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1977 F100 dies after idling for a bit....altitude related?

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1977 F100 dies after idling for a bit....altitude related?

  #1  
Old 09-19-2018, 02:22 AM
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1977 F100 dies after idling for a bit....altitude related?

Ok, time for another couple of questions.

After finally figuring out I’d sheared my distributor gear pin, I’ve got my F100 running again. Sort of.

The truck will start and idle just fine, but after about 5-7 minutes of idling, there is what sounds like a drop in the idle. It’s almost exactly like when an AC compressor kicks on an AC equipped vehicle.
The drop lasts for a count of 1-2 or 1-2-3, and then the idle goes back to ‘normal’. About 35-40 seconds later, the idle sounds like it drops again, for the same length of time.
If I let it just sit there running, the interval between the change in idle slowly decreases to around 15-20 seconds and then the engine will sputter once (sometimes) and then die. The drop duration doesn’t seem to change.
It starts right back up, but the idle drop cycle starts almost immediately and it’s usually dead again within a couple of minutes – no 5-7 minute wait to start the cycle.

Driving around town, everything seems to be good, with the exception of going through a drive thru. When I let off the brakes to creep forward, the engine will sputter and die more often than not, although it doesn’t do it every time, sometimes it’s just fine.

Here’s what I’ve replaced within the last week and a half:
Fuel pump
Fuel filter
Battery
Ignition Solenoid
Coil
ICM
Stator
Plugs

While replacing everything, I set the timing to 7ᵒ BTDC and gapped the plugs to .040”.
I hooked a vacuum gauge to the manifold and adjusted both sides of the carburetor until I was pulling the highest vacuum I could get.

I should note a couple of things: I’m at about 7,200 feet above sea level. The highest vacuum I could get was about 8-9 inches. I’ve read in the forum here that a 1 inch drop is expected for every 1,000 feet increase in altitude. So I figure I’m pulling somewhere near 15-16 inches. Not great, but not too bad?
Also, the vacuum gauge needle ‘fluttered’ extremely quickly by about 1-1.5 inches, from what I could tell. When I say fluttered, it was almost a blur.

My questions are what could be causing the idle drop-to-die cycle? Could it be altitude related?
Are the timing, gap, and vacuum readings ok/recommended for 7K+?
What could be the cause of the vacuum needle flutter, and what would be the recommended fix? I’ve seen breakouts of what the needle behaviors indicate, but I didn’t see anything that listed a very quick flutter in such a narrow range.

1977 F100, 302, automatic, pretty much stock from what I can tell

Once again, thanks for all the help.
-Raxx


 
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Old 09-19-2018, 06:07 AM
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What is your actual vacuum now ?

It is either 8-9'' or 15-16''. It can't be both. (The former is more likely.)

One you know, advance your idle timing to 15* and measure the vacuum again.

Post up the new vacuum number and behavioural change in the engine.

Between what numbers on the gauge was the needle fluttering, at what rpms, and was the idle smooth-ish at this point ?

Have you had the truck a while at this altitude and the current behaviour is 'all of a sudden' ?

Hmm, where did the sheared dizzy pin go to ?
 
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Old 09-19-2018, 07:09 AM
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FMJ-

Thank you for your response.
You are correct. My actual vacuum is 8-9”. The 15-16” was me speculating on the compensation for altitude, and what it would be at sea level.
I will advance the timing to 15* and see what happens.
As for the needle flutter, that was constant at idle. I don’t have a tach, and I’m not sure how to get the actual RPMs without one (?).
The truck came from about 1,000-1,500 feet lower in altitude, so still a high altitude vehicle.
And the shear pin didn’t really go anywhere. The center section was still in the shaft, and both ends were still in the gear itself. I guess I got lucky that way.

I’ll post the new behavior after I’ve had a chance to advance the idle to 15*, but that might not be until late this evening.

Thanks again,
-Raxx
 
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Old 09-19-2018, 08:28 AM
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Your speculation should be close !

I think you'll see closer to 15-16'' vacuum when you've advanced the timing.

If it idles decently when you've done that, take it for a test drive, and post up any difference you felt in driving when you post up the rest.

$10 dollars says you'll be posting up a smiley face !!

That's good news about the pin.

I've attached a pdf with regard to vacuum readings. You can do tests and observations when you're ready.

A tach would be handy, but no worries that there isn't one.

Ok, even at 5,000 ft, the timing was not advanced enough.

Have you had this problem since acquiring the truck or is it 'all of a sudden' ?
 
