71 F100 Hard Start

[majesticpark]

Hey all, I'll do a quick summary here and add additional info afterwards. And uh, just so you know: every project is a story. Enjoy.

My F100 is hard starting. The behavior is as follows: I give a gentle push & release on the accelerator to free the fast idle/choke circuit, turn the key and listen to the fine cranking-amps of my new starter... and I continue to listen for as long as I please, or until I release the ignition. I have to pump the accelerator 2-5 times, turn the key, and at least half the time hit WOT while cranking to catch. After the hard start, I usually need to maintain accelerator to hold idle. I can hold it at any rpm I like above 600, but if I release it drops down to about 600/700 (even with feathering), then slowly winds down to nothing and stalls. I've seen it run for several seconds at ~150 rpm. Now, if I do maintain idle with the accelerator (I've experimented and found even as little as ~800 rpm will serve), Old Red (12yo daughter named him, wife voted in favor) will usually pick up the slack and take over and is even drivable with no balking, spitting, backfiring or bogging while shifting into rpm as low as 900 (that darn np435...). I'll include a list of all things I've done to Red since he came to live with us below, and appreciate any insight you can all offer.

Brace yourselves!

So then. Old Red is a stock 1971 F100 Y-vin with a stock FE 360, Vin-tag certified ford 172 HP @ 4100 rpm. Carburetor has an Autolite brass tag reading D1TF HA, and was rebuilt after purchase about 2 months ago. After the rebuild Red would run for a bit after sitting, and then stop running, backfire like a howitzer and not budge. Came to find out he was flooding like crazy, and this was sourced to the carb not being adequately tightened down. I felt this was due to the wings being warped downward and added a second gasket between the carb and manifold. Tighening the carb to spec with 2 gaskets eliminated flooding.

After that episode, we couldn't get the points to behave properly and the mechanic to whom I defer when I feel like I'm chasing a white rabbit suggested a petronix electronic ignition, which we had installed. Living in Denver, CO, Red came due for his initial emissions test (2-speed idle; @idle <1000rpm and @2500rpm), which he failed several times. First two times were on my watch, and the next three under the supervision of my mechanic. He eventually had to "detune" Red; My understanding was that Red's timing was set to almost 0* BTDC, and it seems the idle screws were opened almost completely. After passing CO emissions, Red was set by mechanic back to 6* BTDC and idle screws were set at about 5 turns out.

After that I thought I'd go at Red with some common sense and a great deal of external consultation (mostly this site, but some others). First thing I did was gap the plugs (no obvious signs of unusual wear and no unexpected rich/lean indications; I had even swapped in new plugs pre-emissions testing, and gapped them) to .035". This seemed to make no difference. I then checked the timing and found the 6* setting. Some reading lead me to believe Red was a bit of a codger and I fiddled with his timing (as low as 3* and as high as 20*, advance unplugged and capped with a BB) and found his sweet spot to be about 12* BTDC. He ran fine in the driveway as high as 20*, but I thought that would bring total advance too high under operating conditions and backed it down through careful listening and observation (with a timing light of course). He was super smooth once running at 12* and drives very nicely there despite his unwillingness to start cold.

OK; plugs seem ok, ignition isn't an issue, and now thanks to a new timing setting he starts great when hot... now what? Well, I got to thinking that maybe he was running too rich (I tend to get going in the mornings when it is about 20-35 F here in Denver at this time of year), so I checked my carb out from the outside. Through trial and error I found that Red wasn't quite warming up in an expected fashion. Turns out during the rebuild I had not properly seated the choke bimetal spring. Correcting this issue lead me to discover his fast idle was set to about 2100 rpm, silly as that might be. I got to tinkering.

Best way I could figure out to set fast idle on the block was to warm the engine and plug the vacuum advance. With the choke opened to max I was able to trick Red into fast idle by gently persuading the throttle open enough to toggle the fast idle circuit by hand. I pulled it all the way open, released the throttle and checked rpm. I eventually settled on 1500 on account mornings sometimes being colder than euphamisms may allow. Still no change in hot starting, shifting or drivability. Next day I go to start Old Red up and... a lot of cranking. Starter makes a fine racket like it should, but no catch.

Right then. I took my vacuum gauge back to work, warmed up the engine, blocked the advance and fiddled with the idle adjustments; I was now of the opinion that maybe Red was running too rich. I seemed to need to hit (gently depress mind you) the pedal to WOT and hold while cranking for Red to start and allow me to warm him up. My limited carb experience indicated that he must be getting too much fuel. Ok: I found vacuum at warmed up idle to be ~12.5" Now, I live at about 5400ft elevation. I was taught (God knows when) that for every 1000ft above sea level vacuum falls roughly 1", ignoring fancy cams and such. Well, that should put me at about... 17". Hm. I messed with the screws by the following method: turn one down a half turn, then the other. I did this for a total 0f 2 turns, and on 2.5 turns on screw 1, vacuum slowly increased. Screw 2 duplicated that result. I kept going, but maxed out at just over 14", but rock-steady; initially the vacuum needle was quite the mover and shaker (steady oscillation +/- for your more data oriented folks). Red and I settled on what turned out (forgive me) to be about 3-1/4 turns out from completely closed (I checked by turning the screws in all the way running and off and averaged the two, then set them to said average as I apparently had not been as consistent as expected, and verified vacuum at a hair over 14").

