mikuni carb tuning advise

Hi guys,
first off, I really appreciate all the help everyones given to this forum. It's a valuable resource!
so, the issue:
From what i've read on here, mikuni carbs are persnickety at best. I have an 86, automatic, Dodge Ram 50, with AC (tho the AC isn't working at the moment) Here's the scoop,
I got the car a month ago from a guy who let it sit for 2 years or so. it was a little sticky once it got running and a friend and I messed around with the three adjustment screws to give it a little more pickup. Then I got a new exhaust and have put about 200miles on it. it recently began to do this deiseling thing that kinda scared me. I was gonna rebuild the carb but chickened out and just used the spray seafoam front engine cleaner that you just spray in the intake. It feels like it's running better but now I'm having trouble adjusting the high and low idle. It runs rough in drive - kinda just an inconsistent idle no obvious misfiring but it almost revs itself, at stoplights mostly, and not in my driveway for some reason (is my car just giving me attitude??), it starts up on the first go both cold and warm. Basically, I feel like i'm just turning a screw then driving it around the block to see if it did anything. I have the manual, so I'm not flying blind, but still feel in the dark. Honestly, it probably would have been better if I hadn't messed with it in the first place, but I did, so, whatta gonna do.
if there's a post about this please link. Any and all advise is appreciated.
If I can't get it within the next few days, I might just take it down to my shop and ask them how much it'd be for them to adjust it.
Thanks!

Is the hot idle running high rpm like 1100-1200 rpm? If it's idling high it'll keep pulling fuel until it starves which will cause the dieselling.

I got the dieselling to stop. I don't have any tools to tune the carb so I'm going off sound and feel, which is why I might just bring it in.
The only issue now is really the inconsistent idle while in drive (like at stoplights etc.) it seems to almost rev up and down a bit. This it was not doing before I started messing with it.
on a less important note but also related,
I'm trying to get the throttle valve dialed in, it seem like there's slack or something, I have to floor it almost all the time. the response is good, there's just not as much power as I'd like. Maybe that's just these trucks tho?
And again, as always, I really appreciate all your help.

go ahead and do a weber conversion. its likely the carb is trashed. float coule be out, needles could be stopped up, anything can cause it. or just an old carbuerator. weber swap or starion FI, thats your best bet for the truck.

that will probably be what I end up doing in the long run. I'm still convinced it was something I did when I adjusted it and if i can get some more life out of the carb it'll save me a few bucks.
That said, if I'm going to replace it anyway I might as well keep screwing with it for a bit.
so here's some photos of the manual and the screws i've been fiddling with. the screw circled in orange is the low idle... I can't figure out which is the high idle.... what am i doing wrong and why is nothing else changing?
Attachment 25081Attachment 25082Attachment 25083Attachment 25084

Image #1 - throttle up for the A/C. If you've adjusted it to the point where it's pulling on the throttle, you've overdone it (this may be enough to cause the throttle 'flutter' you're experiencing - maybe try backing it off)

Image #2 - orange ring is the throttle stop. Has to be set on a hot engine (850-900ish RPM) green ring is fast idle (...? hard to tell as you've got one of those crap carbs with the shroud permanently rivetted to the auto choke) set when cold @1100ish RPM and the yellow ring is the altitude compensator (you won't even notice this thing unless you are driving on mountain roads...)

Check your throttle cable as you should have plenty of adjustment. They have a tendency to break where the end is clamped to the carb which causes the cable sheath to slide instead of actuating the carb. If you have no adjustment @ the fire wall (really unlikely - these cable have stupid amounts...) you can plan B the issue by bending the end of the accelerator pedal arm slightly towards the drivers' seat (1/2" - 1")

DO NOT mess with ignition timing until you know your carb is at least half sorted out. Adjusting stuff like the carb settings are a death spiral of engine problems.

Orange is the idle speed adjustment. According to the Haynes manual, green and yellow are factory set and should never be adjusted, as doing so will make it impossible to adjust anything else accurately. I think the choke fast-idle screw at the bottom of the throttle lever can be reached by going underneath that riveted-on shroud with a long screwdriver, or by opening the throttle enough to get at it going over the top of the shroud. A/C fast idle can be adjusted via the screw in that vacuum capsule in Pic #2, just above orange and a bit blurry.

