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So I'll get in my truck for a cold start, pump the gas a couple times, and start the engine. It'll start, jump to about 1300 rpm climb to about 1500 rpm in a minute, then stay there. After about 3 minutes (give/take) the rpm jumps again to 2000-2200 rpms and will climb to 2600 in another couple minutes. Shortly after that the engine reaches temp, I'll pump the gas one more time and the choke will disengage and rpms drop to 700. My question is, does that process sound normal or should the engine jump to 2600 right away? The delay from 1500 to 2600 makes me think something's going on with a thermo-switch. Any advice is appreciated, thanks guys.
My theory is that the idle mixture is set too rich and the hot (slow) idle has been mucked with to suit.
Remove your plugs and describe what they look like. They should be a dry brown with maybe a tiny bit of carbon on the curved negative electrode.
if they are caked in dry black furry scummy soot, then this confirms my theory.
idling fine doeant confirm that its not too rich.
if the idle air bleed is slightly blocked, this will enrichen the idle circuit and u would need to compensate by turning mixture screw in.
this is ailly as it ia designed to run at ur current setting, so that suggests a cleanout is in order.
idle air bleeds get gunked up all the time. the cause ia usually pcv syatem fresh air supply burping backwards on deceleration and splatting soot all through the carb.
ohh i see ur problem, its jumping suddenly and not smoothly.
ok the diaphragm in the choke opener is also dirty and is waiting for the vacuum to reach the required level to pull it past the gunk. this happens in stages and is a problem.
your entire carb needs a teardown and cleanout.
Doesn't the choke opener use vacuum from the bvsv (once it's warmed up) to pull the fast idle cam to the lowest step (and open the choke as a result). You're probably onto something, as I mentioned, I have to pump the gas pedal once it's warmed up to get it to drop to idle speeds, but I've also heard that that's the design and the carb isn't supposed to drop rpm itself using vacuum. Maybe you know whether that's true or not?
What confuses me about your suggestion is that even if the choke opener isn't working perfectly, why would that effect how quickly the engine climbs to fast idle? It seems to me that whole system is used to drop the fast idle.
It drops to 700 thanks to the choke BREAKER
The other part, the choke OPENER relies on vacuum to hold the choke closed. so if that is sticking, or the diaphragm is burst, it will moved out large proportions at a time. i say its not managing to fully choke as u are gettinga high idle.
I have to disagree with you thommo, the choke lever will receive pressure from the bi-metal coil in the choke housing when cold, when you press the gas pedal twice it releases the linkage and the pressure from the coil will close the butterfly valve almost all the way. This also moves the fast idle cam to the highest step. The bottom of the choke lever inside the choke housing should be pressed against the end of a small rod extending from the choke breaker diaphragm. When that diaphragm receives vacuum immediately on start-up it pulls that small rod towards itself and forces the choke to open slightly more to prevent an overly rich mixture. Hence the name choke breaker. The choke opener is a separate diaphragm that receives vacuum from a bvsv (once it warms up) and the rod extending from the choke opener diaphragm will retract and pull the fast idle cam to the the lowest step. This should drop you down to curb idle. I can try to screenshot the pages from the fsm that detail this process if it would be helpful.
oh i see now. gees thats complicated.
ok the idle that high is pribably a bit extreme. try adjuating the (cold) idle screw back a bit. just examine its function before u do it, sometime tightening the cold idle screw lowers the cold idle.
Sounds like the fast idle linkage is sticky. Spray the linkage with some carb cleaner, wiggle all the linkage, spray it some more and give 'er a try. Now it will speed up a bit (say 500 to 1000RPM) as it warms up but should kick down to idle without touching the gas. Don't know where you live geographically but even up here in the great white north it has to get down to ' freeze the nuts off a brass monkey' for the fast idle to hit 2600RPM. It was nice weather here today (5C, that'd be about 40F) and I fired mine up to go for a cruise, the fast idle never went past 1500RPM.
