Idle Mixture Screw Setting
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Fleet Mechanic
Joined: Feb 2007
Posts: 1,326
Likes: 48
From: Middle TN
Idle Mixture Screw Setting
I have a 79 f250 that I bought as a parts truck but after I got it running and some repair it has possibilities to make a good truck.
Someone has removed all of the emissions equipment.
I carefully rebuilt the carb and made sure even the smallest ports were clean.
The idle mixture screw wants to be way to many turns out when the engine idles at its best. I had to locktite it to keep it from drifting.
If I turn it in to specs (1.5 turns) it runs too rough.
Timing is set at 8 degrees at idle with dist vacuum plugged.
Vacuum reads 16 inches with a slight (1/2 inch back and forth) rapid pulse of the needle. This could be the PCV vibrating?
A slightly rough idle to match (not objectionable and almost unnoticeable)
Head has been completely redone by a top machine shop.
Is this because the emissions have been removed.
How can I compensate or is it ok like it is ?
Someone has removed all of the emissions equipment.
I carefully rebuilt the carb and made sure even the smallest ports were clean.
The idle mixture screw wants to be way to many turns out when the engine idles at its best. I had to locktite it to keep it from drifting.
If I turn it in to specs (1.5 turns) it runs too rough.
Timing is set at 8 degrees at idle with dist vacuum plugged.
Vacuum reads 16 inches with a slight (1/2 inch back and forth) rapid pulse of the needle. This could be the PCV vibrating?
A slightly rough idle to match (not objectionable and almost unnoticeable)
Head has been completely redone by a top machine shop.
Is this because the emissions have been removed.
How can I compensate or is it ok like it is ?
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
The steady movement of the vacuum gauge is normal. However, 16 sounds a little low. If you have to have your idle mixture screw more than 2 turns out to get it to idle smooth, you most likely have a vacuum leak. That could explain your somewhat low vacuum reading as well. So, how to find that leak? With the motor idling, take a can of spray carb cleaner and shoot it at suspected leaks, and if your idle goes up, that is a leak. Common sources of a leak are dried\cracked vacuum hoses and caps, missing hoses and caps, carburetor base gasket, or brake booster if you have power brakes.
There are issues that can arise out of the emissions components being removed, but they won't affect your idle. Unless your truck had a smog pump, and the diverter valve was removed and the previous owner did not cap off the vacuum to the valve. That could cause your leak. Look around the carb and manifold vacuum tree for missing caps. Try the spray-can technique I mentioned above as well.
There are issues that can arise out of the emissions components being removed, but they won't affect your idle. Unless your truck had a smog pump, and the diverter valve was removed and the previous owner did not cap off the vacuum to the valve. That could cause your leak. Look around the carb and manifold vacuum tree for missing caps. Try the spray-can technique I mentioned above as well.
Thread Starter
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Fleet Mechanic
Joined: Feb 2007
Posts: 1,326
Likes: 48
From: Middle TN
Just an update:
I pulled the carb, took the top off and turned the metering rod adjustment screw 1/4 turn in, which raises the rod slightly. The results were incredible. The idle smoothed up, acceleration was brisk and the over all performance was noticeable. It stopped pinging when opening the throttle at interstate speeds. Now I'm wondering what another 1/4 turn would do?
Thanks for the tip, you guys are great!!!!
I pulled the carb, took the top off and turned the metering rod adjustment screw 1/4 turn in, which raises the rod slightly. The results were incredible. The idle smoothed up, acceleration was brisk and the over all performance was noticeable. It stopped pinging when opening the throttle at interstate speeds. Now I'm wondering what another 1/4 turn would do?
Thanks for the tip, you guys are great!!!!
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