Idle Problem

[Jon87V20]

Hi everyone, I need some help here. The truck has been running flawlessly now for quite a while. Today I went through some dirt field just messing around near my house and it's a little bumpy but nothing major at all. When I was going to pull back out on to the street it died on me. When I started it again the idle was pretty rough and seemed like it was missing. As soon as the engine gets a little above or around 1,500 it clears up and runs fine. Just the idle is having a problem. At first I was guessing maybe it was just a vacuum leak but I checked all the connections and they seem to be okay. Something kind of strange though was when I pulled the vacuum line off of the port at the carburator I noticed that it's not pulling any vacuum there and I know it used to. Any advice? Thank you.

[Lt.Del]

first thing i'd check is the PCV valve.  Then perhaps EGR if you have one.  I am assuming you already adjusted your carb (idle screw, air/fuel) and I am assuming you have a 79 based on your screen name, you didn't put what kind of truck in the text. 
Also, a common vacuum robber is the modulator on the trans.  Playing in the mud and hills, make sure your vacuum line didn't pop off that.
You checked plug wires both at the plugs and at the dist.cap?  They do pop loose sometimes.  Timing is set correctly...bolt not loose at distributor is it?  How high did you wind up the rpms 'playing in the mud'?  Didn't red line it did you?  Timing chain didn't jump did it.  Check timing.

[Jon87V20]

I have a 79 GMC K15. It has an RPM Air Gap and a Edelbrock 650 on it. I took off the EGR. I don't think it's the modulator because it still shifts okay. I know it's still connected on top but I'll check down by the tranny to be sure. I'm sure it's nothing valve timing related because it only does it at idle. As soon as it passes 1,500 it runs great. I'm just confused as to why it wouldn't be pulling vacuum through that port anymore?

[Jon87V20]

Okay correction, I just went out and messed with it again and it is pulling vacuum out of that port. So would a slipped chain make it behave that way? It still has alot of power after 1500.

[dumbucket1]

Just ideas but the air filter isnt packed with mud is it? How about your exhaust system?

[joesgarage71]

 Sounds like you have a Q-Jet? maybe your float is stuck in the up postition, tap with a small hammer on the front of the carb. If its stuck it will dump gas in.

[Irish_Alley]

i had a similar problem when i went mudding apparently i stirred up some particle in my tank and ended up clogging the carb up had to take it apart to clean it but i don't think it cleaned up with higher rpm just would bog down. now i know a vacuum leak will cause the same problem and the higher rpms would fix this problem. check around your carb for any leaks. can you hear a leak any were?

[Jon87V20]

No I can't find any leaks anywhere. I have a suspision that the vacuum advance diaphram is bad. I'm going outside in a minute to check on that. I'll update on it later today. Thanks for all the suggestions.

[Jon87V20]

I saw that the vacuum advance was working today. I rebuilt the carburator today as well. Still not idling right. I think it might be the timing chain after all. It was idling good with the timing retarded a bit but it was getting hot and backfiring out of the exhaust. Since the problem began it doesn't speed up when the timing is advanced, it just shuts off which makes me think it's a chain issue.

[TexasRed]

When you suck on the vacuum tube going to the vacuum advance on the distributor, the rpms don't go up?

[Jon87V20]

Hi sorry about the delay in reply. Yes the vacuum advance was working proporly. I took the timing cover off and noticed that the timing chain, although lined up proporly, was ridiculously sloppy. After replacing the chain the engine started and ran great. Unfortunaly I thought I had gotten the front oil pan gasket in without dropping the pan but it had come out. I didn't notice this untill I had already put everything back together. So today I am going to try and lower the pan a bit and put the seal in there. Other than that it seems to be going great. When I had the water pump off and sitting on the garage floor I saw that some grayish looking fluid was puddling up underneath. Is that for the fan clutch? I just moved the truck and it sounded quieter than usual. The fan's probably not working now. I'll find out later. But if that leaks out is the clutch pretty much garbage after that? Thanks.

[BigBadJohn]

could it be something in the ignition/firing?