258 / 4.2L Lean Above Idle
I have a question to pose to the 258/Carter BBD experts out there, so here it goes.
1985 CJ7, 258, Carter BBD, T5, 31's. Nutter'ed & Rush'ed. Rebuilt Jasper about 5k on it.
I'm doing the typical tune-up this weekend and the next. I wanted to try and work smarter, not harder, this time around. I decided to use the factory oxygen sensor bung and hook up a narrowband O2 sensor with a sunpro air fuel mixture gauge. My thoughts were that I could use it for setting the idle mixture screws.
So after installing everything, I set forth on the tune-up. I checked the timing and it's at approximately 7.5-8.0 BTDC. Hot idle is 650 rpm. So it was time to start tuning the Carter. I shut it down and turned the screws all the way in, then 5 rotations back out. I started it up and saw the gauge show full rich (~1.0V from the sensor). So I started turning the screws in until I saw lean-ideal (~0.4V from the sensor). That happened to be about 4 turns out. I was very pleased until I started opening up the throttle.
When I open up the throttle, the gauge jumps to full lean or off (<0.1V from the sensor) and when I let off the throttle the gauge would jump all the way to full rich. I thought this was weird. Shouldn't it be the other way around?
Would this indicate a vacuum leak or a poor seal on the intake manifold? Could the sensor be picking up a leak in the exhaust manifold or a crack in the exhaust somewhere?
I was wondering if anyone had ever looked into using the O2 sensor like this before, and if they had the same experience. I'm mostly asking out of curiosity, but I do seem to have a hesitation if I just slightly open on the throttle.
Thoughts? Thanks in advance guys.
I've used the narrow band sensor for tuning since the mid 90's
Its my opinion (and others) that if your running a BBD with the fuel screws out farther then 3 1/2 turns to get your best idle you have a vacuum leak somewhere. Mine are at 2 1/2. The fact that turning it in put it down to .4v is another indication.
.7 to .8v seems to be a good idle mixture.
If you didn't preform the procedure to set the needle assembly properly you need to do that also. That includes backing out the idle screw all the way to ensure the butterfly's are all the way closed.
When I nutter'ed it, I took care to set the metering pins to the center of their travel. So 4 1/2 turns out is way too far out?
Given the behavior described above, is there any place in particular I should focus my search for a vac leak? (Ported, Manifold, Intake, etc.)
stationary metering pin's
with the carb's metering pin's now set in in a fixed position i wouldent pay to much attention to any readings during throttle movement, only stabilized rpm settings. so you could dial in the idle mixture but "off idle" you are at the mercy of the main jets and fixed metering pin's. i think the stepper motor by design moves most during throttle transitions and then settles down when the throttle is stable. hence the reasoning behind the "nuttered theory". i also agree with the 2.5 turns being close to "maxed out" on "0" vacume leak's from head to throttle plates, any more than that is masking a leak somewhere. i have just got back from oversea's again and will be diving back into my bbd challenges, im retaining the stepper motor function, had the ECU reworked based on ic #5 being arced/shorted and am hoping for better results on my "eye watering" exhaust smell at idle. good luck
Where intake bolts to head
For me I sprayed the intake manifold with non flammable brake cleaner. When I got to where the intake bolts to the head the intake runner for cylinders 4&5 sucked up the brake cleaner as fad as it came out of the can. I replaced a perfectly good carb because of that leak.
Carb cleaner will work too.
Spray everywhere. Where intake bolts to head, carb base (but remember that a direct spray at the throttle shaft may cause a stumble), under intake where mani heater is mounted.
Bla bla bla.
Edit: as a note mine were tight and still almost stalled the engine spraying the intake at 4&5 cylinder. I didn't have the 2 piece gasket. I now have the 2 piece gasket and all the cupped washers. No problems since.
I replaced a perfectly good carb because of that intake leak. Piss me off. I had had the same problems with the reman carb I did with the old one.
