Carb Question

Got my 86 home and it idles pretty rough and pretty high (1,000 rpm +), cleaned the throat of the carb with some carb cleaner and that helped a little. The back throttle plate seems to want to stay closed.... is that right? Is it possible to make this carb work right or will it be a neverending nightmare? I plan on replacing plugs cap and rotor in the next day or 2 and maybe that'll help as well. I like the idea of simplifing everything and getting rid of the rochester and all of it's vacumn lines and putting in a Weber but wonder if the Weber will pass emisions. Here in MD they Check for the cat, test the gas cap, and sniff test the exauhst... no visual underhood anytime I've been there.
Thanks,
Mike B.
86 Cheif w/3.4L, 5spd, np207, 4.10's

Not knowing all the details of your states inspections , It is hard to say. However if the Weber is adjusted properly it should pass the sniffer. I passed in Colorado with my 86XJ and a holley carb.

The Rochester will always be a PITA, but it should run better than what you describe.

The Weber will help immensely in the idle range, both in performance and CO emissions.
But it doesn't have the vacuum modulation and dash pot that keeps the primary throttle from slamming closed when you lift your foot off the gas. This causes a momentary lean/hot condition, and that in turn creates NOx (Oxides of Nitrogen). This MIGHT show up on a sniffer test.

Do a complete tune-up (rotor, cap, plugs, wires, SET TIMING) and check the Cat before you mess with the carb any more.

Your sig says a 3.4. The Jeep was a 2.8; you (or somebody)swapped in a cambird 3.4 and stayed with the factory carb???
If so, then you have an electric fuel pump 'cuz the 3.4 block doesn't have provision for a mechanical pump. Might want to check the elec fuel pump and the filter, to make sure they are flowing fuel properly to the carb.

Yep the 3.4 was swapped in like GM says to, by swapping all the 2.8 brackets, alt, manifolds, ect.. onto it including intake and carb then adding the fuel pump. I'm wondering maybe if the old 2.8 distributor might be shot (not sure if it's the original or not... 265k on it so maybe it could be a little sloppy I hope to find time to fool with it and do a tune up within the next week. Anyone have a spare FSM they would part with?
Mike B

Grab the carb body and shake it. The hold-down nuts have a tendency to loosen from vibration, which cause a vacuum leak and really screws the idle. Also, the carb itself is two pices -- the upper body is screwed to the base with four screws that go up through the underside of the base. These also work loose. Naturally, the only way to tighten them is to remove the carb from the vehicle ... which means you better hope you can remember where all that spaghetti goes when you replace it.

The carb on my '84 4-cyl was so loose that two of the four body-to-base screws were missing (no, I don't know how they could have gotten out of there either, but they were gone).

Good idea, thanks. The Weber I had on an 84 1.9 Trooper I had used to do that about 2 time a year and made it run just like this like it wasn't firing on all the cylinders.
Thanks,
Mike B.

Eagle said:

Grab the carb body and shake it. The hold-down nuts have a tendency to loosen from vibration, which cause a vacuum leak and really screws the idle. Also, the carb itself is two pices -- the upper body is screwed to the base with four screws that go up through the underside of the base. These also work loose. Naturally, the only way to tighten them is to remove the carb from the vehicle ... which means you better hope you can remember where all that spaghetti goes when you replace it.

The carb on my '84 4-cyl was so loose that two of the four body-to-base screws were missing (no, I don't know how they could have gotten out of there either, but they were gone).

Click to expand...

rixXJphx said:

The Rochester will always be a PITA, but it should run better than what you describe.

The Weber will help immensely in the idle range, both in performance and CO emissions.
But it doesn't have the vacuum modulation and dash pot that keeps the primary throttle from slamming closed when you lift your foot off the gas. This causes a momentary lean/hot condition, and that in turn creates NOx (Oxides of Nitrogen). This MIGHT show up on a sniffer test.

Click to expand...

rix, you say that the Weber doesn't have the dashpot to shut down the primary slowly. Any thoughts on this, as to a remedy. I have a 2.8 and it didn't pass the NOx part of the emissions. Took it off the road for a while to work on it, but haven't tackled that part yet. Really don't like the Rochester and would like to switch over, but this would be a problem.

Eagle, where were you two years ago when I was replacing the miles of lines under the hood chasing down the leak that you described! Geezzz! Just kidding of course. Thanks for all the good honest info over here guys. It sure helps.

I take no credit for that one. The idle on the '84 was driving me nuts. I happened to mention it when I was buying some parts at the dealership one Saturday, and the kid behind the counter said "Oh, yeah. Mine did that too, the screws holding the carb down are loose. They all loosen up."

Simple (?!) after that.

tbi

If you're going throught the trouble of putting in a electric fuel pump and have some time on you're hands then put a tbi in or efi if you're daring. There are several articles on doing it. I changed my '85 2.5l to tbi and although I don't get more ultimate power I have increased power across all engine speeds. It lugs much better and doesn't die out at top speed. It wasn't as hard as I thought it would be and it only took one night of soldering to adapt the wiring harness.

I think if I go to FI I'll try and hunt down a Holley pro-jection used... the 1,200 for a new one would be more than I paid for the Jeep
The Weber is becoming more and more appealing to me the more I think about it, though.
Mike B.

