Cleaning stock Mikuni carb

[MidnightPass]

Does anyone have a break down or diagram of the stock Mikuni carb? I am going to buy a Webber, but I am cleaning this one up for now until I sell my car and can buy my Webber. I am stuck on one part and just need to see a diagram to make sure I have everything I need to at least hear it fire up and possibly take it for a test drive around the block.

[FMS88]

This thread has an exploded diagram:
http://www.mightyram50.net/vbulletin/showwiki.php?title=Fuel+Management+System:ASA+Miku ni+Carburetor+Breakdown+Diagram&highlight=mikuni+diagram

and this one has a PDF manual for the Gen 2 Mikuni:
http://www.mightyram50.net/vbulletin/showthread.php/8147-1988-mighty-max-carburetor-vacuum-diagram-needed(actual-photos)?p=73354#post73354

Which part of the carb are you stuck on?

[MidnightPass]

That is perfect, thank you! I am currently stuck on the mixture control valve. When I opened it, the little ball was just sitting in there as well as a cylindrical bushing? I'm not exactly sure where they go. I'm used to fuel injection :P

[FMS88]

That is perfect, thank you! I am currently stuck on the mixture control valve. When I opened it, the little ball was just sitting in there as well as a cylindrical bushing? I'm not exactly sure where they go. I'm used to fuel injection :P

Hmmm. I've never found a ball in the valve and not sure about a cylindrical bushing. :scratchheadblue: Got a any pictures of those two parts as well as the rest of the mixture control valve?

[geezer101]

Ball and brass float in the gallery between the fuel bowl and the accelerator enrichment pump. Lucky they were both in there or the carby would be a block of wood. They are usually the first things to go flying when you take one of these apart and aren't warned about it.

[MidnightPass]

http://www.mightyram50.net/vbulletin/attachment.php?attachmentid=27243&stc=1

So this is how it looked when I took it apart. I am not sure if a spring is missing or how that is supposed to go in there. This will be the first time I have ever started the truck when I get this back together. The old fuel I took out of it smelled like nail polish remover at first, then sweet almost at the end of the tank when I drained it. I am going to blow out the lines, put this carb back on, and put some new fuel in it to see if it starts... Then when my car sells, I'll get the Webber to throw on it.

[xboxrox]

Midnight ~ there might be some photos in my Mikuni albums at my member page ~ please take a look ~ FMS88 and geezer helped me a lot with carb info

[MidnightPass]

Unfortunately, you found the same diagrams I did… which I could not see the little bell or cylindrical bushing in either diagram. I posted the picture of what I’m taking about, but I think it’s awaiting approval.

[MidnightPass]

Never mind, it let me attach it that time ����

[FMS88]

Here's a pic of the Mixture Control Valve's parts:

http://www.mightyram50.net/vbulletin/attachment.php?attachmentid=27246&stc=1

The part in your picture I'm calling the valve body. The valve shaft passes through the small sleeve in your picture, however, the sleeve isn't easily removed from the valve body I have.
http://www.mightyram50.net/vbulletin/attachment.php?attachmentid=27248&stc=1

There's no small ball in the MCV. Maybe someone tried to disable or modify it?

[geezer101]

Oooh okay, that's not inside the carb body. Pictures always clarify things. FSM888 might be right that someone has tried to disable it for some reason. I would start looking for a complete carb in a JY if that is an option. These carbs are hard enough to work on when complete without having to chase missing diaphragms etc...

[xboxrox]

Oooh okay, that's not inside the carb body. Pictures always clarify things. FSM888 might be right that someone has tried to disable it for some reason. I would start looking for a complete carb in a JY if that is an option. These carbs are hard enough to work on when complete without having to chase missing diaphragms etc...

Agree what everyone is saying and showing here + to the best of my knowledge the 1987 Mikuni carbs got even more complicated than earlier versions ~ even tho several 1987 carb parts are same & will fit older versions ~ I never purchased anything but always got great Mikuni info and advice from Alan @ Carburetion Technologies in Tacoma, WA ~
he might have a reasonably priced like new rebuilt carb on the shelf ~ he claimed to have rebuilt over 2,000 Mikuni carbs ~ even with all his experience, he says the Mikuni is a biatch to get right ~ hope you find the best fix and share it here...

[MidnightPass]

Hmm, mine looks similar to that... But also looks slightly different. I am curious if maybe they tried to modify it or something. This is what mine looks like underneath where that ball and cylinder is.