An easy way to tell how bad of a vacuum leak you got is seeing how many turns out your fuel screws are. On a BBD carb anything beyond 3 to 3 1/2 turns out is a vacuum leak that needs to be taken care of.
Well I was able to get out there today and look at all my vacuum lines. I traced out everything and even replaced many of the hoses.
Please note that I have an open air filter and not the stock snorkel. Therefore, the lines to the TAC, TVS, yada yada, are not there.
Currently the setup is, looking down the carb from the from the front bumper and going clockwise,
3 o'clock position - Huge manifold vacuum fitting leading to brake booster. Fitting coming off this is plugged. Can't remember where that used to go to, but its sealed tight.
5 o'clock position - Small ported vacuum nipple connected to the CTO via two lines T'ed. I don't remember the numbering/lettering for these but from the same viewpoint, it would be the top-right and bottom-middle. (You can see the CTO in the lower left of the picture below.)
8 o'clock position - Large carb vents. These tie back into the charcoal canister.
9 o'clock position - Small ported vacuum nipple plugged. If I recall that's where you're supposed to tie into after the nutter for distributor advance. You'll see later where mine is tied into.
10 o'clock position - Big manifold(?) vacuum source for the charcoal canister vapor return and PCV tie in.
12 o'clock position - Manifold vacuum source for the vacuum breaker(?) connected to the choke near the old, unused WOT switch.
2 o'clock position - Manifold vacuum source for the vacuum advance on the distributor.
Here is a shot of the intake/exhaust manifolds. I wasn't sure what you (85CJ74.2L) were referring to with the 2-piece gasket. At any rate, I sprayed all over the place with the brake cleaner and the engine never stumbled at all. So no leak???
Have I missed something guys?
Shaun (Sorry for the huge and ample pictures. I thought someone might be interested in close ups of the vacuum sources on the Carter BBD.)
The blue on top of the metal gasket is the two piece. You have it.
I see you have a stepper BBD.
Take a picture of the carb top holding the choke plate open. I want to see where your stepper pins are positioned since your not using the plug.
Go to my YouTube channel "EugeneKingify" and watch my stepper pins at idle videos. Your pins should be in the same area as mine. Ideally you should have 1/8 inch of shoulder showing when not using the computer to control mixture.
I was thinking that was the only picture I didn't take! I've read the bit about 1/8" of the shoulder exposed and that's what I aimed for but I'm not certain that's where it was when I unplugged the pins. I can take a picture and see what you guys think. If I can remember correctly, which I probably cant, but I thought the "shoulder" was the pointed part of the pins. And as the pins move in (from the front bumper towards the steering wheel) the condition enriches the fuel mixture. Is that correct? So if my pins are closer to the front bumper than they should, I would have to keep my mixture screws backed 4+ turns out to compensate for the pins creating a lean condition. This is probably all backwards ...
I just wanted to say how appropriate your avatar is....since all Jeep builds/restoration budgets turn into a Fibonacci Spiral. ;)
You're vacuum layout appears correct to me, so I think you're on the right track with the carb.
Stepper pin is in correct position.
Matt, Order in chaos, it's everywhere!! (I hope I'm not quoting Jurassic Park at the moment.) but, is my logic correct with the stepper motor pins? I can't remember definitively.
I'm curious as to where everyone else has their pins. Pics?
Have you adjusted your accelerator pump arm correctly?
If you removed the needles from the needle barrel, did you set the gap correctly?
It's not always easy to find a vacuum leak. The one person on the Fourm looked at the same spot 3 times before it responded.
Without the mcu controlling the stepper pins I found the best idle around .75 to .8v.
Grammys jeep has a half nutter bypass. I control the spark part and the 'Puter still operates the carb.
At 4 to 5 turns out something IMO is still not correct. Gasket for Venturi cluster or something........
Did you drive it down the road yet watching the volt meter.
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