Hot Rods 348 start up problem "RPM runs away"

I have my newly rebuilt 348 installed in my 1957 Chevy Napco pickup. It has a tri-carb set-up. I believe I have the timing right (not 180 out) and the valves (hydraulic) are adjusted correctly.

My problem is that the engine, while it does start fairly easily, but when it does, the rpm (at least 3-4K rpm) is way too high, I have to shut it down immediately. It is almost like it is "running away." Pushing on the accelerator increases the rpm. The carbs are set to lowest idle and the choke is off. Even when I disconnect the outer two carbs, it does the same. I don't see anything obvious (vacuum leaks) that would cause this. Any suggestions would be welcome.
don

 

look down the throat of the center carb....are the ****erflys closed?

 

Yes all three appear closed.
don

 

you must have a huge vacuum leak then. It can't rev that high with no air.

 

I would use something nonflamable like brakleen to spray around the bases of the carbs and intake gasket area.
The engine will slow down if you spray it in a leak.

 

Double check the ****erflies. Keep the linkage disconnected from the outer carbs, Make sure ****erflies are not being held open. You may have to bend the acclerator pump rods ( I think that what therc called) to make sure the ****erflies are fully seated, make sure nothing else is holding them up (idle setting), or that the shaft is bent.
When you hook the secondaries back up make sure the linkage lengths are right, and not opening the outer carbs at idle.

 

If it is the first run on the 3X2 make a block off plate for the outer carbs then get it ruing on the center carb then start working on the outer carbs one at a time.

 

Obvious or not, it's getting plenty of air. Most likely its down the throat of the carbs because its also getting enough fuel to sustain your 3000 + Rpms.

Since its new, you really need to suspect everything until its proven to not be a problem. Presumptions without eliminating things will have you chasing your tail.

Blocking off the two outer carbs will help speed up the process greatly.

 

Use a aluminum can gasket under the outer carbs, cheap and quick to cut out a couple and punch bolt holes in. They are thin enough to go right under the carb with out messing with fuel lines and linkage.

Joe

 

Hollow carb stud maybe?

 

Thanks for the responses, but I still haven't found the problem. However, I did notice that the forward carb does not spray any gas when actuated. I'm wondering if this could be an indicator of a problem with that carb and the overall problem?
don

 

Problem? Yes...Overall problem? No.
As stated, block off the end carbs. Back idle screw all the way out so it doesn't touch the idle cam. If it still races, you have a m***ive vacuum leak..Shouldn't be too hard to find at that point.

 

the standard 3x2 cast manifold came with a hollow stud usually in the rear near the distributor that was used for manifold vacuum in the day. I run a 3x2 348 and can ttell you that you should always start a fresh engine with the center only until everything is run in. Block off the outer carbs, deal with them later on.

 

I've pretty much done what everyone advised. I have blocked off the two end carbs, plugged all vacuum lines and still have the same problem. So, I took off the center carb and found the following:
1. The throttle plates were open a bit because of the mechanical linkage between the throttle and pump mechanism being to short. I adjusted that so the throttle plates can close completely.
2. The throttle plates do not have bleed holes. There are "dents" where they should be. I was wondering if I should drill them out (factory defect?), or that is the way they should be.
Thanks,
don

 

I don't think that I have ever seen holes in the throttle plates on a 2GC carb.
Have you tried it since adjusted the linkage?

 

Tried it again, no difference. I'm at the point of getting a new center carb. Thanks though.
don

 

If you drill holes it's going to go even faster as it will be getting even more air.
With the blades closed tight it shouldn't run at all let alone up to 3000 rpms. If the blades are closed and it does the same there's some irrational things happening.

That thing is getting plenty of air to do this, PLENTY of air and the right amount of fuel too.
To get fuel, the air need to get thru the throat of the carb. To get thru the throat the blades aren't closed.
You might want to try a known good carb just to see.

 

If you put your hand over the center carb will it kill the engine or make it idle down?

 

No, you don't want to drill holes in the plates..That's only done to clean up the idle with a big duration cam..Race car stuff.
Like Vick says, if the throttle is closed all the way, how's it getting gas to run? Does it start clean after shutoff , or do you have to hold it WFO?

