I finally got my carb back from rebuild after 8 months of waiting, now im trying to get all the quirks worked out from it sitting for so long. Its an 86 LG4 with 102,000 miles. I had the carb rebuilt, has new distributor cap and rotor, new plugs, new wires and just ripped all of the original exhaust off and ran hooker exhaust the whole way throughout. It fires.right up without a problem but cruising about 2500 rpms it gets occasionally bucky is the best way to describe it. It wont spin the tires even if i try. Im thinking of checking vacuum lines first, any other suggestions of what to check?

 

was the newly rebuilt carb tuned to the motor by adjusting the IAB for proper dwell (mixture control solenoid cycling) at idle?

 

it was tuned to a motor before he sent it back but not my engine specifically

 

That sounds like a lean condition if that helps. Replacing the restrictive exhaust would cause that.

 

tuning it with a dwell meter (or scanner that can read 'live' data) to your motor will likely fix your issues.

if you need help with this post back.

 

yes ill definitely need help. I did adjust the choke on the carb because the choke was adjusted to where it was running super rich

 

do you have a dwell meter or scanner?

 

no i do not

 

you can get an old style analog dwell meter for less than $15.

dwell meters were used to measure the dwell, or 'closed' time on old points style ignition systems. GM provided a diagnostic lead for the LG4 that could be read with this, then widely available, meter.to measure the rate of cycling of the mixture control solenoid within the carburetor. they provided a diagnostic link to connect to one, a green wire near the blower motor (coming from the fender well - source of the ECM wiring harness)

if you have a multi-meter you can do a few other checks first

is the MCS clicking with ignition on? you should hear it click 10 times a second even with the engine not started (although the ECM will stop if after about a minute if it sees no ignition pulses indicating the motor's running)

you can back probe the B and C terminals on the TPS connector (middle and bottom) and confirm around 0.4 V at idle throttle and 4.0 V at full throttle. and also confirm a smooth change in the voltage as the throttle is pressed with no gaps.

choke completely open when warm?

a dwell meter will allow you to fine tune the IAB for mixture at idle. it's a fairly rudimentary process. a scanner that reads live data from the ECM like VAC, TPS, MCS Cycle is much handier as it also functions a a diagnostic tool, but more expensive. I have an old autoxray.

see if you can borrow one or the other.

 

It does click when you turn the ignition on, also when the guy rebuilt the carb he put new sensors in it. It does still have very old vacuum lines which I plan to order and replace. The choke was cranked the whole way shut and was running so rich it would burn your eyes running, I turned the choke the whole way open. I adjusted the choke plate back two clicks today to see if that made any difference and it is bucking and popping a lot worse now then when I had the choke adjusted the whole way open

 

if running richer doesn't alleviate the problem, it may not be caused by a lean condition.

you can try driving it with the MCS unplugged, allowing it to run full-rich. for just a little while anyway.

if no change (other than some sluggishness), I'd focus now on the ignition system first.

double check your plug wire routing, disconnect each wire and ensure it 'snaps' back on properly to the cap and plug terminals.

inspect your cap and rotor.

double check your timing, it should be set to around 0-4 degrees advanced with the EST disconnected from the distributor. the timing light should show it advanced to around 20+ at idle with the EST re-connected.

if this is the original distributor, or just really old, it might be time. a mis-fire at the pick up may cause the tach to bounce.

 

I unplugged the mcs and the idle dropped from about 1000rpms to around 750rpms and the engine sounded better I didn’t drive it though

 

I replaced vacuum lines and the idle fell down to almost 500rpms so that fixed my rpm issue, but now i guess i need to order a dwell meter to check the iab dwell cause its still not running right

 

I got a dwell meter, hooked it up and let the car warm up, it was set to almost 60% swell. I got it down to 35% with it idling about 850 in neutral. It definitely is running better as it doesn’t smell like raw fuel and I don’t here popping coming out of the exhaust anymore

 

I meant 35 degrees not %. However I still have the surge at anything but wot, not sure what to check next.

 

im seriously at a loss on this stupid thing. Im not 100% sure the dwell meter i bought is working properly or not. No matter how much i turn the IAB it doesnt change the reading on the dwell meter. if i change it from 6cyl to 8cyl it barely changes the reading. i have it plugged in positive to green cable and black to ground on the battery. i capped or replaced all my vacuum lines but i still cant get this thing to work right

 

when the system is within the narrow range of mixture for the IAB to affect it the dwell meter should waver slightly about +/- 5 degrees or so as the ECM adjusts the mixture AND turning the IAB in (reducing air-richening mixture) should cause the dwell to increase (more time down in the jets, reducing fuel).

assuming the internals of the carb are adjusted correctly....you can only really check the range of motion of the solenoid without opening the carb-you should be able to measure 1/8" inch of travel with a rod inserted into the IAB opening (take the IAB out). you should be able to click the solenoid 'hat' with a small rod up and down. IF GOOD:

put the IAB back in and about 4 turns out from bottomed

next check your idle mixture screws. they should be around 4 turns out from seated.

set everything at four turns out and try the dwell meter again. if no luck try:

set idle mixture screws at 2 turns out, run the dwell meter again with the IAB from 2 turns out to 7. let it set for a minute at each half turn to see if the dwell changes.

if no change thru the range of IAB turns out, add half a turn out on your idle mixture screws and repeat. iterate until the mixture screws are about 7 turns out.

the goal is to get the dwell to respond to changes in the IAB and see the 'wavering' needle, ALL with the engine WARM.

you want to, hopefully end up near four turns out on everything, maybe 3-6 depending.

remember that the ecm only remembers it's 'last known good' settings. when that carb was swapped on those settings no longer apply and if the O2 sensor doesn't see the mixture cross the correct 'threshold', it doesn't know which way to adjust the mixture and the dwell will just hang somewhere.

hope this makes sense...