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Old 09-19-2018, 08:34 AM
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Normally we set ignition timing with a timing light, and measure manifold vacuum as a general indication of engine health and tune. Compression, valves, etc. One problem that is very common with older engines is the crankshaft vibration absorber inertia weight (where the timing marks live) slips off its register or "clock". It is bonded to the basket with a rubber sandwich, which rots over time. Consequently if you were to set the timing with a light at say, 8° BTDC or whatever, the actual true ignition timing might be something like 3° ATDC, who the hell knows etc, the engine runs terrible etc, and everybody scratches their head.

It is true that at higher elevations the average manifold vacuum is decreased but considering a stock engine in good tune and timing at sea level should pull close to 20" somewhere around 16" is what you'd want to see. Instead of timing the engine to an arbitrary number on the damper try using a piston stop and verify that the TDC "0" mark on the damper is still accurate. This is important because everything timing wise depends on this point being accurately located. If the damper is defective it cannot absorb crankshaft vibrations either. There is a specific RPM where the crank reaches what is called Resonance. This is a Bad Thing(tm) for a crank and will eventually cause cracks and may even break, also contributes to oil leaks I'd wager, buggers up main bearings etc.

Another thing you could try along the same lines is adjust distributor timing for the highest steady vacuum, which should be around 15" or 16". Then check and see what the damper timing # indicates with a light. If it's some screwy number that doesn't make any sense, it's another clue that the damper is lunched. It's important that timing is set correctly as it directly affects the manifold vacuum through the carburetor. A weak manifold signal means fuel isn't being pulled through the venturies fast enough, poor fuel atomization, poor idle and off idle acceleration yadda yadda yadda.

If everything is in good shape and tuned up right a key metric to observe is that a vacuum gauge hand is steady. Higher is always better than lower in a stock engine, but steady is crucial. You'll have to investigate why if this is not the case. Generally a rapidly bouncing needle or rythmic drop or fluctuation is indication of some defect that only affects one cylinder. Keep studying the vacuum gauge charts, you'll get it figured out.
 
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Old 09-19-2018, 08:38 AM
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Oops ! I forgot the attachment.
 
Attached Images
File Type: pdf
Vacuum readings.pdf (134.1 KB, 8 views)
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Old 09-19-2018, 10:35 AM
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If you sheared the pin the dizzy may be in bad shape mechanically. i.e. dragging.
 
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Old 09-19-2018, 10:38 AM
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Thanks for the advice, everyone.

I should have stated that after pulling the dizzy, I found TDC by first finding the compression stroke, then using a chopstick (it was what was handy) and cranking the engine ever so slightly back and forth until I was sure it was on TDC. After doing that, and stabbing the dizzy, I checked the timing marks. As for what I could see, the timing marks are still accurate to within 1*. Some previous owner had marked TDC with a paint pen, and the edge of the paint pen mark aligned with the top “long” edge of the timing mark pointer. So at most, it was off by the width of a paint pen mark...about 1* to either side of the TDC line.
Due to the rusty, hard to read nature of the marks, I hand cranked the engine around until the marks were facing down, scrubbed them with a wire brush and painted all of them yellow. While I was down there I did have a look at the wheel (is this the harmonic balancer?) where the crankshaft vibration absorber inertia weight is mounted. Everything seemed good, but it was not a “tear-down and remove “ inspection, just a visual and grasping it and trying to see if I could get it to wiggle/move type inspection. Everything seemed tight and in good order.
I then set the timing using a timing light. After setting the timing, I hooked up the vacuum gauge to the manifold and adjusted both sides of the carburetor until I got the highest vacuum.
Using a vacuum gauge is new to me, so I'd read here about 'tuning' (setting the lean/rich mix) a carb using one. From what I could find, more of the guides said set timing, then adjust the carb for the best vacuum.
I didn't catch coming back to adjust the timing to improve the vacuum.
I did run the timing light again after fussing with the carb, simply out of curiosity to see if anything had changed. From what I read, it was still sitting at 7*.

I just got off a 15 hour shift and need to grab some sleep before getting up in about 6 hours for tonight's shift.
I excited to see what happens when I advance the timing to 15*, but I'm more likely to make some stupid mistake at the moment, so it will have to wait. If I can't get it done before I have to head into work, it will be done when I get home tomorrow morning.
Working a 12+ hour night shift is not so fun, sometimes.

I will post what I find, though. I REALLY hope it's a smiley face!

