Sudden rough idle

[Rustytruck]

Your 2 idle mixture screws turn them in until they lightly bottom out. count the turns going in so you can set them back after you remove the screws. now remove those screws. spray carb cleaner those holes using the carb cleaner with the tube attached. Don't go crazy with it just a couple of quick blasts now screw the screws back in all the way and then back out each screw the proper turns you wrote down. You could also have problems with a leaky power valve in the carb Have you had any back fires through the carb? always search for vacuum leaks with an idle issue.

 

[1970 Palmer]

You have two idle "mixture" screws. When you adjust them, are they reacting the same on both sides? When they are adjusted to the smoothest idle, they should be out approximately the same number of turns. If one does not react, or if you have them turned out quite differently it is a sign of a vacuum leak, or you have the carb idle circuit gummed up. You need to diagnose the problem before you start changing parts. It's very easy to clean the carb idle circuit if that's where you narrow down your diagnosis. Today's ethanol diluted fuel is a big cause of fuel contamination problems.

John

 

[DeepC73]

float level too high or needle and seat failing. hows your fuel pressure.

Which needle and seat?

You have two idle "mixture" screws. When you adjust them, are they reacting the same on both sides? When they are adjusted to the smoothest idle, they should be out approximately the same number of turns. If one does not react, or if you have them turned out quite differently it is a sign of a vacuum leak, or you have the carb idle circuit gummed up. You need to diagnose the problem before you start changing parts. It's very easy to clean the carb idle circuit if that's where you narrow down your diagnosis. Today's ethanol diluted fuel is a big cause of fuel contamination problems.

John

I plan to drop a few drops of fuel into the mouth of the carb while it is idling to see if it smooths it out. If it does, I know its in the idle circuit. I will keep you posted.

 

[DeepC73]

I have checked out the carburetor (Holley 350). The idle mixture screw on the passenger side does nothing if I turn it all the way in or out. Could this be it?

 

[1970 Palmer]

Rusty, YOU BEAT ME! LOL

I have checked out the carburetor (Holley 350). The idle mixture screw on the passenger side does nothing if I turn it all the way in or out. Could this be it?

Yes, your getting closer. Do as Rusty suggested and remove the idle screw, and use a little compressed air to see if it helps. Don't loose the cork around the screw that's pressed in. If no improvement, I would pull the carb apart and clean it out, and install a new carb kit. Make sure the diaphragm in the power valve is still good.

John

 

[1970 Palmer]

Thi

Which needle and seat?

I plan to drop a few drops of fuel into the mouth of the carb while it is idling to see if it smooths it out. If it does, I know its in the idle circuit. I will keep you posted.

This will will not help your problem. A simple way to richen the idle mixture for testing purposes is to just slightly close the choke manually (slightly) with your hand. If it smooths out, it was too lean. Stock, you have a split plain intake manifold, which is way you have two adjustable idle circuits. You are basically synchronizing them when your adjusting to two small screws. If one side of the carb is plugged up it will show up by no adjustment, and no improvement in idle smoothness. As previously mentioned, a bad power valve will show up the same.

John

 

[DeepC73]

Could a clogged idle circuit really cause the engine to bounce that much. ?

 

[DirtDonk]

Not sure exactly what you mean by bouncing, but if you mean a really rough idle, then yes it can. Especially if only four cylinders are getting fuel.
It should not matter to a running engine while you’re driving, but it can make a huge difference when you pull up to a stop. It’s just the idle circuit, so it does not affect the rest of the running conditions by much.

Since checking and cleaning the plugs and things did make it run a little better, chances are you needed those things tweaked a little anyway. So they made a carburetor problem worse.

Did you happen to have any small backfires or spitting through the carburetor that you remember? That might’ve destroyed an old power valve. If that’s the case then the cure, or at least a partial cure is to replace it.
Which of course means cleaning the carburetor while you’re at it so to make sure there aren’t any other problems.

 

[DeepC73]

Not sure exactly what you mean by bouncing, but if you mean a really rough idle, then yes it can. Especially if only four cylinders are getting fuel.
It should not matter to a running engine while you’re driving, but it can make a huge difference when you pull up to a stop. It’s just the idle circuit, so it does not affect the rest of the running conditions by much.

Since checking and cleaning the plugs and things did make it run a little better, chances are you needed those things tweaked a little anyway. So they made a carburetor problem worse.

Did you happen to have any small backfires or spitting through the carburetor that you remember? That might’ve destroyed an old power valve. If that’s the case then the cure, or at least a partial cure is to replace it.
Which of course means cleaning the carburetor while you’re at it so to make sure there aren’t any other problems.

I have a rebuild kit I bought last year in a drawer. I plan to pull it and start rebuilding it tonight when I get home. I haven't had any backfires through the carb that I recall. However, in it's current state, when I repeatedly press the accelerator while reving the engine, it does let out a few small backfires/rumble from the exhaust pipe. I checked and see no fuel on one barrel of the carb at idle, but it does deliver fuel in both barrels when I press the accelerator, like you said.

