Rough idle

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sbh126

sbh126

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I've had a rough idle on my 383 ever since i got it. It seems to rock back and forth like it has a misfire but when I pulled the spark plug wires one by one I couldn’t find a cylinder that wasn’t firing. I adjusted the carb yesterday and it runs a little smoother but still not quite right. I thought I might have a vacuum leak and was trying to find it with the propane test. I wasn’t quite sure what I was looking or listening for so to test it I pulled the vacuum line off the brake booster to see how it ran with a vacuum leak and how it ran with the propane next to it. Well when I pulled the line the rpm’s went from 650 to 950. Isn’t the idle supposed to drop with a vacuum leak? Another thing to be noted is that when I pulled the line and the rpms jumped up the engine seemed to run the same. I don’t know a lot about the engine other than it was rebuilt at some point and it’s in my 68 Polara.
 
Hi sbh126,
It could be a number of things.

When i got my car it had a rough running 360, turned out the valves were set too tight, (not fully closing) which resulted in a shaking rough running engine. Have you hooked up a vacuum gauge yet? If so, what does it read. Is it steady or does the needle move a lot?

if you create a vacuum leak on an engine the RPM should drop then when you do the propane test (near the vacuum leak) the RPM should increase. When it does, you know you've found your leak. Try going around the base of the carb, check all vacuum lines for cracks and check around the intake. I have previously used WD-40 or carb cleaner to spray to find vacuum leaks but i think i read somewhere that you can just spray water as well (never did it myself) which would be safer than a combustible.
 
did you replace your spark plugs/do a tune up yet? if not spark plugs can fire but they maybe in hard shape and not fully fuctioning to there best of abilities
 
I dont have a vacuum gauge but I am planning on getting one today. Ill let you know the reading when I get it. I was trying to check the timing and it seemed to jump around quite a bit. Would this cause the vacuum to go up and down? I have an aluminum intake that I was planning on putting on so maybe I'll just go ahead and do that and see if it fixes the problem. Could you explain the problem you were having with your head? If the valve springs were to tight wouldnt it cause the valves to close tighter? Wouldn't the hydraulic lifters correct the problem?
 
since the car was new to me i didn't have any idea about the internals of the 360. It was a 74 block but it had been rebuilt.

Turned out it had solid lifters and it had adjustable rocker arms. When i adjusted the valves the first time (assuming it had hydraulic lifters) the pre-load i had set kept the valves so far open it wouldn't even start the engine, it would just spin free. That's when i figured out i had an issue.

So after posting some pictures on fabo i figured out it had solid lifters. Solid lifters need what they call a valve lash, which is a cushion of space between the rocker tip and the top of the valve stem at full valve closure (know as zero lift). How much this cushion of space is, depends on the camshaft profile.

When the previous owner adjusted the valves he must have put it a little too tight, therefor letting some of the valves open before they should. This can result in valve overlap where the exhaust isn't fully closed when new air and fuel is let into the combustion chamber. This can result in a rough idle and a shaking engine.
 
i would wait with the intake until you have hooked up a vacuum gauge to the engine. When you do have it, hook it up to a constant vacuum source like full manifold vacuum. Don't hook it up to the ported side of the carb, you won't get a reading at idle.
 
Running problems are hard to really nail on line. The best thing I could tell you is the basic setup to see if it's something big or not.
First, set the idle speed to 600 with the vacuum advance hose off the distributor and plugged.
Second, set the initial timing to 12° before top dead center. Each hash mark on the timing cover tab is one degree. So go about two marks further than the "10°BTDC" mark.
Third, hook up the vacuum gage and while adjusting the idle sped screw as needed to keep the idle speed at 600, adjust the two mixture screws until you get the highest vacuum reading. This takes time. Start by turning them both in 1/8 turn each, alternating one at a time. Give 30 seconds of idle time after each 1/8 turn for the vacuum level to stabilize. Once you have the highest vacuum, speed the idle up to 650-700 and plug the vacuum advance back in. See if that evens it out. You can also hit the top of the carb with carb cleaner. Dont do this while it's running, and i'd wait and do it after it's warmed up. Make sure to spray the little tube deal down the air bleeds and emulsion tubes.
 
Vacuum is at 16 at 600 rpm. I'm having trouble with reading the timing. It seems all over the place. If I have a loose timing chain would it cause this? A guy I work with told me he had a similar problem with a chevy 350 and it ended up being because he was using a damper off of a 305. Could I be running itno a similar problem? What could cause the timing mark to jump around so much?
 
sbh126 said:
Vacuum is at 16 at 600 rpm. I'm having trouble with reading the timing. It seems all over the place. If I have a loose timing chain would it cause this? A guy I work with told me he had a similar problem with a chevy 350 and it ended up being because he was using a damper off of a 305. Could I be running itno a similar problem? What could cause the timing mark to jump around so much?
Click to expand...
timing chain would have to be more than a little loose, but yes it could do that.balancer prob. no you would feel a vibration big time,besides most bb balance internally.ex cast crank 400s and440s. do a comp. test might be a valve
 
................ur distributer is prolly worn out..........take off the cap.....rotor.....then see if the shaft wiggles side to side...or maybe a spring came off a weight.....kim.....
 
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