Idle Speed

[texasmike]

Here's my dilemma on my stock engine/carburetor setup. I have the timing set at 11 degrees btdc @ 600 rpm (no dist. advance) and when i connect the distributor to manifold vacuum, the rpm jumps way up. I have backed the idle screw off to the point that it is no longer touching the throttle arm and it's still about 250 rpm too high. Is there any way to decrease the idle without backing off the timing, say to around 8 btdc (no advance)? Everything seems to run much better with manifold vacuum, after removing all the emissions junk. Thanks.

 

[SirRobyn0]

Here's my dilemma on my stock engine/carburetor setup. I have the timing set at 11 degrees btdc @ 600 rpm (no dist. advance) and when i connect the distributor to manifold vacuum, the rpm jumps way up. I have backed the idle screw off to the point that it is no longer touching the throttle arm and it's still about 250 rpm too high. Is there any way to decrease the idle without backing off the timing, say to around 8 btdc (no advance)? Everything seems to run much better with manifold vacuum, after removing all the emissions junk. Thanks.

Well I'm going to try to mostly just answer your question, but also want to say that an 80's era 305 unlike the 350 and earlier versions of the 305, these engines were literally built with that emissions stuff in mind and it maybe very hard if not impossible to get it to truly run right, but of course that is subjective.

Check for vacuum leaks, don't over look the throttle shaft. Make sure the throttle isn't hanging on the fast idle cam. Make sure the mechanical advance isn't hanging in the distributor.

Regardless of the timing no air is no air and air will stall the engine when the idle screw is backed off.

Are you still running the OEM Q-jet and if so is it a feedback carburetor designed for computer control? The telling thing would be an electrical connector on the carb in the air filter area (that's the mixture control solenoid).

Lastly don't forget that the original 305 distributors came with a lot of advance both in the vacuum and mechanical side, more so than a stock 350 distributor. If your running the OEM distributor you are likely getting a ton more total timing than say a 350 set to 11, for base timing. You may find that you have to back that down some to avoid pinging and other issues.

 

[Bextreme04]

I agree with Rob. If you have the idle screw backed all the way out and the engine is still running, you have a vacuum leak from somewhere. That could be internal air intake leak(runners leaking into the engine valley) or could be any number of vacuum lines or gasketed surfaces.

Adding to the timing conversation, you need to know how much vacuum advance and total mechanical advance your engine will provide. Then you need to figure out how you want to use those things to give the engine the timing it wants at every performance point you care about. If the distributor is designed to use ported vacuum, you will NEVER get it to run great on manifold vacuum. There are a few timing related threads on here that describe why that is.

 

[fast 99]

Loose or twisted throttle shaft? Secondaries not closed if Q jet or 4V.

Fast idle cam stuck.

 

[SirRobyn0]

I agree with Rob. If you have the idle screw backed all the way out and the engine is still running, you have a vacuum leak from somewhere. That could be internal air intake leak(runners leaking into the engine valley) or could be any number of vacuum lines or gasketed surfaces.

Adding to the timing conversation, you need to know how much vacuum advance and total mechanical advance your engine will provide. Then you need to figure out how you want to use those things to give the engine the timing it wants at every performance point you care about. If the distributor is designed to use ported vacuum, you will NEVER get it to run great on manifold vacuum. There are a few timing related threads on here that describe why that is.

Internal leaks of the intake manifold gasket into the valley can sure be tricky to find. I just went though that on my 305. Ended up pulling the manifold to see if there even was an issue, and sure enough!

 

[Ricko1966]

The screw on the throttle linkage is the throttle stop screw,it keeps the butterflies from getting stuck in the throttle bores,if this screw is cranked in too far,you rob fuel from the transition circuit.Your Idle speed should be mainly controlled with the screws at the base of the carburetor. Yes everyone uses the throttle stop screw to adjust speed but that's not its primary function and can cause drivability problems. Anyway you should check for vacuum leaks as many others have recommended but also familiarize your self with your carburetor it's different functions and adjustments. For all I know your Idling on the fast cam.
You've gotten some very good advice already. Someone touched on total timing,which you want to check. You should have approximately 36 degrees we can tweak it later. Total is base timing plus centrifugal,no vacuum advance hood up.You use timing tape or a dial back timing light to check it. Vacuum leaks,spray carburetor base and manifold perimeter,with carb cleaner you will hear the engine pick up if you find a leak. 2nd stage of vacuum leak checking, remove every vacuum hose from the carburetor and plug all the carburetor ports,hopefully you idle correctly then. Now start hooking things back up,1 at a time while listening to your engine. If it speeds up considerably when 1 line is hooked up,you've located a problem. I won't be surprised if you find something unhooked. And I do not agree with removing egr and it probably will make your truck run worse. The solenoid on truck quadrajet is just for a 2 stage accerator pump and won't affect idle.

