cut out with sustained throttle

Brownie Mobile · Oct 29, 2014

I've been trying to Google this for the past 30 minutes or so before someone mentions that. I've found a few but people just seem to be chasing their own tails. It's a 302 4 barrel in am 85 f150 but the motors an 87 mild cam. So I was listening to her rev at idle. Had it at about 1500 holding steady, went to raise it feeling for 2 k rpm and it starts to cut out I held my foot steady and it stayed shitty. Went to increase climbed rpms very badly same problem up to 3 k ish rpms then it smoothed out and climbed just fine. Went to replicate the results but this time I went straight for 3/4 throttle it took a second to get out of the funk then climbed like it should. I'm leaning towards fuel pump issue or possibly vacuum leak. What do you guys think? Thanks.

alwaysFlOoReD · Oct 29, 2014

I'd be inclined to put a rebuild kit into the carb. I just did that to my 77 f250 motorcraft carb and went from 8.5 to 11.5 mpg. It also smoothed out a stumble under part throttle. I'm no carb guru but there are idle circuits, part throttle circuits, full throttle circuits, power valves, etc. depending on the design of the carb. A kit could solve a problem and will have all the correct settings included in the instructions.
Good luck,

Richard

Brownie Mobile · Oct 30, 2014

Yes normally this engine would have that but the po stripped this engine down to the very basics. It had freaking points and I just rebuilt the carb a few hundred miles ago. I'm going to try some things today and see what happens.

alwaysFlOoReD · Oct 30, 2014

There is a circuit that would affect what you describe. I believe its a hole just above the closed throttle plate position. Make sure that circuit is clean. Again, I'm not a guru and the correct advice may be different.

alwaysFlOoReD · Oct 30, 2014

Also, does it backfire? That would be a lean condition and support the vacuum leak.

RonD · Oct 30, 2014

Fuel pump issue wouldn't recover at higher RPMs, if pump isn't keeping float bowl full at 2k then it would be worse at 3k rpm.

Yes, are you getting pinging/knocking during stumbling(lean) or more exhaust smoke(rich), this would indicate carb issue, not eliminate carb issue though, lol.

Are you running vacuum advance on distributor?
You might be getting too much advance too soon, at higher RPM it catches up to spark advance.
If so, plug vacuum advance hose and try reving without vacuum advance, see if stumbling takes longer to show up or shows up sooner.
Or use timing light to check advance at higher RPMs, but vacuum level is different under load vs sitting in driveway, so hard to adjust spark timing this way, but might at least ID a problem with advance.

Very good read here on Ford mechanical spark advance distributors: http://www.fordmuscle.com/archives/2000/03/timing/

Pay attention to the vacuum source choices, full vacuum and ported or timed vacuum are available on most carbs.

Brownie Mobile · Oct 30, 2014

Ok it's 36 outside go to start her up and tried doing the same I did last night ( yeah I know revving a cold engine ya da ya da ) was perfect revved steadily up to 4k when I decided I didn't want to go any higher with cold oil on my cylinders. So this is a warm only problem. And it felt like it had more pep on the road. I'm going to let her idle come up to temp see if I get the same results make sure it's consistent.

RonD · Oct 30, 2014

You will always have more pep, and lower MPG, with cold outside air, although it is harder to take advantage of that with a carb, the more pep part.

The cold air is denser so more fuel can be added to each cylinder while still maintaining the 14:1 ratio.

This could mean your mid-range problem is a rich mix issue, since nothing changed on the carb and engine ran smoother, only difference is you added more air to the mix, denser air = more air.
But it could have also just been the choke since you didn't warm it up.

Cold engine has a cold intake and cylinders, so when fuel is pulled from the jets by vacuum it doesn't vaporize well, much of the fuel just wets the surfaces, choke causes more fuel to be pulled from the jets increasing the amount of vapor that reaches the cylinders allowing engine to start

Exgie · Oct 30, 2014

Does it start doing it when you accelerate?

Accelerator pump?

Brownie Mobile · Oct 30, 2014

I don't notice it as dramatically while driving. But it seems to be on increased throttle yes. Some good news and bad. Went to warm her up like I said decided to swap out my pcv and hose while it was warming up pulled off my air filter and my choke was closed..... I really only choke this puppy if it's been sitting for a few days. So I had the choke set to wide open but the butterfly was stuck opened it manually and wd 40 the linkages. Freed up I thought voila my problem fixed was just a stuck butterfly. So I tried it out and my 1500 to 2 k problem was gone but now if I stomp on it quick it wants to die. Gradual Is fine but I hit it pretty quick and it kind of coughs a little bit and starts to climb. If I do that and then let it drop down to idle suddenly it doesn't want to stay where I set my idle it stays a couple hundred rs lower let me know what you think that's what I had time for today .

RonD · Oct 30, 2014

Accelerator pump is either not pushing enough fuel into engine or pushing too much.
When you open the throttle plate quickly, the vacuum drops so carb's jets can't produce enough fuel.
The accelerator pump sprays extra fuel in to compensate until air flow and vacuum match up and jets are back to normal metering.

Also do discount vacuum advance.

If choke butterfly was sticking check the choke's cam on throttle as well, when choke plate closes there is a cam on the throttle that changes as well, so idle is set higher for cold engine.

Brownie Mobile · Oct 30, 2014

RonD said:
Accelerator pump is either not pushing enough fuel into engine or pushing too much.
When you open the throttle plate quickly, the vacuum drops so carb's jets can't produce enough fuel.
The accelerator pump sprays extra fuel in to compensate until air flow and vacuum match up and jets are back to normal metering.

Also do discount vacuum advance.

If choke butterfly was sticking check the choke's cam on throttle as well, when choke plate closes there is a cam on the throttle that changes as well, so idle is set higher for cold engine.

Went crazy on Google so I couldn't resist going out and trying some things. I adjusted the accelerator pump linkage tried it on all 3 settings no difference. Messed with the air control no difference. Messed with timing no difference. I think you're right from what I read the best test is to pump the throttle and see how much fuel is coming out with the engine off. It was kind of piddling out at low throttle ans just under half it seemed to really pick up which explains the problem hopefully that'll clear it up thank you guys. Accelerator pump coming on the 1st. Oh I will need to ID this carb not sure what model it is where would I find that? It's an edelbrock

alwaysFlOoReD · Oct 30, 2014

I just rebuilt the carb a few hundred miles ago

What carb did you rebuild?

Brownie Mobile · Oct 30, 2014

alwaysFlOoReD said:
What carb did you rebuild?

Edelbrock makes a universal kit so I just picked that up. There was quite a few it covered and I didn't think to look. Didn't seem relevant at the time.

RonD · Oct 31, 2014

Most edelbrock carbs have an ID number stamped on the base plate facing out, right below one of the hold down bolt holes.

And just as a reminder to those reading this thread
This is "the good ol' days" some wish for when they have a problem with their fuel injected, distributorless engine vehicle, lol.

Being younger at the time was GOOD, lol, but carbs and distributors were a PITA, monthly issues as they aged, give me EFI and distributorless any day.
Only reason people think carbs and distributors were "easier" is because they had to adjust or work on them all the time so they had to learn about them.
The newer self adjusting systems just don't need the constant attention so you don't need to learn about them..........until there is a problem, then you need to learn about how they work.
First time you worked on a carb you screwed it up, I did, I have put many a distributor in 180 out, F'ed up firing order on cap, even forgot to put rotor in and cursed the engine for not starting, lol.
"Good ol' days" yes, but not because of carbs or distributors

cut out with sustained throttle

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