Hello again everyone,

I had posted this over in Tech help, but i have updates on it that i figured it could use a new post..

So - long story short, on Monday I went to my buddys house in my '78 (350, TH350) to mock up the mounts for the side pipes i am putting on. Finish up the mockery, helped him bleed his rat rod's breaks, and went on my merry way..
I back out of his driveway, throw it into "3", and give it a little "showboat" rev, nothing huge, just to around 3000rpm (yes, the engine was already warmed up when i did this - should have been no harm no foul.. no tires were spun or anything like that)

As i roll up to the first of 2 stopsigns between our houses, i notice that my RPM are not dropping at all.. i mash on the brakes and get it to stop, sitting at 3200 rpm... i cycle through all of the gears to see if there is any change, and there isnt... so since i can see my house I decide to make it the rest of the way into my garage. When i left the stop sign, i let my foot off the brake and the tires almost spun from it..

So - car is in my garage now, and im trying to diagnose wtf is happening.. heres what i have looked at:

1) Cables binding at carb - I went through accel and trans kickdown cables and they are both fine - the springs are not catching, nothing out of the ordinary here.
2) Trans fluid is good (i know this isnt a symptom, but i checked it so its in my list)
3) Throttle plates are closed and functioning
4) Choke is working as it should
5) Sprayed carb cleaner on EVERYTHING looking for a vac leak - cant seem to locate one.. I even sprayed along the manifold where it mounts to the block - no dice.
6) disconnected accel cable to make sure that wasnt my issue, still no luck

So - about the carb.. i bought it from Holley back in december.. Holley 650 600 cfm single intake. I figured maybe a float is sticking or something.. so i beat it up a little to see if i could get it to unstick... no dice..

To get RPM that sit instantly at 3200, Air has to be introduced from my understanding.. do you guys have any other ideas before i go tear into my Edelbrock Permformer intake and have to re-gasket everything? What can i try?

 

Just a guess...will a power valve with debris caught in it keeping it from closing cause high idle?

 

Hmm.. Thats a good thought!! Question is - where is the power valve on my holley.. bottom side of the metering plate?

 

On my 600 I remove the front bowl and the plate that the metering valve is screwed into.


BEFORE you take the carb apart......for some ?reason? one of the primary butterflies would "catch" in the throat and not close completely causing a high idle. I never would have discovered the problem until I tapped the butterfly with a screwdriver tip and it snapped closed

 

Just out of curiosity, what happens if you block off all of the air going into the carb? What I mean is, try covering the intake with a ziplock bag or something to completely restrict the airflow, and see if the idle dies down or the engine shuts off completely. Me thinks that would tell you whether you have a leak somewhere or not, and allow you to eliminate an air leak completely.

 

Lars already showed that a open power valve has nothing to do with idle circuit (unless there is a hole in it). Look elsewhere.

Are you absolutely sure the secondaries are closing? All the way? There should be an adjustment screw for the secondaries. Unfortunately, its under the carb which will require removal and turning it up side down. Is there any reason someone would have played with that screw?

Float levels have little to do with idle speed. To prove that, you can raise & lower the floats while running and note no change in engine speed.

How about the carbs links? The one going to the accelerator pump. The one hooked to the secondaries. Off & On link binding?

I doubt its a vacuum leak issue but maybe Those are usually more consistent. Your issue is inconsistent.

 

I'll give that a check tomorrow when I pop the carb off.. thanks!

Ok, this is the first test I have done that has yielded some results.. placed a ziplock bag over my carb after started and it choked it out!
Note to future readers- dont do it with a ziplock.. carb sucked in so hard that I was fishing plastic ziplock bags out of my carb for the last 30 minutes.. use something rigid.

So this eliminates manifold vac and tells me that it's a carb issue- correct?

No binding in the linkage.. I dont think anyone played with my screw, especially since It was in my eyesight the whole time i was at my buddies house and it didnt start happening until i left there.
What do you mean by mine is inconsistent though? It happens every time i start the car now - instant shoot up in rpm.

 

Inconsistent because you drove to your buddies house it was fine. Left his house afterwards and it changed RPMs. That's not consistent.

IDK

Its not vacuum. Its not gas pedal cable. Its not timing.

Next question: When its doing its mood swings, and the air cleaner is off, can you grab the throttle and push the RPMs down?

 

Sorry, I didn’t mean to cause you the extra hassle

 

A wet hand towel works well. You know the one. Its pink and frilly. Maybe some lace edges. Your wife has it hanging in the bathroom with a sign that says:
Do Not Touch.
You hate that thing. It fits any carb, any size, any time.

 

Three things I would check:
1 Make sure secondary throttle blades are fully closed
2 Make choke linkage is free
3 Make sure fast idle cam is in it's proper position

 

I just tried that out, no dice.. I'm gonna pull the carb up tomorrow and really ensure that plates are free and all

Hey, no harm no foul! I appreciate your advice because I made progress with it.

Oh boy.. I'm not touching that damn thing!! Am I the only one that gets the "you spend all of our money on that damn car" lecture? Lol!

Will try it all tomorrow once I get h ok me from work.. thanks!!

 

Either one can cause your problem though the stuck idle cam is where I would look first.

 

quick update for everyone - hit the throttle while watching my choke plate and it was not opening to a full 90 degrees... it was close, but not quite all of the way - so that is part of my issue... still seems like its running high though. going to grab a timing light and check it out, bring it back down to reality.

What should my timing be at? .. i am thinkin around 9, because i have a nice mild cam in the car (i unfortunately dont have specs on it - was in the car when i got it)..

 

From what you described your fast idle cam is sticking. Most people start around 10º I believe though for a street engine I use a vacuum gauge and adjust for the highest reading and then back it off just a taste.

 

Timing should be 36 degrees total @ 3000 rpm. Disregard what the timing is at idle.....you do not drive at idle. If the distributor is stock though....36 total will get you about 14-15 initial.......the 36 number total is a proven number. Once the timing is set with the vac hose off......then check to see how much the vac advance canister pulls.....it should pull about 12 degrees.....refer to Lars papers on this for a more detailed explanation.

Your carb is either being held open by the choke, a piece of debris, or bent blades....there is nothing else that will hold it open if the throttle spring is tight. If none of that then you have a massive vac leak as stated in a previous post....

Jebby