Struggling with my sons LG4 Rochester.

Car was bought out of estate after sitting for four years. I removed carb and purchased rebuild kit from a local NJ dealer. I purchased the "vintage" Burrows and Thexton Tools to dissassemble and bench asswemble..

I rebuilt the full ignition, 100% AC Delco - Cap, Wires, Coil, Module, Plugs (plugs are gapped to 0.045)
Rebuilt the carb after cleaning it with joy soap in my ultrasonic heated parts cleaner that I use for my 2 cycle lawnmower and chainsaw carbs.

I have the 1986 GM service manual.

I carefully reassembled and bench set all the adjustments (IAB, Lean/Rich, MCS, TPS, etc. per the factory specs with the gages and tools.
Checked the vacuum choke break (good)
Checked the TPS on the bench with a power source and on the car running. Adjusted the voltage to 0.54 per Shop Manual. At full open it reads 4.0V.

All vacuum lines are new.

Removed the AIR, EGR and the EFE system and capped off the one port on the carb.

Restored the Cannister EVAP system with all new lines and 4 way control valve.

Checked my manifold water temp sensor (good) and traced the circuits good
Checked my O2 Sensor (Good) and traced the circuits good

JB Welded the bowl plugs

The car is not behaving consistenly.

When cold, it is an SOB to start. Crank Crank Crank (I put new fuel filter with check valve and a new Carter Fuel Pump as the one on car was leaking when purchased.

I can tune it to 700 RPM cold and running about 4 deg BTC (spec is 0 but doesn't run any better.

Float was set with the "L" gage. It seems real close spec. Setting on the rich and lean screws are good and the plunger travel of 4/32 is spot on.

IAB was set with gage and idle air screws were set 3 turns out

Checked the MCs dwell per the FSM and at hot idle and at 3000 RPM, I can get it to dwell around 30....it bounce between 28 and 33.

The big issues:

1. Very hard to start cold
2. Huge BOG on WOT on road...car just nose dives.
3. Smells rich, had some carb fires
4. At cold start-up, cannot get the fast idle to engage.
5. A startup, car will hunt around the idle. As low as 550, as high as 1000, Then, it will settle after a few minutes to the 700 where I set it but it does not have a smooth idle. It moves around +/- 50 RPM.

I am running car without dash so I will get the VSS signal code. I don't think that has any impact on timing or MCS (am I mistaken)
I now have had the TPS error Code 21 happen twice in last two days. I clear it and then I can drive car and it does not come back. It seems when I take it out and jump on it a few times, it has set.

Any help is greatly appreciated.

I am starting to appreciate why people hate this setup. Throw money and time and read and follow manual and it seems impossible.

I have checked dwell, all sensors and circuits with continuity.

NO OBDI codes show up except the VSS signal missing and now two times the TPS voltage high.. But carb was running like crap before I got that code. I have now ordered a new TPS in case it has a dead spo6t but it seemed to be linear betweeb 0.54 and 4.0 V.

 

cold cranking: ensure the booster pump is squirting fuel into the throat, you should be able to visibly observe it

VSS: this will affect the behavior of the idle bump solenoid, may want to disengage it temporarily to rule it's possible affects out

choke linkage: manually operate the linkage and ensure everything works as intended

double check the vacuum break. pull the small vac line off, press in the plunger, hold your finger over the nipple. the plunger should stay in until you release your finger.

 

OK, Thanks for follow-up.....

Just went out to car at 9:15 am today 3/2/22

It is 37 degrees in New Jersey
Car is in unheated barn so temp is sub 50 degrees at least

Remove air cleaner.
Primary Butterfly was open vertical.

