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christofoo · 10-02-2012, 11:23 PM

EDIT: Solved.

This is an old-school carburetted Toyota 4A-F engine (1989). I realize I may just be griping about the nature of the beast. Fuel injection is so much more consistent. Althought it is carburetted, it does have some intermediate emissions tech on it, O2 sensor and presumably stepper motor, 3-way cat.

But I doubt it ran like this when it was new. Maybe there's something worn I can fix or adjust. If you think I should let it go just say so.

It idles rough. After setting the idle speed (supposed to be 650) it wanders between 550 and 750. I had to adjust the idle speed set screw after it started stalling at idle a few days ago, that got me into tune up mode. The cause may just be that I reduced the octane from 89 to 85 on the last tank, and now it wants to be tuned for low octane. But regardless, now I'm in tune-up-mode, and it has been idling rough for a long time. Some other misbehaviors, or maybe just old carb behaviors: 1) it may hesitate if I try to accelerate at low RPM while cold; 2) after sitting in the driveway a week, I have to pump the accelerator 5-7 times before it will start, sometimes I have to pump the accelerator a couple times after it sits overnight.

I checked the timing with my timing light. Actually I'm not sure if I changed the timing because I moved the distributor and then remembered to remove and plug distributor vacuum lines. So the timing is now correct, whether it was right before or not. I cleaned the indicator plate for the first time, so for all I know I misred it when I tuned it ages ago.

Compression on this engine was perfect when I checked it 13k ago, although I feel like it runs a little smoother when I use high-mileage engine oil.

I had the mechanic clean and tune the carb about 3k ago, didn't make much difference that I noticed. He did say I should rebuild the carb.

Ideas?

Could it be a bad / poor O2 sensor?
Could it be valve clearance? When I checked that 13k ago, they were in spec, but not consistent one with another. 5-8 mil range on intake. 8-10 mil range on exhaust. Should I go back in and adjust them so they are all consistent?
Can I rule out a vacuum leak? The brake boost is alive before I start the engine after a weekend of resting. I got a "stethascope" from a length of 1/4" hose, plus ear muffs, and couldn't hear any hissing anywhere in the engine compartment, except what I think is air going through the throttle (I can hear it with the stethascope off coming from the air filter vicinity, but can't find any locality for it with the stethascope).
Time to listen to my mechanic, rebuild the carb?
I've been over and over and over the distributor cap, rotor, wires, and spark plugs. Pretty sure I've replaced all of them inside of the last 15k, not with any noticeable change. Spark gaps are all 43 mil, as speced, and they all seem to have about the same coloration. However, if I (carfully) unplug the spark wires one at a time, I notice that cylinders 1 and 2 strongly affect idle speed, but the effect is quite subtle for cylinders 3 and 4.

10-02-2012, 11:23 PM	#1 (permalink)
christofoo Master EcoModder Join Date: Mar 2012 Location: Salt Lake City Posts: 292 00C - '00 Toyota Corolla 90 day: 43.54 mpg (US) Thanks: 147 Thanked 190 Times in 73 Posts	Carb rough idle EDIT: Solved. This is an old-school carburetted Toyota 4A-F engine (1989). I realize I may just be griping about the nature of the beast. Fuel injection is so much more consistent. Althought it is carburetted, it does have some intermediate emissions tech on it, O2 sensor and presumably stepper motor, 3-way cat. But I doubt it ran like this when it was new. Maybe there's something worn I can fix or adjust. If you think I should let it go just say so. It idles rough. After setting the idle speed (supposed to be 650) it wanders between 550 and 750. I had to adjust the idle speed set screw after it started stalling at idle a few days ago, that got me into tune up mode. The cause may just be that I reduced the octane from 89 to 85 on the last tank, and now it wants to be tuned for low octane. But regardless, now I'm in tune-up-mode, and it has been idling rough for a long time. Some other misbehaviors, or maybe just old carb behaviors: 1) it may hesitate if I try to accelerate at low RPM while cold; 2) after sitting in the driveway a week, I have to pump the accelerator 5-7 times before it will start, sometimes I have to pump the accelerator a couple times after it sits overnight. I checked the timing with my timing light. Actually I'm not sure if I changed the timing because I moved the distributor and then remembered to remove and plug distributor vacuum lines. So the timing is now correct, whether it was right before or not. I cleaned the indicator plate for the first time, so for all I know I misred it when I tuned it ages ago. Compression on this engine was perfect when I checked it 13k ago, although I feel like it runs a little smoother when I use high-mileage engine oil. I had the mechanic clean and tune the carb about 3k ago, didn't make much difference that I noticed. He did say I should rebuild the carb. Ideas? Could it be a bad / poor O2 sensor? Could it be valve clearance? When I checked that 13k ago, they were in spec, but not consistent one with another. 5-8 mil range on intake. 8-10 mil range on exhaust. Should I go back in and adjust them so they are all consistent? Can I rule out a vacuum leak? The brake boost is alive before I start the engine after a weekend of resting. I got a "stethascope" from a length of 1/4" hose, plus ear muffs, and couldn't hear any hissing anywhere in the engine compartment, except what I think is air going through the throttle (I can hear it with the stethascope off coming from the air filter vicinity, but can't find any locality for it with the stethascope). Time to listen to my mechanic, rebuild the carb? I've been over and over and over the distributor cap, rotor, wires, and spark plugs. Pretty sure I've replaced all of them inside of the last 15k, not with any noticeable change. Spark gaps are all 43 mil, as speced, and they all seem to have about the same coloration. However, if I (carfully) unplug the spark wires one at a time, I notice that cylinders 1 and 2 strongly affect idle speed, but the effect is quite subtle for cylinders 3 and 4. Last edited by christofoo; 12-11-2012 at 03:05 PM..