 

[Rustytruck]

Rusty, YOU BEAT ME! LOL

Yes, your getting closer. Do as Rusty suggested and remove the idle screw, and use a little compressed air to see if it helps. Don't loose the cork around the screw that's pressed in. If no improvement, I would pull the carb apart and clean it out, and install a new carb kit. Make sure the diaphragm in the power valve is still good.

John

Sorry just trying to help where I can. I don't see how I can beat you. I type with 2 fingers, miss spell a lot and have to proof read 3 times and still get it wrong and half blind and after posting when I think its right I often have to go back and edit after its posted. Its one hell of a chore for me to beat you guys.

 

[DeepC73]

UPDATE!
I rebuilt the carburetor. It did help a bunch, I mean a bunch. I would say it’s 90% back to normal (the way it was before this happened last Sunday). None of the idle passages were clogged at all. It is still a little off. The passenger side idle mixture screw barely cause any change when turned all the way in either direction. Also, the idle screw is turned all the way out, not touching a thing and it still idles a tiny bit too high, so it seems. I will put a tachometer on it tomorrow and set it. It isn’t bouncing anymore, but still not quite right. It runs and drives fine. I’m going to do a compression test tomorrow if I get time. I have no ideas what else could be causing it.

 

[Rustytruck]

Make sure the float isnt too high. best to set the carb with a vacuum gauge adjust idle rpm to 750 rpm after warm up. then hook up vacuum gauge to manifold and adjust idle mixture needles to the highest steady vacuum needle reading then readjust the idle back to 750 and check needles again. the vacuum needle should be extremely steady. put a hose on the distributor vacuum can and suck on the hose make sure the vacuum canister isnt ruptured.

 

[svastano]

Sounds like dirt hit the carb. Have you changed the fuel filter? Maybe run some seafoam or the like thru the system to see if it helps at all?

 

[TruFordBlu]

How does it run going down the road ? How is the wide open throttle performance? If those are ok and it's just an idle issue it could be something as simple as a clogged air bleed or debris in the idle passage(s). The tiny passages usually clog first. I have an attachment that allows me to slowly meter in propane at the top of each venturi. It's a good way to check idle mixture quality and adjusment. The top of the venturi is where you look for extra fuel at idle. There should be no fuel dripping onto the top of the throttle plates until the main circuits come in play as the throttle is opened. And remember that when you check for vacuum leaks that you have to also check the device at the end of the vacuum hose for a leak. Example is a choke diaphram with a hole in it or a trans vacuum modulator hose that is cracked at the trans. Us old guys love carbs cause they're "simple". But there's alot going on inside those things.

 

[DeepC73]

How does it run going down the road ? How is the wide open throttle performance? If those are ok and it's just an idle issue it could be something as simple as a clogged air bleed or debris in the idle passage(s). The tiny passages usually clog first. I have an attachment that allows me to slowly meter in propane at the top of each venturi. It's a good way to check idle mixture quality and adjusment. The top of the venturi is where you look for extra fuel at idle. There should be no fuel dripping onto the top of the throttle plates until the main circuits come in play as the throttle is opened. And remember that when you check for vacuum leaks that you have to also check the device at the end of the vacuum hose for a leak. Example is a choke diaphram with a hole in it or a trans vacuum modulator hose that is cracked at the trans. Us old guys love carbs cause they're "simple". But there's alot going on inside those thing

 

[DeepC73]

Thank you EVERYONE, and I do mean everyone. You guys on here are awesome. I rebuilt the carburetor, but it was still kinda doing it. I decided to check the distributor and found that the fast idle lever was seized up so I bought a new distributor. It still didn’t fix it. I started looking closer at the carburetor because it made the biggest difference when I rebuilt it. I discovered that the passenger side was dripping fuel in the barrel pretty steady which was flooding it causing it to stumble. I pulled the carburetor again and reset the passage block. It is running much better. It’s still not as good as it was but really good. I just need to go in and set the timing and adjust the idle screws.

 

[m_m70]

I decided to check the distributor and found that the fast idle lever was seized up so I bought a new distributor

what's a "fast idle lever" on a distributor??

 

[DeepC73]

what's a "fast idle lever" on a distributor??

Vacuum advance is the correct term I believe.

 

[m_m70]

Vacuum advance is the correct term I believe

lol. it is. I personally love setting up carbs. getting everything tuned to work together is very therapeutic for me but I know it's not for everyone! Good luck with this one make sure you post up the final fix once accomplished. Helps us all out in the end!

 

[DeepC73]

lol. it is. I personally love setting up carbs. getting everything tuned to work together is very therapeutic for me but I know it's not for everyone! Good luck with this one make sure you post up the final fix once accomplished. Helps us all out in the end!

So I am having a problem with my newly rebuilt carb. It’s a Holley350. It is dripping fuel out of the main throttle passages during idle. It only happens randomly. What could be causing this?