fuel mileage drop problem solved!

just an update on the recent radical drop in fuel mileage i experienced after doing a rebuild on my Carter BBD (87 YJ 4.2L) from about 17-18mpg to something like 5-7mpg! there were no obvious signs of excessive fuel consumption (black smoke clouds) or leaks in the system, and she seemed to run and start ok. turns out to have been rather simple and rather complicated at the same time. when i took the carb apart i noticed that the stepper motor didnt move very freely - the shaft appeared to be sticking in the bushing or something. i pulled the shaft back out so that the pins were in the forward position and put it back together. the next day i did the nutter which recommends placing the pins in the full forward position. now when i got this beast the CTO had been broken off as well as the O2 sensor being disconnected, so i had a pretty good hunch that the stepper motor had been inactive for a long time (hence the corrosion and the sticking). thus i hoped that taking the computer out of the loop would bring the fuel mileage back up (or something like that). i also bypassed the charcoal canister (bad purge valve), retightened the carb mount nuts and the manifolds, and rechecked that there were no vacuum leaks. well none of it worked - and needless to say with gas going for $3+ at the pump it is alarming to practically be able to WATCH your gas gauge drop as you drive down the highway! then a few days ago i remembered that having the stepper motor pins in the full forward position richens the mixture. so i figured - what the hell i'll put em in the full lean position and see what happens. and VOILA! gas mileage is back to ~17-18 mpg and she runs like a top! so CTO and O2 no good -> stepper motor doesnt move -> i 'rebuild' carb and inadvertently put the pins in full forward position -> stepper motor cant move because corrosion/old/POS -> disconnect computer ->

stepper cant move b/c no signal (obviously) -> restore pins to full lean position - problem solved! whew. anyway i just wanted to post about this because while i was researching on this list it seemed to me that i ran across alot of folks posting about their problems, then getting some responses and possible solutions and then nothing else. it would be nice to hear what exactly the problem turned out to be and how exactly it was solved (or not) - at least more often. so here ya go. cheers all and thanks for all the help. bennett

Reply to
bobvonbob
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Thanks. We all like to find out what happened after a thread like that, especially if it is a success story!

Earle

Reply to
Earle Horton

If you still have the 'mixture' screws factory blocked so you can't set the carb up properly, running it too lean like that will burn holes in your pistons.

You should put them in the center and hope that is the best for where the factory set the carb's mix at.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >
Reply to
Mike Romain

BRILLIANT! Instead of using an Air Fuel mixture guage to AT LEAST get in the ballpark for good power and MPG's, you want him to put it "in the middle and hope".

Reply to
Simon Juncal

I thought so, thanks.

Seeing as he still has the metal plugs blocking the mix screws and no working stepper motor 'and' the factory setup had the pins in the center of their travel when the mix screws were originally set, would you have a better suggestion?

And I have yet to see any back yard mechanic have an 'air fuel mix gauge'. The CJ's are Carburated Jeeps that were manually tuned since they came out in 1949. Only since the sad demise of mechanics and the rise of 'technicians' that only know what a computer tells them is wrong do you have tools like that.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
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Reply to
Mike Romain

It's not a computer Mike, everything that lights up and is attached to sensors isn't necessarilly a computer... It's a scientific measuring device for combustion temps that can tell you if you're running too lean or wasting gas (and POWER) by running too rich. It's a HELL of a lot more accurate than your gut feeling, and it's WAY WAY freakin more accurate than your "hope" technique.

If "computers" and scientific data providing guages caused the demise of mechanical yokles and non-adapting-stuck-in-the-past technophobes then the world is a better place...

Reply to
Simon Juncal

I'm guessing Mike's reply will be something along the lines of "Back in the early days of computerized carburetors it was commonly possible to manually tune the carbs for better power, mileage and lesser emissions than allow the early computer controls to do the adjustments"

That statement would be true..

Generally the only time a properly adjusted carb needed attention was when it was rebuild time which was every couple years.

