Engine running away after carb rebuild....

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Cuda416

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Rebuilt a Holley 1920 to replace a damaged BBS on top of our 170. Had lots of problems with the BBS, likely caused by vac leaks etc. The new carb is much "tighter" and the car starts right up, but then runs like it's wide open throttle (though it's not) until we cut it off. Float adjusted properly (tested the float beforehand) and nothing coming out of the vent tube to feed the engine from the top. Where else would the engine be getting that much fuel from this carb to run out of control?

Even though the BBS gave us problems starting, the engine ran semi smooth and didn't run away like this

Any ideas are appreciated.

-=C
 
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You may be looking for the wrong solution to your problem. Fast idle like that typically indicates too much air, not fuel. I would look for vacuum leaks.
 
You may be looking for the wrong solution to your problem. Fast idle like that typically indicates too much air, not fuel. I would look for vacuum leaks.


Thanks, we'll double check for them but would it just be a fast idle or like wide open throttle?
 
Thanks, we'll double check for them but would it just be a fast idle or like wide open throttle?
Depends on the size of the leak. The bigger the leak, the faster the idle. Just like opening the throttle blades. The more air introduced, the faster it runs.
 
Sounds like the blade is not closing all of the way. Loosen the carb and cycle the throttle to center on the manifold.
 
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OP, fire it up and then put your hand over the carb, if it doesnt die.... it's got a vacuum leak.

Try some water in a spray bottle or carb cleaner and spray the base gasket and lower carb gasket. If the idle speed drops... there is your leak.
 
I would agree with air/ vacuum leak. check all hoses and if you can get it to calm down to a safe level to run, squirt water from a pump squirt bottle, or even take a length of hose.....vacuum or fuel hose, and hold one end to your ear, then probe around with the other end, listening for the leak

^^LOL I was typing as you^^
 
Thanks gents.

According to RustyRatRod, something is wrong..... Whew, i was really hoping it was just too close to the moon at the time and the problem would only manifest itself at night when the moon was out.. Gosh darn the luck... Thanks Rusty!

So, based on his sole assertion, I've done some more homework and it seems there are several ways a vacuum leak could cause this. Learning all the time. if we don't find anything obvious, we'll likely need to take the carb off and tear it down again I think to make sure things such as the power plunger/economizer are working properly etc. Watched a couple of videos from Mike's carburetor shop on these. I'll let you all know what we find.

-C
 
Here's the deal;
Air entering the intake from under the throttle blades will not cause a runaway............ wait for it............................... because there is no fuel in that air! Too much air entering under the plates, and if the throttle is set right in the first place, will just cause a stall.

Only too much air on top of the plate can cause a runaway, cuz it will pull fuel from the transfer port. And that can only happen if the throttle plate is not closing properly, or if it is opening by itself!
But in neutral, A whack of timing will also cause a high rpm.

So check your Vcan plumbing and your fast-idle cam, make sure the throttle return spring is properly installed, that the KD return spring is properly anchored, and make sure there are no big holes drilled thru your throttle plates. And yes if she's fat in the first place then a leak into the PCV system can also cause an elevated idle, but not a runaway!
Good luck
 
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Here's the deal;
Air entering the intake from under the throttle blades will not cause a runaway............ wait for it............................... because there is no fuel in it!

That's where I was confused initially, but apparently a low vacuum condition can cause the carb to "add" fuel into the venturi because it acts like it's under a load, or so I've seen/read. Sounds odd but it can apparently happen with these. I'm just not sure it's going to cause a run-away this severe.

That said, I am more then open to other ideas.

-=C
 
Does a 1920 even have an internal return spring..NO ITS DOESN'T. It actually has a default spring that opens the carb via the accelerator pump linkage. Make sure your linkage is set up with a return spring that can overcome the accelerator pump spring and the carb is fully closed. That is the strangest design as if you lose your throttle linkage/return spring, the carb will go ~40% open!
 
Does a 1920 even have an internal return spring..NO ITS DOESN'T. It actually has a default spring that opens the carb via the accelerator pump linkage. Make sure your linkage is set up with a return spring that can overcome the accelerator pump spring and the carb is fully closed. That is the strangest design as if you lose your throttle linkage/return spring, the carb will go ~40% open!


