Rochester 2gc idles high - stumped given what a simple carburetor it is. - Hot Rod Forum : Hotrodders Bulletin Board
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Old 07-06-2006, 09:52 PM
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Rochester 2gc idles high, stumbles on partial acceleration - can't find a vacuum leak

Guys I'm stumped on this. The automobile is a 1981 DeLorean DMC-12 (90 degree 2.8 V6)and I'm trying to correct the issues that K-jettronic mechanical fuel injections introduce to these cars by fabbing up my own intake manifold and running a Rochester 2GC on it. The autozone p/n is 3-21 and this carb in particular is a handy replacement for the 2 bbl found on 265 Chevy V8's. My success with the 265 is what lead me to purchase another for this project.

Anyway, so I have the carburetor mounted and I've tracked down the handful of vacuum leaks I had at first. I cannot find anymore at this time and I've gone through half a can of carb cleaner looking. The car runs great wide open...well, great as a 2.8 V6 can allow I suppose. It stumbles at part throttle if you slowly transition to WOT from idle. Jumping on it seems to let it get along ok. The idle itself is about 1800 rpm and that is with the butterflies completely closed and the choke full open. If I tighten down the fuel mixtures, it idles down but it starts sputtering and carrying on until it finally chokes down.

A brief description of the carburetor - there is a single idle screw on the driver's side (well, driver's side if it were in a 55 Chevy). There are two idle mixture screws on the front at the base, there is a single threaded port up and to the left of those screws. Above that port and towards the right is the inlet for the fuel. On the pasenger side there is the bimetal choke widget, on the back is a single vacuum port for accessories. I'm not certain what function the port above and to the left of the mixture screws serves. My haynes manual doesn't indicate that it even exists.

This particular carburetor does not have the dual idle screw setup that I've read about. My haynes manual indicates that I should simply adjust the imaginary one in case my idle is too high...which has become something of an issue (grin). I'm curious about its EGR details and if that might be where my trouble is at. In my application the entire underside of the carburetor except for the very sides and corners are exposed to the intake plenum.

I can post pictures of the carburetor but since the manifold is all kinds of ugly, I won't be showing it off. I originally intended to use a quadrajet for this application but decided the 2bbl would work better. It sits a little higher than I can use though so I am putting together a different manifold using a different design and a real sheet metal break, hehe.

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Old 07-07-2006, 03:29 AM
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rochester idle issue

it sounds to me like your timing is too far advanced, if you had to remove the
distributor to replace the manifold that might be where it happened. i have
had it happen to me too many times that no matter how good you mark things
they don't end up in the right place. if that checks out double check your
throttle cable and make sure, that with your idle screw backed all the way
off of the stop, that the butterflies completely close, and with your pedal
all the way down that they are completely open. i've worked on a bunch of
those carbs but my memory is fritzing out on how the curb and fast idle
adjustments are set up. if none of this helps, post a reply to me.
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Old 07-07-2006, 06:54 AM
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dmc12mk3,

I think the fast idle screw is behind the choke-stove and at the very bottom. You will need to almost stand on your head to see it.
The port above the idle mixture screws is for the vacuum advance. Since this is a "homemade-a-bago" installation I am not sure of how you are advancing the timing so that would explain the stumble as you go up in RPM. I do not think the EGR is causing any of these symtoms because it makes the engine have a vacuum leak and the car need to run with the mixture screws wide open and you say you don't have to do that.
The one place I would check is if your throttle plate is fully closing. Maybe one of the plates got bent and now that bend holds the carb off idle. Just a place to look.

