Carb - Transmission - Idle Question

[gtbags]

I've got a '77 Bronc with the 302 and C-4 transmission. Rebuilt the engine 4 years ago and it has run well ever since save for some carb issues. I put a Holley Truck Avenger on it and have been fairly pleased with it since.

The thing is, I'm still having a little trouble getting the idle setting right. There seems to be a huge difference between the idle when it is in park/neutral and when it is in gear. So much so that when I have it set to where it seems about right in gear, the RPMs go really high whenever it's in park. The truck's not currently at my house or I could give better numbers on what I'm seeing, but I'm thinking right now when it's warmed up it idles at around 900 in gear, but like 2000 in park.

So what RPM should I see at idle in gear and at idle in neutral/park? Is it normal to see the RPMs drag down so much when in gear or could there be a transmission issue causing it to load the engine so much at idle? The transmission seems to perform fine, but it is old.

I'm hoping to get up to work on it this weekend, so just looking for some ideas/thoughts to start with.

I appreciate any comments.

 

[MachDream]

I've got a '77 Bronc with the 302 and C-4 transmission. Rebuilt the engine 4 years ago and it has run well ever since save for some carb issues. I put a Holley Truck Avenger on it and have been fairly pleased with it since.

The thing is, I'm still having a little trouble getting the idle setting right. There seems to be a huge difference between the idle when it is in park/neutral and when it is in gear. So much so that when I have it set to where it seems about right in gear, the RPMs go really high whenever it's in park. The truck's not currently at my house or I could give better numbers on what I'm seeing, but I'm thinking right now when it's warmed up it idles at around 900 in gear, but like 2000 in park.

So what RPM should I see at idle in gear and at idle in neutral/park? Is it normal to see the RPMs drag down so much when in gear or could there be a transmission issue causing it to load the engine so much at idle? The transmission seems to perform fine, but it is old.

I'm hoping to get up to work on it this weekend, so just looking for some ideas/thoughts to start with.

I appreciate any comments.

Lets say its warmed up and you have the idle set at 900 or so in park and its idling fine. then if you put it in gear at that point what happens?

 

[gtbags]

Lets say its warmed up and you have the idle set at 900 or so in park and its idling fine. then if you put it in gear at that point what happens?

I want to say it still draws the RPM down even lower, making it sound like it wants to stall. I'll check that out for sure this weekend. It could just be that I didn't let it warm up enough when I originally set the idle and so everything is set too high.

I'll know a lot more after messing with it this weekend. Any other tests to run like that, or other things to check out in the beginning? Any idea what the neutral idle speed and drive idle speed should be after everything is warmed up?

 

[MachDream]

I want to say it still draws the RPM down even lower, making it sound like it wants to stall. I'll check that out for sure this weekend. It could just be that I didn't let it warm up enough when I originally set the idle and so everything is set too high.

I'll know a lot more after messing with it this weekend. Any other tests to run like that, or other things to check out in the beginning? Any idea what the neutral idle speed and drive idle speed should be after everything is warmed up?

this isnt gospel...but in neutral/park idle should settle in about 850-1000 rpm in there. if i were you doing this i would focus on getting the idle set while in park first. get it set between 800-1000 so it sits and idles nice and continues to idle when warm. Then, when warm...how is throttle response while in park / neutral? will it rev up smooth? if it does idle nice, revs fine, doesnt spit and sputter at all...then while idle-ing..drop it onto Reverse or drive and see what happens. if it bogs way down wanting to die..you have a torque converter/vacuum line/tranny problem. post results here and we'll go from there! good luck fella i know how much of a pain this can be!!

 

[MachDream]

so did ya test it?

 

[gtbags]

so did ya test it?

Sure did. It made all the difference just to get myself thinking through it and get your input to get the wheels turning. When I first got it warmed up it was idling around 1800 and would drop to 800 whenever I put it in gear. I lowered the idle to 900 and it would die whenever i put it in gear. Raise the idle back up, put the vacuum gauge on it and messed with the idle mixture screws the way Holley tells you to do to maximize the vacuum. Still didn't help much and it was running pretty rough.

Started spraying starting fluid around the manifold to see if i had a leak somewhere. The engine was reacting to me spraying it, but it didn't really seem to matter where i sprayed it as long as it was around the manifold and not up at the main intake. Didn't take too long after that to find a freakin 3/8" vacuum port hidden under the back carb bowl that the rubber plug/cover had fallen off of. I capped that sucker (figured I'd give the vinyl caps a shot instead of rubber), ran through the idle mixture sequence again and set curb idle to around 700. It's like a whole new truck. There's a 100 RPM difference between park and drive now. Can't believe it was that easy and cheap, and can't believe I ran it so long the other way. Good lesson learned.

I appreciate all the help.

 

[MachDream]

Sure did. It made all the difference just to get myself thinking through it and get your input to get the wheels turning. When I first got it warmed up it was idling around 1800 and would drop to 800 whenever I put it in gear. I lowered the idle to 900 and it would die whenever i put it in gear. Raise the idle back up, put the vacuum gauge on it and messed with the idle mixture screws the way Holley tells you to do to maximize the vacuum. Still didn't help much and it was running pretty rough.

Started spraying starting fluid around the manifold to see if i had a leak somewhere. The engine was reacting to me spraying it, but it didn't really seem to matter where i sprayed it as long as it was around the manifold and not up at the main intake. Didn't take too long after that to find a freakin 3/8" vacuum port hidden under the back carb bowl that the rubber plug/cover had fallen off of. I capped that sucker (figured I'd give the vinyl caps a shot instead of rubber), ran through the idle mixture sequence again and set curb idle to around 700. It's like a whole new truck. There's a 100 RPM difference between park and drive now. Can't believe it was that easy and cheap, and can't believe I ran it so long the other way. Good lesson learned.

I appreciate all the help.

nice work!