Sudden rough idle

[m_m70]

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?

float level set too high is something to check

 

[BOBS 2 68S]

What m_m70 said. Float level, also fuel pressure. Pull the needle and seat check for dirt.

 

[DeepC73]

I pulled the needle and seat last night. I blew it out and it continued. The fuel is not coming out of the sight screw on the side so I don't think the float is too high. I just rebuilt it. The float needle and seat are new. I may install the old one tonight and try it. The fuel pressure is not high at all. What else would cause the main barrels to drip?

 

[m_m70]

The fuel pressure is not high at all. What else would cause the main barrels to drip?

do you have a fuel pressure gauge hooked up??
I assume you have replaced the fuel filters. If not, replace those.

I re-read this entire post. After your rebuild, do both the idle mixture screws affect the idle?

Have you used a vacuum gauge to adjust mixture screws?

In my experiences, dirt has been the biggest issue.

Did you replace the power valve with rebuild?

 

[DeepC73]

do you have a fuel pressure gauge hooked up??
I assume you have replaced the fuel filters. If not, replace those.

I re-read this entire post. After your rebuild, do both the idle mixture screws affect the idle?

Have you used a vacuum gauge to adjust mixture screws?

In my experiences, dirt has been the biggest issue.

Did you replace the power valve with rebuild?

I do now have adjustment with the idle mixture screws. I did a total rebuild, yes the power valve was replaced. I also checked it last night. I replaced the filter attached to the carb and added a micron in-line filter. I do not have a fuel pressure guage in the system. I am planning to buy one on the way home today. I doubt that is a problem, but it’s possible. I replaced the fuel pump about 9 months ago. As I said, while it is idling fuel is dripping into both barrels. I am going to lower the float setting dramatically today and see if that cause the dripping to stop. If it doesn’t then I plan to re-install the old float needle/seat for comparison.

 

[m_m70]

OK.
Did you use a vacuum gauge to set the idle adjustment?
What are your rpm's at idle?
What do you have your timing at?

 

[DirtDonk]

Just how much fuel is dripping out? Is it just a minute amount, but enough to see? If so that's kind of normal for some carburetors. It's not "correct" I don't believe, but not all are able to atomize it all especially at idle. You'd think it would work out in the design of carburetors in general, but I've seen lots of them (even brand new ones) give a visible droplet or ten while the engine was running.
I'm just thinking that if your fuel is not high enough to come out of the level plugs (or be visible in clear sight plugs) then it's not being forced into the venturi under pump pressure. If it was we'd be seeing it coming out as a too-high level in the bowl. And if bad enough, would continue to fill up to the point it comes out of the float bowl vent(s) as well.

I'm certainly not an expert on Holley carbs, so I'm probably missing some important point in this discussion. Just seems to me that if the float needle was stuck open you'd be getting fuel out of the top in addition to being forced through the normal channels and into the venturi.
But a float level that's just slightly too high for your setup? Yes I would think that could cause some extra dribble while idling. Could the sight plug(s) be too murky or clouded to see by any chance? Seems to me that most of them are a bit cloudy after awhile, but maybe yours is too far. Have you tried shining a light through it just in case? Maybe what you're seeing as a low level (surface meniscus not showing) is actually too high of a level where the visible line is above the plug instead of below it.
Not sure if that's possible here, but figured I'd mention it anyway.

Paul

 

[DeepC73]

My sight for my bowl is a screw that you remove to see the level. I lowered my float level quite a bit. It eventually stopped dripping. I then slowly set the level to just below the sight screw hole, like the Holley website said to do. The dripping did not return as of yet. However, now the idle mixture screws have a lot less effect when I turn them all the way in. It is running and idling fine at around 750 rpm’s. I need to check the timing again, but there are no marks on the balancer. I’m going to use a TDC tool to find the mark for TDC and check the timing and put on a new timing sticker.

 

[DirtDonk]

Yeah, frustrating even when there are factory marks on the damper. But with so many things changed over the years, sometimes they're there but you just can't line them up right.
Timing tape is a life-saver for sure. Find your zero, put on the marking tape and you're good for the next few decades.

Paul

 

[m_m70]

However, now the idle mixture screws have a lot less effect when I turn them all the way in.

So it still runs with the idle screws all the way in??

 

[DeepC73]

So it still runs with the idle screws all the way in??

Yes sir, it still runs with the screws all the way in. It actually runs better.

 

[Rustytruck]

Maybe check the power valve suck on it and make sure it doesnt leak. there is a number stamped on it maybe it is too high. the number is where it sees vacuum and opens it like 6 , 6.5, 7 or what ever the higher the number the sooner it opens.

How To Tune The Power Valve In A Holley Carburetor

How to ensure your Holley carburetor is using the right power valve for your engine. Holley carburetors have a power enrichment system that provides fuel to the main power circuit during heavy loads or under full throttle situations. The vacuum operated power enrichment system is controlled by a...

www.holley.com