hesitate on acceleration / low vacuum?

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timk
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hesitate on acceleration / low vacuum?

Post by timk »

Hi. A few weeks ago I bought a 2wd F100 with 360 & C6 auto which starts easy and runs ok (if a bit rough) except for a really bad hesitation on initial acceleration. Reading the existing threads here I see that this is quite a common problem so the first thing I did was start going through the advice already posted, and while there is definite improvement it is still not right and I have also compiled a couple questions of my own.

[As I was typing this it started getting really long so I'm going to put my main question right up front so those who get bored reading don't miss it:
I can't get more than 15 inches of vacuum and the needle is bouncing a lot -- what's up with that?]

First up I did a full electrical tuneup because there's no point chasing carb settings if the ignition is not up to snuff. So, new plugs & wires & cap & rotor & points & condenser. Set the points gap to spec with a dwell meter and set the initial timing to 8 degrees btdc with a timing light. I also replaced the aftermarket inline fuel filter because I saw mention of one guy who had a silt-filled filter causing the same hesitation.

Oh, and I should mention up front that I have to wait until next month before I can pull the carb and rebuild it on the bench, so I'm just trying to do what I can with it in the vehicle for now.

I'm new to Fords and therefore to the Autolite 2100, but I've worked on enough vehicles to immediately suspect the accelerator pump when this symptom is encountered so I looked into that first. I can see it squirting raw fuel down both barrels when I move the throttle linkage by hand, so I know it's at least not completely dead with a destroyed diaphragm or something like that, but I don't know how to determine whether it's squirting the full amount that it should. The rod which controls it is in the 2nd hole from the end at the pump arm and the 3rd hole (of 4) from the bottom at the other end. I'm thinking of maybe some trial-and-error with the other positions to see how it reacts but haven't done that yet. I did watch a youtube video on rebuilding an Autolite 4 barrel and the guy in that mentioned that his old diaphragm felt stiff and the new one in the rebuild kit felt more flexible -- could a stiff & old but intact diaphragm be pumping some but not enough gas?

One other often-referenced item is the vacuum advance on the distributor. To verify that it actually advances the spark when vacuum is applied I disconnected the vacuum hose from the carb and hooked up a little Mityvac hand pump that I use for bleeding brake lines. When I apply vacuum directly to the advance unit on the distributor and watch the effect with the timing light I see that it is indeed advancing. Much like with the accelerator pump I don't know if it's operating to the full extent that it should, but it is at least doing something. I looked for a spec stating exactly how much it's supposed to advance the ignition but couldn't find one -- any help there?

Then I went to setting the carb idle screws, which I found to be in a horribly unbalanced state -- one was 2.25 turns out and the other was slightly over 3 turns out. I expect that was at least part of the cause of the rough idle. So I turned them both in all the way and out two turns to get a starting point, and here's where I couldn't get the result I was looking for: Everything I read indicates that I should be able to get near 20 inches of vacuum and a steady needle but I can't get more than 15 and the needle bounces pretty dramatically. I have my vacuum gauge hooked up to a 3-port vacuum block at the rear of the passenger side of the intake manifold. One port has a hose going to the brake booster, another has a hose going down to what I would guess is the transmission, and the third port just had a cap over it. So that's where I hooked up my gauge. (Oh, and just for extra testing I also hooked my gauge up to the hose from the carb to the distributor -- when hooked up there it shows zero at idle and then jumps to 15ish when the throttle is opened, which seems right given its intended function.)

Any suggestions on how I should proceed? (Short of pulling the carb and rebuilding it -- I'll be doing that in April but am trying to get the truck to run as well as I can until then.)

And if you made it this far, thanks for reading and thanks in advance for any replies. This appears to be a really solid forum and I look forward to growing from being an all-questions newbie to hopefully being a contributing member of the group.
1971 F100 Ranger XLT Camper Special w/360 & C6 automatic
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sargentrs
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Re: hesitate on acceleration / low vacuum?

Post by sargentrs »

Here's a good reference for interpreting your vacuum gauge readings for potential issues. http://www.secondchancegarage.com/public/186.cfm I would first rule out any vacuum leaks. Try plugging off the brake booster, vacuum advance and transmission vacuum lines then check again. If no improvement, spray carb starter around the external areas above as well as the carb base, heads, intake, etc to see if you notice an increase in rpms when sprayed. Make sure you hit the back of the intake. If no noticeable leaks, hook up your vacuum gauge to the manifold port you mentioned, disconnect and plug the vacuum advance, and tune your timing and carb for the best, steady vacuum reading you can get. Then hook up your timing light, rev it up to 2500 rpms, and see how your max timing comes out. You're targeting 35*-38* all in. If you can achieve that, lock your distributor down, plug everything back up and see how she runs. My 302 must have had some issue with the timing marks, harmonic balancer or something. I timed it for 38* max timing and the initial fell in at 22*. Sounds fishy but she ran great. The factory specs for timing are a "guideline". Every engine is different and it wants what it wants.
Randy
1970 F100 Sport Custom Limited LWB, 302cid, 3 on the tree. NO A/C, NO P/S, NO P/B. Currently in 1000 pcs while rebuilding. Project thread: http://www.fordification.com/forum/view ... 22&t=59995 Plan: 351w, C4, LSD, pwr front disc, p/s, a/c, bucket seats, new interior and paint.
1987 F-150 XLT Lariat, 5.0/C6 auto.
70F250CamperSpecial
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Re: hesitate on acceleration / low vacuum?

Post by 70F250CamperSpecial »

Timk.

Did you ever get this issue resolved? I am having same problem with my '70.

Thanks
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colnago
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Re: hesitate on acceleration / low vacuum?

Post by colnago »

Well, the original post is almost two years old, so he's probably not looking for an answer any more. But hopefully this can help 70F250CamperSpecial.

I have a '67 with a 352. Very similar issues. The vacuum needle bounces around when idling, but settles down when I rev the engine. No matter what, I couldn't get the needle to be smooth on idle. Also, on really cold mornings, the engine has to heat up before the C6 would shift from 2nd to 3rd. I ran a compression test, and found that #1 and #3 were way below the others (like, half of the other six cylinders; not just low, but useless). Finally, the engine backfires through the carb when cold.

So, I figure I have two burnt or bent intake valves. This is causing the jumpy needle, and prevents the engine from creating enough vacuum for the C6 when it's cold outside. I'm doing a rebuild on another block in the garage, and hope to finish that and swap it in this spring. Hopefully, the problems will go away after the swap (then I'll only have new problems to deal with!).

By the way, I thought about just rebuilding the heads, but a friend convinced me that if the top end was worn out, so was the bottom end, so I might as well do it all.

Hope this helps,

Joseph
"Sugar", my 1967 Ford F250 2WD Camper Special, 352FE, Ford iron "T" Intake with 1405 Edelbrock, Duraspark II Ignition, C6 transmission, front disc brake conversion.
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