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Troubleshooting slowdown or low rev stalls

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macnewbie

Troubleshooting slowdown or low rev stalls

Hello,

I am a relative newcomer to the forum, slowly learning about my new-to-me 1986 XLT Lariat truck. I have slowly been working down a list of a few things that require some attention.

Among them are engine stalls that happen when decelerating (for instance when making a turn) or at low revs from a stop.

My truck has the 351 HO engine with the Holley 4180 carb. Reading online, it seems likely that the root cause is either a vacuum leak or something going on with the accelerator pump.

I am mechanically inclined and have access to a well-stocked shop, but I have never done this before - how do I go about troubleshooting to figure this out?

Thanks!

Juan

Juan

1986 F-150 XLT Lariat 2WD, 351w HO

Gary Lewis

Re: Troubleshooting slowdown or low rev stalls

Administrator

Juan - Welcome! Glad you joined.

However, you may have missed the email that asked you to post first in the New Members Start Here folder. The reason for that is because we have our guidelines posted there. We ask you to do that to ensure you've had a chance to read them because we hold everyone to them.

So, can you assure us that you've read them?

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

macnewbie

Re: Troubleshooting slowdown or low rev stalls

This post was updated on .

hmmm. I think you are confused Gary -I did post an intro thread.

Juan

1986 F-150 XLT Lariat 2WD, 351w HO

Gary Lewis

Re: Troubleshooting slowdown or low rev stalls

Administrator

I am confused.

Thanks for pointing that out.

On the stalling, the two issues would create opposite situations - a vacuum leak would cause it to be lean and a leak in the accelerator pump (or the power valve) would cause it to be rich.

So let's explore. A vacuum leak usually causes the idle to be high and hard to bring down to normal. Do you have that problem?

But a bad accelerator pump or power valve will usually cause the exhaust to stink. And the truck will be easy to start cold but hard to start hot. And awful MPG.

So, do you have other symptoms?

Gary, AKA "Gary fellow": Profile

macnewbie

Re: Troubleshooting slowdown or low rev stalls

I don't have a tach installed (yet - it's on the list), but the idle seems normal. The choke works as expected on a cold start, then a blip of the throttle brings the revs down.

I would not say the exhaust stinks particularly bad, but the MPG is dismal. As of now, I have only observed the stutter/stall when accelerating from a idle (as in from a stop, or from slowing down to make a turn).

Juan

1986 F-150 XLT Lariat 2WD, 351w HO

Gary Lewis

Re: Troubleshooting slowdown or low rev stalls

Administrator

I'm going to tag Bill/85lebaront2 since he ran a carb/ignition shop for decades and knows far more about it than I do.

Gary, AKA "Gary fellow": Profile

85lebaront2

Re: Troubleshooting slowdown or low rev stalls

Administrator

Ok, let's start with which carburetor you have, original would be a Holley 4180C which is a special emission compliant carburetor. If I remember correctly the idle mixture screws are in the throttle body and sealed from the factory.

If the power valve is bad, mixture will be extremely rich which will cause a stumble from idle as the accelerator pump adding fuel to an already rich mixture momentarily overloads the engine.

Another source of over fueling is the secondary metering body, some versions had square corners that would warp allowing unmetered fuel into the secondary idle circuits.

Since the idle mixture screws are a pain to get to, and if never adjusted will need the anti-tamper plugs removed. A quick check, create an air leak (disconnect the brake booster or PCV valve, if the engine speeds up and smooths out you are too rich.

Unfortunately, my experience with "factory remanufactured" carburetors is not good (besides I know what I used to sell to the core suppliers like J.N. Grillo and sons). As a result I can't recommend a good source.

Bill AKA "LOBO" Profile

"Getting old is inevitable, growing up is optional" Darth Vader 1986 F350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator Wife's 2011 Flex Limited Daily Driver 2009 Flex Limited with factory tow package Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413

macnewbie

Re: Troubleshooting slowdown or low rev stalls

Thank you Bill.

Yes it is the 4180C. I will try the suggested tests later today or over the weekend and report back.

I also just realized that on my truck the air pump is still there but there is no belt. I tried turning the pump by hand and couldn't but not sure if it is because it is seized or because all the hoses/check valves are still connected.

Any thoughts on whether the air pump situation could be connected to the stalls? "Layers of the onion" as Gary likes to say. 😭

Juan

1986 F-150 XLT Lariat 2WD, 351w HO

85lebaront2

Re: Troubleshooting slowdown or low rev stalls

Administrator

It shouldn't have any effect on running as it doesn't inject air into the intake side. Some of the early air pump systems had a dump valve that put air into the intake manifold under high vacuum conditions.

Bill AKA "LOBO" Profile