"Simon Juncal" wrote in message news:N9CdnaBtM4iRtUDZnZ2dnUVZ snipped-for-privacy@rcn.net...

Reply to
billy ray

umm... appears i set off a little bit of a firestorm here... anyway, yes mike the screws are still blanked off - until sunday at least when i will be pulling the carb to change out the rounded mounting nuts with new hardened ones. i am hoping that adjusting the idle screws will even out my idle somewhat - it still lopes a bit from say 500-600 rpms... BUT wait mike, i thought that the idle mixture screws only affected the mixture AT IDLE! so running lean on the highway or at > ~1200 rpm wouldnt be affected by their position at all... correct? in any case she's been running like this for about 5 of the last 6 months without any noticable issues. additionally, i would suspect that if it was running so lean that i might hole a piston i'd be noticing a little backfire on the overrun etc etc. oh yeah - i DONT have a carb mixture measurement device, whatever it is... i have seen vacuum gauges for doing carb 'timing' adjustment on motorcycles with multiple carbs but otherwise i dont know any other way of adjusting the damn things besides seat-of-the-pants, old-school, shade-tree, guestimation. not to say that that's my preferred method - just the only option i currently have. if anyone with a carb thing-a-majig wants to come and help me tune this sucker on sunday by all means - come on by! cheers, bennett

Reply to
bobvonbob

What billy ray said....

Plus, the OP is trying to make a carb behave by adjusting the pins on the 'stepper' motor. This means he has something like 4 or 5 different static places or 'steps' to put the pins on.

If he had the idle mix screws in the equation, my answer would have been different than telling him to put it back 'like the factory' originally set it with the pins in the center.....

I guess if all you know is a computer printout, this doesn't make any sense to you, but putting it back 'factory' makes perfect sense to me....

Too bad computers seem to make people forget the basics....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
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Reply to
Mike Romain

When you get your screws open, (You can flip the carb upside down and cut 1/8" off the block where the screws are or drill in from the side to pop the blocks out or if they are tin, just use a sharp punch to get them out) you can put the pins back in and set the mix like a 'normal' carb.

Just FYI, the idle mix screws do affect the mix at idle, but that tends to have an effect all the way up. The stepper pins affect the air mix through the whole throttle range. They work (have effect) at speed also, via input from the O2 sensor and computer.

I go for a best lean idle mix. I start with a warm engine and the screws both out about 5 turns and set the rpm at 600- 700. I then turn them 1/4 turn in each, then give the throttle a rev, then another 1/4 turn. I continue while revving it between turns until the engine get a lean stumble. You will hear it. I them back them out 1/4 turn from there and she just purrs.

As you are doing this the rpm will change up normally. You are supposed to stop and drop it back down ending up at 600-650 final. 500 is pretty low.

The emissions sniffer likes them to have that lean rumble so I leave it like that, 1/4 turn lean on both screws, come emissions time.

Getting the mix right will do wonders for the vacuum.

Mike

bobv>

Reply to
Mike Romain

thanks mike - you are the master of these 258/BBD setups as far as i can tell. thanks to everyone else as well - i'll be tearing the carb off tomorrow and will report back after. BTW mike - STILL no ported vacuum off the carb - thinking its a gasket issue. we'll see soon enough. one thing tho - if i'm not getting any advance from 1200-2500 rpm (or whatever the window is before the mechanical takes over) then why is she still running so well? not that she's gonna win any stump pulling contests or anything... cheers, bennett

Reply to
bobvonbob

I was thinking on my setup and I believe if I have the stepper pins all the way in that they can physically be jammed, that is one step farther than the computer can set it the way the 'nutter' says to make the computer 'full rich'. I also think I lost about 5 mpg if I am right. I think I set the pins one click out or where the carbon mark on it stopped which indicated the travel the computer had it doing and got my nice 23 mpg or 11L/100 km for us Canadians, eh.

The engines are very forgiving for timing... Right now, when I goose the throttle, my vacuum timing snaps to full instantly. I can watch it move.

