Mikuni Choke Plate Stuck open?

[AdamW]

Hey folks,

I haven't posted here too much yet, but have found some great advice so far and really appreciate it! Here's a question:

The Mikuni carb on my '88 2.0 Ram-50 has caused me nothing but trouble. The truck doesn't have much power until A) it's warmed up a lot, or B) you really gun it on the highway. It was worse before I discovered the secondary wasn't opening AT ALL; rerouting the secondary's line to a port with more vacuum seemed to help a good deal (there are lots of capped-off vacuum ports on this carb). I replaced the fuel filter and catalytic converter too--the converter especially was a big help. I also replaced the bowl vent diaphragm some months ago (which was visibly damaged), but that didn't do much good. I get about 23 mpg hwy, so figure everything's somewhat OK.

I don't know too much about the parts of a carburetor, but here goes: the primary and secondary each have a butterfly valve way down in the bowl, and the primary also has a choke plate way up at the top. The other day, I noticed that this choke plate is wide open all the time, even at idle. It seems to me that it should be almost closed at idle, and then open more and more as the truck accelerates, right? This would explain why it sounds fine at high speeds, but idles roughly (sort of a "wheeze") at stoplights and such.

Does this sound like something that's easily remedied? I know the throttle linkage attaches to and locks/unlocks this choke plate, since I was doing so with my hand (and then the plate would move freely--though still not close on its own). Maybe there's a problem with this linkage?

Thanks, Adam

[pennyman1]

the choke plate will only be closed when the motor is cold, then open as the motor warms up, then stay open. the wheeze sounds like a vacuum leak or loose hose to the air cleaner. tighten down the bolts on the intake and the carb to eliminate possible vacuum leaks.

[AdamW]

Thanks! I'll check & tighten the bolts later on today.
Do you know why the choke plate might be open when it's cold?

[pennyman1]

Choke is not setting up - you must push the pedal to the floor once to set it - it should go almost fully closed. Once it starts, tap the pedal lightly - it should open slightly and idle should drop to 1400 to 1600 rpm. The speed will drop down to curb idle once engine is warm. If it does not set the choke, spray down the linkage for the choke with carb cleaner - it will clean off the gunk on the linkage, and lube the moving parts. If it still doesn't set, let us know and we will give you further guidance.

[AdamW]

Thanks for those tips, Pennyman1! I tightened bolts and set up the choke as you suggested, and it idles MUCH better now! The truck doesn't like running cold, and this really helps with that. Around-town driving is much more fun.

The only issue left now are the "dead spots" when I'm on the highway and trying to get up to speed. I'll step on the gas, and the truck will bog down momentarily--easing up on the gas and/or making sure the secondary is open helps, but it's hard to find that sweet spot. My guess is a vacuum leak somewhere, or not enough/inconsistent vacuum to both primary and secondary.

[AdamW]

Thanks, Pennyman1! Setting up the choke as you suggested helped tremendously--the truck idles much better now. It never liked to run cold, and I think this was the reason.

The only trouble now are the "dead spots" that occur when trying to accelerate on the highway. The truck will bog down and I'll have to REALLY step on it to get any response--secondary kicking in helps, of course. I know vacuum to my secondary is questionable, so that may well be the problem. Of course, the vacuum lines on these carbs are a mess out of the factory, so there could be pinhole leaks that I just can't find.

[pennyman1]

check all the vacuum lines especially the ones going to the metal box on the drivers inner fender - that contains a sensor for the computer controlled carb. Also check the diaphram for the carb secondary, they are known to fail and cause the flat spot you are describing.