Dreaded Mikuni Carb question....

[Dweezil8675]

I have an '83 MM with the 2.0 in it. I have rebuilt the Carb recently and it was running great. It lacked power in the mid-high rpms so I fixed this by pulling the secondary vacuum diaphragm off completely. I replaced it with a spring running from the secondary linkage to the mount that held the diaphragm. I blocked off the one vacuum port as well. No leaks there. I have also retimed the engine to 5 BTDC, new plugs, wires, distributor cap, rotor and ignition coil . I also did a valve jet adjustment as well. All of them were off and now they are correctly gapped to.025mm per the Chilton manual. So the engine is timed and ignition is all new. I have checked vacuum lines with no leaks. When I rebuilt the carb I learned 2 things, take my time and to never rebuild a Mikuni ever again! So before I fixed the secondary diaphragm I had it running great, just no power. I now cannot get this thing to idle correctly to save my life. I'm guessing the adjustments you should not touch have been messed with before to compensate for the crappy running carb. So now that it is all rebuilt and correct I have no idea where to start on all the adjustments. Does anyone have the factory settings for the screws that should not be touched? or am I kind of up a creek with no paddle on this one? I also verified that my spring "fix" works also, it is not opening the second barrel at all until you turn the main one so far. BTW, super simple fix if anyone wants pictures to basically 'delete" the secondary diaphragm. I have looked at the other posts on here as well about adjustments and I cannot seem to find the factory settings for the screws you should not touch. I'm sorry for the long post but I'm hoping the wizards on here can help me out a little bit. Thanks for any help

[geezer101]

The spring mod has been tried many times and will 'work' (the engine will run) but the reason why Mikuni designed these carbs with vacuum actuated secondaries is to allow metering of the secondary throttle. Anyway we need more info on the idle issue - what is it doing exactly? Idling high, rough etc.

[Dweezil8675]

I knew I couldn't be the first one to figure out the spring trick.....damn I thought I was cool hahaha. I just need this truck to pass smog here in California then im taking the title to Ohio and putting a weber carb on it! And getting rid of all the damn vacuum lines and emissions junk. So for now the trucks idle is really high, around 2300 rpm. I turn down the idle screw and once it gets down to about 1400 I try to go a little more then it falls on its face and dies out. It seems to not be idling to rough at the higher rpm but once I try to back it down any lower then 1400rpms it does get rough and then it peters out. it does all this with the truck at operating temperature and the choke is off when I am attempting to "tune" the carb.

[geezer101]

Check the air mixture screw - wind it in all the way and wind it out 3/4 to 1 1/2 turns. Next question - is the auto choke working? The choke butterfly has to be loosely closed when the engine is cold and be at least 3/4 open at normal operating temp. There is a fast idle adjust screw on the bottom of the throttle linkage (it has a captive spring in it). This one can be tricky to adjust and if you are careless the spring will disappear into the netherworld. Check the throttle cable as well - make sure it isn't loading up the throttle somehow. These carbs are easiest to set when they're off the engine (you can physically see the throttle butterfly gaps and set the stops before it's installed) Yeah the Mikuni is not fun to work on and definitely shouldn't be the first carb you ever tackle unless you're really confident or you are out of options...

[Dweezil8675]

I have rebuilt carbs before so I got the manual and took my time when I rebuilt it. everything in it was set to the specs it called for. The auto choke is working I can see the butterfly working so that is good. I did what you suggested and got it to run! went back after it was idling and double checked the timing and vacuum lines. I replaced a couple of vacuum lines and I did away with the whole spring set up on the secondary. I put the diaphragm back on it and it seems to be working ok, still not the best mid-top end power but what can I expect from a 2.0 right? Hopefully it will pass smog now so I can put the weber on it and not have to worry about this in the future. I still need to dial in the mixture but for now it goes down the road so Im happy with it. I appreciate the help! i was losing my marbles trying to make it run again.

[geezer101]

:thumbup:

[Dweezil8675]

So I smogged it today and he tuning is spot on, a little lean but good to go. BUT it still failed....apparently my egr isn't working? where can I get one of those that doesn't cost 200 bucks?

[pennyman1]

take it off and clean out the passages - there are actually 2 egrs - the main one and the sub egr - clean them both and the passages in the intake

[Dweezil8675]

I cleaned those today before I smogged it to be sure, but the guy who smogged it said he didnt get any response from it with a vacuum on it. He suspects it to be the internals of it not holding a vacuum or something like that. Does that sound right?

[pennyman1]

they do fail sometimes after 33 years. But it also could be something else in the vacuum lines off the other side of the egr.

[pennyman1]

http://www.ebay.com/itm/EGR-Valve-for-1985-Mitsubishi-Mighty-Max-part-MD110900-/321692615268?hash=item4ae65fc264:g:GygAAOSwpDdU~2c m&vxp=mtr This might work to replace it, but check it out by part number for sure if its cali

[85Ram50]

pennyman1 That page says the EGR listed is for an 87 in spite of the title. That part on rock auto is $174.79. So in case it has to be specific and I remember that California is usually very specific...
Rock Auto says the 83 and 84 are the same. So I looked up an 84 as that is as far back as they go here and it looks like from this page the part number for the Cali EGR is MD026556. The page seems to list all the different EGRs as solenoids for each motor from 84.
http://oemmitsubishiparts.com/1984-mitsubishi-mighty_max-base-gas_2.0_l_4-transmission_standard_trans_speeds-emission_system-egr_system/

Have you tried removing it and cleaning it? It should be possible to make it carbon free and that should work for a smog as long as the valve operates. Changing the oil just before you drive in for the smog is also helpful on old motors. Literally change the oil and drive straight to the smog station.