 

Taking a stab here but do you have any idea what the center carb is off of? Is it an original 3x2 carb or just some junkyard piece? I ask because after getting my "junkyard" carb setup running I found out several things! LoL!
IF you don't have a vacuum leak, The first I would look at is the air bleeds in the carb. A 348 ci engine will have a completely different vacuum pull than say a 235 I-6. Mine acted totally opposite and wouldn't idle anywhere below about 3000, like a HUGE vacuum leak. Someone told me about the air bleeds and I changed the booster/air bleed tree and it fired right up. That was I course after yanking the intake 3 TIMES searching for a m***ive vacuum leak!!

Give her a shot!
Jimmy


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vacuum leak combined with a leaky carb , need both the fuel and the air to make it pull that high of rpm . 2000-2500 is about the normal m***ive vacuum leak I have seen without adding more fuel . check the float levels in all and double check to see if the throttle plate bushings on the carbs are not worn out which will cause the air leak also check the carb bases for cracks . . and one other question did you rebuild these?? if so double check the gaskets I have seen sometimes a wrong gasket will create a air/fuel leak .

 

This is the acid test for a vacuum leak.

 

If the carbs have been rebuilt, do you know if the actual throttle blades were removed from the throttle shaft? Typically, they shouldn't have been. If they were they can be a ****** to properly reinstall. With the idle speed screw backed out and the throttle completely closed they need to fully close off the throttle bores. But they also need to be fitted properly and evenly on both sides so they don't tend to stick when the plates are closed.

Remove the throttle body from the bowl and back out the idle speed screw. Make sure the throttle plates are properly tightened to the throttle shaft. Hold it up to a bright light with the throttle plates closed and look for light leaks between the plates and the throttle bores, especially a leak on one side of the carb but not the other. And while you're at it double check for the correct gasket between the throttle body and the bowl.

 

All the talk about vacuum leaks.... this has to be a leak "above" the throttle plate because it can pull fuel from the venturi using the main metering circuit (because where else could fuel matched up with airflow come from?). Look for loose throttle plate, too-high float level, the already-stated mechanical issues which cause throttle to stick open (or otherwise admit too much air past throttle blades).

It is not likely to be a leak in the manifold or below the throttlw plates unless the carb was significantly over-rich in which case you will "discover" that over-rich stalling problem after you fix the vacuum leak

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Great responses, many thanks, appreciate the rich source of ideas. I know this is a tough one to pin down remotely (it probably could be many things)

When I put my hand over the center carb, I can feel the strong "****ing in"of my hands. Then the engine quickly dies. Does that mean that is the carb itself? If so, then I will look at the identified "above the throttle plate" issues.

This issue has really got me down (I have had all three carbs out three times each), I was really out of ideas.
This is another example of informed people willing to share their expertise. With all the time and money I put in this engine build and swap, and being so close, I was on a real downer to not be able to resolve this problem. Your input has given me direction and hope. I don't want to come across as "sappy,"I just wanted to let everyone know that their input does makes a difference.
don

 

That indicated that the engine is drawing a lot of air (not surprised) and that either the blades are not closing all the way or,they are undersized (?). If you have the two end carbs blocked off and no vacuum leaks, the center carb is allowing too much air to the engine.

A gasoline engine is "throttled" by those blades in the carburetor, hence the name throttle.

 

Thought I would close this thread by thanking everyone for their input and letting you know the resolution. Ends up that after multiple dis-***emblies and a rebuild of all three carbs, I replaced the center carb with one from Summit. That helped, now I could run on the center carb by itself. Finally I dis***embled the front carb (again, because it would cause it to race up again when connected) and found that the venturi well inserts were missing. Replacing them solved the problem, I can now run on all three carbs. Again thanks for the troubleshooting ideas. They got me where I needed to go.
don

 

Good on you man !
You'll be able to run thru those carbs blindfolded now

 

Thanks for letting us know the outcome. It's difficult enough to get a vehicle to run on one carburetor let alone 3.

 

Cool man! You are a genius with 2G's now!! LoL!

Well since I'm not, what are the Venturi well plugs?


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