Another Mikuni question

[Cartman2112]

I have an issue with my 'good old' Mikuni carb on my '85 2.0L automatic. After replacing the secondary vacuum diaphragm and a bunch of leaky vacuum hoses I finally got this thing running great except for one PITA problem. After the engine warms up, if I punch it from a dead stop it falls flat on it's face for about two seconds and then takes off like a rocket. Even if I take off from a slow rolling start it will hesitate until I kick it down into first gear and then it takes off. Does anyone have any ideas?

Also, can I remove ANY of the spiderweb of vacuum hoses without killing this thing totally? I'm not in the position to look at a carb swap at this time.

Thanks for any insight that you can offer!!

[xboxrox]

In my experience any of the rubber software parts could be suspect ~ accelerator pump diaphragm, distributor vacuum advance diaphragm, fuel pump diaphragm ~ assuming the ignition spark is strong & fuel filter has been replaced ~ make sure there are no major vacuum leaks especially from the vacuum advance diaphragm ~ tighten any loose fasteners ~ hope the fix is quick & easy
:thumbup:
P.S. does the transmission have any vacuum operated diaphragms..?

[Cartman2112]

Thanks, I'll check all of the hoses again. The transmission linkage should all be mechanical. Could the PCV valve or the charcoal canister be causing issues. I know they haven't been looked at in years.

A little background: this truck sat for about 5 years after my Dad quit driving. I got it running pretty good but am still going through everything part by part to try to get in running as well as I can without having to do any major overhauls. I do know that the number four cylinder is oil-fouling the plug. Probably the rings but I can even try to tackle that yet!!!

[geezer101]

I'm going to put money on the accelerator enrichment pump diaphragm being shot or the jet is clogged. It's the small square assembly on the side of the carb with the lever arm pointing straight down. It gives the primary throat on the carb an extra shot of fuel when you jump on the throttle. Look down the throat of the carb with the choke butterfly held open and twist the throttle linkage a few times (engine is off) - you should see a straight jet of fuel shooting across the throat of the venturi. If it's a trickle or hardly and sign of fuel being pumped out, it's dead or had debris blocking it. You can try taking the pump cover off and unblocking it but there's a risk that the gaskets will be crap. Use a screwdriver that is is good condition and fits the screws firmly as the screws on the Mikuni are prone to stripping heads :shakehead: The accelerator enrichment is not a separate part and should come included in an overhaul kit (can be replaced without removing the carb but I'd be thinking about rebuilding it completely) Also check the fuel filter and eyeball what the fuel is like as the tank can accumulate water condensation and contaminate the fuel if it's been sitting for a while (plus fuel goes funky if it's old...)

[Cartman2112]

Thanks Geezer, I'll check it this weekend!!

[geezer101]

Good luck. Hopefully this is the problem and you'll nail it. Once that pump is working properly it will get up out it's own way at a standing start :thumbup:

[xboxrox]

Thanks, I'll check all of the hoses again. The transmission linkage should all be mechanical. Could the PCV valve or the charcoal canister be causing issues. I know they haven't been looked at in years.

A little background: this truck sat for about 5 years after my Dad quit driving. I got it running pretty good but am still going through everything part by part to try to get in running as well as I can without having to do any major overhauls. I do know that the number four cylinder is oil-fouling the plug. Probably the rings but I can even try to tackle that yet!!!


PVC VALVE ~ should be clean & allow air flow in one direction OUT away from the crankcase (if plugged it can cause the crankshaft front seal to leak) engine will idle smoother with a clean PVC valve ~

BVV ~ carburetor bowl vent valve does have a diaphragm ~ I replaced the BVV in my Mikuni carb ~ not sure if would be the cause of engine stumble but it could cause the truck to smell gasoline - ish and fuel foul stinky pollution ~ it's operated by an electromagnet attached to the carburetor ~ since your truck otherwise runs strong I will guess the carburetor float adjustment is correct & the needle seat does not leak ~

LOOK HERE: http://www.mightyram50.net/vbulletin/album.php?albumid=615 <----- BVV BOWL VENT VALVE INFO

[Cartman2112]

Thanks Geezer and Xbox, I'll try to check everything out tomorrow.

Geezer - one question. Should I try your recommendation with the engine cold or hot?

[geezer101]

Thanks Geezer and Xbox, I'll try to check everything out tomorrow.

Geezer - one question. Should I try your recommendation with the engine cold or hot?


Shouldn't make any difference if the engine is hot or cold. There's a square rubber diaphragm inside the pump assembly with a metal 'piston' fastened through the middle of it. Once the rubber is stretched out of shape, it won't pump fuel through the housing as efficiently. If it's just a clogged jet (this jet is cast into the body of the carb and cannot be removed) you can get a fine piece of wire like one from a wire brush and poke it through the gallery in the pump housing. A blast of carb cleaner won't hurt either.

[Cartman2112]

I haven't been able to check out the carburetor or the vacuum hoses yet but noticed a broken hose when I was checking the fluids yesterday. It's the large one that runs from the air cleaner to the secondary air control valve. Could this have anything to do with the hesitation issue?

