Adjusting Mikuni Carburetor Choke

[BJH324JH]

I bought a Mikuni Carburetor from the junkyard so I could mess around with it and not damage mines. Examining the carburetor I noticed that the choke on the junkyard carburetor is adjusted differently. Like the accelerator is actually touching that thing with the number 19 stamped on to it. It has 19 stamped on it and appears to have lines for something. Here is a picture:

18310

Now, if you take a look at my carburetor that was adjusted at the carburetor shop, you can see that the accelerator is not touching that thing with 19 stamped on it. I am guessing this has something to do with the way my truck is acting up.

18311

I started up the truck to see if it would touch the lever with 19 stamped on it and it still wouldn't. I don't know if this is supposed to be touching or not, but it seems that on the junkyard carburetor it is touching. I thought to myself, this might control the idle because my truck shakes like an earthquake when I cold start it. So I decided to test if the idle is the culprit. I did this by loosening the accelerator cable and tightening it just enough so that it gives it a little bit of gas. I think this is supposed to be automatically done by that lever with the 19 stamped on it. After doing that I fired the truck up and she started on the first crank and now that horrible shaking is gone. :)) (I will put back the accelerator cable as it was because I don't want the truck to always have the accelerator on. This is not a fix, only a test.) With further examination of my carburetor I noticed the idle screw is threaded in all the way, making it so that the accelerator doesn't touch the lever. Thanks to pennyman1 (http://www.mightyram50.net/vbulletin/member.php/5-pennyman1) and messing around with the junkyard carburetor, I am almost certain that the choke is not adjusted correctly. I think this is the case because I notice that the choke butterfly doesn't move at all and what moves it is the lever thing and it can't move it because it's not touching the accelerator in the first place. If my understanding of this is correct, that is what makes it go into high idle. Now my question is how do I adjust this choke?

[BJH324JH]

I have no idea why the pictures aren't showing up, but here are direct links to them:

Picture #1:
https://s23.postimg.org/law7zwyuj/20170127_205153.jpg

Picture #2:
https://s29.postimg.org/pn7nn4i4n/20170128_000252.jpg

[BJH324JH]

Just checked the idle screw and it's not threaded in all the way. It's difficult to work on the carburetor with it bolted onto the truck. Sorry bout that.

[geezer101]

Does your JY carb have an electric choke? I have never seen an electric choked Mikuni before...

[pennyman1]

all US carbs are thermowax element chokes - with some kind of electric connections to it. Geronimo's failed at 70K miles in 3 years - that's why he has had a Weber since 83. If you can determine which screw is for the choke adjustment, it should contact the cam that has teeth on it. When you rotate the carb throttle to full open when cold, you should see the linkage move the screw toward the cam when it returns to closed position.

[geezer101]

Can you provide a full shot of the throttle side of your carb? There is something off here (IMO). The first pic you provided has a plastic cover where the choke tensioner gear should be. If it is a weird electric choke then it may be a case of simply undoing the retaining screws around the cover and rotating it.

[BJH324JH]

Can you provide a full shot of the throttle side of your carb? There is something off here (IMO). The first pic you provided has a plastic cover where the choke tensioner gear should be. If it is a weird electric choke then it may be a case of simply undoing the retaining screws around the cover and rotating it.

Okay, here is a picture of what I think you are asking. I loosened the three phillip head screws on the black plastic thing and it does indeed rotate, and in doing so the butterfly choke moves. I can see it move from the top of the carburetor. Turning the black plastic thing clockwise, closes the butterfly choke, turning it counter clockwise, opens it.

18318

[geezer101]

Thankyou for the pic on a number of levels. #1 I have never seen an electric choke on a Mikuni, and #2 verifying that the adjustment technique is the same as an electric choked Weber carb. I think you have a relatively rare carb in your hands and I would then wonder about the manifold you removed it from. Was it from another 4G54 engine and do you remember what vehicle? This could be very handy resource material for future reference :)

[geezer101]

Thankyou for the pic on a number of levels. #1 I have never seen an electric choke on a Mikuni, and #2 verifying that the adjustment technique is the same as an electric choked Weber carb. Have you worked out how to hook up the vacuum solenoid next to the choke cam gear? I think it would have a thermal switch somewhere telling it when to release it at operating temperature (my guess anyway) I also think you have a relatively rare carb in your hands and I would then wonder about the manifold you removed it from. Was it from another 4G54 engine and do you remember what vehicle? This could be very handy resource material for future reference :)

[BradMph]

Is that the egr sub valve you talking about. Another troubled Mikuni item. Besides the wax pellet, carbon would crust all over that egr valve on them darn carbs. I was constantly getting a sticky peddle from it. Had a dealer also be told to replace my carb by corporate. Dealer said carb wasn't a smog item, lmao They were very wrong and main office reamed them a new one. Funny how nice a dealer is after a good push up the butt from the big bosses at corprorate.

[BJH324JH]

I'm going to guess that 4G54 is the same as G54B or was that a mistake. Anyways, I pulled it of a 1987 Mitsubishi Mighty Max. Here is a picture that I took of the vehicle safety certification label:

18320

According to the VIN number breakdown:

Country: Manufactured in Japan
Make: Mitsubishi
Vehicle Type: Truck
Restraint System: 4001-5001 LBS.
Car Line: Truck 2WD (PRE-1987)
Series: Low
Body (Mitsubishi): ?
Engine: 2.0 G63B
Model Year: 1987
Plant: Nagoya-2
Transmission: ?

[BJH324JH]

Thankyou for the pic on a number of levels. #1 I have never seen an electric choke on a Mikuni, and #2 verifying that the adjustment technique is the same as an electric choked Weber carb. Have you worked out how to hook up the vacuum solenoid next to the choke cam gear? I think it would have a thermal switch somewhere telling it when to release it at operating temperature (my guess anyway) I also think you have a relatively rare carb in your hands and I would then wonder about the manifold you removed it from. Was it from another 4G54 engine and do you remember what vehicle? This could be very handy resource material for future reference :)

I have been fiddling around with the vacuum pump and hooked up a hose to what I think you are describing. When I create a vacuum with the vacuum pump, I notice that this little arm hook thing moves in and pulls in the cam gear with the teeth on it. I turned on the truck to see if this little arm hook thing would move and it didn't. I inspected the hoses and found that they were cracked and torn. So I removed all the hoses to check them for leaks with my vacuum gun and some water. My vacuum gun can also pressurize. I plugged one end of the hose and attached the other end to my gun and pressurized the hose, then I put the hose underwater in a little bowl of water to check for leaks. If you aren't familiar with this technique, its basically the same way you check inner tubes for holes. Works pretty well. I might make a tutorial on this. After checking all the hoses for leaks, I noticed it still would not hold vacuum and thus not be able to move the arm, so what is happening? Will it begin to hold vacuum once the truck reaches temperature?

Some pictures of what I attached the vacuum pump to, to make the little arm hook thing move. (Vacuum = Arm moves in, No Vacuum = no movement of arm.)

NO VACUUM:
18321

VACUUM:
18322

[geezer101]

It's likely the diaphragm in the vac solenoid is split - similar fault that occurs with the secondary throttle vac actuator. My bet will be this vac solenoid is normally hooked up to a thermal switch like the unit you'd find screwed into the base of the thermostat housing in the inlet manifold (got a feeling this unit operates the fast idle function). Once the thermal switch reaches operating temp it will allow a vacuum circuit to actuate the solenoid and disable the fast idle function (I'm taking a stab at this but I could be wrong...)

[BJH324JH]

The vacuum hoses are routed wrong and currently in the process of replacing and putting them back where there supposed to go. Will update when I finish.