2.0L with stock carb struggling to idle

[Mitsuzero]

hello, i have an 87 MM i recently repaired the transmission on and have been driving around to test the repairs, the vehicle had been sitting almost a year after i acquired it and probably was sitting a few years before that. on a cold start up it'll fire right up but the RPMs with the choke and fast idle cam seem excessive, i'll tap the throttle and it'll come down slowly where it'll idle roughly until it dies. i can keep it running by periodically tapping the throttle when the RPMs start to slow down. i suspected a vacuum leak somewhere since i had similar behavior from my other truck. other than the issue with idling this truck runs perfectly and i can drive it indefinitely until i have to stop and idle. the truck has the stock mikuni carb and i checked all the vacuum lines for cracks or splits. I'm hoping it's not plugged idle circuits in the carb since i've been told the Stock carb is a handful to take apart and put back together correctly. could it be the silver box on the driver's side fender? i have yet to open that thing up

[1fastcolt]

did you drain all the gas in the tank and replace it with fresh?

[Mitsuzero]

no. the gauge was reading pretty low so i dumped 5 gallons of fresh gas in. do you suggest i drain the tank completely? or just add more fresh fuel?

[1fastcolt]

at this point i would fill it up with fresh... and put in some additive from parts store to help dilute the old gas.. maybe a can of carb cleaner and spray the heck out of the carb while its running... (without too much to make it die). might take a day or 2 of driving to clear out that old gas.

[Mitsuzero]

i guess that's what i'll do. i put some B-12 Chemtool in the tank and ill probably dump carb cleaner into it. good thing i enjoy driving the truck

[xboxrox]

Gas that contains ethanol (corn alcohol) gels clogs into clumps of yuck garbage ~ I don't think these older classic vehicles were designed to run on crap gas ~ since I began using non-ethanol gasoline my truck runs twice as nice as before so much better smooth idle better acceleration too ~ if you want even more performance visually inspect the catalytic converters for blockage & check brake drag & clean regrease adjust the front wheel bearings & replace all fluids engine oil transfer case transmission differential lubes & replace all filters especially the air filters ~ PCV if not clean and functioning properly will also cause a rough idle ~ don't forget the wiper blades ;p

Test the air casing snorkle hot air valve used for faster engine warm up -- any vacuum leak in it's diaphragm will cause it to remain open forcing hot air into the engine -- cap off the vacuum connection under the carb which defaults this valve to stay in the closed position which causes the air intake to ingest outside colder air -- sneaky thing !!

[Mitsuzero]

those are also very good points, it'll be getting new tires and an oil change, a new air filter is also on the list. i haven't checked the cats yet but that'll be something to investigate if a full tank of new gas doesn't cure it. the only other fluids that i haven't changed is the differential fluid and the antifreeze

[xboxrox]

Ggreat thanks for acknowledging our posts ~ I forgot to mention testing checking the 3 types of ignition advance initial centrifigal vacuum use a cheap Summit Racing laser tachometer to set engine idle when setting initial timing which must be done at mfg rpm

Your truck will love you back thumbs up !

[Mitsuzero]

would a conventional timing light work or would it have to be a laser tachometer?

[xboxrox]

Use a timing light that has the brightest light flash possible ~ might need a darkened place to see the timing marks (clean the timing marks on the timing cover & maybe put a dab of white fingernail polish on the pointer end) a Made in China laser tachometer from Summit Racing helped me set the engine idle RPM speed before, during & after setting the timing (it shoots a red beam onto a piece of white reflective tape that you paste onto the harmonic balancer) ~ make sure you know exactly which carburetor screw is the idle adjustment screw (it took me several years to find it, no joke) the engine should be at normal operating temperature when setting the timing...

Initial Timing: set at 5° or 10° degrees BTDC @ designated idle RPM (refer to the lable on the engine hood)
Vacuum Advance: advances timing whenever the throttle is opened quickly (observe and confirm that it does using the timing light) a bad diaphragm or vacuum leak will prevent advancement & the engine will bog down when the accelerator pedal is quickly put to the metal (zoom zoom nope)
Centrifigal Advance: begins as the RPMs are increased and will advance the timing the most, something like 36° BTDC (refer to specifications)

Timing specs are somewhere but not on my fingertip (look em up)
If you have a timing light that also displays RPM thats all you need

[Mitsuzero]

i'm in luck, i've got one with a built in tach, and i have a white paint pen so i'll have that on hand. i have not tested the vacuum advance but road tests seem to suggest that it's working, initial timing might be off still. i think what i'll have to do is get the tank completely full of gas and then investigate the timing settings. thank you for all your suggestions, i've seen you've tackled the Mikuni carbs when most just discard them.

