Hey guys. I have a 351w H.O. with the stock holley 4barral carb. This holley is a little different than what I usually see. The fuel/air mixture screws are allen head screws on the base plate onder the fuel bowl. There are two on the front and two on the back. I got the truck idling good adjusting the front screws but where do I set the Back screws. I assume they are for the secondary circuit but not like you can floor it down the road and make an adjustment. LOL. It seems like the engine gets rich when under hard acceleration. Cylinder 3 and 4 and 7 and 8 get black and miss but regain spark upon releasing gas pedal to cruise speeds. Do these screws have an initial adjustment? I have them 2 1/2 out like the front. I hope there is someone out there that has messed with one of these carbs and can help. Holley.com doesnt have info on this type carb. Also could catalitic converter be restricted? How can I tell?

 

Never heard of screws like those in the back. Also never have seen one that is a allen screw head. You may have a carb that someone has modified and added those screws, or put a different baseplate on. I don't think the factory ever had a 4 corner idle adjustment. If they work, that's what they would affect the idle only.

You can tell if the exhaust is restricted by using a vacuum gauge plugged into the engine. Do a search on the web for "vacuum guage readings". There are a couple of good sites that show the motion of the needle on the gauge for various problems.

 

My 4180 has allen mixture screws in the base flange...

I find it strange that only the spark plugs in the back half of the block get fouled.
You must have a light duty 250 with under 8500gvw if you have cats.
But your symptoms do point to an exhaust restriction.

Is it rich the whole time you have the throttle wide open? The annular boosters in these carbs do a very good job of atomization.
If it was richer at idle I would suspect your powervalve was blown.
Check and make sure the correct one is installed.
I found that if anything my carb (in stock form) was lean, verging on knock except when the powervalve was open.

On Holley's site, here: http://www.holley.com/data/TechServi...%20Listing.pdf
Across pgs. 15 & 16 the 4180's list numbers and calibrations can be found.

Have a look at the '80's Mustang site FLEMWORLD .
Under '85 How To's there is an article archived that details the differences w/ a 4160 and modifying the 4180 for dragstrip performance, at least it gives you a better idea how these carbs are set up.

 

Well the engine runs good as long as you drive relatively easy. its like when you would imagine the secondarys are opening when the problem occures. Someone has rebuilt this carb cause the holes where the adjusting screws are look tike they had plugs over them and they are gone. Problem definately seems like its only under power like hard acceleration. It is wierd that it is only the back plugs getting fuel fouled since those cylinders are on seperate planes on the intake. the exhaust is not stock but its a two into one cat then into a flowmaster out into two again. I had thought that the cat had just enough restriction to affect only during a high flow condition but the truck runs great down the highway at 70 mph. Im going to turn the secondary screws all the way in and see what it does today. man those things are hard to get to. I'll post what happens later.

 

Years ago I picked a double ended flexible shaft carburetor adjusting tool from the KD or Lisle rack at the auto parts store. It had a hex shaped handle that slid along the spring with one slotted and one hex end.
$10 spent saved so much frustration!
You may not be able to see the screw but if you mark the handle you can count turns.

Maybe the person who rebuilt the carb before lost the metering ball for the secondaries, or installed too light a spring?

 

I run an 1984 vintage 4180 on my modified 351W in a 1981 F100. Most of these carbs
had four corner idle adjustments but some of the last 4180s had only the front idle
adjustments. My carb is pretty much stock with the exception of the power valve
which is a single stage 6.5 and one size larger jets. My truck has an aluminum Edelbrock
intake and long tube headers to dual exhaust with no cats. I adjusted my carb with
a vacuum gauge and set each of the rear idle screws one turn out and then adjusted
for the best idle with the guage and front screws which adjusted out approx 2 turns.
A 1987 vintage 4180 that I have has only the front idle ajustment screws. I haven't
run it yet on the truck, but plan to in the future. As was mentioned in a previous post
on this thread, these trucks have very retrictive exhaust systems in stock form which
does effect the carb.

 

Does your truck have catalytic converters as well?

 

Shows you how much I know. The book I have doesn't show it either. But I have only worked on the aftermarket holleys. That 4180 is a really weird one.

 

Well to all the guys and gals following this thread....I have messed with these old trucks long enough to know that I have a carb prob now. These are simple machines. Its a new ruck to me and someone has rebuilt this carb. I grew up messing with old 70s trucks. Drove old 360s and 390s and 351m trucks in the mid to late seventies. always tuned them the way I like them. Timing by ear and fuel mixture a little on the rich side. Fuel is always there when you need it. The secondary circuit is flooding the engine. I readjusted the Idle mixture screws to no avail. All it affects is idle performance only. the more you turn the secondary screws out the more you have to turn the primary in. and vise verse. I can make the truck run awsome Idle to normal driving. Its when I apply power when it Fails. Its not the cat cause I would have more serious problems under normal conditions. The secondarys open and I have hell conditions. I wouldnt mind rebuilding the carb but there are much better carbs out there. going to get an aftermarket 600 or 650 cfm holley or an edelbrock carb. what are your thoughts on this? Thanks for all the input.

 

I did even go as far as making sure the left bank exhaust butterfly valve is open. Its frozen in the open position. thank goodness. got to be carb.

 

I'm running a 0-80457S now...

Over 600 cfm Holley's needs dual feed inlet, and my state mandates all that emission stuff that's there on the passenger side of the intake manifold.

 

My truck doesn't have cats and it has very free flowing mufflers. The 4180 is a good carb. The best thing it has going are the primary annular boosters which most Holleys
don't have. The 4180 needs to be carefully rebuilt using a quality kit and very good
cleaning. The Motorcraft kit has all the bench setting instructions with it and there
are very good web resources on rebuilding and tuning it.

 

Aftermarket is the way I would go myself. Much simpler to work on and tune. Plus you don't know what's been done to the one you have. It's much harder to get a carb going after someone has messed with it.

 

Agreed!

 

I live in rural america so there arent any inspections or emission testing here. Leaves me open to any carb I want. Should I stick with holley or get an edelbrock. The edelbrock is more affordable and easy to work on. Thanks for all the input.