Mixture seems to lean out while cruising but not acceleration/decel and idle

crazypete · 12-08-2002, 04:29 PM

I'm starting to storngly suspect that the o2 sensors are toast. These are fairly new, bosch units that I got in april. When I did my carb conversion, I just ran some power wires down to them and took the signal wire up to a pair of mixture guages.

At idle, they're up in the "orange zone", but the pass side one is always 2 "bars" lower no matter what. After ...oh....5-7 minutes, the guages suddenly bottom out, them bounce back, then bottom out. Almost like theyre fluctuating. Idle continues without problem and the vacuum reads an unsteady 16 hg's, goes up to a solid 20-22 at fast idle (1200).

Now when I accelerate, no problem, tons of power, decel, no problem, no backfire. The guages ride in the orange zone during accel, but when I start cruising, they bottom out to zero consistently. They come back to normal when I put in the clutch. The car does seem to...well....cruise a bit choppy, but just a bit.

I'm not running egr and it seems as if the crossover in the exhaust came loose again, I'm welding it tonight. I'm running 70/72 jets and 6.5 powervalve, floats dead on and a mechanical fp.

An ideas on why this leans out at cruise? Those jets should be plenty enough.

Thanks for any carb advice, you crazy racers!

66StangGuy · 12-12-2002, 04:17 AM

check your pcv valve connections and hoses. i had the same problem in my 66. i added the o2 sensor and gauge..same one you have. anyway turned out the hose was leaking next to the carb and under steady vaccume it would leak and cause a lean condition. identical to what problems you had. take it completely apart and check for vaccume leaks and any bad clamps letting air in.

crazypete · 12-12-2002, 12:24 PM

yeah, it seemed to idle at a steady 19 hg's of vacuum before, then I found a serious defect with the carb (secondary plates were jammed shut by the primary throttle linkage. Even at wot, max secondary motion was at most 30 degrees). I sent my 570 back and they sent me a "flowtested" 670 instead. Now the idle seems lumpy. I'm not using any of the carb ports at all. My pcv is using the sealed 91 system without pcv valve since the edelbrock performer does not have a pcv passage at the back of the intake. I have the valve cover oil filler stem air tube hooked to the air cleaner dish underside. That should create some minor airflow through the tube. Do carbs have internal leaks? If I didnt know any better, I'd say this carb has an internal leak

12-08-2002, 04:29 PM	#1
crazypete Registered Member Join Date: Sep 2002 Location: Boston Posts: 592	Mixture seems to lean out while cruising but not acceleration/decel and idle I'm starting to storngly suspect that the o2 sensors are toast. These are fairly new, bosch units that I got in april. When I did my carb conversion, I just ran some power wires down to them and took the signal wire up to a pair of mixture guages. At idle, they're up in the "orange zone", but the pass side one is always 2 "bars" lower no matter what. After ...oh....5-7 minutes, the guages suddenly bottom out, them bounce back, then bottom out. Almost like theyre fluctuating. Idle continues without problem and the vacuum reads an unsteady 16 hg's, goes up to a solid 20-22 at fast idle (1200). Now when I accelerate, no problem, tons of power, decel, no problem, no backfire. The guages ride in the orange zone during accel, but when I start cruising, they bottom out to zero consistently. They come back to normal when I put in the clutch. The car does seem to...well....cruise a bit choppy, but just a bit. I'm not running egr and it seems as if the crossover in the exhaust came loose again, I'm welding it tonight. I'm running 70/72 jets and 6.5 powervalve, floats dead on and a mechanical fp. An ideas on why this leans out at cruise? Those jets should be plenty enough. Thanks for any carb advice, you crazy racers! __________________ 91 GT Carb conversion, holley 600 double pumper, edelbrock performer intake, FMS "C" drop springs, march 1000 underdrive pullies, crane 1.7 roller rockers, GT-40P headers, bassani x pipe, american thunder catback, FMS 4.56's, msd aL6, trunkmount battery, A/C eliminator kit, 3000 stall tci streetfighter, AOD with transgo kit, A+ servo, 300M hardened lockup shaft, kevlar bands and 28,000 gvw trans cooler, 3 core radiator, 300 lbs stripped with a full interior

12-12-2002, 04:17 AM	#2
66StangGuy Registered Member Join Date: Feb 2002 Location: Diamond Bar Posts: 258	check your pcv valve connections and hoses. i had the same problem in my 66. i added the o2 sensor and gauge..same one you have. anyway turned out the hose was leaking next to the carb and under steady vaccume it would leak and cause a lean condition. identical to what problems you had. take it completely apart and check for vaccume leaks and any bad clamps letting air in. __________________ 66 mustang-292-Holley 4160-World Castings Windsor Sr.-Hooker super comp headers-MSD box & Coil.3.55 gears. B&M Z-Gate-Black leather racing seats with 4pt harnes.

12-12-2002, 12:24 PM	#3
crazypete Registered Member Join Date: Sep 2002 Location: Boston Posts: 592	yeah, it seemed to idle at a steady 19 hg's of vacuum before, then I found a serious defect with the carb (secondary plates were jammed shut by the primary throttle linkage. Even at wot, max secondary motion was at most 30 degrees). I sent my 570 back and they sent me a "flowtested" 670 instead. Now the idle seems lumpy. I'm not using any of the carb ports at all. My pcv is using the sealed 91 system without pcv valve since the edelbrock performer does not have a pcv passage at the back of the intake. I have the valve cover oil filler stem air tube hooked to the air cleaner dish underside. That should create some minor airflow through the tube. Do carbs have internal leaks? If I didnt know any better, I'd say this carb has an internal leak __________________ 91 GT Carb conversion, holley 600 double pumper, edelbrock performer intake, FMS "C" drop springs, march 1000 underdrive pullies, crane 1.7 roller rockers, GT-40P headers, bassani x pipe, american thunder catback, FMS 4.56's, msd aL6, trunkmount battery, A/C eliminator kit, 3000 stall tci streetfighter, AOD with transgo kit, A+ servo, 300M hardened lockup shaft, kevlar bands and 28,000 gvw trans cooler, 3 core radiator, 300 lbs stripped with a full interior

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