yotashoofy

I have an 87 4x4 pickup w/ 22R. I just put a new carb in it and it now hesitates really bad when I hit the gas. I didn't have this problem with the old carb. Double checked all vacuum lines. Accelerator pump seems to be working - it is spraying fuel into carb when the motor is off and I hit the throttle. Is there a way there a better way to test the AAP? Should I be looking at something else?

L5wolvesf

new carb brand model info?
old carb brand model info?

Should I be looking at something else? Have you gone over all the connections (vacuum, electric, mechanical etc.) at least 2 times?

slow-mo

Are you sure you connected the vacuum line to the distributor correctly as well?


The FSM (factory service manual) has a procedure for testing the AAP (auxiliary accelerator pump). The vacuum controlled AAP only works up to a set engine coolant temperature, then vacuum to diverted off the AAP via the BVSV (bi-metal vacuum switch valve). After that, the mechanical Accelerator Pump works in conjunction with the venturi.

yotashoofy

have checked for proper vacuum hose connections multiple times. I'm absolutely certain they are correct. Yes my bad on the AAP, it only assists acceleration when the engine is cold. My problem is all the time.

I have been tinkering with the truck all weekend and have become even more frustrated. (checked the float level, timing, etc..) I replaced most of the vacuum lines, except the brake booster line. As I test it more, it seems the hesitation is really a low-end problem. Once the rpm's get up to about 2000 it goes away. I think it is not getting enough fuel as I can hear it sucking air when it struggles.

The original California carb was replaced with an “Autoline C4036” remanufactured unit. This is the only carb available for my California truck. (all the auto parts distributors sell the same one) The original carb had a problem where the float bowl would drain out overnight and it was very hard to start. I would have to turn it over for about 40 seconds to get fuel back up in the carb. It was rebuilt twice and the issue continued to persist. It was assumed there must be some kind of crack or something in the body of the carb. Eventually, It wouldn’t pass smog for running too rich. So I parted ways with that carb and bought a new one. The reason I mention all this is I’m wondering if the rich running carb was masking a problem that the new carb is exposing? I’ve been trying to adjust out the problem with the fuel mixture adjustment. Normally one would lean out the mixture until the engine started idling rough and then back it out until it is smooth again - then call it good. For me, after backing it out the engine will stall or nearly stall upon de-deceleration. So I have to back it out about 2-3 more full turns before that problem goes away. Then upon continual adjustments, I'm able to get the hesitation to a minimum, but never completely go away. I'm running out of things to try. Any ideas would be appreciated.

Thommo Thompson

So its hesitating only when u rapidly ask it for more power??
What if u gently ask it for power, and let it catch up, is it ok in that scenaro?
Its very likely that the old one was always leaking fuel into the manifold, which could mask a distributor advance issue.
Perhaps just check that the distributor rotor turns freely left and right and springs back freely. If not, its seized and the extra fuel form the old leak couldve helped the retarded ignition.

Any backfires?

yotashoofy

Thanks for the reply! It certainly hesitates with the gentelest request for power at the low end all the way up to about 2500 rpm. It hesitates almost indefinitely until the RPMs are up. If I rev the engine before letting out the clutch I can get off the line without lurching. Then keeping the rpm high can shift through gears with minimal hesitation. I'll check the distributor rotor. Thanks.

yotashoofy

Rotor shaft is free and returning to center after compression of spring. On another test ride today I noticed the issue is not present for the first minute or two of operation. Hopefully this rings a bell for somone? Any help is appreciated.

Thommo Thompson

yes thats interesting
only happesn after slightly warm, and sort fo becomes less apparent above 2500 rpm unless u really stab it above that... correct?

