Idle steadily worsening

[dzbowtie]

Hey guys,
My truck has developed a rough idle over the last couple weeks. This started one day and went away but has progressively worsened.
To start with, I have a basic vortec headed 350 with Holley 600 carb. Nothing special, small cam, aluminum intake, daily driver.
I have always been able to pull the choke halfway or so, pumped once and the truck fire right up. Lately, I have been having trouble getting it to stay running once fired, until it warms up a bit. Then, it will idle between 700-900, surging up and down. I have never had this issue. It had idled steady around 800 in park.
When in gear, it usually idled right at 500. However, now when placed in gear, the rpms drop to 150-200 and almost dies out. When I tap the gas, there is a delay, then a surge where the fuel hits.
While driving home, the truck almost felt like it wasn't running on all cylinders. The rpms felt like they still wanted to surge. When I parked the truck, and since it was late at night, I felt for a cold header tube, but they were all hot.

My initial thinking is a carburetor issue. I sprayed some carb cleaner down the butterflies. I checked the vacuum lines and they all seem good. No cracks or open lines. I have put some carb cleaner in my fuel also, thinking maybe this will run through and help if there is an internal carb issue. I doesn't seem to have made a difference yet. My fuel cap was loose also, so maybe there could have been some water get into the tank from heavy rains, but again, this has never seemed to be an issue before.

My spark plugs and wires have not been changed in about 8 yrs but I haven't had an issue before. I have not had a chance to put a timing light on it yet. And I'm sure it couldn't hurt to replace them anyway.

Aside from this recent issue, the engine has always ran like a top. But this rough idle and surging is frustrating. I want to focus on the carb, but would take any help at this point.

I appreciate any suggestions

[Getter-Done]

I would start with pulling All the spark plugs.
Don't stop at the first bad one you find. (if any are bad)
Lay them out 1-3-5-7 and 2-4-6-8.
That way if or when you find a problem you will
remember which one went where.
Then Run a Compression Test while the plugs are out.
Then from those results change things one at a time.
This way you know what fixed your issue.
Just my approach. Hope this helps.

[geezer#99]

Simple stuff first.
Check your timing.
At 600-700 rpm but no higher than 750 rpm. Set your initial at 10 degrees. Adjust your mix screws on the carb.

[Dead Parrot]

Partial list of suspects not already mentioned:
Bad suction lines from tank to frame line or from frame line to fuel pump.
Failing fuel pump.
Plugged fuel filter.
Bad batch of gas - did the problems start shortly after your last fill up?
Button in top of dizzy cap worn.

You mentioned a loose gas cap. Any chance someone added bad stuff to your tank?

If you have dual tanks, switch to the other one.

[GASoline71]

I'd start with replacing the fuel filter first.

Gary

[dzbowtie]

I appreciate all of the responses guys. I checked my vacuum and it was holding steady at 18 in. I could not find any vacuum leaks or bad tubing (checked in daylight this time).

The carb would spray steady streams and idled ok ( marginally satisfactory lol) when warm. Again, when put in gear, it wanted to stumble and then pick itself back up.

I looked at the vacuum advance on the hei. I could not get it to move when I sucked on the tube. There was a popping sound from the diaphragm, but no movement. I could move the advance by finger. I'm going to replace the advance and get back with you all.

[cadillac_al]

I can't get AC plugs to last 2 years. I would think 8 year old plugs would have problems unless they are Bosch Platinum's.

[Getter-Done]

I always use Autolite plugs for my carbureted engines.
The are designed to burn oil.
I have all-ways had better luck with them.

[dzbowtie]

I have replaced the vacuum advance on the distributor. Tomorrow I'll start it back up and check the timing.
Would you guys recommend using ported vacuum or manifold vacuum on the advance? It was originally set on ported, but I have read a lot suggesting manifold.

[Getter-Done]

Ported vacuum

[geezer#99]

Manifold vacuum!

[68gmsee]

Originally Posted by dzbowtie

I have replaced the vacuum advance on the distributor. Tomorrow I'll start it back up and check the timing.
Would you guys recommend using ported vacuum or manifold vacuum on the advance? It was originally set on ported, but I have read a lot suggesting manifold.

Hope the suggestions above fix your problem. Replacing the defective vacuum advance is a good first step.

As for using either ported or manifold I'd try each one to see which one works better for you. Some say ported but in my research on the subject years ago, I found more preferred manifold vacuum. I've used both, and to be honest, on my stock engine just regular driving, I couldn't tell much difference in performance.

[dzbowtie]

Got the timing light out and warmed up the truck. With the vacuum advance swap, the truck already idled better. The initial timing was set at 12* and total at 34*. I also bottomed out the idle mixture screws on the carb and started over, only having to turn out about 1 3/4 turns each. I did switch to manifold vacuum and adjusted the idle screw. This gave me approx 800 idle and 500 in gear. No surging. Everything seems back in order. Thank you all for your help. This board continues to impress!

[68gmsee]

Great! Thanks for posting the fix. Should help someone else in future.