 

[Bextreme04]

The screw on the throttle linkage is the throttle stop screw,it keeps the butterflies from getting stuck in the throttle bores,if this screw is cranked in too far,you rob fuel from the transition circuit.Your Idle speed should be mainly controlled with the screws at the base of the carburetor. Yes everyone uses the throttle stop screw to adjust speed but that's not its primary function and can cause drivability problems. Anyway you should check for vacuum leaks as many others have recommended but also familiarize your self with your carburetor it's different functions and adjustments. For all I know your Idling on the fast cam.
You've gotten some very good advice already. Someone touched on total timing,which you want to check. You should have approximately 36 degrees we can tweak it later. Total is base timing plus centrifugal,no vacuum advance hood up.You use timing tape or a dial back timing light to check it. Vacuum leaks,spray carburetor base and manifold perimeter,with carb cleaner you will hear the engine pick up if you find a leak. 2nd stage of vacuum leak checking, remove every vacuum hose from the carburetor and plug all the carburetor ports,hopefully you idle correctly then. Now start hooking things back up,1 at a time while listening to your engine. If it speeds up considerably when 1 line is hooked up,you've located a problem. I won't be surprised if you find something unhooked. And I do not agree with removing egr and it probably will make your truck run worse. The solenoid on truck quadrajet is just for a 2 stage accerator pump and won't affect idle.

You DO NOT use the idle mixture screws to set idle RPM. You set the timing first, then you get the engine hot and set idle mixture screws for peak RPM and then lean them ~50-100rpm while adjusting the idle screw to keep it around 750rpm. Once you have set the idle mixture to slightly lean of peak, then you set the idle screw to get the desired hot idle. Once timing, hot idle, and idle mixture are all set, then you need to set choke and high idle. Once it is thoroughly cooled down, you should be able to put the pedal all the way to the floor once and then slowly let it off and have your choke flap fully closed and on the high idle cam on the passenger side. You can then start it up and set your high idle to ~900-100rpm on initial start. This will then settle to around 1200-1300rpm when it gets warm and then should then be able to kick off the choke and settle the idle back to hot idle rpm.

Are you certain that you don't have it on the high idle cam? Did you mess with any of the choke stuff while removing the "emissions junk"?

 

[fast 99]

Internal leaks of the exhaust manifold gasket into the valley can sure be tricky to find. I just went though that on my 305. Ended up pulling the manifold to see if there even was an issue, and sure enough!

Know you meant intake. What I have done in the past, close off the pcv with a plug. Then, engine running close off the other side where the breather is attached with your hand. Let it run for 5-10 seconds. Slowly remove your hand. If there is pressure it's ok, if vacuum pull the intake.

 

[SirRobyn0]

Know you meant intake. What I have done in the past, close off the pcv with a plug. Then, engine running close off the other side where the breather is attached with your hand. Let it run for 5-10 seconds. Slowly remove your hand. If there is pressure it's ok, if vacuum pull the intake.

That's right I meant intake lol. I will edit that post so it's less confusing to future readers thanks for catching that.

That test your talking about. I've done that very thing, remember the Dodge magnums with the belly pans on the intakes? That's how I'd test to see if those gaskets were gone. On my square it would pass that test, never the less I did have a leaky intake, in my case it was sucking a good amount of oil in.

 

[Ricko1966]

@Bextreme04 you are 100 percent right about the mixture screws, I got a couple of things tangled in my brain, I'm glad you caught it. I had the exact opposite situation going on in my head because it is so common. Where someone's trying to make it idle with the stop screw instead of the mixture screws.

 

[texasmike]

Thanks everyone, I really appreciate all of your comments, thought and help.