That MAS screw was adjusted all the way closed then opened 1 1/2 turns on my 86 Mikuni truck... My mechanic friend Vernon did this with much confidence saying he knew this was correct... So far my truck has been running good & today I pumped the gas pedal twice which helped make a really quick start (no more cranking x10 revolutions waiting for something to happen)

*any screw on a Mikuni with a spot of paint on the threads is a factory sealed screw and shouldn't be tampered with unless you absolutely have to. The Mikuni is an evil unforgiving device and any 'friend' offering to 'make it run better' should either have workshop certification, a few years experience with these @sshole contraptions or be willing to lose a limb if (well, when...) they mess it up. Normally someone will massively drop the idle so it stumbles, then try to fix it by advancing the ignition timing. By then, you are lost. It doesn't run, you can't get it to fire and are left unable to even get base timing. When you watch someone undo this kind of mess and the first time they hit the key and it fires straight up, you feel like you are witnessing divine intervention :biggrin:

Yeah, i'm sure we did more harm than good, as all the screws have now been moved from any factory setting.... haha. The friend builds hotrods so there was some experience there. Still, we should have left it alone.
The timing is fine (spark plugs, rotor, cap all new) and the car starts no problem, which i take as a win. Honestly it's not running poorly, but there's just this very slight revving thing on idle while in drive and only when warm. i'm assuming its related to the carb as it wasn't doing this before. i've started going through replacing vacuum hoses that look old and might replace the PCV valve if for no other reason than to rule it out.
it's very possible that the cleaning that I did gummed up an internal valve or knocked some piece of grease and dirt loose that was actually keeping the whole thing working smoothly.
Thanks to all who have chimed in and have validated my frustration with the tangled mess that is a mikuni carb.

check the float level dude. my 78 f150 had a high float level and it had a slightly surging idle. also check the choke, accelerator pump, and power valve if it has one. is the fuel pump surging???

Ok, I played around with it. Readjusted the spark gap, which fix the misfire and messed with carb screw adjustments, and think I got it to stop running funny. Idle sounded good, no more revving etc. I was so excited I drove it around the block for a few min. Then I went to put gas in it and she died on the way home. Just flat out died mid turn, wasn't getting gas. Walked home got tools, drove over. thought it might have been spark, tested spark and coil; electric, seemed fine. I shook the fuel filter and took the air filter off, then she started. Could this have been a carb issue also? I replaced the fuel pump and filter a month ago when I got it and the filter didn't look like it had anything in it. Once I got it running (and had a tow on call) I just drove her to my shop, it ran a little rough the whole way over. I figured I should probably have them do a once over on the engine and test my vacuum lines etc. as i don't have those tools.
ok. question:
It's still a little sluggish. could this be my problem? https://www.youtube.com/watch?v=lrBq3eKu6wQ
anyone have issues with a secondary valve? I haven't even looked to see if I have a secondary valve.
Thanks for all the chatter, you all are so helpful. I do hope this thread is helpful for folks having issues with their carb.
This is the first car I've ever worked on, so thank you for teaching me a new skill!

Maybe the fuel filter is clogged? They'll starve out an engine if they're blocked up. I have experienced cars with lots of sediment in the fuel tanks and have had an engine conk out from a sudden surge of debris passing into the fuel lines. Also take note that el cheapo fuel filters are sometimes held together with a glue that doesn't handle ethanol blended fuel which can cause it to dissolve and wreak havoc on anything upstream (carb, fuel pump etc) The elements in them can drop out allowing unfiltered fuel to pass through them along with gummed up glue. Not good... Try a new filter and inspect all of your hoses.

The Mikuni carb has a secondary actuator for the throttle which runs from internal metered vacuum - it's the biggest vacuum dashpot/solenoid on the carb and has a small elbow vacuum hose on the back it. If you have literally no power from about 2000 rpm upwards, this could (and usually is...) the issue. Full throttle, but lacks power across the rev range could indicate a bad vac advance on the distributor (may only be gummed up from crap...) or a general tuning issue. Flat or stumbling off idle may indicate a bad accelerator enrichment pump (this is a carb issue). You can visually see if this is working with the engine off by removing the air filter and rapidly opening the throttle. You'll see a single shot of fuel from the inside of the primary throat if it's working (no fuel could mean either a blockage in the jet or the pump diaphragm has failed)

* a helpful tip - it is actually easier to go through the wheel well to access fuel lines than try from the engine bay. Remove the drivers' side wheel and make sure you have the truck supported on solid ground with axle stands - safety first!

just an update if anyone's following:
Carb was shot. carb shop said it had old water damage from before I got it, which makes sense because when I resurrected it, it had some water in the tank. Put a refurbished mikuni on there and it works great! starts up perfectly almost every time and idles solid. Now I get to continue with the fun cosmetic stuff!
Again, thanks for all the help guys!!

:thumbup:

Hey, it worked! ;) Did the shop give you the dead carb? It's worth taking one apart just so you can get to know the dark magic lurking inside of a Mikuni without having to sacrifice a functioning one...