If so. it could still be fuel fuel related. Warmer engine atomises fuel better.
So my experience would point me towards the new carby and how it is setup.
Can you confirm that the carby is idling on the idle jet alone.
While idling, look down the carby throat Primary side, and make sure its not driping from the main jet. the onl fuel going in should be not visible , it should be comng in under the throttle plate.
If you can see droplets coming out the top "main" jet, in the middle of the venturi thing at idle, then its not idling on the correct circuit, which could indiate a couple of things.
1. idle circuit blocked.
2. idle circuit completely inactive due to solenoid or deceleration cutoff system faulty or simply not connected.
3. vacuum leak, which makes the operator wind up the base idle to start getting venturi over the primary jet, when in fact it should be running on static pressure, (vacuum) sucking in fuel below the throttle plate.
the idle circuit actually runs low rpm performance up to about 2000 rpm, not just idle.
It is in fact reffered to by toyota as the SLOW jet not the idle jet,
Have a look at those and tell me if u find anything.

Thommo Thompson

adn id say the reason its not happening during first minute or so is that the choke is doing its thing.

Thommo Thompson

but dont take my words as gospel, could be barking up the completely wrong tree!!
Ive focused on what people on the net said far to many times when it was somethign simple like, NO FUEL, haha

yotashoofy

Thanks, Thompson. I will take a look at that tonight.

yotashoofy

UPDATE: I end up finding a vacuum leak around the base of the carb by spraying everything down with acetone. I pulled the carb off, inspected the gasket, and realized the gasket was covering a vacuum port on the manifold for the pcv. I assume this is what was causing it to suck through the plate. I also did some research online and found there are two gaskets types for my vehicle. So I just simply cut out the hole to make it right. After re-installing, the leak was confirmed to be remedied by spraying it down with acetone again. The issue with the motor stalling when letting off the accelerator is gone (only when the idle adjustment is leaned out). However, it still studders on the low end. Again, I continued to mess with the idle mixture adjustment. I found it seems to run a bit better when the adjustment was completely leaned out. Another thing I noticed as I put more drive time on it is that it seems to completely go away for short periods while Im driving.

So then I took your advice (thanks again Thompson!) - looked down the throat of the carb and sure enough there if fuel dripping out of the upper jet when idling. I checked again for leaks - nada; solenoid - works great; idle jet - not sure. I decided to mess with the idle mix. adjustment some more to see what happens. I turned it out about 4 turns (from the dead bottom where it was adjusted to) and fuel stopped coming out of the upper jet. But the studder issue seem to get worse. So I played with it more. Turned in a few turns and saw the fuel start coming out of upper jet again. Turn it out and the fuel stopped. Then I thought I would try to tune it by setting it to the leanest point I could go without fuel coming out of the top jet. In doing this I was able to turn the jet all the way in and fuel was no longer coming out of the top jet. So again, more proof that the issue is somewhat erratic.

My understanding is the best method for adjusting this carb is to turn out the idle speed 3.5 turns and the mixture 2.5 turns. Set the idle speed. Then start turning the mixture in until the engine starts running rough. From there slowly turn out the mixture nob until it runs smoothly again. I have tried this repeatedly and can never hit the spot where the idle gets rough. In fact, I can turn the nob all the way in (where fuel starts coming out of upper jet) and it seems to run a bit better.

QUESTIONS: Is my understanding for proper tunning of this carb correct? Is it possible there is a very sensitive sweet spot in the idle mixture adjustment that would eliminate my problem and I just haven't found it?

Thommo Thompson

The best method for any carb is to take 6 mths off work and obsess over it haha. But seriously, there is no text book approach that will work every time. Every situation is different.
So now that uve identified the vacuum leak as the source of the new slow circuit issue, u want to be sure that the idle jet is std correct size and that u didnt just buy a carby with whatever jets happened to be on the shelf.
Im not sure i expect u to Open the new carb, but for future refference, these are the jet sizes.
Carb - Aisan
Model - Twin Choke
Ventury 1 - 26
Ventury 2 - 40
Mainjet 1 - 114
Mainjet 2 - 280
Idling jet - 65
Idling air bleed - 104

