Car idles rough

[Guest Billy's Toys]

My 69 Chevelle SS with a 396 idles and runs rough . The problem is that its taken in to much gas causing it to burn way to rich. This is a recently rebuilt motor just put in with a new Holley carb. The motor was dynoed tuned at 479 HP before being put in car. It was running great before getting this problem that's gotten worse. I removed the gas from the "new" tank and replaced with fresh 93 octane . I also replaced the sparkplugs at this time but this did not help.

Any Ideas ? thanks

Edited January 6, 2014 by Billy's Toys (see edit history)

[Steve Braverman]

I doubt that a 479hp 396 will ever idle smooth.

[GLong]

If the car ran good and reasonably smooth when the engine was first installed, and you believe the engine ran rich causing the current rough running, then the problem is most likely a problem with the carburetor. probably a float is filling with gas and getting heavy, or there is something stuck in a float needle and seat causing the float bowl to over flow and flood the engine.

I'd take it back to whoever did the engine work and let them sort out the problem.

I also would not drive it at all until you get it sorted out. An over-rich mixture will wash all the oil off the cylinder walls, and get past the rings. The un-lubricated cylinder walls will wear quickly and the new rings will not seat.. AND the extra fuel will wash down the cylinder walls, past the rings, and dilute the engine oil.. making the oil contaminated and not able to properly lubricate the engine.

Either problem will mean either a very short life for the new engine, or worse a destroyed engine.

Greg L

[Guest Billy's Toys]

If the car ran good and reasonably smooth when the engine was first installed, and you believe the engine ran rich causing the current rough running, then the problem is most likely a problem with the carburetor. probably a float is filling with gas and getting heavy, or there is something stuck in a float needle and seat causing the float bowl to over flow and flood the engine.

I'd take it back to whoever did the engine work and let them sort out the problem.

I also would not drive it at all until you get it sorted out. An over-rich mixture will wash all the oil off the cylinder walls, and get past the rings. The un-lubricated cylinder walls will wear quickly and the new rings will not seat.. AND the extra fuel will wash down the cylinder walls, past the rings, and dilute the engine oil.. making the oil contaminated and not able to properly lubricate the engine.

Either problem will mean either a very short life for the new engine, or worse a destroyed engine.

Greg L

That would make sense Greg. It was running great then started running rough. It is surely getting flooded out with gas. It started to smoke and smell of burning gas and when I pulled the plugs they where carbon up "black" and smelled like gas.

Unfortunately I bought the car 4 month ago with all the work done on it already so Im on my own on figuring it out. Like I said it was running nice when I got it before it started to get this problem. Maybe I should replace the carb on it and see if the problem goes away. Any suggestions on what make and size carb I should go with.

Again, thanks a lot for all the help.

[GLong]

It's a bit of a pain to figure out, but take and post a photo of the carb, so we can figure out what flavor/make/model of carb you have.

Carbs are fairly easy to work on. and the problem can be as simple as a 1/16" piece of grit lodged in the rubber tip of the needle/seat and letting fuel leak past the seat.

With these older cars with carburetors, points and condenser etc. It rarely is a good idea to just start putting new parts on it to try to fix it. it is MUCH better to learn how it works and fix it..

For example, if you put another carb on it, then another piece of grit messes up the second carb's needle and seat, then you have wasted the money on a carb and the problem is still there: a contaminated fuel system or a bad fuel filter that is letting grit [rust flakes?] get to the carb .

So I'd recommend finding the make/model of your carb, find a good carb kit, mainly for the directions. and look into the carb, find what the problem is..

Example: I had a '64 TBird that sat for several years. when I started it up about a year ago, the carb was leaking and flooding.. This model carb, has a removable float bowl with a thin paper gasket to seal the bowl against the carb body, the paper had shrunk over the years and pulled in, leaving a big gap for gas to pour out of..

BY THE WAY: ALWAYS have a good fire extinguisher nearby when messing with the fuel system! Nothing like a surprise fire to ruin your day !

​

[GLong]

To continue, or go online and find an expanded view of your carb and figure out how it works.

Carbs can be fixed, unless they get run over by a truck, burnt and melted or beaten with a hammer. They were designed to be repaired and rebuilt.. Unlike fuel injection parts and pieces.

Post a few photos, or some carb info.

Greg L

[joe_padavano]

Carbs can be fixed, unless they get run over by a truck, burnt and melted or beaten with a hammer. They were designed to be repaired and rebuilt.. Unlike fuel injection parts and pieces.