HossHofer-Who's TBI? I thought only MPI was available thru GM.

1986xj - I reading the literature, it seems the Holley FI is for small blocks at about 5.8 liters. Do you know if it works for airflows as low as a 3.4? IIRC, it's a 'four bbl-' type design, so I think the Edelbrock 2-pc 4 bbl manifold would be a necessity.

Cure for a carb that's not passing NOx:
1. Richen the idle mixture (it's the high temps of a lean mixture that create the chemical reaction to produce NOx's).
2. Make sure the cat is in good shape to then REMOVE the CO's that result from the enrichened mixture.
3. On the 2.8, the air pump injection into the exhaust manifold needs to be in top shape; this further reduces the CO level.
4. On the 2.8, make sure the EGR (Exhaust Gas Recirculation) circuit is operational. I'm not exactly sure how it works, but emissions go to hell when it's faulty.
5. Play with the timing: Adjust for optimum emissions results, even tho the engine probably won't run at its smoothest/most powerful output.
6. Piston engines ALWAYS run better when it's cold and damp; try to arrange your test for a cool, rainy morning or evening (we only have three of these a year in Phoenix ), but still allow the engine to warm-up so that the choke opens and the carb heat shuts off.

The normal 'damper' for a closing throttle is a vacuum-controlled dashpot: it 'cushions' the slamming throttle, then gradually allows the throttle to close.
If I were working on a pre-'72 non-emissions regulated carb, I might install a generic dashpot that approximated this action. Even a small block of foam rubber would have a cushioning effect that *might* be adequate to prevent an NOx 'spike' during the test.

I don't know about Weber, but there are books on the shelf at AutoZone and such, specifically for working on Holleys. Should be some info on tuning for street-legal emissions as well as track performance.

BACK TO THE ORIGINAL CARB QUESTION:
1. The Rochester 2SE (or E2SE for Kalifornia) was never meant to handle the CFM of an engine that is 16% larger (cid) and larger valve faces for even better flow.
2. The 3.4 *might* be sucking so much air at 700rpm idle that the carb idle circuit isn't adequate; therefore, you're having to adjust the throttle open to get an air-fuel mixture through the primary venturi instead of just the idle circuit.
At the front bottom of the carb is the idle mixture screw. It has a weird 'D' shaped head, it was factory-sealed at the ideal (for a 2.8) idle mixture, and there is only about an inch of room in front of the screw head to try and adjust it.
The 'D' head screwdriver/wrench thing is available in the specialty tools section of Checker/AutoZone/etc.
Then you have to pick out the epoxy adhesive (using dental picks) that is sealing the screw (on an engine this old, it may already have been removed).
Due to lack of straight-in approach with a screwdriver shaft, you have to apply the removable 'D' head to the screw and turn it with an open-end wrench. Adjust for whatever combination of smoothness/rpm/emissions you feel is best. BUT, the ignition system needs to be in fresh tune and timing before messing with this.
A real PITA, but this might be the problem with a 3.4 trying to breathe through a carb intended for a 2.8.

-Good luck. -Rick

rixXJphx said:

HossHofer-Who's TBI? I thought only MPI was available thru GM.

Click to expand...

Read again -- HossHoffer has a 2.5L (I-4). The Jeep I-4 was carb in '84 and '85, then became throttle body injected for '86 thru '90 before Chrysler changed everything over to Mopar multiport injection in '91.

Just out of curiosity, has anyone tried converting a 2.5L TBI for use on the 2.8L? The displacement is almost the same, is there any reason why this wouldn't work for normal street driving (excluding WOT drag racing and heavy towing, of course)?

The 3.4 will never idle smoothly, nor will you realize the power upgrade over the 2.8 fully with the original carb. Bear in mind also that the loping, lumpy idle is a function of a considerably more radical cam than the 2.8 had.

I battled this same problem for two years until I bit the bullet and swapped in a Holley TBI designed for the 4.3 liter S-10 and ran it with a cannibalized wiring harness and ECM from a 2.8 s-10 with a blown motor.

I wish I could give you a complete write up on it, but I did it mostly with the guidance of a former contributor who had gone there before and basically led me by the hand.

The truck did not pass NY emissions with the carb, even with the help of the inspecting shop's mechanics who tune race cars fiddling with all sorts of adjustments. Even a rebuild of the E2SE from a very reputable carb shop didn't help.

The Weber may pass the sniffer, though I think it's unlikely, but technically it shouldn't pass because the emissions equipment won't match what is represented on the underhood EPA emissions sticker.

Here in NY that's the first thing they're "supposed" to do. They open the hood look at the sticker, and if the cuffs and collars don't match they close the hood and issue a fail.

My XJ passed emissions with flying colors once switched to TBI, so the inspecting shop agreed to overlook the modifications to the system.

Not much along the lines of guidance but maybe you can glean something useful from my experience.

Check out www.customefis.com.

This guy can make you a TBI system for the old 4-cylinder, for about 800 US$. I wish he had that setup before I ordered my Weber kit. It costed me almost 550 US$, so for a little for dollario, I could have gone to EFI.

Rgds