Your questions below:

a. Actuated booster pump and it had a strong squirt of fuel - I had already put new plunger -blue rubber for ethanol - and new plunger backing spring from the rebuild kit so I was pretty certain it would be good but now that is confirmed.
b. The Idle Bump Solenoid is currently not on the car as I could not get to the idle screw when it was on. I understood that the "bump" solenoid only gets actuated when AC was running on this car. Currently AC is disabled as system is depressurized. I plan to rebuild that next. Is there any other mode that the Idle Bump Solenoid actuates on a T-5 Car? I bench tested the Idle Bump Solenoid and when energized it pushes out. I adjusted its travel as when I got the car, it was not bumping the throttle at full extension. I used a wrench to make it longer.
c. I got in the car and pushed pedal to floor. Primary butterfly closed to a approximate 1/8" gap.
d. Removed vacuum line to choke break and actuated and held with finger. It held goody. I released after 45 seconds. FYI, I replaced all vacuum lines and while running, I observed vacuum break holds good under its ported under carb vacuum and the rod is adjusted good to the secondary butterfky. I was wondering if the choke break it is too slow to release when I go WOT and that could lead to the "bog". But it does smoothly (but slowly) release when I actuate WOT under hood. Assume it is operating same when driving.

After doing the above, I tried start car. It cranked for 15 seconds with no start. Then it shudder and light pop through carb. More cranking and it finally kicked off but died. I cranked again, got it to start but it rumbled sub 500 rpm. I went under the hood and held throttle to about 1200 and it ran but I could sense an underlying rumble.

Electric choke warmed up and butterfly opened to vertical. This took about a minute I assumed she was good to idle but when I let go of throttle, it coasted down and died.

Above summarizes what took all of 15 minutes. Returned inside and wrote this reply to you.

My unexperienced opinion is my carb is flooded. Last night I had a fire under the secondary butterfly. I wrote my original thread after frustration.

I have a float gage BT 8420A coming hopefully soon but I feel the float level is a red herring as we spent a lot of time measuring when we disassembled and reassembled on Sunday.

I am concerned that the primary bowl plugs are leaking when the car sits and with the new vacuum lines throughout the system, I have a strong vacuum signature and i could be sucking fuel through these bowl plugs. However, after cleaning the carb in my UT tank with joy and hot water,. I dremel brushed all 6 plugs and put new ethanol resistant jb weld on all 6 ad let cure for 24 hours per instructions before reassembling.

I have no reason to think my TPS is bad as I bench cycled and live cycled it and it seemed to respond smoothly from 0.54V to 4.0V. I ordered a new one anyway (I can return it). My concern now is on two occasions after road testing car and going WOT (with the BOG every time), the ECM set the code 21 which I understand it TPS voltage high.

Look forward to your thoughts on the above events.

PCC

 

I guess to be fully disclosing of all that I did....

Two differences I made to the carb during the rebuild process:

1. The carb as we got it did not have the black cup under the MCR connector. I asked the company that I purchased the rebuild kit about that and was told many times, these cups were removed on rebuild nd not reinstalled. They sold me the correct used cup (new not availale) and I installed when I rebuilt the carb. I was told that the cup cuts down on fuel slosh in the primary bowl. Since son's goal is to autocross, I thought this would be useful. When I disassembled carb (after rebuilding and dissatisfied road testing), the cup did have fuel in it which led me to be concerned about flooding and float being too high, but we confirmed the dry float level and reassembled.

2. The existing needle and seat was an enclosed assembly and there was no pin connection to hang the needle from the float. The rebuild kit supplier told me that was an aftermarket needle/seat unit. Told me I should used the new needle and seat in kit, and my choice whether with or without the wire hanger as many rebuilders do not use the wire hanger. I originally installed the new needle and seat without the wire hanger as the existing enclose needlle/seat did not have hanger. Then when I dissassemble carb after dissatisfied road test to check the float, I put the wire hanger in.

Currently carb is assembled with MCS black cup and wire hanger.

Also, the base gasket that came with rebuild kit is slight;ly different than what was on the car. Under the primary section between the two idle air jets, the gasket has less material than the existing gasket. Carb kit supplier told me that the new gasket is correct and that neither shape would make a difference as all the ports that the old gasket came into contact we utlimately go to manifold. ther than that section, the two gaskets are identical and of equal thickness that is about 1/4 thick. There was another base gasket that was very thin material but I assumed it would not provide a good thermal break even though its shape matched the carb base silohuette the closest. The instake manifold has a machined depresion in that area of the so I doubt the orginal gasket could tightly seal that area.