The emissions sticker says to have the vacuum off and plugged and to have it at 1600 rpm when setting the timing. I was under the impression the mechanical advance was full on at this point and the vacuum varies it from there.

Stock it has manifold vacuum to the timing, not ported.

Have you read around this site?

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It is a good one.

Mike

bobv>

Reply to
Mike Romain

ahhh - i see... yes that makes sense. any advice on how to find the proper position if there is no indication on the stepper motor shaft? i dont think the thing will even move if i was to hook up the computer, the shaft is so stiff. another question: i set my timing originally to 8-9d BTDC at 1600 rpm, but then after reading some posts on this list i reset the timing to

8-9d BTDC at 'idle' (approx 700rpm) - i have noticed somewhat of a drop in power at higher rpms, but there are so many variables i'll have to sort it all out tomorrow when i reset the carb gaskets... but what rpms should i set the timing at? cheers, bennett
Reply to
bobvonbob

I set my timing at 650 rpm.

I 'think' it is one 'step' from full jam on the pins. I need to look at mine so I will have look at where it sits. I will try to do that today, but I am not moving well due to a change in drugs so I will have to see what the pain level is. (got smucked up in a car accident a couple years back)

The pins 'don't' move linearly or easily, they move on a 'stepper motor'. You have to rock them to get them to go to the next step.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >
Reply to
Mike Romain

Ok, I just checked mine and thought about what I did to it last time. It's been a while....

As far as I know or have seen anyway, the computer won't bottom out the pins the same distance you can by hand.

Most I have seen are one or two steps out from full physical depth when the computer puts them rich and I ran/run mine actually 2 steps out which was as far as my system would put it so I tried it there. It has worked fine there with lots of top end power. The plugs show as 'normal' burn. I have done several of my friend's carbs also and leave the pins as far as the system will put them usually. Maybe it's different factory emissions that makes the difference or a different year computer.

I wanted to try and burn that 15% ethanol mix crap they are forcing on us, so I went full rich by hand. My gas mileage dropped to 10 or 12 mpg.... and it still wouldn't run on that crappy mix. Back to the 2 steps out and it purrs. 2 steps makes 1/4 turn difference on the idle mix screws to get the nice stable 600 rpm.

I also tried rich because my engine was running hot. Made no difference so went back.

Anyhow, I just tried mine and I can move the pins by rocking them with a mini screwdriver on mine.

I have taken mine and many other's who I have done this to in for emissions and we all pass with great numbers so the setup must be damn close to perfect or as close as those guys that only know how to read scanners can come....

On the ASM 2525 test and I got 589 NOx and 16 ppm HC and 0.11% CO. That is with no cat converter too!

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >
Reply to
Mike Romain

thanks mike. i assume you mean that you are moving the pins while the motor is mounted to the carb... hence the small screwdriver... yes? i will try this but my shaft is pretty damn stubborn so we'll see. i have a feeling it hasnt really moved in years. will be tearing the carb off and checking the gaskets so i will let you know how it all works out. cheers, bennett ps sorry to hear about your accident. a few years ago? hope its not too bad.

Reply to
bobvonbob

Reply to
L.W.(Bill) Hughes III

You can't on that one Bill. The stepper has two pins that control an air passage each into the venturi so taking it out would leave a massive air leak. They don't suck through the add on piece.

Mike

"L.W.(Bill) Hughes III" wrote:

Reply to
Mike Romain

Reply to
L.W.(Bill) Hughes III

Yes, my pins will move when I rock them with a mini screwdriver twisted against the case and the start of the V shape. I took out the spring clip that holds the air filter for more room first. They didn't want to just 'move', it took a few rocks to get them up on the next step.

And 3 years ago, the one time in years I wasn't in a Jeep but in a car with plastic doors, I got T-boned on the passenger side and I was in the passenger seat. Got dinged bad enough, but I am still walking and talking and Jeeping, although not on many rough trails...

Mike

bobv>

Reply to
Mike Romain

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