HUH? I'm guessing you've never worked on an EFI vehicle. They can easily be fixed - and sometimes without even getting your hands dirty!

[GLong]

I guess I should say that you can resurrect a carburetor that actually has a mechanical or contamination problem without having to buy a new carb.

With EFI, which I've had plenty of experience with, If the computer is messed up, sure, you can reset, reprogram etc etc. But my question here is, why does it need resetting, or reprogramming?? Some mystery electron run rampant through the circuit boards ?

But if you have an EFI with contaminated fuel, water in fuel etc and it sat for several months.. Well, get out your credit card, it's gonna hurt!

I've resurrected many carbs that looked like junk.. but I've never resurrected a bad injector, corroded connector, or trashed fuel pump.. and if the computer gets wet, especially in salt water, get out your Credit Card,,

How many EFI cars survived submersion in Hurricane Sandy without buying new computers etc?? And how many '60's and earlier cars submerged in salt water can be made to run again without buying more than a gasket set?? At least until the body rots off the frame..

Greg L

[joe_padavano]

I guess I should say that you can resurrect a carburetor that actually has a mechanical or contamination problem without having to buy a new carb.

With EFI, which I've had plenty of experience with, If the computer is messed up, sure, you can reset, reprogram etc etc. But my question here is, why does it need resetting, or reprogramming?? Some mystery electron run rampant through the circuit boards ?

But if you have an EFI with contaminated fuel, water in fuel etc and it sat for several months.. Well, get out your credit card, it's gonna hurt!

I've resurrected many carbs that looked like junk.. but I've never resurrected a bad injector, corroded connector, or trashed fuel pump.. and if the computer gets wet, especially in salt water, get out your Credit Card,,

How many EFI cars survived submersion in Hurricane Sandy without buying new computers etc?? And how many '60's and earlier cars submerged in salt water can be made to run again without buying more than a gasket set?? At least until the body rots off the frame..

Greg L

I'll respectfully have to disagree. First, the ECU is rarely if ever the cause of a problem. Yeah, lots of them get replaced by "mechanics", usually unnecessarily. I've had really good luck cleaning injectors by flushing them with solvent while pulsing the coils (I have an OTC box that will cycle the coil - no different that buying specific tools for your carb and not expensive since I got it used on ebay). Replacing all eight injectors in my truck cost about $200 for new ones. That may be more than a carb kit and float, but not that much - have you priced carb kits lately? I'll agree that the in-tank fuel pump is a PITA to replace, but I understand why it's there, and frankly my truck has 250K miles on it with only an injector replacement and fuel pump replacement.

On the other hand, the 4GC on my 62 Olds (which had sat for 30 years) had an accelerator pump problem that multiple rebuilds (including pump replacement with a factory-style cup) could not fix. I suspect it was caused by rust in the pump well fill passage in the cast iron carb base, but unfortunately that hole was factory drilled then the outside plugged, so I could not easily clean it. I finally bought a new carb when I got tired of fooling with it.

Neither of these are definitive proof, but each method has its pros and cons. I don't crap on either one.

[GLong]

to the original poster: Hey Billy, did you find out what is wrong with the engine or carburetor ??

My last comment on carb rebuilding vs testing, rejuvenating and buying parts for EFI systems.

The only special tool I have for a carb overhaul is a #2 straight blade screwdriver and a pair of needle-nose pliers.

Gaskets-only carb kits are dirt cheap. Recently, for a '64 TBird, 390v8, the gaskets were $8, The whole kit $45.

The carb that you couldn't get the accelerator pump to work on seems like it still needed more work.. not the carb's fault.

I'm sure that those with a love of electronics will always love electronics. But all the specialized test equipment for the various injection

systems I've worked on over the years now sit collecting dust.. maybe there is a market for the stuff in Arizona and California, plenty of old injection vehicles out there.

Personally, i've never worked on an intact carb that I couldn't get to work right, some had worn out throttle shafts and had to be rebushed. Some very early carbs come under the 'driven over by a truck' , they had potmetal bodies and fell apart.

But I've had many EFI computers fail, many injector bodies go bad electrically, bad MAP sensors, bad temp sensors, bad grounds. Alternators putting out too high voltage, erratic throttle position sensors. And the winter salt environment makes it all worse.

I'll keep my #2 straight blade screwdriver handy..

And pray that my late model truck doesn't start to have any electrical problems with the injection system.

