Brake booster changes idle?
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Junior User
Joined: Mar 2017
Posts: 68
Likes: 1
From: Nelson, BC
Brake booster changes idle?
Hi there,
I'm going through a truck I've just bought from a bozo that played around with EVERYTHING! It's a 82' F150 300 4speed.
I've just started going through the carb. It was idling very splatty and surging pretty bad. I was able to jam something in the carb linkage just to get it home. With the choke jammed the idle was high and there was plenty of power.
Now that it's home I'm reading threads on here and tuning the carb. Here's what I've done so far.
-Removed choke jam... hehehe.
-Backed out the idle screw until throttle plate closed.
-Turned mix screw in until it stopped and backed it out 1.5 turns.
The idle improved but was still pretty chunky.
I started pinching hoses to see if anything changed. When I pinch the brake booster line the idle smoothed out more. I went for a drive around the block and the truck was bucking pretty bad when I was steady on the throttle.
Does pinching your booster line affect your truck?? I'm going to go check my booster check valve. Has anyone ever seen a vacuum leak from their booster's internals?
Thanks!
I'm going through a truck I've just bought from a bozo that played around with EVERYTHING! It's a 82' F150 300 4speed.
I've just started going through the carb. It was idling very splatty and surging pretty bad. I was able to jam something in the carb linkage just to get it home. With the choke jammed the idle was high and there was plenty of power.
Now that it's home I'm reading threads on here and tuning the carb. Here's what I've done so far.
-Removed choke jam... hehehe.
-Backed out the idle screw until throttle plate closed.
-Turned mix screw in until it stopped and backed it out 1.5 turns.
The idle improved but was still pretty chunky.
I started pinching hoses to see if anything changed. When I pinch the brake booster line the idle smoothed out more. I went for a drive around the block and the truck was bucking pretty bad when I was steady on the throttle.
Does pinching your booster line affect your truck?? I'm going to go check my booster check valve. Has anyone ever seen a vacuum leak from their booster's internals?
Thanks!
Thread Starter
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Junior User
Joined: Mar 2017
Posts: 68
Likes: 1
From: Nelson, BC
The clamp on the carb end of booster hose looks good and I'm pinching right next to clamp already. I'll do what you're saying. Maybe some air was slipping through that clamp.
What happens when you pinch the booster line on your truck?
What happens when you pinch the booster line on your truck?
Thread Starter
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Junior User
Joined: Mar 2017
Posts: 68
Likes: 1
From: Nelson, BC
Beauty! My truck definitely runs better with the hose pinched so I guess that means the booster or grommet around the check valve has a leak. Bummer! I just scrapped my parts truck that had a good booster.... such is life.
Thanks a lot, Reamer.
Could anyone else please pinch their brake booster line and report any change in idle (or lack of change)? I want to be 100% sure.
Thanks again, Reamer.
Thanks a lot, Reamer.
Could anyone else please pinch their brake booster line and report any change in idle (or lack of change)? I want to be 100% sure.
Thanks again, Reamer.
Logistics Pro
Joined: Jan 2001
Posts: 3,789
Likes: 26
From: Connecticut
your welcome
What's you got to do now is unplug EVERYTHING from the carb and plug off all of them, now plug everything in one at a time.
plug something in and it runs rough, there's another leak....EGR is another big problem spot.
What's you got to do now is unplug EVERYTHING from the carb and plug off all of them, now plug everything in one at a time.
plug something in and it runs rough, there's another leak....EGR is another big problem spot.
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Lead Driver
Joined: Jul 2009
Posts: 6,477
Likes: 19
From: Sheffield, AL
The reason the idle air fuel mixture screw had to be ~3 turns out is to compensate for a vacuum leak.
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Thread Starter
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Junior User
Joined: Mar 2017
Posts: 68
Likes: 1
From: Nelson, BC
Ordered a booster from Amazon for $70 (Canadian). Just waiting for the drone to arrive. I'll post the results.
With the booster pinched I'd have thought I could get the truck running right. I got the idle decent but there wasn't any power and it was bucking a lot. I guess I better do some reading and leave the carb to another thread?
With the booster pinched I'd have thought I could get the truck running right. I got the idle decent but there wasn't any power and it was bucking a lot. I guess I better do some reading and leave the carb to another thread?
Lead Driver
Joined: Jul 2009
Posts: 6,477
Likes: 19
From: Sheffield, AL
Kelly D, you quite possibly may have more than one vacuum leak. On the intake manifold, just below the carburetor is a piece [vacuum tree] with multiple vacuum ports. This piece screws into the intake manifold. Depending on the options, not all of these ports are used. The unused ones will have a rubber cap. Check all and make sure they are not damaged or missing. There is one particular one, which when looking at the "vacuum tree" from the passenger fender side, is nearest the firewall and closest to the engine. This rubber cap is out of sight and nearest the exhaust [heat]. Over time, it degrades. Recently, I have found this rubber cap to be in very poor shape on two trucks equipped with 4.9L engines. Symptoms were higher than normal idle speed, somewhat rough running and bucking while under light throttle. On both, the carburetor air fuel mixture screw had to be out 3-5 turns just to idle acceptably.
What I said, Unplug and cap everything
Thread Starter
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Junior User
Joined: Mar 2017
Posts: 68
Likes: 1
From: Nelson, BC
Swapped the carb and got her purrrrrring like a kitty today. When I remove the pinch on the booster the truck's idle revs up and then simmers down to a lumpy idle.
Looking forward to installing the new booster and moving on with my life....
Looking forward to installing the new booster and moving on with my life....
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