacuum reading but as soon as it returned to idle the needle began to flutter again. Strange that the vacuum reading was stable when I first started the car. Could this have something to do with the choke being on?

Another thing I noticed is that there was a small amount of gas leaking from the large flat brass nut at the rear of the carburetor. The gas was dribbling down onto the intake manifold. There are two of these large nuts on the top of the carb, seen in the photo above at lower right and left of the Holley, seemingly related to the balance tube between the float bowls. Ok, my ignorance of carbs is showing through as I don't know what these nuts are for.

So, I took a flat bladed screwdriver and gently tightened this rear nut down, which stopped the minor gas dribble. I know Holley carbs are prone to leak gas, as from time to time I've seen small amounts of gas on the manifold, mostly coming from the area around the accelerator pump. I had this carb rebuilt two years ago, and the re-builder advised me to tighten the small bolts holding the float bowls to the carb body from time to time, which I have done.

I didn't have time to start pulling the spark plugs to check their condition. Now I'm wondering if I should bother as maybe my problem is carb related? Also, I when I pulled each wire off the distributor there was an RPM drop.

Your thoughts?