of those, of interest is the idle jet and idle air bleed. An idle circuit air bleed that is too big will give u the issue u experience more so than an idle jet that is too small.
some of the cheaper carbs arent quite perfect. I can personally say ive had to muck with brand new carbs to get them to work.
U really want to take the carb off, look at the throttle plate, and set the idle screw so that the throttle plate is only just letting the slightest amount of light theough. Smaller than a bees dick even.
put it back together and try not to touch the idle speed screw from there.
start it and revv it for a couple of minutes so that ot heats up.
If it doesnt idle, its likely the idle mix isnt open enough.
however
Realising the old carb had been running very rich, the spark timing will have to be advanced compared to what it used to be because there is a weaker mix being allowed in now.
So try to get it running by winding the idle mix all the Way open, and attempt to get it idling at about 800. Then set the timing at 0 degrees with vac advance hoses unplugged and blocked. Then plug hose back in and check where timing jumps to. At idle it should be up close to 10 or even 12 if ur lucky. If its not at 12, maybe advance it a little more so that it is but dont go further than 5 degrees base timing (vac advance unplugged)
finally, with the timing at 10-12 ish with vac advance on, try to lean in the mixture screw a little bit.
If it dies bring the screw back out a bit.
see where this gets u

yotashoofy

Thanks Thompson for your thorough reply. Im definitely feeling like I need to take 6 months off work right now! If not to fix this thing to recover from the emotional pains! After I catch up on rest from multiple nights of wrenching until 2 AM, I think I might pull that carb apart to check the jets. I have also had some suspicion over the jets. At this point I'm rather convinced the problem is internal to the carb. Thanks again for the great feedback!

Thommo Thompson

Before u take it apart just check that advancing the timing doesnt smoothen it out

slow-mo

As a side note, the OEM Aisan California spec carburetors have a larger Main Jet than Federal spec carbs, since it is a Feedback carburetor. The California carbs have the Electronic Bleed Control Valve (EBCV) to regulate A/F mixture via the oxygen sensor.

California uses 1.19 jet.

Thommo Thompson

oh dear i hate feedback carbs. a half assed fuel injection system haha.
and ur in california so u could be dealing with that too.
usualy the only real problem with feedback type is when theres an exhaust leak, and the extra oxygen hits the O2 sensor and the car runs rich. But there is also the possibility that the computer is either broken, O2 sensor is broken, or someone has bypassed it (permenatnly grounded the solenoid) so that the solenoid is always completely open when the ignition switch is on. This doesnt affect the idle circuit, but but does affect the main primary circuit. it doesnt affect secondary either. However u said this only started with the new carb, so im going to ignore it for now.

yotashoofy

well, I took that carb apart over the weekend. What a tird! Found a vacuum orifice obscured by a hanging chad on the gasket, and it looks like someone took it a part and dropped it out of the back of a truck on the freeway. Big gouges in the Al, gouged jets, etc.... The air bleed port in the main barrel looks like someone hammered a screw into it. What a shame. None of the jets had numbers on them, and the only jet guage I had was too big for the 4 banger jets. (auto parts store was closed at 2AM) After fixing the gasket, I put it back in and it still ran like ˟˟˟˟. So finally I put the old carb back in and it runs like a champ.

I originally replaced the carb because it failed smog for running rich and I have had an ongoing problem where the float bowl drains dry after about 2 days. So I figured it needed a new carb any-ways and it should help smog. I do have a minor exhaust leak. It seems to go away when its cold, but maybe its still leaking a little. I'll put effort into the exhaust and take it back for a smog retest with the old carb. My best bet to fix the float bowl problem is to probably buy a used one and rebuild it myself. Not going to give the remanufacture bastards any more of my money.

Thommo Thompson

If the exhaust leak is on the manifold then that is definitely the cause or one of the causes of it running rich. If u cant fix the exhaust leak, (braze it or replace it) then shorting the earth wire from the fuel control solenoid on the carb to earth, (black wire) will just make it run lean but will still pass any test, and maybe just have a smidge less power around the 2000 rpm area.

Kilo392k

My carburetor was doing the same thing draining after 2 days. I took it apart and found out that the CHECKBALLS were not installed there is on the auxiliary accelerator pump and one on the main mechanical carb pump.