fuel mileage drop problem solved!

bobvonbob · 2006-08-10T18:48:01+00:00

just an update on the recent radical drop in fuel mileage i experiencedafter doing a rebuild on my Carter BBD (87 YJ 4.2L) from about 17-18mpgto something like 5-7mpg! there were no obvious signs of excessive fuelconsumption (black smoke clouds) or leaks in the system, and she seemedto run and start ok.turns out to have been rather simple and rather complicated at the sametime. when i took the carb apart i noticed that the stepper motor didntmove very freely - the shaft appeared to be sticking in the bushing orsomething. i pulled the shaft back out so that the pins were in theforward position and put it back together. the next day i did thenutter 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 O2sensor being disconnected, so i had a pretty good hunch that thestepper motor had been inactive for a long time (hence the corrosionand the sticking). thus i hoped that taking the computer out of theloop would bring the fuel mileage back up (or something like that). ialso bypassed the charcoal canister (bad purge valve), retightened thecarb mount nuts and the manifolds, and rechecked that there were novacuum leaks.well none of it worked - and needless to say with gas going for $3+ atthe pump it is alarming to practically be able to WATCH your gas gaugedrop as you drive down the highway! then a few days ago i rememberedthat having the stepper motor pins in the full forward position richensthe mixture. so i figured - what the hell i'll put em in the full leanposition 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' carband inadvertently put the pins in full forward position -> steppermotor cant move because corrosion/old/POS -> disconnect computer ->stepper cant move b/c no signal (obviously) -> restore pins to fulllean position - problem solved!whew. anyway i just wanted to post about...

Aug 10, 2006 24 Replies

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bobvonbob 19 years ago

hey thanks mike - had to work today so probably not getting to it till tomorrow. sorry to hear about that accident - if it makes ya feel any better i really think you help alot of people on this list - i KNOW you and your previous posts have helped me a alot! so thanks again. not to short-change anyone else either - thanks all. bennett

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Mike Romain 19 years ago

The passages are internal. You see the pins going into the center of the venturi cluster and as they pull out an air gap happens. This is just the wrong type of carb to plate over which is why the 'Nutter' goes into the 'full rich with the mix screws full lean first' routine. It would be nice to just plate the sucker, but....

Mike

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

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L.W.(Bill) Hughes III 19 years ago

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Mike Romain 19 years ago

Well that's really odd....

That is 'Not' the Weber-Carter BBD we are playing with!!

It's close, but the electronic parts are totally different. That electric module on the back has 2 moving on the 'step' pins coming out that go into the venturi. The one you show just opens passages and likely can just be plated. Oh if it was only that easy...

Apples and oranges.

Mike

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

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L.W.(Bill) Hughes III 19 years ago

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