Greg L

[Larry Schramm]

I agree with Joe on ECM's usually not being the problem. I know of some that were built with cold solder joints that could fail over time, but those are the exceptions and those problems happened years ago. You can usually find that problem by GENTLY TAPPING the ECM with the palm of your hand and if the car stumbles or stops, then it is probably the ECM. Other than that, new software programming usually changes some fuel/air parameters for performance or emissions.

[rons49]

What about the intake manifold gasket or carb gasket? What is vacuum reading? The bolts at either of those components could have expanded and need a retorque & be drawing in air from the outside and running rich. Don't forget to verify that the choke is not staying closed. BTW, I just had to replace a PCM on an '03 Impala. The car ran great for days, then out of the clear blue while driving, the engine shut off. Hook up a reader and "rock" the PCM housing and watch the meter go nuts. But, of course not an issue on the ' 69.

Edited January 10, 2014 by rons49 (see edit history)

[GLong]

A vacuum leak lets air into the intake system, this would make an engine run lean, not rich. A choke sticking closed or very slow to open could be the cause and certainly would be the first thing to check, maybe something as simple as the wire to the electric choke element is loose or fell off.

Greg L

Hey Billy, any update ??

[junkyardjeff]

If it has a Holley I would check for a bad needle valve and seat first and then the power valve next,fire it up and remove air cleaner and look down the carb and see if you can see any fuel dripping and if it is its the needle valve which can be done without taking the carb apart but the power valve will need the front fuel bowl and metering plate removed.

[ol swede]

If it is a newer Holley they're supposed to have "blowproof" power valves

[junkyardjeff]

I have one that is supposed to be blow proof but had to change one and I think I will need to do it again,it also might need a different power valve due to not having alot of vacumn if the motor has a radical cam.

[ted sweet]

sounds like a blown power valve

[Larry Schramm]

BY THE WAY: ALWAYS have a good fire extinguisher nearby when messing with the fuel system! Nothing like a surprise fire to ruin your day !

​

I keep two in the garage. A dry chemical and a CO2 and have used the dry chemical once. Great investment in common sense!!

[Marty Roth]

Quote Originally Posted by GLong View Post

BY THE WAY: ALWAYS have a good fire extinguisher nearby when messing with the fuel system! Nothing like a surprise fire to ruin your day !

I keep two in the garage. A dry chemical and a CO2 and have used the dry chemical once. Great investment in common sense!!

Is HALON still available? I always carried a few of these in each car, but sold the cars with the extinguishers as gifts, and have not seen that type recently.

The benefit was that they left no residue, but removed the oxygen, and thus the ability to support combustion.

[GLong]

New halon or halotron or some new version is available. You can find halon extinguishers for aircraft use. But I usually buy halon extinguishers off of ebay, Some can be reasonably priced. They often are ones past some expiration date, but still hold a charge,

The main thing about halon, is that it will travel to a fire, even if it is in a location that is inaccessible to a powder or liquid extinguisher. The effectiveness of the Halon is hard to describe. The air being drawn to a fire brings the Halon to the fire.

A dry chemical extinguisher is cheap, but in many instances just about worthless.

If you have say, some old cloth covered wiring catch fire under the dashboard, just how effective will a dry chemical be? it needs to cover and smother a fire.. The halon will travel right to the fire, even if you just generally aim it at the floor under the dash.

I carry only Halon in my collector cars, and keep several around the shop and house, one in each car or truck.

My neighbors barn just burnt to the ground, for the lack of a good extinguisher. Thankfully nobody was hurt, only loss was the stucture and a bunch of tools and machinery.

Greg L

[ojh]

I bet you made up some nice new fuel lines to connect the carb with, right? What happens a lot is that trash from making the new fuel lines go right to the needle & seat so the float cannot close them fully. I have a hot rod shop and see this a lot. We always wash the new lines out, blowing with air won't get the job done. You'll have to pull your needle & seats out of the carb and spay the contact area with carb cleaner and they're good to reinstall. If you use a 5/8ths wrench to loosten the nut and leave the screw tight to the nut & bring the assembly out it won't affect the height adjustment, spray them and screw them back in and tighten the nut. If you break the screw loose from the nut then you'll have to adjust the float level.

It'll run rich for a few moments until it burns the fuel that is too high in the bowl, once it does that you need to double check the float level, theres lots of threads on how to do that.

Depending on how long it has been run with excessive fuel in the bowls you should check your oil to see if raw fuel has contaminated it.