Good Luck
Scholman
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Old 07-07-2006, 03:47 PM
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Thanks for the replies. I checked with a mirror and pulled the other 2gc I have off the shelf and confirmed that there is not second idle air bypass screw. I realized later on last night that the port I didn't know what it did was simply a partial throttle vacuum. I've got the vacuum advance for the distributor hooked to it now. I'm pulling up the firing order specs and will check the timing on it later on this evening. Thank you for your help so far.
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Old 07-07-2006, 06:08 PM
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dmc12mk3,

Here is a site to check out so you will understand which rod I am talking about.
http://www.tocmp.com/manuals/Carbs/R...02GPart202.jpg
To set the cold fast idle speed you bent rod number 47. The choke pull-off will cause the idle cam to drop once the choke has heated up.

Scholman
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Old 07-07-2006, 08:27 PM
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Looks like the timing is excessively out of spec. Without the vacuum advance it is running 'after' tdc. With the advance it comes to about 12-14 btdc. So I think I'm on the right track though it seems like the idle trouble would be the other way around on this guy. Anyway, I've got a new set of plug wires on the way and I need to swap out the spark plugs. Once I'm done with that tomorrow I should have a little better understanding of where I'm at.


btw- I drove it around to put about 50 miles or so on it this evening. The idle dropped to ~1500 and at part throttle the car bucks a lot. WOT is great and if i got part throttle without a load it runs fine as well.

I'll let ya'll know how it behaves after I fix the timing.
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Old 07-08-2006, 10:49 AM
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Here is where I'm at:

I have confirmed that the plugs and wires are good (actually, just replaced the wires and watched them arc the plugs). I have set the static ignition timing to 12 deg. btdc that is specified on the engine (however it requested that it be set with a 750 idle..I set it at twice that), I have hooked the vacuum advance up to the ported vacuum source and took the car out for a drive. I put a vacuum gauge on the car while it idled and it is dead center of the green area (50). Runs fine except that now I idle at 2000rpm and unless I jump on it, it hesitates/stumbles/dies off idle. If I hold the throttle about 10 - 15 percent, it bucks like crazy regardless of rpm but it lessens at higher revs. Makes it difficult to drive around town and show the car off...to which, showing people that you have a car that matches your kitchen appliances is really all it has going for it.

Oh and I confirmed that the choke is not causing any of my issues. It is hot enough here that I do not need to use it so it is fully off to begin with. The idle screw is backed out so far as to not even be touching anything to affect the idle. I can look down the throat of the carb and see that the butterflies are closed.


Ideas? I think I'm whooped on this one.
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Old 07-31-2006, 08:00 AM
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just my two cents worth

you may want to make sure your carburetor is put together right. meaning th three mounting screws that screw the butterflies to the actual carb. i kinda stripped one of mine last night. and when i push down on that side of the carb it idles down when i let off it idles up and stumbles and i have the same exact jerking and pulling once it`s good. when i press on the brake it wants to shut off. so check if your carb base has a leak. you may not be able to know but here me out on the . where it is loose is the worst place for it to be loose. acts as an open vacuum port. so u have to compensate by unscrewing the idle mixture screws to mix with the air. to give u a clean mixture. now when u give it more gas and the vacuum droppes one you goto take off. it lets off of the sucking fresh air and richens up therefor your jerking . i got the same problem gotta find a small tap to fix mine. any suggestions on what to do. drill it out fill in with "quick steel" drill out again and tap? or find a helicoil small enough??
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Old 07-31-2006, 02:11 PM
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if you set the timing at 1500 it is probably still not right, the centrifigal advance will be in play at this rpm, that is why it says to set at 750 rpm.probably still slow on timing, the idle speed I don't have a clue if the blades aer closed it just about has to be a vacuum leak somewhere.
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Old 07-31-2006, 02:53 PM
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I thinks you've got a big vacume leak somewhere. For the engine to idle at 1500 with the throttle blades completely closed means the engine is sucking air in from somewhere, it needs air to burn for 1500 rpms. Find the leak, set your throttle blades so they are just cracked a hair and fire it up, set the timing to 10BTDC and dial in the mixture screws-they should end up at about 1.5 turns out +or- 1/4 turn. The vacume leak may be at the carb or manifold but I'm sure you've got one. Bob
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