Rough Idle after Tune-up

[1425956]

Hi all. I have an 84 GMC Sierra Classic SWB with a 305 engine and lock-up TH350. This truck is new to me (have had for 3 weeks) and has 74,000 original miles on it. Because the truck has been sitting for the last 9 years, I decided to freshen some stuff up, tune-up and fix random leaks. Below is a list of items and parts that were replaced or repaired.

-Dropped the trans to replace the front and rear seals and replace the trans filter, fluid and gasket. (Was leaking bad and is still leaking!)

-The original owner had someone replace a leaking tank and they didn't know what they were doing. He/she got the straps too tight on the replacement tank but failed to replace the fibrous material between straps and tank. The relatively new tank developed a hole from a crease so I replaced it. The replacement tank was clean and rust free inside and out.

-I replaced the fuel sending unit because the fuel gauge bounced around and the original strainer was trashed.

-I replaced the original A/C Delco plugs, wires, cap and rotor with new Borg-Warner parts (except plugs, they are Delco). All wires were re-routed back through the original wire holders/looms and are away from the exhaust manifolds. All 8 spark plugs looked like cardboard.

-Drained the rear end oil, replaced pinion seal and rear diff cover gasket and refilled with 80W-90 gear oil.

-I changed the engine oil and filter.

-I removed the fuel filter located inside the Q-Jet and installed a new clear in-line fuel filter.

-Replaced the exhaust flange donut gaskets.

-I noticed the rubber hose connecting the trans vacuum to the intake was nearly deteriorated so I replaced it.

-Someone removed the smog pump and capped the A.I.R. on the manifolds.

In addition to a continued trans leak, my engine is now idling rough. However, I drove the truck 2 hours from the place of all of this work to my house and it ran better than ever!

My first thought was that since there was a vacuum leak, part of the improved performance was due to the vacuum advance now able to work because of a fixed vacuum leak. I know there are two different types of vacuum, above and below the carburetor.

I decided to run to town yesterday (after my truck sat all night after the 2 hour trip home) and that is when I first noticed the rough idle. I started it and let it run for a few minutes, then headed to town when my truck decided to die in the middle of the road. It cranked right back up but was surging real bad. Once I get the engine rpm past 1K rpm's or so, it takes off. However it does not have the pep it did on the trip home.

My first thought was I knocked a vacuum line loose but I could not find anything loose after an inspection this morning. I used starting fluid to spray lines with the to see if I could locate the vacuum leak. When I sprayed the passenger side of the engine, the idle did smooth out a bit but I could never really define the area. I took each line off the carb and plugged it to see if I could locate a problem there. Blocking the various vacuum ports on the carb did not change anything.

The research I did today led me to possible trans leaks on detent cable and dip stick grommet. I did not replace either of those seals/gaskets but I will do so. My bigger concern is the engine running smoothly. I am fairly proficient with SB Chevy's and have built several of them with success.

I can hear the valve in the P.C.V. rattling but I think that is normal. I have not looked at the E.G.R. valve yet. There is also a hissing noise that sounds like it is coming from the carburetor but it may be the normal air movement.

I am not really interested in removing all the emissions equipment if it is not necessary. I want to keep the truck as original as I can and keep everything operational. BUT, I want dependability and maximum mileage.

I am really at a loss...Any ideas? I double checked all wires to make sure they are plugged in and they are. What am I missing?

[1425956]

Don't everyone jump in all at once!

So I found a fuel leak coming from under the electric choke area. The smoke that was coming out of the exhaust was black so I knew it had to be fuel. I took the carb off, bought a kit and float and took it all to a guy in town that has a good reputation to rebuild Q-Jets. He seemed to like the fact that the carb had never been adjusted on before because the plug had not been removed on the top.

After I get it back in a day or two, I will post pics and updates.

Somehow, I feel like I am talking to myself!!!

[Rumble]

i'm paying attention, just didnt have any anwers for you, i have the same problem with my tbi 454 right now

[Restrorob]

Your not talk'n to yourself.....But I tend to once in a while !

Soon as I read hissing sound from the carb a internal vacuum leak in the carb came to mind, Also these carbs are known for worn primary and secondary throttle shaft wear that can cause a hiss. Make sure both shafts are snug (no side to side movement) in the base plate.

I'll be watching for your update when you get the carb back.....

[Max1775]

If that doesn't do the trick, try a vacuum gauge and see what you get. At least you will be able to tell if you have a vacuum leak.

[rcsweet21]

I had the same problem with my 350 after I did a tune up replaced dist cap and plugs and wires it would run great at 50 mph but it kept getting worse what I found was that I didn't tighten the ground strap in the cap tight enough and it kept arcing across hope this helps

[1425956]

Thanks for all of the responses. I will check the ground strap when I get my carb back tomorrow and install it. After I saw the fuel dripping on the passenger side of the carb, it made sense that the idle smoothed out after spraying that side with WD-40.

Next, I have to figure out the trans leak. That is driving me crazy. I spent about $300 last weekend just getting the maintenance caught up and fixing 9 years worth of dried out gaskets and seals.

After cleaning the trans lines where they go into the trans, kick down cable area and the dipstick area with carb cleaner, I ran the truck for a few minutes and the areas I cleaned stayed clean. It still dripped fluid from the hole on the dust shield. I hate to drop the trans again, especially since I won't be able to get it up on a lift to do it this time.

But, I have done so much, with so little, for so long, I can do anything with nothing! It will be nice to just be able to drive the truck and enjoy it for a while.

I'll update tomorrow on the carb status and then I'll dive into the trans leak this weekend and update on it as well. Thanks for all of the input and keep it coming.

Lots of experience on this here forum!

[pig rig]

just for futuer referance you can take a chunk of hose and put it to your ear and listen for vacum leakes it helps pinpoint them

works real good when the trucks running and you can hear the hiss but it sounds like its coming from every where

[1425956]

**UPDATE**

Ok, so I picked up the rebuilt Q-Jet from the shop, installed a new base gasket and torqued the bolts to 13 ft/lbs. It fixed my problem, plus some. I did not realize how poorly my truck had been performing until having a properly working carb. In addition to a lot of gunk and tarnish, there was apparently a part just laying in the bottom of the carb. I believe he said it was something related to making the back 2 barrels work. The afore mentioned hissing noise is now gone and my truck idles well.

The one thing I have noticed is a very slight hesitation from idle. The mechanic set everything as close to where it should be without having the truck to fine tune it. I also noticed that after sitting in line idling for 10 minutes with the A/C on yesterday, the carb seemed to load up pretty bad. As I went to take off, my engine was bogged and it seemed overloaded with fuel...But quickly cleared out.

I have not adjusted anything on it yet as I am still researching info on how to properly do so. I think these 2 small issues can be remedied with some minor adjusting.

I certainly appreciate all of the responses and help. Hopefully, this information will help someone else in troubleshooting.

Thanks again!

[Restrorob]

Good Deal !