idle speed

[Sovereign]

Hi all,

I finally got my hands on a laser tachometer so I could set my idle speed properly. This is part of my efforts to baseline the truck before I start making changes or major repairs. The truck was and is running well. I just had my idle set based on feel.

I followed the instructions on the emissions tag under the hood and with the engine warmed up and vacuum advanced disconnected and plugged, I set idle speed to 550rpms with the truck in gear. Timing was at 4-6* advanced so I left it alone.

I plug the vacuum advance back into the distributor and idle speed in gear goes up to 750 rpms.

When I move back to park, idle speed is now 1050-1100 rpms.

Does this sound right for a normal idle speed? The truck is running fairly smooth at all these speeds. It drops into and out of gear without heavy jerking. To me, it's just higher than I was expecting.

I did not check timing after plugging the vacuum advance back in. I was running out of time and didn't think to check that. I do know the previous owner installed a normal HEI so I do not have ECS control any more. Could it have lighter springs that are advancing the timing too much. Assuming that idle speeds I am seeing now are too high that is.

I have not adjusted the idle mixture screws from where they were before I did these idle speed adjustments to see if that has any effect. I do not have any external vacuum leaks. As I said, the truck is running well.

Thanks

[kwmech]

Double check the vacuum line routing. OR you may have a bad thermal vacuum valve. There is one valve that is supposed to increase the idle if the engine gets too hot to facilitate coolant circulation. That one may be bad

[Sovereign]

Thanks I will look into it.

You agree the idle speed is too high though?

[kwmech]

Originally Posted by Sovereign

Thanks I will look into it.

You agree the idle speed is too high though?

Yes, it should be about 5-600 in gear and maybe 650-700 in neutral

[Dead Parrot]

Two questions:
Stock or mostly stock motor?
Type of carb?

Many carbs have two ports for use with the vacuum advance(ported and manifold) Sounds like you are using the manifold vacuum port. While you want to set initial timing with the vacuum advance disconnected, you want to set idle speed with the vacuum advance connected. Just reset your idle speed with everything connected and you should be good.

Once you get idle speed set, then adjust your idle mix screws for best idle speed, then turn each in 1/4 turn. Reset idle speed after you finish, if needed.

[Corts60]

Originally Posted by dead parrot

two questions:
Stock or mostly stock motor?
Type of carb?

Many carbs have two ports for use with the vacuum advance(ported and manifold) sounds like you are using the manifold vacuum port. While you want to set initial timing with the vacuum advance disconnected, you want to set idle speed with the vacuum advance connected. Just reset your idle speed with everything connected and you should be good.

Once you get idle speed set, then adjust your idle mix screws for best idle speed, then turn each in 1/4 turn. Reset idle speed after you finish, if needed.

x2

[Sovereign]

The truck is stock except for the normal HEI distributor and removal of the emissions components. The charcoal canister remains though. I connected the vacuum lines according to the emissions diagram on the core support. I can double check that I identified the correct port for the advance.

Unless I read the emissions tag wrong, it says to set idle with the vacuum advance disconnected and vehicle in gear.

Thank you.

[kwmech]

So are all the emissions equipment on there or not?

[Sovereign]

Only the charcoal canister remains. No other emissions components are connected.

[Sovereign]

Also, you'll are correct regarding the vacuum source for the advance. I had it connected to where the emissions diagram said. However, there was a delay valve that I either removed or that wasn't there before. Once I plugged my vacuum gauge into the port, it was obvious I was on a manifold source since I had full vacuum present at idle. I've moved the advance to a ported source.

I also adjusted the idle mixture screws while there. I now have 16 inHg of vacuum at an 830ish rpm idle while warm. Truck still chugs a little in gear but sounds healthier than before even though I was at 15 inHg for vacuum before adjusting the idle screws. Kind of amazing how such a small change can have a profound affect.

Time to drive it and see how it handles hot restarts etc.

Thanks!

[Corts60]

I think you should shoot for 8-10 degrees initial timing bud and that engine will be a lot happier.

[Sovereign]

I appreciate the suggestion. The truck drives very good now. A little shakey while sitting in gear compared to when I had the advance hooked to a manifold source. Not sure if it was idle speed or advance though.

Hot restart is not great. I'll need to investigate that some more. I swear it was better before these changes I did today.

[truckin 79]

What year truck and motor do you have? The 4-6 timing seems really low to me. Does it jump around from 4-6 or is it one or the other? The timing mark should stay fairly consistent on whatever timing number it is set at.

[Sovereign]

1986 305. Timing is steady. It just depends on how you look at the timing tab on the front cover which is why I say 4-6*.

Thanks

[Sovereign]

Sorry. I thought I had my signature filled out but I did not. Should come up now I hope.

[geezer#99]

Is your timing tab at the 12 o’clock position on your timing cover?

[Sovereign]

Pretty close to it, yes.

[geezer#99]

You’re looking down the tube on the timing cover then?

[Sovereign]

There is no tube. It's just a timing tab with the notches cut out. Biggest notch is 0 and then smaller notches from there.

[geezer#99]

Ok, I was thinking your timing tab/balancer mark are mismatched resulting in your initial timing numbers being quite low. You might actually have much higher timing. You’re hard starting when hot is a symptom of too much timing.
Check this link for info on the different set ups.


https://www.crankshaftcoalition.com/..._TDC_lines_SBC

[Sovereign]

I appreciate the information. I'll take a look at the link later.

I'm fairly confident the engine is original. The timing cover is most certainly a stock unit. That doesn't mean the timing marks aren't off but I doubt it.

[truckin 79]

I am surprised but your truck is supposed to be set at 4-6 btdc, you were right. I would not really set your idle by the radiator support sticker. That would just be a suggestion after all these years. The engine is 37 years old and has carbon build up and wearing engine parts. There is not a perfect setting especially on a carbed engine where metering the atomization of fuel changes because of altitude, load and many other factors. I have a O2 gauge on my 79 truck and that thing changes from 13-15 all the time. My truck idles about 1000-1,100 and in gear about 600-700. If truck was happy before you messed with it then dont fix what aint broke as my uncle used to say. Check your spark plugs that will tell you a lot about how truck is running. I think we all chase little issues instead of just enjoying the ride. Good luck

[Sovereign]

All good points. You're right, it's not worth the effort to make it perfect or like new. I just want to make it better and before I start spending lots of time or money making changes, I wanted to know where I am currently. Plus, it's a relearning experience. It's been a long time since I have had to deal with a carbed engine.

Thanks