2F Rough Idle ?'s and Compression Test Results (Long) (1 Viewer)

[S4Cruiser]

Still learning my way around my new to me 40 and have new questions/need some help.

I'm working with a rebuilt 2F with ~40k miles, '77 Toyota Federal Carb (desmogged) and 60 series large cap distributor w/coil/ingnitor.

I've corrected the PO's vacuum line connections on the carb per Jim C's direction, new plugs, new OEM PCV valve, OEM air filter and new Koyosan mechanical fuel pump (diaphram in the original one was failing).

Didn't idle that bad when I got the truck but idled high. I bought a tach to attempt to set the idle per the FSM at 650 rpm with no luck. It idles very rough and erratic when set near 650 (or as close to it as I can get it). Sounds like it wants to die then will randomly rev up to 1k or so. The truck runs fine at speed and while accelerating. When coasting to a stop it wants to almost die then rev back up 1k or so with no throttle and then will eventually settle back to 650 or so running rough.

Could this crappy idle be due to a vacuum leak? I tested manifold vacuum at idle and it was all over the place due to running rough. It held steady around 20 at 1k rpm.

Also:
Had been meaning to do a compression test so figured I'd start there. Truck had been driven around the block, parked for 30 minutes and then I did the test so not sure if it was warm enough. Disconnected the coil, removed all plugs, tested each cylinder with the throttle open (cranking over what sounded like 5 times or so).

Does this sound like the proper way or should I redo differently?

Here are my numbers -

1 - 140
2 - 130
3 - 130
4 - 135
5 - 130
6 - 140

I assume these are ok but not great. Thoughts?

Any idea on where to start/what to check to help cure my crappy idle??

for the help!
-Jon

 

[Pin_Head]

Compression is OK.
Rough / wonky idle:
Could be a vacuum leak.
Could be a non-functional idle fuel cut off solenoid.
Could be improper / non functioning desmog.
Could be a carb in need of rebuilding.

 

[S4Cruiser]

Compression is OK.
Rough / wonky idle:
Could be a vacuum leak.
Could be a non-functional idle fuel cut off solenoid.
Could be improper / non functioning desmog.
Could be a carb in need of rebuilding.

Thanks Pin_Head. Here is what I tried...

Could be a vacuum leak - with the truck running, I held a peice of cardboard on top of the carb and the truck died. Did not rev up so could this rule out a vacuum leak?

Could be a non-functional idle fuel cut off solenoid - couldn't tell if I could hear it click or not. Is it a noticable click or subtle (i.e. do you need your ear up next to the carb to hear it or can you hear it from the cab?)

Could be improper/nonfunctional desmog -
Attached is a pic of the way the carb was desmogged with the rough idle (see below).

I removed the cap on the air horn port (power valve?) and connected it manifold vacuum and this seems to have solved my rough idle. I was able to tune the carb to idle at 650 and not die when coming to a hard stop.

My question though is it correct to connect the port on the air horn to manifold vacuum or did me doing this mask a potential faulty fuel cut idle solenoid?

 

[Mr. Toad]

Could be improper/nonfunctional desmog -
Attached is a pic of the way the carb was desmogged with the rough idle (see below).

I removed the cap on the air horn port (power valve?) and connected it manifold vacuum and this seems to have solved my rough idle. I was able to tune the carb to idle at 650 and not die when coming to a hard stop.

My question though is it correct to connect the port on the air horn to manifold vacuum or did me doing this mask a potential faulty fuel cut idle solenoid?

You turned off the power valve by connecting it to manifold vacuum. It's supposed to be off for most desmogs.

Edit: The power valve is the one at the top of the carb, facing the radiator. It has a number 8 near it. In your picture, it is plugged.

The other hose shown connected from the throttle positioner to manifold vacuum should have no effect on idle quality.

 

[jodtan]

got experienced same symptoms, make sure measure/adjust valve on idle. Engine hot especially exhaust. Thermal expansion is terrible on this old Toy. that's my .02 cents....

 

[Pin_Head]

I don't know about the vacuum leak test you described. I doubt it would detect a manifold leak.
Take out the idle solenoid and check it. Spray some carb cleaner in the hole in the carb and blow it out with compressed air.
The power valve needs some kind of vaccum to close it or it will run rich constantly. This should have no effect on the idle mixture, so I am at a loss to explain your joy. Maybe your engine was stalling from being too rich? I'm not sure that it will ever see vacuum low enough to open hooked up to manifold vacuum. It doesn't take much vacuum to keep it closed. The engine control computer used to run it by venting to atmosphere.