[FMS88]

Hmm, mine looks similar to that... But also looks slightly different. I am curious if maybe they tried to modify it or something. This is what mine looks like underneath where that ball and cylinder is.

Yes. It's definitely been "modified." The small valve is missing and I bet the ball was jammed in the valve's guide to prevent a vacuum leak. This isn't good. If someone tried to disable the MCV, what other modifications might they have tried that could compromise the carb? If you can't get the Weber soon or you're intent on trying to get the Mikuni to work, plug the rectangular opening in cylindrical part of the MCV valve body with JB Weld (visible in your latest pic) and the other smaller holes with RTV, then reassemble. That should plug the passages from the air horn to below the secondary throttle and any vacuum leaks caused by disabling the MCV.
The MCV has a very limited purpose; it only opens momentarily when the throttle closes quickly to prevent an overly rich mixture. If the rest of the carb is fine, it should start and be drivable despite an inoperable MCV.

I overlooked your post #6. Why was the truck sitting so long?

[geezer101]

The Mikuni is a wretched thing to work on. Lots of diaphragms, control circuits, vacuum lines and components that are hard to replace and/or source. Add to it years of service and dealing with vacuum leaks around throttle shafts, adjustments made by PO's that didn't know any better and a huge number of model variants with altitude compensation and internal/external EGR valves etc and it is no wonder some guys abandon these trucks as being in the 'too hard' basket.

[MidnightPass]

Yes. It's definitely been "modified." The small valve is missing and I bet the ball was jammed in the valve's guide to prevent a vacuum leak. This isn't good. If someone tried to disable the MCV, what other modifications might they have tried that could compromise the carb? If you can't get the Weber soon or you're intent on trying to get the Mikuni to work, plug the rectangular opening in cylindrical part of the MCV valve body with JB Weld (visible in your latest pic) and the other smaller holes with RTV, then reassemble. That should plug the passages from the air horn to below the secondary throttle and any vacuum leaks caused by disabling the MCV.
The MCV has a very limited purpose; it only opens momentarily when the throttle closes quickly to prevent an overly rich mixture. If the rest of the carb is fine, it should start and be drivable despite an inoperable MCV.

I overlooked your post #6. Why was the truck sitting so long?

I am not sure exactly why it was sitting for so long, nor how long it was sitting for. I know it sat for at least six months, but other indicators say it was longer than that even. I was told the guy I bought it from was selling it because the carb needed work and he didn’t have time to work on it. It Did not run for at least those 6 months though. But from the smell of the fuel I drained from the tank, I would say it sat for even longer than that.

[FMS88]

I am not sure exactly why it was sitting for so long, nor how long it was sitting for. I know it sat for at least six months, but other indicators say it was longer than that even. I was told the guy I bought it from was selling it because the carb needed work and he didn’t have time to work on it. It Did not run for at least those 6 months though. But from the smell of the fuel I drained from the tank, I would say it sat for even longer than that.

Hopefully the PO is right and it just needs a carb and fuel system cleaning. How did the rest of the engine appear and test? Okay I hope.

[MidnightPass]

Hopefully the PO is right and it just needs a carb and fuel system cleaning. How did the rest of the engine appear and test? Okay I hope.

With the carb how it is, I was able to run the truck off of ether for basically however long I wanted, but the engine sounded beautiful ��

[MidnightPass]

Also, I figure it should be a good thing to mention, the truck would crank up and run for about 2 seconds then die and not start back up for about 2 to 5 minutes… Then do the same thing again.

[FMS88]

Also, I figure it should be a good thing to mention, the truck would crank up and run for about 2 seconds then die and not start back up for about 2 to 5 minutes… Then do the same thing again.

Good to see that it will fire up and run even briefly. Still, I'd test the compression just to be sure it doesn't suggest another reason it was parked. Not sure about the failure to restart. My first guess would be an ignition problem (coil, ballast resistor, distributor, timing). Check what you can before replacing parts and especially the coil/distributor wiring. The PO might have tried to get creative with the ignition system, too.

[MidnightPass]

Yeah, that’s a possibility. But everything looks stock for the most part. The guy did an electric fuel pump, but that’s about it in the engine bay.

[geezer101]

Do this little test - after cranking it, quickly check the ballast resistor on the coil to see how hot it is. If it is hot enough to burn your fingers it has failed (obviously take care not to burn your fingers!) If the coil looks original, replace both the coil and ballast resistor. There are plenty of OEM style coil/resistor assemblies on ebay but try to upgrade to a transformer type ignition coil as they deliver a more stable spark at higher rpms and the output voltage is also higher which improves burn of the air/fuel charge.

