My engine vibrates at idle more than I think it should. This is the only SBC I've ever had experience with so I'm not sure what is normal, but I think it is enough that it warrants investigation. It has done this for as long as I can remember but I didn't want to spend time on it until after I had checked the timing. Now that I have, and I just adjusted my carb idle jets again (holly 4150) I have it as smooth as I can get right now. What is the next place to look?

This is a 75 l-48 manual, stock except for headers, I recently replaced the clutch. No change in vibration when the clutch is fully disengaged (pedal to the floor) so I don't think it is a transmission/driveshaft/differential/ujoint issue.

Also, I suspect I may have a vacuum leak somewhere as my headlights take a couple seconds to raise/lower. Not sure if this would affect it.

 

A vacuum leak could easily be the cause and usually is if it's just a ruff idle. Pull the main vac line off and plug it then check the others one at a time to see if you have any improvement.
If you can't find it, I would move to a compression test. That can lead you to a tight valve lash to a wiped cam lob at worst case scenario.

 

Thank you, I will check vacuum first and then the rest.

I did notice today that one of the zip ties holding the plug wires up had melted and the wire boot had melted through also. the wire was sparking against the headers! Fixing that didn't fix the vibration but it sure goes faster now that it's back on eight cylinders

Need to use metal ties.

 

You can also get the corvette wires and run the wires under the headers. This will eliminate the wires from flopping over and touching.
If she's running hard and fast, that's good! Pretty much eliminates a bad cam lobe.

 

Not if it's got a Holley carb. Sounds like a vacuum leak.

 

Did you replace the plug wire and the plug? There might be damage to the wire where its not getting enough juice till you load it up, same with the spark plug. The plug could be gummed up where it might not work soo good at idle. If you havent replaced these, I would do so even if that is not the complete problem.

 

Yup, I'm going to replace that wire. The plugs and wires are less than two years old and the wires are threaded under the engine but one or two still come dangerously close to the headers without ties. I pulled one of the spark plugs to check fuel mixture and it looks a healthy tan color, not sure if I should check all eight or if they're generally the same.

Vacuum check time...It's a shame it is raining for days, I might put it off until later this week.

I unfortunately don't have the original q-jet. There was a holly on the car when I got it.

 

Always a good idea to check and change the plugs. Also sometimes the power valves in a Holley can leak causing strange idle problems.

 

RONG!!!! I know the OP has a flat tappet cam, so I dunno if it is capable of the same behavior as I faced with my ROLLER cam in a '89 truck short block I had installed a TPIS ZZ9 cam in my '72 vert.....it ran fine for maybe a few months then developed a raggedy assed idle, that drove me krazy for some years, every once in a while it would smooth out and be fine, but most of the time it was a damn mess, I went through all the injectors, induction, timing, dizzy replacment, caps, rotor buttons, wires, wire ends are Jacobs ceramic boots....so a friend sent me a L98 cam that sat on my shelf for almost a year before I got tired of the poor fuel economy when gas went to 4 bux.....so I decided to pull the rad/fans etc....and change the cam...the Cloyes double roller chain was in perfect condition, I pulled the lifters out perfect condition.....I pulled the ZZ9 cam out and about crapped, 6 of the lobes looked like the metal had delaminated.....full of tiny pits and scars.....but yes the overall shape of the lobes looked normal.....the L98 cam has been in there for some years now, and runs smooth as silk......

SO, an intermittent bad idle CAN be the result of a bad CAM, damn if I understand WHY it would smooth out once in a while, but that's my off the wall experience with it.....

 

Okay so I just got a vacuum test gauge and gave it a try.

I hooked it up to the brake booster outlet on my holly carb, let me know if this isn't correct.

Crank test. PCV removed/plugged. Vacuum booster hose disconnected from carb, vacuum gauge connected there instead. Distributor wires disconnected. Carb idle screws turned to close. Reading was 6hg during crank.

Idle running test. Same connections but ignition hooked up, idle adjusted back to "normal" at 1200rpm (i'll get to this later) and running. It was already fully warm from driving to buy the gauge. Reading was 17-18hg.

Engine revved to around 2000rpm, same hookups. Reading was ~21hg. To around 3000rpm, over 21hg. I forget the exact reading now.

I restored brake booster line and hooked vacuum gauge up to distributor vacuum advance line with a T so it was still connected. Seems to match previous readings, I see it go up well into the 20s during revving.

So when I bought the car the idle was set to about 1200rpm. I didn't realize until a year or more later that it should be around 800rpm. If I adjust it lower it runs very rough. What kind of problem does this indicate? I any case I'm pretty sure I'm getting a ZZ4 crate before the fall so I'm not *too* worried but I'd like to understand what is going on before I go changing it all around.

 

Does this sound likely given the information above? My engine has 78k miles on it. I got it at 71k so no idea what kind of life it has had. It does seem to run smoothly above idle speed though it would be nice if it had another 100hp

 

Any chance you can post a short video with sound of it at idle?

 

Your idle at 1200RPM: The distributor is too far advanced. You should borrow a timing light.

You can retard the advance by loosing the hold down clamp and turn the distributor clockwise, this will slow the rpms down and then you use the idle screw on the carb to set the idle up to where you want it.

 

I actually just checked the timing this week, I have another thread on it here. I don't think the timing is the issue, unless my TDC indicator is wrong. I may mess with it again though just to see what the lowest stable idle I can get is.

I think I can post a video... I'll give it a try when I get home.

 

I don't have a video yet, but I just went through the holly carb tuning procedure (http://forums.holley.com/entry.php?4...n-Holley-Carbs) from scratch, here's what happens:

Close both idle mix screws, then open one and a half turns to start
Start engine, it dies immediately
Increase idle rpm screw
Start engine, it moves between 1200 and 1000 every couple seconds, sounding like it starting to stall a bit then catch itself.
With vacuum gauge connected, increase idle mix richness until vacuum peaks at 21hg. Quite a few turns. Repeat for secondary mix screw. Engine is now idling at 1800rpm
Turn down idle rpm screw, rpms drop until 1200 rpm then the screw stops having any effect. It turns freely.

If then go and lean out the idle mix screws the engine will run unsteadily at 1000rpm and die anything below that.

 

It seems there is some noise on the passenger side. If I listen to the exhaust at the mufflers the drivers side sounds normal but the passenger side sounds like it isn't firing on one of the cylinders or is firing weakly. I can sort of feel a pause if I hold my hand up to it while the left side feels consistent. I did a leak down test today and got similar results on all cylinders. I am going to replace the plug wire that had melted against the headers. Hopefully that's it, as it is on the same side as the noise.

 

also replace the plug. Also check the prongs inside your distributor cap, make sure there is not burn marks.

 

I replaced the damaged wire and the two cruddy plugs but no change, still makes the noise and vibrates. I took two videos I just have to find a place to upload them. The noise is definitely coming from the passenger side, it's most noticeable from listening to the exhaust but it doesn't feel like I have an exhaust leak.

I'll check the distributor next, thanks.