hesitation/stumbling at 4k rpm?

[FordPrefect]

Hey, guys!

I took the car out camping this weekend, about a 3 hour drive down the freeway, and had a blast. (topped it off with oil before I left)

Unfortunately, this morning, before I left for the return trip, I noticed it idled a little low, and maybe lacked a little power. I attributed this to cold temperatures. (still full of oil)

But on the way home, when I hit the freeway, it felt like it wasn't completely steady. The tach jumped just a little now and again, and I could feel a slight change in power when it occurred. This increased steadily, until it became quite noticeable.

Now I could tell when it was happening. It was *always* at 4k rpm. Slower was fine, and faster was fine. I stopped at a gas station to look at everything, and found the oil level had gone down maybe half a quart, and the carb had vibrated slightly loose again, so I took care of both those things and took off.

Pretty much the same story. Slight problem at 4k rpm, but not too bad. I kept it up around 4.5k to 5k rpm, and it was great. When I hit the next big city and had to slow down, the problem got so bad it became difficult to accelerate back past 4k, and the hesitation became really annoying, so I hit another gas station, and let things cool off while I investigated.

Carb was still snug, and it was still full of oil. Nothing else appeared amiss.

The problem didn't seem quite as bad on the way home, but I kept it above 4k most of the way. I also noticed when I got home that the tach was jumping a bit while it idled in the driveway.

I'll take a look at it tomorrow, but I wondered if you guys have any ideas? The only place it appears oil might be escaping is by the dipstick tube or the oil vapor separator 'elbow'. I pulled the breather hose from the air cleaner for the last bit of the trip, but it didn't seem to make much difference.

[vandor]

If it has electronic ignition check the pickup coil wires inside the distributor.

[FordPrefect]

Checked it out today, turns out the points weren't opening at all. Reset them, along with the timing, and now it's purring along wonderfully. Kind of curious as to why they'd get that way and the timing get so far off after a drive like that though.

Thanks for reading!

[Fi8spider]

The rubbing block on the points can wear out as it slides around the lobes on the distributor shaft, as a result the point gap will reduce. Nothing unusual about this.

A slight smear of grease on the lobes will help prolong wear on the rubbing block.

Part of my oil and service routine when checking the inside of the distributor cap and points.

Regarding the timing, maybe it was good enough before but better now? How many degrees did you have to move it to get the setting back to normal?

[FordPrefect]

After it ran great last night, today it started happening again. Bogs down and misses from 3-4k rpm.

Timing is still great, points gap is still the same... only thing I noticed are hairline cracks on the outer two 'fingers' on the distributor cap. I'll put a new one on and see if it improves.

As for the points themselves, these are new ones I put on a couple of months ago, and I had lubed them at that time. I expected some wear, but not quite so quickly.

The timing was very far off the other day, I have a cheap light, but it appeared 30+ degrees advanced before I put it back to 10. That seems pretty far off to be caused from driving.

[Fi8spider]

It does seem weird that the timing moved so much.
Cracks in the distributor cap wouldn't be helping much, see how a new one goes.

Some things that come to mind if the problem persists:

If you have a cam mounted dizzy check that the rotor button is sitting snug in it's notch on the dizzy shaft, check for wear or damage inside the rotor button where it mates with the dizzy shaft's keyway. Check to make sure the drive gear pin is pinned in nice and has no play in it.

If the distributor is block mounted check that the splines are ok where it meshes into the oil drive gear, worn oil drive gear maybe?

Bad spark plug and or coil leads will make the car do strange things too.

[bradartigue]

You say the dizzy cap is cracked, are you sure the distributor shaft is straight? If not it will move the points all over the place until they eventually have no gap. The fun part is this also burns up your coil.

[FordPrefect]

Hmm, I guess I might pull the dizzy when I get a chance and see how it looks. Right now I've put an older cap on there, but I haven't driven it yet, so I'm not sure if it will help.

Thanks again for your advice, guys. I'll let you know how it goes.

[FordPrefect]

Kind of bummed out that it still stumbles at around 4k rpm (which is kind of the sweet spot in the power band, I think).

Everything is dialed in as far as I can tell, points, timing, etc. It sort of seems to come and go, one test drive will be perfect, and the next it really struggles.

I guess I should pull the dizzy out and check the advance and everything? It's a centrifugal advance, which seems like it would be pretty problem-free. And if the dizzy shaft was warped, wouldn't it be more of a consistent miss?


At idle, the tach doesn't seem to be completely still, it kind of dips every so often, which makes me a little curious.

[azruss]

the centrifugal advance can stick. on older dizzies the weights can wear a groove in the bottom plate that can either restrain movement out or not allow the weights to return. I have seen dizzies where the return springs come off the post and allow the weight to move to full advance as soon as the motor revs.