Yup, fair statement, but this is with my operating the carb by hand and not depending on the linkage at all yet.
 
Exactly, you need some sort of external return spring. Rig one up and test again. With your linkage closed by hand (fighting the slight accelerator tension) its still revving up?
 
Exactly, you need some sort of external return spring. Rig one up and test again. With your linkage closed by hand (fighting the slight accelerator tension) its still revving up?

Not sure I am being clear, or maybe I am missing something. I am manually operating the throttle, by hand, and the engine is running away. Weirdest thing...

We'll test again tonight, and yes, I'll rig a return spring cuz it's just a good idea :) and see how it goes. But like I said, with me operating the carb by hand, it goes into runaway almost immediately.
 
Check the throttle linkage. Changing carbs, did you get something backwards? Where is the throttle stop when it's racing?

My 64 has the old style linkage (rotating tube/bellcrank, not a cable) and one time I hooked if up incorrectly and it went full throttle right away.
It sounded like this:
RRR-RRR WHAAAAHHH!
 
Check the throttle linkage. Changing carbs, did you get something backwards? Where is the throttle stop when it's racing?

My 64 has the old style linkage (rotating tube/bellcrank, not a cable) and one time I hooked if up incorrectly and it went full throttle right away.
It sounded like this:
RRR-RRR WHAAAAHHH!

Ours has the same style linkage, I hate it...
 
Just with the carb sitting on my bench, the throttle blade is at about 45 degrees. I can close it manually and itll pop back open to 45.
 
Just with the carb sitting on my bench, the throttle blade is at about 45 degrees. I can close it manually and itll pop back open to 45.

Ok, I understand now. Ill take a look at when I get home. @Bodyperson suggested maybe the throttle isn't closing from interference. It might be I just "think" I'm closing it and it's hitting something.
 
That's where I was confused initially, but apparently a low vacuum condition can cause the carb to "add" fuel into the venturi because it acts like it's under a load, or so I've seen/read. Sounds odd but it can apparently happen with these. I'm just not sure it's going to cause a run-away this severe.

That said, I am more then open to other ideas.

-=C
Only if the throttle blades are waaaaay too far open.
I tell you what, when the low-speed circuit is working right, you can take the whole top of the carb off, while the engine is idling, and it will continue to idle nicely. If the boosters are flowing the throttle blades are waaaaaay far open.
 
I just gotta ask this, don't take it the wrong way; Have you verified that the throttle blades are closing when you believe that you are manually closing the throttle?

Haha, don't worry man, I got a pretty thick skin. It was @pishta who got me to think clearly. Normally, a carb "rests" in it's closed state. I wasn't thinking about the fact that this thing has no return spring built in and what you are suggesting is EXACTLY what I did. Tt didn't matter that i was running it manually because I was operating it backwards. I A$$umed the carb was closed in it's resting state when it was in fact wide open. Big old face palm moment to be sure!

Once I pulled my head out of my arse and did it the correct way, with a return spring connected I might add, it worked great. +1 for my kid rebujilding the carb right. Didnt' find any vacuum leaks either, unless you count the one in my ear.

Thanks for the tips, suggestions and most of all patience. I learn something every time I ask a question here. A serious group of awesome folks.
 
Good result.
To help people's understanding, modern gas engines have a throttle plate which closes off tightly. The idle speed is controlled by the Idle Air Control (IAC) which is an electric motorized valve which leaks air around the throttle plate. You can speed up the engine by pulling a vacuum hose, say the big one to the brake booster. The IAC will go full-closed, but the engine will still idle too fast. But, the engine will run fine since the engine controller will just add the required fuel. If you run that way long (~30 sec), the computer will likely trip a code "unable to control idle speed". When my 1996 Plymouth 2.4L was idling way too fast, even w/ the IAC port plugged, I found the air leak by spraying water around until the speed changed. The intake manifold gasket had become brittle and lost chunks, causing a large air leak.
 
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