350 High Inconsistent Idle

[captainwhisker1]

Hey guys, I'm having some trouble with getting my 350 to idle right.

It's a '76 4 bolt block with a really recent complete rebuild. 1405 Eddy carb, 2101 Performer intake, 462 heads, Lunati .420/.442 lift cam, gear drive, and long tube headers into a 2.5 inch stainless exhaust. It has maybe 200 miles on it and ran really well, then my issue started only about 50 miles ago.

My issue is, when the truck is out of gear it idles roughly at 800-1200 rpm, but when I drop it into gear, it drops to 600 rpm (what I'm aiming for) and is pretty smooth. Before my issue started, the rpms would barely drop (maybe by 100) when I put it into gear. It's also stalled on me 3 times today already, every time it was in gear. It runs really well with throttle applied, good power, it's just the idle acting up.

The fuel pump is maybe 4 years old, the carb hasn't even seen it's first birthday yet, The carb was dialed in before the problem flared up, so I don't think it can be a carb issue. I even messed with the idle adjustment a little after I got home but it didn't seem to help. I haven't noticed or heard any vacuum leaks, but I haven't got it tested. The only other thing I haven't checked yet is the ignition system, which is still the original HEI system that came with the truck. Could this be the problem, or do I need to be looking elsewhere?

Any input will be appreciated. I had a video but it says it's an unsupported file type.

 

[Chevy 88]

I would check for vacuum leaks with a smoke gun (you can make one very easily using a cigarette). Maybe the vacuum line to the trans? Also, I would toss another carb on there just as a test. Are you sure your fuel is good? No water in the tank or old fuel? Ya, check the timing for sure, and make sure the distributor hold down is not loose.

 

[captainwhisker1]

I would check for vacuum leaks with a smoke gun (you can make one very easily using a cigarette). Maybe the vacuum line to the trans? Also, I would toss another carb on there just as a test. Are you sure your fuel is good? No water in the tank or old fuel? Ya, check the timing for sure, and make sure the distributor hold down is not loose.

Smoke gun was one of my first thoughts, I'd like to try and get that done soon. Fuel is good, I drive it frequently so it gets filled about once a week, I also try to put treatment in it once a month. I would try and throw another carb on but I don't have another one that's in useable condition. Thank you!

 

[legopnuematic]

Might give a quick check with a wrench on the intake and carb mounting bolts, I've had them loosen up a little (or the gaskets settled/compressed some).

 

[Ricko1966]

A quick squirt with carb cleaner well usually find a vacuum leak quick and easy. Hey @legopnuematic remember last week when we were talking about installed vacuum gauges. He'd know now if the vacuum level had dropped.

 

[PrairieDrifter]

Electric choke? Check choke adjustment.

 

[84GMCSierra]

I would check for vacuum leaks with a smoke gun (you can make one very easily using a cigarette). Maybe the vacuum line to the trans? Also, I would toss another carb on there just as a test. Are you sure your fuel is good? No water in the tank or old fuel? Ya, check the timing for sure, and make sure the distributor hold down is not loose.

My 84 Sierra 2500 has been acting weird just lately. I never thought about the trans modulator line. Mine has started running about 1500 - 2000 rpm's while driving down the street and I have to "punch" the gas pedal to get it to slow down. Also, no "umph" if I need to pass. Is this a timing issue?

 

[Ricko1966]

My 84 Sierra 2500 has been acting weird just lately. I never thought about the trans modulator line. Mine has started running about 1500 - 2000 rpm's while driving down the street and I have to "punch" the gas pedal to get it to slow down. Also, no "umph" if I need to pass. Is this a timing issue?

Do you get your umph back by manually downshifting when you go to pass. When did this problem start? What transmission is in the truck? What transmission was originally in the truck when new?

 

[1STLS1]

I would carefully look down the inlet of the carb while it's running to see if it's pulling any fuel from the venturi and make sure your still in the idle circuit.

I had one older Eldelbrock 1406 that refused to idle and fouled out plugs. I ran the engine with the top of the carburetor off and saw bubbles coming from the floor of the fuel bowl area, the casting was either porous or had corroded and was leaking into the idle circuit. I JB Welded the fuel bowl floor just to see if it made a difference and verified that was the issue.

Good luck

 

[PrairieDrifter]

My 84 Sierra 2500 has been acting weird just lately. I never thought about the trans modulator line. Mine has started running about 1500 - 2000 rpm's while driving down the street and I have to "punch" the gas pedal to get it to slow down. Also, no "umph" if I need to pass. Is this a timing issue?

That's a choke adjustment issue.

 

[xm20k]

1405 is a 600 CFM manual choke. 1406 is 600 CFM electric choke.

If 1405 is the cable hooked up or is it wired open otherwise it's flopping around and closing.

If it is a 1406 and electric it's not setup correctly and being kicked shut with throttle movement or engine vacuum is pulling it closed because it is not adjusted to be fully open.

I run a pair of 1902s 500CFM AVS2s with manual chokes the front one is wired fully open, and the rear is on a cable I had the front one come undone and it would idle erratically as the choke would flop open and closed till, I realized it came undone.

 

[84GMCSierra]

That's a choke adjustment issue.

I was wondering about this. There's a guy from Southwest carburetor going to be near my work on Saturday to do some adjustments. Thanks for the info.

 

[84GMCSierra]

Th

Do you get your umph back by manually downshifting when you go to pass. When did this problem start? What transmission is in the truck? What transmission was originally in the truck when new?

Eres something wrong with the shifter rod as it's hitting the frame so it wont go into the lower gears, so need to fix that. Original TH400 in the truck. I'm having carb issues so having it looked at on Saturday the 3rd, and get it back up to speed, so to speak.

 

[84GMCSierra]

Do you get your umph back by manually downshifting when you go to pass. When did this problem start? What transmission is in the truck? What transmission was originally in the truck when new?

I found out the shifter rod is bent so it's hitting the frame, probably from the moron that sold me the bad engine initially so need to fix that as it wont go into the other gears easily. The trans is the original TH400

 

[Ricko1966]

Does the transmission down shift on its own when you floor it to pass? Whatvwork has been done recently before the problem started happening?