Thanks again,

-Raxx
 
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Old 09-19-2018, 11:12 AM
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OK, sounds good. Be sure to disconnect and plug vacuum advance port in the carburetor, and set the timing to specification at a slow idle, no more than 500 RPM say. The centrifugal weights start to advance the ignition timing just off idle and will lead to an inaccurate (retarded) timing setting otherwise. Being mindful of the radiator fan plane - then run the RPM up to 1000, 1500, 2000, 3000+ etc and make sure the mechanical advance moves smoothly up and down. It should top out around 36° to 37° BTDC without vacuum advance. Where the timing tops out is important, initial timing not so much.
 
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Old 09-19-2018, 08:02 PM
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FMJ,
Thanks for the attachment!

beartracks,
This is the first I've heard of the dizzy possibly dragging....how would I check this? From what I've read on sheared pins, it seems to be something that just sort of happens...some guys have posted stuff like, "Oh, I had to replace three pins in six months four years ago, but everything's been great since."

All,
I didn't get a chance to mess with the timing before having to go into work, so it won't happen until tomorrow morning at the earliest or maybe even tomorrow afternoon. I'll post what I find as soon as I get some results.
 
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Old 09-19-2018, 08:07 PM
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Oh, and Tedster9,

Thanks for the tip about plugging the vacuum advance on the carb. I 'sort of' know that (it's been about 30 years since I've really done anything with a carbureted engine). I can't honestly say if I did that or not when setting the timing...
 
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Old 09-19-2018, 08:27 PM
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It's easy to forget! Normally, it won't matter if the RPM is low enough, but "belt & suspenders". That's where the novice gets sideways. Idle RPM @ 800, half the advance is pulled in already when trying to set it.

Initial timing isn't really too critical, something more than 10° BTDC and less than starter kickback° is fine. The timing you're interested in is on the far end, where it tops out at, and that it isn't sticking somewhere along the way, or the shaft & bushings all wallered out etc. These kinds of defects will cause the engine to have to be detuned, to keep it from pinging.
 
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Old 09-20-2018, 01:54 PM
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@ raxxter

There's a good chance that your carb is too rich seeing as the truck has been elevated another +/- 1,500 ft.

When you're ready, set it up with an AFR gauge, or read the plugs (google when and how to if need be.)

You'll know if the dizzy is dragging or not when you look at your 'timing curve'.


@ Tedster9

His total mechanical timing will, and should, exceed 40*. Maybe around 44* given the elevation.

The idle timing is just as important as the 'all in' timing, and the timing curve is customised between those 2 numbers.

The spring strength determines how quickly the mechanical advance starts, so if half the timing is already pulled in at 800 rpms, there's a problem with the spring settings.

Good call on disconnecting the vacuum advance. I forgot to mention that.
 
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Old 09-20-2018, 02:45 PM
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Originally Posted by FMJ. View Post


@ Tedster9

The idle timing is just as important as the 'all in' timing, and the timing curve is customised between those 2 numbers.
Yeah I just trying not to get way off in the weeds too much on ignition timing, the novice generally seems to want to obsess over the initial distributor timing whether it's 4° or maybe 6°, though verifying the top end is just as important, put it that way.

The novice often isn't likely to disassemble and customize the curve in the internal distributor advance so it makes more sense to set the far end of the curve and let the initial timing fall where it may in order to achieve the OEM total mechanical advance or whatever it will run best. Somewhere between 10° and 20° BTDC initial timing is fine, the exact number is not critical. The only factor limiting this is starter kickback basically. What is critical though is that the timing is set at least close to specification up and down the RPM band.

If he can get 40° or 44° BTDC at altitude without knock or detonation that would be good, the general idea is always to have as much advance as possible short of ping or detonation. Most V8 are very happy around 36° to 38° BTDC around 2500 - 3000 RPM. This would be without vacuum advance. Higher compression engines tolerate less of course.

The spring strength determines how quickly the mechanical advance starts, so if half the timing is already pulled in at 800 rpms, there's a problem with the spring settings.
With the vacuum advance connected it will start to pull in quite a bit of advance just off idle, the primary spring, maybe a little bit. Yes I freely admit to exaggeration, to make a point!
 
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Old 09-20-2018, 11:47 PM
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Hi Tedster9,

Ah, ok, I see now where you're coming from.

I know you know your stuff, so when I read the posts I thought WTF !? (As you can imagine. LOL)

LOL, it wasn't so long ago that I was focusing on 6* idle because that's what my Haynes book said, and totally oblivious to the entire ignition timing system.

 

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