I have a '87 F350 with a holly/motorcraft 4180 carb. In the morning it starts fine but won't idle smooth and loads up until it chokes out. After warming up it will idle, but still not smoothly. When rebuilding the engine, I think the mechanic turned at the idle scrw a little bit, (not the idle set screw) I'd like to try to back off the idle mixture a little if possible. Can someone direct me to where this screw is and which way to turn it? Thanks
Tony

 

Tony G

If they are accessable on that late of the Motorcraft/Holley carb the will be under the front fuel bowl pointing straight out.

I don't know if the idle mixture screws had any kind of anti-tamper covers on them for that year.

You need a vacuum gauge to adjust the idle mixture screws properly. Though you may try turning them out ( counter clockwise ), one at a time, a 1/4 turn at a time and see if the idle smooths out any. If you hear no difference within 3/4 turn thats probably not the problem.

 

Do you have any other ideas about what might be causing a problem such as this? It smell very rich in idle, but I can get as high as 15+mpg on the road.

Will there be 2 idle mixture screws? or why would I need a vacuum gauge?
Thanks
Tony

 

Tony,

Those carbs actually had four mixture screws, two under each of the front and rear fuel bowls. They probably will have tamper plugs to keep them from being adjusted. They take a special tool to adjust them and I don’t have clue where to tell you to find one.

I would suspect that the choke is not functioning properly from the way that you describe the symptoms. Make sure that you have voltage to the choke with the motor running (there will not be voltage if the motor is not actually running) and that the choke is opening as it is supposed to. Also make sure that it is not closing too much.

You should also make sure that the air is coming up from the exhaust manifold when the motor is cold. Otherwise you could be icing up the carb and making it run rich until the motor warms enough to kept it de-iced.

The best way to set the idle mixture is with a vacuum gauge. You adjust each mixture screw to the highest vacuum reading. It is much more accurate than trying to see the small changes in engine RPM. If you are having to adjust the mixture screws a lot then you should periodically reset the engine idle speed back to the spec then continue adjusting the mixture screws.

Good luck,

Mark

 

Don't know about the 87, but the OEM 4180 that was on my 79 460 donor engine only had the mixture screws on the front.

It had no anti-tamper covers on it.

I was all over that carb trying to get it to run decent and never could. But it was a 79 model that was good and worn.

 

Well,
I was able to make it through last winter. I had headers on the truck so the absence of warm air coming up from the headers may have caused some of the problem. I've had to replace the engine and got a salvage engine from a '87. The engine is definately '87, but the carb wasn't on it and came later. Anyhow, neither of the carbs I have use 4 screws, but neither were original either as both have a throttle kickdown for the automatic and my truck is standard. I'm planning to try to adjust the carb now while setting up the newer motor. The carb that came after the engine is quite dirty and would need a dismantled cleaning. The screws are capped on that carb but on the carb that is on the truck, they are uncapped. I'm wondering where to get a tool to adjust the idle misture screws. It appears that they are very small torx screws. Maybe I could find the right size and then use a flexible shaft that has a changeable head if none is available.

Tony

 

The 4180 mixture screws are in the front of the carb base. They are sealed behind brass pugs and the carb must be removed and the plugs drilled out to access them.