Aisin carburetor idle circuit routing (1 Viewer)

[cbmontgo]

I am working to diagnose an idling issue on my trail rig and am fairly convinced that the idle circuit has a blockage somewhere. Can someone help explain to me the routing of the idle circuit from start to finish? Carb is a December 1969 production 2-barrel Aisin. Idle mixture screw will not kill the engine when fully turned in.

As far as troubleshooting goes, I have checked and removed the idle mixture screw and looks fine. I have tested the solenoid and it clicks and works fine. Vacuum is 14" hg at 7,000' elevation. I know that is a little on the low side, but I still don't think that the idle fuel should be non-existent.

See attached pics and let me know if I'm moving in the right direction.

 

[inkpot]

It is hard to detect, but if someone has previously closed off the idle mix screw way to hard, it can damage the seat/casting that the needle fits into. Then it will never close off completely. I have come across this twice over the years. If you take a can of carb cleaner with an extension straw and blow it into each passage way then you can tell where they go and if they are/are not open. John

 

[Pin_Head]

The reason you can close the idle mix screw all the way without stalling the engine is because your idle speed screw is set too high which opens the primary throttle plate far enough to pull fuel out of the transition slot, which is immediately above the idle fuel port and connected to it. The typical reason for this is that you have a manifold vacuum leak and need to increase the idle speed to compensate for the vacuum leak.

Take a peek inside with a flash light and I bet you will see fuel running out of the transition slot. This condition is also typically associated with bogging down off idle because there is no fuel left at the transition slot when you open the throttle further.

 

[cbmontgo]

Thanks much, guys.

So I assume that the attached picture is correct in regards to the location of the transition slot? Pinhead, you are spot on about bogging down off idle. Is the purpose of the transition slot to deliver fuel during the transition from idle to throttle? Should be slot be covered by the throttle plate at idle?

If I adjust the idle speed screw so that the throttle plate covers the transition slot, wouldn't I be back to square one because the idle fuel port is not delivering fuel? It seems that it should be delivering idle fuel through this port even with a vacuum leak?

Also, where does the transition slot receive its fuel? Separate from the idle circuit?

 

[Pin_Head]

1. Yes.

2. Yes.

3. Yes

4,5. The idle port does deliver fuel all the time. It just isn't enough fuel to let it run with the air leaking in because of the vacuum leak has no fuel. When you open the throttle plate the manifold vacuum drops, slowing the rate of air leak and now more air is going through the carb and picking up fuel from the transition slot.

6. The idle port and transition slot are connected together as part of the same hole so they are both part of the idle circuit and get fuel from the same place. The idle port is the end of the line and the slot is upstream. That is why you can shut off the idle port and it makes no difference because all the fuel is coming out of the slot.

7. Fix your manifold leak and you will notice the difference in performance.

 

[cbmontgo]

Thanks much, Pinhead. I am going to look into addressing the vacuum and see if it persists. I'll be sure to follow up.

 

[cbmontgo]

Update on this issue for future readers...

After much carburetor rework and adjusting, the issue was some sort of blockage in the idle circuit in either the air horn or the body. I gutted the carb internals and put them into a spare carburetor air horn and body that I had recently dipped and cleaned, keeping only the cast-iron base so that I could use the same vacuum ports as before. The idle circuit is clear and she idles perfectly now.

 

[Pin_Head]

So it was "idling" off the fuel coming out of the main nozzles. You should have been able to see the fuel dripping. I'm not sure about a '69, but a common source of junk plugging the idle circuit is pieces of the O ring for the idle fuel cut off solenoid.

 

[FJ40Jim]

The problem may have been (probably) was an air leak at the idle circuit between the warped main body and warped air horn. The older the F carb, the worse this problem is. It is not a problem on 75-later 2F carbs.

Just sayin...

 

[cbmontgo]

Thanks much, guys. This thread really got me digging deeper and I learned a lot more than I knew before.

 

[emac]

I know this is a very old thread, but I wanted to thank the posters for the pictures and descriptions. Last week, my son had to put a couple of gallons of ethanol fuel in the 40 and it started to act up at idle. I think some varnish must have been dislodged and found its way to the idle circuit. I took the top off and blew things out, but I think it is deeper than the top. This thread helps me visualize the idle circuit. I also found the plunger rubber cover was in a couple of pieces. Will that cause an air leak?

 

[Pin_Head]

Not an air leak in the sense of carburetor function or manifold vacuum.

 

[thebigredrocker]

Emac I think there are kits that have leather plungers. Possibly "Mike's" I recall reading FJ40Jim comment on rubber plungers and got the impression he prefers leather with today's RFG.

 

[thebigredrocker]

Yes, that is the original 1972 carb.