I'll try spraying it with carb cleaner again, something may be binding. But Old83, you say you don't have to tap the gas to get it to drop to curb idle? That's good to hear, I've read both that it'll drop on it's own and that you need to tap the gas. Wasn't sure which is right. I'm going to play around with the vacuum line leading to choke opener, I thought I had it setup the way the fsm described with the line leading from bvsv splitting between choke opener and the vacuum port on that side of carb. When I applied vacuum to choke opener diaphragm it would pull the choke linkage open so I figure it should work.
but should kick down to idle without touching the gas.
My following information is based on my '87 with the Aisan California feedback carburetor.
On a cold engine, the idle speed will NOT decrease RPMs until one of two things: the BVSV directs vacuum to the Choke Opener to disengage both the Fast Idle Cam & Automatic Choke. Or second, as the engine is warming up and the Automatic Choke progresses through it opening phase, the gas pedal is blipped or depressed, the Fast Idle Cam will move down to the lower "steps".
The Automatic Choke and the Fast Idle Cam are indirectly connected. The choke can move (open up) independently from the Fast Idle Cam. The force of the Primary Throttle Valve resting on the Fast Idle Cam is too much for the spring of the Automatic Choke to rotate the cam to the successive "steps" of the cam. Only until the gas pedal is blipped or depressed does the force of the choke spring allow it to rotate the Fast Idle Cam. At that time the Fast Idle Cam is not touching the throttle valve, so the cam is free to rotate.
Once the engine reaches operating temperature, the BVSV then diverts the vacuum from the Auxiliary Accelerator Pump to the Choke Opener. In the following pics, Toyota calls our BVSV, TVSV. It serves the same function.
jees thats horrible hey. look how many failure points there are. i much prefer my mitsubishi wax element choke.
but that is an awesome diagram, is that from the workshop manual or a 3rd party?
The Aisan choke itself is a simple Bi-metal wound coil that automatically closes when it gets cold, hence the Automatic Choke nomenclature and not an electric choke.
Originally Posted by Thommo Thompson
is that from the workshop manual or a 3rd party?
That is taken from Toyota's own carburetor handbook. The handbook covered all the carburetors they used for the 80's vehicles.
Yup it looks messy but its worked (for me) since 1983! The linkage and operation is the same as 'Slow-mo' posted. The 83 manual doesn't seem to put as much detail into it as the 87.
I've said it before that a lot of issues with this and other OEM carburetors from the 70's to the time of wide spread use of EFI is simply the emissions stuff on them. Ford used a lot of 'Weber' carburetors (made by Holley) in the seventies and they were absolute pieces of .... and the cars were too!
So today I needed to take the truck out of the garage and I took note exactly what it did.
Ambient temperature...-4C. Overnight was about -8C (27F / 18F)
Pump gas three times...truck starts and within 1 -2 seconds is 1200RPM. After about 15 seconds idle is up to 1700RPM. Runs this way until temperature gauge needle is almost at the ' T ', about 5 minutes. (This is an SR5 gauge cluster). Idle dropped down by itself to 1200RPM and ran at this speed as long as I was moving stuff in and out of the garage, about 10 minutes. At this point I drove it down my lane and back (about a 1/4 mile both ways), idle was then 'normal', 750 - 800 RPM. The needle on the temperature gauge really didn't past the 'T' until I drove it up and down the lane, so it may not have been warm enough to knock it to idle, again I seem to recall it will (and that's cause sometimes it stalls, oddly in the summer) and that's after maybe 15 - 20 minutes. Idling in my driveway for 15 - 20 minutes isn't something I normally do and its usually in the winter when again I have to haul something else in or out. I know that in cooler weather (10C / 50F) the truck hasn't completely warmed up by the time I get to the first stop sign and stalls sometimes. This is probably because the choke has been kicked off by depressing the gas...the other condition 'Slow-mo' pointed out.
Most of the issues with the fast idle / choke I found are with the pivot on the fast idle cam and various gunk's / dirt's / corrosion / zinc fuzz on the cam follower and/or the steps on the fast idle cam. The choke valve pivot shaft tends to get sticky too.
Last edited by Old83@pincher; 12-08-2017 at 04:19 PM.
11-26-2017, 04:02 PM