[xboxrox]

I used the covid lockdown time to tackle the Mikuni MR50 member geezer101 messaged me a lot of advice...
Oh my bad, the white paint is needed mostly on the crankshaft pully TDC mark which was really hard for me to find as well as see ~ it looks like the factory made an indent on both sides of the pully belt rim like a hit with a chisle mark
Be suspect of anything made of 35 year old rubber (tubing ends hose ends vacuum diaphragms brake lines carburetor parts CRACK CITY
I bet your truck will run just fine after all the love it's getting now (lucky truck)
Aloha Bro

[SubGothius]

I also found I had to pretty much hose down the pulley and timing indicator with Brakleen, think I also hit it with a degreaser gel spray and let that soak a while first. Clean as a whistle!

[Mitsuzero]

so i managed to start driving the truck longer distances, i completely filled the tank with fresh gas and threw some MMO in the tank , the truck runs very nice at highway speeds , it seems to jerk or shudder a little if i let off the gas and coast in gear. the idle has improved slightly and i was able to have it idle for 3 minutes , although it wasn't the smoothest of idles. I'm going to see what the timing light says and if it's good to go i'll keep driving it and hoping the fuel additives break down any crap still left over

[1fastcolt]

glad its a lil better.. hopefully it will get alot better.

[Mitsuzero]

New update, I've discovered that the vacuum advance diaphragm on the distributor will not hold a vacuum, how difficult is it to find a new one?

[xboxrox]

Mitsuzero it seems that (now) none of the auto parts stores stock or sell a vacuum advance control for our distributors..! My memory is shot but I think it was O'Reilly's that sold me a vacuum advance for my 1986 truck a few years ago (my install photos below) I was able to replace the vacuum advance control diaphragm part without having to remove the distributor by using an offset Philips screwdriver...

SUMMIT RACING SELLS IT: https://www.summitracing.com/parts/smp-vc268

Here is a link to the 1987 part that fit my 1986 truck and it might fit the distributor on the 2.0L engine as well... There is a Tel No. at the page bottom to contact Standard Parts Company & hopefully they can source the part...
https://www.standardbrand.com/en/ecatalog?partdetail=VC-268

[Mitsuzero]

Thank you so much, I had forgotten about summit! I actually found the same part on Amazon after some searching, I think they'll get it here faster than Summit but I'm going to have to cross my fingers and hope it works. I'll update the thread after I get it

[xboxrox]

Thank you so much, I had forgotten about summit! I actually found the same part on Amazon after some searching, I think they'll get it here faster than Summit but I'm going to have to cross my fingers and hope it works. I'll update the thread after I get it

Outstanding..!

Member Fingers brought up an important point about setting [initial] ignition timing: "Disconnect the vacuum hose from the distributor. BOOM! You're in "timing mode".

Not only remove but PLUG the distributor vacuum line end because a vacuum leak can cause the engine RPM to change and RPM must be set before setting timing... I suggest double check the whole RPM & timing process twice as a sanity check...


Fingers MR50 thread here: http://www.mightyram50.net/vbulletin/showthread.php/9480-My-2-0-86-Mighty-Max-is-running-really-rough?p=84834#post84834

[Mitsuzero]

yeah timing will definitely need to be adjusted

[Mitsuzero]

i've been doing some more research and i think the vacuum advance i have on the way is not going to work, the distributor for the 2.4 is different than the one for the 2.0. i may be forced to buy a whole distributor just to solve the issue with the damaged vacuum advance diaphragm

[xboxrox]

Mitsuzero I am confused & I don't know which year trucks got the 2.4L engine AFAIK the 1987 and 1988 trucks have either 2.0L or 2.6L engines..?

Your truck or your distributor is a 2.4L engine or distributor..?

You should be able to find a vacuum advance for the truck w/o having to buy a whole new distributor...

Try calling STANDARD PRODUCTS For Catalog assistance: (718) 729-4357

[Mitsuzero]

My apologies, I misspoke, the distributor between the 2.6 and 2.0 are different and I have the 2.0 which is why I'm doubting the part I bought will fit. I'm going give them a call and see what they have because the refurbished distributors are getting them somewhere

[xboxrox]

Should fit

Take a good look at your part, does it look simular..?

[Mitsuzero]

well so far I received the part and it doesn't fit unfortunately. I pulled the distributor off of my other 2.0L truck and timed it, it runs but the idling issue isn't solved. I have not found any further vacuum leaks so I'm stumped. the truck seems to experience bucking or surging which is most notable in the first two gears, even with a steady, constant throttle input.