 

any thoughts??

 

any thoughts??

 

Try blocking the throttle open all the way all night after running it and see if theres fuel in the intake the next morning. Also the choke should close tight and then the vacuum pull-off should open it as the engine starts. Also you say the idle mixtures are out 3 turns? That seems to far to me usually start at 1 1/2 turns out and adjust from there but also the mixture control setting has to be adjusted along with the idle mixtures

 

I see the word "autocross" in your post. I'd ditch the Q-jet for this application totally. Some will disagree but that carb doesn't like a lot of rough use. Find a spreadbore Holley.

 

Hello All,

Thanks for the follow-up. I tinkered with it tonight made some very positive progress.....

With a cold car, I went through the electric choke setting and made sure the blades were closed. Then realized the choke vacuum break was not pulling back on the primary blade as it was out of adjustment. My bad because I never touched it, I assumed it was set good before rebuilding. Bad assumption.

I went over all the vacuum connection again because I replaced every inch of tubing and capped off the port for the EFE which no longer exists. To my surprise, the cap was off the EFE port so putting that on changed the vacuum. Duh.

The results of all the above, on a cold start after sitting for 12 hours, the car started on the first crank with one kick of the pedal to squirt the pump and set the choke....beautiful.

.I then set the fast idle which was too low. It was set to around 1100 then it went up to 1200 as the electric choke opened up fully.

I kicked it down and it was curb idling at 550. I was able to bring the curb idle reliable up to the 700 RPM spec. I am running 4 deg BTC. I am suspicious the car might have an RV cam as the intake was removed and painted and the motor has a slight lope so I did not want to advance it more. I have a new cam and lifter that are going in from Comp Cams and the tech told me that cam has 5 deg of advance in it already. So, I will assume the current cam could have advance as well.

Then I went through the GM System Diagnostic and Performance Diagnostic and car checked good with dwell range on MCS.

Next, I pinned the TPS and it does not seem right. It is very erratic and I can';t get it turned below 0.8V and as I move the throttle it jumps around and goes up to about 2.6V. This was surprising because on the bench with my DC power supply set at 5.0V, I was able to bench adjust it to 0.54 and it swept up to 4.0V. However, the TPS set a code 21 on the last two hot test drives. It did not set one tonight, but it was reading erratic when I checked it and could not be adjusted to the 0.54 volt spec at idle.

I have a new TPS arriving tomorrow. i will check this one again in the morning on a cold start. Then if it does not look any different, I will check it with my power supply for my sanity.,

I will replace the TPS and Bench and HOT test it to see how a new TPS behaves.

Even with a sketchy TPS, the car is running a hell of a lot better now, it does not seem rich and the bog is almost not there.

I will reply back with the results after installing the TPS, Assuming it works out good, I will adjust the IAB valve and mixture screws per the FSM procedure and see how it runs.

I might be done tomorrow night....fingers crossed..

PCC

 

i bench set the car to the GM FSM. The IAB is set per a special gage (BT Partt 8253) which gages of the idle puinger. Then the idle screws are to be set at 3 turns out from seated. I understood that the idle screws are only adjusted if the IAB valve runs out of adjustment pursuing an MCS dwell of 30deg. Otherwise, I understood the idle screws are not further adjusted.

I am doing more reading. on this. I order the Roe Rochester Book from Amazon and it is coming tomorrow.

PCC

 

the ecm uses the vss signal to engage the bump solenoid as a 'dashpot' to dampen the throttle return to curb idle under deceleration. this shouldn't have any affect under other operating conditions.

if the dwell is responding correctly then the carb settings are good enough.

when you remove the TPS check the spring underneath. they can weaken with age and allow the TPS to move up and down in the cavity and provide confusing readings. I've installed a second spring under there before.

the TPS can also check good on the bench as yours did but fail when it is a little sideloaded when installed.

your cold start issues may all be due to choke operation and adjustment. hopefully you have that dialed in.

WOT bog can be ironed out by adjusting the secondary air valve tension, providing the pull off is working correctly, and your pull off seems to be fine.