Still, Red does not want to start. Today and the day before the overnight low was about 20*F, and startup ambient temp was 36*f and 29* respectively. Both days I put the key in, gently depressed and released the accelerator, turned the key to crank... and cranked, and cranked. No start. Yesterday I experimented. One gentle pump pre crank: no difference. Two: a pop and crank. Three: a pop, jumble and... crank. Four: a jumble'n'go but I let off the key too soon and Red went back to bed on me. 4 again, key held fast: Red caught! but when I let go the key he idled for a few seconds at ~1000rpm (?!) then slowly petered out and stalled at ~200rpm. Hm, right then. 4 pumps, cranked and caught then feathered the accelerator and released the key. Red caught and stayed, but I had to hold him at a minimum ~800 rpm via accelerator for about a minute. Then he idled at ~700... didn't I set that fast idle?

Fine, yesterday is gone, today is anew. I repeated the same process as before, except I tried going from 4 pumps to 5 pumps on a crank without accelerator. I had the hood up and filter off. Wouldn't you know it, but Red spit flames out of his carb at me. I went back to 4 pumps with the accelerator, held it for a minute or so, released Red idled at ~1100 rpm.

So suffice to say my truck has me stumped. Admittedly I've not had much experience with carbureted engines. I would greatly appreciate advice and guidance as I dearly love this truck, despite the hours it has required of me to bring it back to reliability.

[DuckRyder]

Sniper EFI?



Ok, that probably isn’t that helpful... give this refresher read:

https://www.hemmings.com/magazine/mus/2 ... 44991.html

It sounds like you have it mostly covered. 3.25 turns out sounds about right, 12 degrees sounds about right. 2-3 pumps sounds about right.

Is this engine stock? Carb has hot air choke? (this would be as good of excuse as any to post pictures of the carb, engine, transmission rear end interior and exterior )

With the engine off does the choke blade close completely when you press and release the accelerator? Does it then open slightly when the engine cranks (there is probably a specification for this)?

[majesticpark]

Thanks Duckryder; I've been over that article several times, as well a few others that come up often as a resulting of searching variations of "71 f100 carb/choke/start/fast idle/etc."

The engine is stock; I'm not a big fan of the painfully small exhaust manifolds and excruciating-looking manifold-to-collecter piping. They look like soda straws to me. It is a hot air choke: I have the coolant line snugged up against it, retaining screws a bit past just-tight so as not to back out, I even took the hot-air line from the passenger exhaust bank and swapped in a new hi-temp rubber(ish) line to replace a collapsed corroded section of that particular tubing.

I believe the choke plate is closing completely but when I go to start this morning I'll pull the filter just to check, and take the opportunity to get some photos and possibly video. Of course since we're diagnosing the engine, I'll need to have any interested parties thoroughly examine the body and such

The choke vacuum pull-of did get adjusted as part of the rebuild; the set screw was cranked all the way in, preventing the pull off from ever touching the choke catch. I'll observe to see that it is in fact opening the choke a bit.

I think that's everything you asked.

[colnago]

Well, I would never call DuckRyder wrong, but I see things a bit different.

You said that you have warped wings on your carb, so you used a second gasket. Sounds like you are set up for a massive vacuum leak. You're compensating for it by having your idle screws out too far (personally, I think anything more than 2-1/2 full turns out is too much). Easy enough to check by spraying carb cleaner around the carb. If the idle changes, you found your leak. The warped wings also make me wonder if there's anything else bent on the carb. If the carb is warped/damaged, why did you rebuild it and not replace it?

If you do have a vacuum leak, everything else needs to wait for this to be fixed.

Joseph

[DuckRyder]

Joseph makes a good point about the warped base plate. Definitely check that...

2.5 turns out is generally pretty good, I was allowing some leeway for the altitude. But tuning at altitude is something that I have not done and carb theory was a long time ago...

Let us know how the choke and pull off is working after your observations. It probably also bears observing when warmed up to make sure it is fully releasing...

[majesticpark]

Hey guys (generic salutation, that),

Sorry I missed your replies. I’ve got some pics and video that need attention from a couple mornings back where my sunrise temperature was in the twenties. I haven’t reviewed the video closely but I recall feeling the choke pulloff wasn’t quite right.

That said, Joseph makes an excellent point regarding warpage and my laxadaisical regard for it. What little mechanical acument I am possessed of tells me bent surfaces that once were flat are bad news. I did try and test this with a heap of carb cleaner followed by some fancy, unstable and cantankerous starter fluid; it gets my TJ going when nothing else will.

I did not note any perceptible change in rpm with either volatile, but I think it bears mentioning that I have doubts about my skill as a diagnostic mechanic.

Now then: I’ve had nagging sense of can’t-get-right ever since we rebuilt the carb. It was -after- the rebuild that the biblical flooding occurred, and I discovered the warpage (about .006” of cancavity as far as my perception and a feeler gauge allows). I figure that the carb had finally had enough once dismounted and like the cylinder head of my ‘05 Honda Element (don’t judge too harshly, Red has cost more in two months of parts than that little k24a4 consumes fuel in almost a year) it just got all twisted up once relieved of the pressure of its much-beleaguered bolts.

That line of thinking has left me with a vague sense of unease about the slim figure of my carb. Is it right? Are all the finely engineered pathways, passages and clearances what they should be? I suspect that it would be hard to diagnose yes or no without impressively accurate measurements on all components involved. Again however, self-doubt sufficiently displaces confidence in my assessment.

I’ll try and get my videos (I just can’t stopnmuself from collecting data) up this weekend so that I can submit them to your (appreciated) scrutiny. I’ll also post the pics I took, but those are more for my ego than any diagnosis.

Anywho, thank you both for weighing in. I hope to provide more information to go on. My gut says there may be a whole lot of not quite right in Red’s carb and that the cumulative problems are dogging me. I hope we’ll figure it out.