Shakes @ idle in D or R

[farmerfred]

I got my truck to idle nicely today (8 BTDC @ 600 RPM). It also runs well. The problem is at idle when you're in drive or reverse, the truck shakes. No adjusting of the timing or the carb would take care of this. When I put a vacuum gauge on it, the needle fluctuated between 9-15 in Hg. When I revved the motor, I'd get 10-22+ in Hg., still fluctuating. I'm thinking it's worn valve guides. Sound reasonable? Other possibilities?

[my67C10]

if you have a brake booster it might be going out.

[farmerfred]

How can I test to see whether it is the brake booster?

[Boog]

Pull the vacumn line going to the brake booster and plug it and if the engine smooths out the booster is leaking. Replace the unit. Inspect the pcv valve and all vacumn lines. Is the engine also down on power?

[Robert1970C20rstbukt]

Pull the line off the booster while the engine is running and plug the hose, if the engine smooths out then the diaphragm in the booster is shot.

[farmerfred]

OK. I found out how to check the booster in the the FAQ. It's at http://67-72chevytrucks.com/vboard/s...d.php?t=182131 in case anyone's interested. I'll have to check it tomorrow. Any other ideas?

[farmerfred]

Thanks, Robert1970C20rstbukt. I'm hoping it is the booster rather than the valve guides.

[corn]

Did you by chance forget to hook the vacuum hose back up after you timed it? Shaking at idle sounds like a vacuum leak to me. Also is the idle set in gear or in park? 600 in park will drop quite a bit when pulled into gear...

[farmerfred]

Actually, I keep forgetting to remove the vacuum hose when I set the timing. When I remembered and removed it, the timing didn't change. The vacum advance doesn't kick in until higher RPMs, I discovered.

I thought there was a vacuum leak, too. I replaced the grommet and hose to the PCV valve this morning. They definitely needed to be replaced, but it didn't help anything.

I've tried setting the idle while the truck was in drive, but that didn't help.

I'm beginning to wonder whether I did something to the booster hose when I removed the valve cover on that side. It would be great if it were something that simple/cheap.

[my67C10]

Originally Posted by cornfed63

Did you by chance forget to hook the vacuum hose back up after you timed it? Shaking at idle sounds like a vacuum leak to me. Also is the idle set in gear or in park? 600 in park will drop quite a bit when pulled into gear...

if the idle is set at 600 in park, then it should drop the rpm's down to about 400 rpm when its in gear. it should be at 800 in park, and 600 in hear (from what ive been told). I hope the problem is cheap and simple, good luck.

[farmerfred]

I've been thinking more about this. I see how a leak in the booster would lower the vacuum the engine produces. I don't understand why it would cause fluctuating vacuum. Can anyone explain it to me?

[68C15]

Originally Posted by farmerfred

I've been thinking more about this. I see how a leak in the booster would lower the vacuum the engine produces. I don't understand why it would cause fluctuating vacuum. Can anyone explain it to me?

now you are on the correct trail. a "bouncy" needle would be guides while a "wandering" needle would be fuel mix. a vac leak would just be a low reading as would be timing out of whack.

[jhow66]

Try this:
http://www.centuryperformance.com/vacuum.asp

[68C15]

very nice link jhow66. this should be studied by more people.

[Misstur_big]

How does it run off idle?
Is it a automatic?
Your guage would point me towards valves but these day's bad valves are rare due to hard seats etc. What kind of engine and what year is the engine.
Without more info I would lean more towards vaccum leak. Possibly intake.
Spray some Card cleaner along the manifold edges while the engine is idling and see if your idle smooths out. If it dose you have a manifold leak. Pinch off your brake booster line and see if it smooths out. Be very careful doing this and dont do it with a hot engine. Carb cleaner is flamable.

[farmerfred]

Thanks, Jhow66. That's the page I was looking at when I thought it was worn valve guides. Some of the other needle conditions looked similar to me, though, so I wasn't sure.

[farmerfred]

Thanks 68C15. I wasn't sure I understood what you meant by bouncy versus wandering. In my case, the needle flips back and forth so quickly that I had to really focus on each end one at a time to get readings. I think this is "bouncy."

[farmerfred]

How does it run off idle?

When I'm pressing the gas pedal, it runs just fine. Coasting is good, too, actually. The only problem is when it is at a dead stop while in drive, like at a traffic light.

Is it a automatic?

Yes. Don't know what you were thinking, but I was wondering whether the transmission might be involved here.

Your guage would point me towards valves but these day's bad valves are rare due to hard seats etc. What kind of engine and what year is the engine.

I don't know. I have no reason to think it's not a stock 350, but I don't know.

Without more info I would lean more towards vaccum leak.

Thanks. I'm going to pursue this today and will report back.

[Boog]

Fred, what condition are the plug wires in? Just sitting here thinking about it and I had one of my trucks do the same. In park it idled just fine but put it in drive with foot on brake and it ran awful. In my case it was 1 plug wire was arking over to the exhaust manifold when the engine was in gear. I couldn't hear it but I waited until dark out and had the wife start it up and put it in gear while I looked around under the hood. That spark shows up real good then.

[farmerfred]

Thanks, Boog. Actually, I've been thinking about replacing the wires for some time now. They don't look that great, and the previous owner bunched them together with wire ties, which I didn't think was a good idea.

[73stepside]

what is your tach setting at in gear? it sounds like you have your idle adjusted with it at 600rpm in park. try putting it in gear and adjusting the idle that way and see if that helps any. 500-600 rpm in drive/reverse is where it should be setting at if it has a reletive mild cam.

[mtndewmaniac66]

Originally Posted by farmerfred

Actually, I keep forgetting to remove the vacuum hose when I set the timing. When I remembered and removed it, the timing didn't change. The vacum advance doesn't kick in until higher RPMs, I discovered.

This is telling me that your vaccuum advance is plugged into the wrong circuit. The idle, or lower rpm circuit is located BELOW the throttle plates, where the high end circuit is located ABOVE the throttle plates. Remove from the current vac circuit and place the tube onto the port located on the throttle plate and you should feel and see the difference. BTW, check for vac signal also, since they have been known to plug up via carbon or "varnish" (dried gas deposits) A can of carb cleaner with a spray straw works wonders to free up a plugged corcuit.

[jhow66]

If the needle is flipping back and forth at regular intervals you have one cylinder that is dead-fouled plug, bad wire, burnt valve, broke spring, cam lobe down or a vacuum leak to that cylinder. To see if this is your problem make sure all cylinders are working.

[farmerfred]

Thanks, mtndewmaniac66. I think I've got the vac advance plugged into the right place, though. I'm using the ported side. The other one has constant vacuum, and that's where I've been attaching my vacuum gauge. BTW, the carb is an Edlebrock 1406. You might be right about the carbon deposits. I had to clean out one of the idle mixture screws already.

[farmerfred]

I was becoming convinced that it was worn valve guides, but you might be right. How can I check to make sure that each cylinder is working? I have an HEI distributor, which I think means I can't just pull the ignition wires off of one plug at a time. How can I test this? I appreciate the help.

Originally Posted by jhow66

If the needle is flipping back and forth at regular intervals you have one cylinder that is dead-fouled plug, bad wire, burnt valve, broke spring, cam lobe down or a vacuum leak to that cylinder. To see if this is your problem make sure all cylinders are working.