[geezer101]

That hose is just another emissions recirculation pipe. No vacuum in it and it feeds into the air cleaner and not the manifold so no affect on carb operation.

[Cartman2112]

The enrichment pump (or jet) seems to be the issue. I saw no gas when I twisted the accelerator linkage. Also found a loose electrical connection to some sensor on the air cleaner and several dry rotted vacuum hoses.

[xboxrox]

Hello Cartman ~ good work finding & fixing the problems ~ that disconnected hose will make a weird rather loud sound while driving so better connect it to the air cleaner ~ you must remove the carburetor to replace the accelerator diaphragm UNLESS your are able to remove & replace all 4 screws that hold it together ~ I did not listen to my mechanic friend & trying to do the job without removing the carb BROKE the black plastic PHENOLIC throttle body center section of the Mikuni ~ it broke in such a way that the accelerator diaphragm could never be installed again ~ when bolting the carb back onto the manifold USE plumbers teflon tape WRAP a little around the threads of any bolt that won't torque tightly ~ there is a measurement & adjustment for the throttle butterflies .065" gap to venturi hole ~ do that adjustment while the carb is removed ~ if the engine ran good otherwise KEEP HANDS OFF and just replace the accelerator pump diaphragm and reinstall the carb ~ use only one gasket under the carb otherwise it might have a vacuum leak ~ I assume the depression diaphragm that opens the 2nd butterfly works ok (?) pull a vacuum on it to see if it opens the secondary throttle
Good Luck

[Cartman2112]

I've got a couple of parts carbs lying around. Hopefully one will have a usable diaphragm...

[xboxrox]

I've got a couple of parts carbs lying around. Hopefully one will have a usable diaphragm...

New is best ~ use a high power LED flash light in a darkened room to inspect the used diaphragm for holes & cracks

[geezer101]

^they don't always split or develop pin holes. Most of the time they have stretched from regular wear and tear. You really notice the difference between a tired carb and one that has just been rebuilt.

[Cartman2112]

Got a crazy question... has anyone ever tried to retrofit a Holley/Weber 5210C to the 2.0L?. I know someone that has a newly rebuilt one. It's another water choke model but it appears that you may be able to connect it to the lines running to the heater core. Any comments?

[geezer101]

It appears the throttle set up is the same as the Mikuni and the fuel bowl faces forward, but the water choke looks like a headache. For the coolant to flow it has to return to the intake manifold and there isn't a provision big enough to adapt the 5210 to it (well, not with the existing coolant barbs) It looks like it could be modified to take an electric choke assembly :shrug: Haven't seen anyone use the Holley 5210 but it's very similar to the Weber...

[Cartman2112]

One of my donor carbs appears to have an accelerator enrichment pump diaphragm that is in good condition. It had recently been rebuilt when I got my hands on it. The previous owner broke the mounting flange when installing it.

Now I have another issue. Vapor lock.

I've never run into this problem until this year. Temperatures have been no hotter than a normal summer and the engine temp is running in the normal range. If I drive it for over about five miles the only way I can get it to start is to open the hood and let it cool off for about 15-20 minutes and hit it with some starting fluid. Once the engine cools off completely (or during a short trip) it fires up right off the bat. Any ideas??

[xboxrox]

Ignition timing might be too far advanced causing the vapor lock

[geezer101]

Open the fuel cap and depressurise the fuel tank. My bros Mitsubishi Scorpion (Challenger/Saporo) suffered from vapour lock as well. Crack the fuel cap and it would run straight away. Major PITA in summer...

[Cartman2112]

Thanks Geezer. I'll try that the next time.
Xbox - I've got to take it to the shop next week so I'll get my mechanic to check it out.

My mechanic is old school. Worked for the local Pontiac dealership for 30 years but has a warm spot in his heart for anything Mopar or Mitsubishi. He can work on just about anything but he told me "that damned Mikuni carb came the closest to kicking my ass as anything I have ever worked on".

[geezer101]

Thanks Geezer. I'll try that the next time.
Xbox - I've got to take it to the shop next week so I'll get my mechanic to check it out.

My mechanic is old school. Worked for the local Pontiac dealership for 30 years but has a warm spot in his heart for anything Mopar or Mitsubishi. He can work on just about anything but he told me "that damned Mikuni carb came the closest to kicking my ass as anything I have ever worked on".


...and I'd believe it. I have no idea why the old Mikuni/Solex all alloy body carb was hated on. They are so much nicer to pull apart and work on than the 32-35DIDTA resin bodied carb. They're full of voodoo and dark curses - they make a 4 barrel look like a kids toy :shakehead:

[Cartman2112]

Quick question, what is the difference between the 32-35DIDTA and the 32-35DIDTF carbs?:scratchheadblue:

[geezer101]

TBH - don't know. There are wax pellet choked carbs, thermo spring choked carbs, electric choked carbs, carbs with single and multiple electric solenoid air circuits and carbs with inbuilt EGR inlets. None of them are fun to work on :shakehead:

[Cartman2112]

I was curious as to whether or not the 'F' carb was a feedback unit and the 'A' was non-feedback.

[geezer101]

That's a good question - "A" for analog and "F" for feedback? :shrug:

[Cartman2112]

What year did they start using the feedback carbs?