Howdy folks! What could cause an excessively high idle....and by "excessively high," I mean around 2k rpm?

Here's the deal: I was cruising back home on Route 2. I upshifted from 3rd to 4th, and the revs went sky-high (well, to around 3500) and wouldn't come down! I pulled over and blipped the gas pedal...nothing. I popped the hood and jiggled the throttle pulley (???) on the back of the carb. Nothing, still revving like crazy. So, I shut off the motor, which continued to diesel on for a good 10 or 15 seconds.

I checked the throttle pulley, and it wasn't binding or anything--in fact, it returns to its stop with no problem. I restarted the motor, and the revs shot to 2000 and stayed there. I got home without any problems, but that dieseling kinda scares me and I want to fix the problem before something vital goes "boom!"


What should I look at first? Keep in mind that prior to the incident I described above, the truck was running great.


Oh, the specs: '87 Raider, 2.6l 5 spd, stock carb.


Thanks!

remove the 2 water hoses going to the carb choke and blow tru them with compressed air hook them back and try again make sure the rad is full

Will give that a try...thanks!

I would adjust the idle screw on the carb first off. I would put money on you probably being able to fix it just by adjusting the idle.

Thanks for the idea. I tried that after inspecting the throttle linkage for binding and was able to knock about 300 rpm off the idle, but it's still not right. Also, the truck was idling fine prior to the incident on the road.

I'll take all ideas--I don't want to "fix" the problem by throwing parts at it!


Edit: How much adjustment room does that screw (SAS1?) offer? Do I run the risk of backing it out of its hole completely?

I say the idle screw because I had something similar happen on my silver truck that has the stock carb on it. It started idling high all of the sudden, and would rev up outrageously high till you had to turn off the truck. Adjust the idle screw after a couple of test drives worked for the fix. Not sure about the screw coming all the way out (I guess it could) but there is a good amount of adjustment that can be done.

Okay, had the day off today so I grabbed a spare junkyard carb and gave it a closer look. It seems that my current carb isn't releasing the high-idle cam, or at least I *think* that's what it's called. It's a small, claw-shaped bit on the back (firewall side) of the carb, directly behind the throttle linkage. The choke butterfly in the carb isn't budging from its near-vertical position.

The idle screw had no effect. Looking at the junker carb, there's a bunch of travel on that screw, but not that much effective adjustment distance. I've managed to get the idle down to around 1500 - 1600, but the truck still diesels for a good 45 seconds or so before it finally dies.

Can this situation be fixed without removing the carb? The junker carb is just that--it has so much guck on it, it looks like it's been dipped into a tar pit.

What should I try next?

Unless you have the carb manual, don't mess with the idle screws on a Mikuni.

The problem sounds most like a vacuum leak somewhere under the throttle plate, like a cracked or loose vac hose or brake booster hose. You get a lean out with that, too, which heats up the carbon deposits in the combustion chamber to DAMN HOT, like hot enough to act as a glow plug and keep the engine dieseling. Look for leakies with a piece of hose in your ear. You'll hear the whistle at the leak.

IIRC, the fast idle thing has a vac control, too, in the dash pot on the back dside of the carb.

Thank you! So my next order of business is to fire up the Raider, grab some vacuum hose, jam one end in my ear and start poking around with the other?

I'm pretty carb-dumb: my last carb'd vehicle was a 1991 Jeep Grand Wagoneer, and one performed delicate adjustments on it with a 6-lb. sledgehammer. Something tells me the Mikuni wouldn't take kindly to that treatment.

When you say "under the throttle plate," do you mean *inside* the body of the carb, or somewhere around the base? Again, I have the junker that I'm using for a model, but it doesn't have any vacuum lines on it. I suppose I could track where the ports are and go from there, right?

Thanks for the help!

Y'know, I was just thinking some more about this...

I tried the "entire can of carb cleaner down the carb's throat" approach today, and noticed that when the RPMs bounced back, they reached a certain point and then began creeping higher gradually. That sounds a lot like a vacuum leak, right? The "slowly creeping higher" bit, I mean. A stuck float wouldn't act like that, would it?