*the crank/start/stop and delayed start are classic symptoms of a bad ballast resistor. The coil will refuse to fire once the resistor has overheated and the cooling off time of 5-10 minutes is enough to encourage the coil to run a few cycles before it overheats again. Hopefully this is a solution for you...

[MidnightPass]

Do this little test - after cranking it, quickly check the ballast resistor on the coil to see how hot it is. If it is hot enough to burn your fingers it has failed (obviously take care not to burn your fingers!) If the coil looks original, replace both the coil and ballast resistor. There are plenty of OEM style coil/resistor assemblies on ebay but try to upgrade to a transformer type ignition coil as they deliver a more stable spark at higher rpms and the output voltage is also higher which improves burn of the air/fuel charge.

*the crank/start/stop and delayed start are classic symptoms of a bad ballast resistor. The coil will refuse to fire once the resistor has overheated and the cooling off time of 5-10 minutes is enough to encourage the coil to run a few cycles before it overheats again. Hopefully this is a solution for you...

that definitely does sound like the symptoms I have, but how is my needle and diaphragm not being there going to effect it? Same symptoms?

[geezer101]

You can try to plug the small vacuum ports in the mixture control valves with something temporary like adhesive wall tack (stuff kids use to put posters up on their walls) and start it to see if the mixture valve is the issue. Not being complete can (and probably is..) causing internal vacuum circuit issues. Unfortunately this is the sort of thing you're going to be up against when someone who doesn't know what they're doing monkeys around in an engine bay. Could be the fuel pump set up, blockage in the carb or fuel lines, failing ICM in the distributor, bad coil and ballast...

To test the theory of no fuel, get a clean kitchen sponge that is big enough to cover the throats of the carb (one without the scotchbrite pad stuck to one side) rinse it in fuel, wring out the excess and hold it down over the carb while starting it (do not open throttle). There will be enough fuel vapour in it to run for at least 15-20 seconds. Safer than tipping or spraying raw fuel down the throats and no risk of accidentally flooding the carb and intake with an overdose of fuel.

[MidnightPass]

You can try to plug the small vacuum ports in the mixture control valves with something temporary like adhesive wall tack (stuff kids use to put posters up on their walls) and start it to see if the mixture valve is the issue. Not being complete can (and probably is..) causing internal vacuum circuit issues. Unfortunately this is the sort of thing you're going to be up against when someone who doesn't know what they're doing monkeys around in an engine bay. Could be the fuel pump set up, blockage in the carb or fuel lines, failing ICM in the distributor, bad coil and ballast...

To test the theory of no fuel, get a clean kitchen sponge that is big enough to cover the throats of the carb (one without the scotchbrite pad stuck to one side) rinse it in fuel, wring out the excess and hold it down over the carb while starting it (do not open throttle). There will be enough fuel vapour in it to run for at least 15-20 seconds. Safer than tipping or spraying raw fuel down the throats and no risk of accidentally flooding the carb and intake with an overdose of fuel.
Once my Talon sells and I have more money to put into the truck, I will order the new carb and follow the instructions on here for converting to a Webber carb. I know for 100% it is a fuel issue already since it runs perfectly on small doses ether for as long as you keep spraying enough to keep it running. I am a little worried about the ballast or ICM now though, but those should both be good if it runs fine on ether, right? The engine wasn’t knocking or ticking or anything when running on the doses of ether. That’s my main reason for buying it. That and, I have a 4G63t engine sitting on a stand that will be going in it eventually. Oh, and also the fact that I only paid $800 for it with no rust.

[geezer101]

The mixture control diaphragm should only be affecting internal fuel circuits from an open throttle/closed throttle condition. Have you confirmed the fuel bowl is actually filling or the needle and fuel float has jammed? With the engine off can you see a small stream of fuel in the primary side carb throat when opening and closing the throttle by hand? What is the fuel pump flow like? Can you get the carb to pull a lot of fuel through the throats by blocking them off by hand and getting someone to crank the engine. I'm beginning to wonder are the fuel inlet lines crossed over, blocked filter or fuel line, debris in the tank clogging the pick up or something else goofy like that.

Has this engine run since you bought the truck and are there any other symptoms things you've noticed that seem off?

[MidnightPass]

I got the Weber carb on there and she’s finally running again after who knows how long. I’m going to go ahead and do a tune up now since she’s breaking up a little bit at idle(plugs, wires, distributer cap and rotor, oil and filter).