 

I tried this and nothing changed. I cranked the idle up to 1500rpms thinking maybe the o2 sensor was getting hot enough, dwell moved down closer to 30 degrees but wouldnt adjust any further. I bought a $300 scan tool that does obd1 and obd2 and it wont communicate with the computer. It loads about 40% ses light flashs shift light comes on then it loads to 100% and says unable to communicate. Im really at a lose with this thing

 

if it has been sitting, perhaps some water in the gas? just a shot in the dark...

 

Gas has been treated, and its already had almost a full tank of gas run through it

 

sorry i missed your earlier posts

try plugging every vacuum port on the carb and manifold to rule out a leak. I usually recommend this right off but it's been a while since this has come up.

what scanner do you have? at some point one would expect the newer scanners to no longer reader the old ALDL data feed from these antiques. My 15 year old Autoxray won't accurately read the dwell on my 84.

 

i already plugged every vacuum port on the Carb to try and rule that out. Also i got an Innova OBD1 and OBD2 scanner. its odd that it loads to 40% and it flashes the lights on the dash, then it hangs up, shoots to 100% and says unable to communicate with ECM

 

check the fuel filter?

I have been having issues with a stock LG4. CEL would come on, code 44. Car just wouldn't run right. I swapped out the o2 sensor and that didn't help. I took the fuel line apart and part of the fuel filter had come apart and was stuck in the fuel line.

 

i replaced the fuel filter that was in the inlet of the carb, unless there is also one on line futher back?

 

I swapped computers to eliminate the computer, no change. i put a new CTS in it, nothing changed. I drove it to the gas station and filled it up with 91 octane, drove it home and still could not get the dwell to adjust at all. It still has a surging at low throttle, when you push it over 1/4 throttle it runs fine. It does however have an overheating problem when drive it. When it sitting idling it is perfectly fine, once you drive it though it starts to heat up. It does have the air dam installed and the fan and thermostat are opening and kicking on. I have unhooked the O2 sensor with no change in dwell or engine response, i tried chocking the engine out with a rag, with no dwell response, I turned the idle up to 2000rpms to make sure the o2 sensor was hot enough, and no dwell response. Now does it make a difference that my dwell meter starts at 20% and ends at 60% or am i still shooting for between 25-30% dwell, because no matter what i do i cannot get it below 35% dwell. Im about to the point of ripping it off and putting a mechanical carb and vacuum advanced dist on the car

 

I'm thinking that the car is not going into closed loop. To try and make sure it goes into closed loop im going to switch up my fan switch from a 180 to a 210 fan switch. Im also going to swap the stock O2 sensor for a heated O2 sensor. Since the car has open exhaust its possible that the o2 sensor isnt getting hot enough to read properly. Im not sure what temperature the car is supposed to operate at to tell it to go into closed loop but im thinking 180 might be too low for the fan

 

the ecm won't go into closed loop until the water temp is high enough, can't remember the temp, BUT you can fool the CTS by shorting the leads on the connector which will show as 'full hot' the ecm.

it also won't go into full feedback mode until the O2 sensor is hot enough and begins to send voltage data. changing to a heated O2 is a wise idea if you have non-factory exhaust manifolds on.

below the CTS 'warm' temp the ecm will command a warm up mode and maintain a richer mixture. as soon as that temperature threshold is reached the ecm will begin commanding mixture changes based on its 'last known good' data. once the O2 warms up it will begin to re-write that data.

so between warm temp and O2 temp the system will not respond to actual changes in lean/rich conditions (rag over carb will have no effect) and the dwell readings will be pretty steady but still change with vacuum, rpm and TPS. once the O2 starts behaving you'll see the dwell change in response to ACTUAL changes in lean/rich.

not saying a heated O2 will fix it, but it's certainly a worthwhile addition for an exhaust that's been modified and will engage full feedback mode quicker than a non-heated one.

 

So i put the heated o2 sensor and higher fan switch in and wouldnt you know the computer actually took over and started controlling dwell. I was about to adjust the dwell to close to 30 degrees with it varying between 25 to 35, however when driving it still has a bad stutter under light throttle, almost like a misfire. I hooked up the timing light and set it to 6 degree with est unhooked and about 21 degrees hooked up, still stuttering. I noticed when i was adjusting the timing with the gun that the mark would actually bounce back and forth a few degrees, im assuming something is going bad in the distributor?

 

it's normal for the timing to move around some. the ecm is constantly adjusting it. when you check it with the EST disconnected it should be steady, although less advance.

a misfire can be most noticed under light throttle from a cruise condition. cylinder pressures increase while the mixture is still trying to richen up from a lean cruise. if the pickup coil is going bad you may see the tach bounce.

have all the plugs, wires, cap and rotor been replaced?

good deal on the dwell

 

yes i put new ceramic wires, plugs cap rotor and fuel filter in it when i got the car right before i sent the carb out to be rebuilt

 

Update. I removed all the computer controlled stuff. I went to vacuum advanced distribution, edelbrock performer intake and holly street demon carb. Put everything in and set timing, thing fired right up and sounded better, took it for a drive and it still surges at light throttle