The keyster kits usually come with a rubber AP plunger, which does not seem to play well with RFG. Either re-use the OEM leather plunger (after cleaning & soaking in motor oil) or get a plunger out of another kit. IIRC, the plunger by itself is NLA from Toyota.
Combining the Keyster kit (main jets, slow jets, power valve, power jet, idle needle, aluminum crush washers...) with a cheapo parts store kit (gaskets, AP plunger...) gets the bestest of everything.

 

[emac]

Thanks guys. It is the external rubber dust cover. It is one of Trolholes carbs. It is about a year or two old. Damn chinese rubber! There are OEM rubber grommets on my 77 that were painted, sandblasted, almost forty years old, and they are still being used.

 

[emac]

Well, I took the top off of the carb for about the fifth time tonight. This time I blew air through the hole going down to the idle mixture screw, as well as a couple of others in the base of the carb. All is well again. These carbs seem very sensitive to the slightest debris in the idle circuit. Idles perfect. I cant thank this forum enough.

I know, I could have just stopped after searching and finding this thread, but I thought it may help some new folks by bringing it back. Carb problems seem to be very common.

 

[Scotty2]

YES, thanks for resurrecting this blog, because I have the same problems with idle. My '73 FJ40 F155 2bbl rig was moved from 8000' elevation Colorado to my new home at 8' elevation NE Florida. The carburetor is a Model 2J23 from 10/72, per the stamping and VIN plate on the truck. So I decided to rebuild the carb and re-jet it at the same time.

I followed some iH8Mud guidance on the re-jetting. Here's the changes:
Main Primary - Was 106, Now 114
Main Secondary - Was 180, Now 180
Power Jet - Was 60, Now 60
Slow Jets - Was 60/80, Now 50/50 (or 80/50...hard to read)

Idle problems commenced. This rig has the CA emissions control stuff on it, and the rail that runs alongside the valve cover makes it difficult to properly tighten the carb mounting nuts. Bought some crowfoot sockets and that helped a bunch. But I still must have a vacuum leak somewhere, as the Idle Mixture Screw has no effect. I've read a lot of advice on here about this problem, and mucho thankso to Pinhead et al.

Gonna take my carb cleaner and fire extinguisher to the driveway today to investigate further. If I find no problems with vacuum leaks, then I will once again tear the carb apart.

What is the best source for an accelerator pump that is made from leather not rubber? (I read that this can cause problems from ethanol attacking rubber accelerator pumps, etc.)

Please weigh in with any wisdom on likely places to find vacuum leaks (so far I've read intake manifold top AND bottom, distributor advance, and of course tubing. I've replaced all my vacuum tubing as it was all cracked and dried.

Having a CA emissions model, I got sidetracked with the little VSV module on the drivers side inner fender. It was completely rotted out inside, so I replaced all the vacuum lines in it.

Oh yes, I need to add one more little goof up I made. Somehow during the rebuild of the carburator, I LOST the "pump discharge weight". Gremlins apparently ate it, because I swept the garage and couldn't find it anywhere. Uggh. So, I tried to compensate by adding a second ball on top of the first one to provide the missing "weight". Is this a fatal error? Could it be causing my idle problems? (Doesn't seem to be in the idle circuit...)

 

[65swb45]

1. Valve adjustment?
2. Base timing?
3. Dwell if points dizzy (not point gap)?
4. Booster and PCV disconnected during systems check?
5. Vacuum reading?

www.marksoffroad.net

FAQ

 

[mattressking]

Does your accelerator stop pin have the weight and check ball? I see it's missing the o-ring on top. Otherwise it would need the pin, spring and checkball from a 2F carb.

What folks tend to miss is that fuel coming out of the butterfly shaft means there is a fault in the carburetor leading to a overfueling/leak issue.

Is your ICS pin chopped short? Something is causing the fuel to continue to flow or overfuel so it leaks down the barrel onto the shaft.

 

[Scotty2]

1. Valve adjustment?
2. Base timing?
3. Dwell if points dizzy (not point gap)?
4. Booster and PCV disconnected during systems check?
5. Vacuum reading?

www.marksoffroad.net

FAQ

Thanks in advance for your help! Well, I spent another day chasing my tail it seems. First, I sprayed and resprayed carb cleaner on anything and everything vacuum related w no idle change. I just don’t think I have a vacuum leak!

So, I decided that the most likely culprit was the carburetor I rebuilt. I took it off, tore it down to parade rest, soaked it in Gunk, rinsed it off, sprayed carb cleaner in every port. Nothing seemed clogged.

Put it all back together. No change.

As for your questions:
1. Valves have never been touched because it ran so good and strong. I did check compression and all six cylinders were strong and similar.

2. I can get it to run roughly at about 2-3 btdc. 7 (the bb) kills it. So the dream of setting it at 10 like everyone on ih8mud advises is impossible right now.

3. No points. The PO installed some electronic innards in the dizzy.

4. I did NOT disconnect the brake booster or the PCV during testing. Didn’t know I should!

5. 17 or 18 in hg at 800 rpm. I can’t get it to run 650 rpm!!!

This thing is driving me mad.