What is this supposed to do / Partial throttle misfire diag.

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07-29-2018, 02:04 PM
Yosemite_Sam

2 Attachment(s)

What is this supposed to do / Partial throttle misfire diag.

So I have this partial throttle misfire I've been chasing on my '88 2.0 2wd. At first I thought it was spark, so I just threw a bunch of new parts at the spark system because it was all original and I figured it would all be preventative maintenance (plugs, wires, cap, rotor, coil). When that didn't work I decided to do all the vaccuum lines (which also didn't work). With the air cleaner off I found this thing in between the carb and the valve cover. It looks like a check valve to me, but I can't blow through it either way and when I tested it with vaccuum it would bleed off in both directions slowly. If anybody has some info on how this little check valve thing is supposed to work or any ideas on what else I should look at I'd really appreciate it. Pretty soon I'm going to throw a rebuilt carb at it if I can't find anything else.

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Attachment 20931
07-29-2018, 02:58 PM
geezer101

I think it's a vacuum restrictor. From what I can tell it's connected to the choke release assist/brake on the electric choked Mikunis. The wax pellet chokes have internal vacuum galleries. Meh, I could be wrong. Been messing with these things for years and I still haven't unlocked all of their secrets :shrug:
07-29-2018, 06:10 PM
StarquestMan

I wanna say early 87's used a vacuum check valve and then all later ones used a restrictor/orifice thingy. They were color coded to tell what they were and what way they go on but I don't remember the colors.
07-29-2018, 10:32 PM
Yosemite_Sam

Ok, I thought it might be a vacuum restrictor. I jist wanted to make sure. I suppose I'll have to go for the rebuilt carb. At this point I don't know what else the misfire could be.
07-29-2018, 11:08 PM
FMS88

That’s a vacuum delay valve to the 1st stage port for the choke pull off. From your description it’s operating correctly. You could try using a hose without the valve. If that has no effect with a hot engine, then the delay valve isn’t the issue.

Have you checked the vacuum advance, distributor advance plate, TPS and the high altitude compensator to see they operate smoothly and within specs?
07-30-2018, 12:03 AM
Yosemite_Sam

Quote:

Originally Posted by FMS88

Have you checked the vacuum advance, distributor advance plate, TPS and the high altitude compensator to see they operate smoothly and within specs?

Nope, I had thought about checking the vacuum advance and the distributor, had no idea that this thing had a TPS or a high altitude compensator though. I'll take a look at all that when I have some time.
07-30-2018, 12:19 AM
StarquestMan

Quote:

Originally Posted by FMS88

That’s a vacuum delay valve to the 1st stage port for the choke pull off. From your description it’s operating correctly. You could try using a hose without the valve. If that has no effect with a hot engine, then the delay valve isn’t the issue.

Have you checked the vacuum advance, distributor advance plate, TPS and the high altitude compensator to see they operate smoothly and within specs?

I thought i would mention that the TPS seem to be a common failure (at least in my case) with these carbs of the 3 carburetors I have all of them tested bad and I had to order a new one.
07-30-2018, 12:22 AM
StarquestMan

It seemed to help the throttle response, calibrating the new one is a bit tricky as I believe you need an ECI tester to do it properly (unless im remembering wrong?)
07-30-2018, 12:31 AM
Yosemite_Sam

What's an ECI tester?
07-30-2018, 11:48 AM
FMS88

Quote:

Originally Posted by Yosemite_Sam

What's an ECI tester?

The ECI Tester is a glorified voltmeter that makes it easy to test the carburetor and computer functions under various conditions. Look at the engine, chassis, body manual for the 1989 Raider in the site's manuals section. The Fuel System / FBC System chapter explains how to use the ECI and the expected test voltages.

The chapter also explains how to test and adjust the TPS as well as other FBC components using a good digital ohm/voltmeter. The ECI Tester won't tell you much more than you can determine with the voltmeter.
07-30-2018, 11:57 AM
FMS88

Quote:

Originally Posted by FMS88

The ECI Tester is a glorified voltmeter that makes it easy to test the carburetor and computer functions under various conditions. Look at the engine, chassis, body manual for the 1989 Raider in the site's manuals section. The Fuel System / FBC System chapter explains how to use the ECI and the expected test voltages.

The chapter also explains how to test and adjust the TPS as well as other FBC components using a good digital ohm/voltmeter. The ECI Tester won't tell you much more than you can determine with the voltmeter.

One other thing, the specs and procedures for the Raider's FBC System are the same for truck. This includes both the 2.6 and 2.0 engines.
07-30-2018, 02:51 PM
Yosemite_Sam

Ah, ok. Thanks for the explanation. I had a feeling it would be some kind of DMM. I'll take a look at that manual. I would have thought that a truck of this erabwould just have a throttle switch if anything, but I guess if it's a potentiometer it could cause what I'm feeling.
07-30-2018, 05:03 PM
StarquestMan

Part number is MD612803 if you need one. I don't think any aftermarket ones are available unless I'm wrong. I couldn't find any at any parts store when I looked.
07-30-2018, 07:29 PM
StarquestMan

Part number is MD612803 if you need one. I don't think any aftermarket ones are available unless I'm wrong. I couldn't find any at any parts store when I looked.
07-30-2018, 07:31 PM
StarquestMan

Quote:

Originally Posted by StarquestMan

Part number is MD612803 if you need one. I don't think any aftermarket ones are available unless I'm wrong. I couldn't find any at any parts store when I looked.

oops lol
08-02-2018, 12:08 AM
Yosemite_Sam

I did some looking. I checked out the wiring diagram and the TPS is a potentiometer, so I'll be checking that to make sure it is operating properly. Is the TPS a common enough problem to bother with a DIY on testing it?
08-02-2018, 12:10 AM
Yosemite_Sam

A DIY on how to do it without any factory level tools that is.
08-02-2018, 07:33 AM
FMS88

Quote:

Originally Posted by Yosemite_Sam

A DIY on how to do it without any factory level tools that is.

All you need is an ohm meter to check the TPS and you can do it with the carb on the engine. The manual in the FBC System section describes what to do and the resistance measurements you should get. Besides minimum values you want the ohms to increase gradually and evenly as the throttle is opened and closed through its entire range. Dead spots or erratic changes in measurements are signs of a bad TPS.
08-02-2018, 10:32 AM
Yosemite_Sam

Quote:

Originally Posted by FMS88

All you need is an ohm meter to check the TPS and you can do it with the carb on the engine. The manual in the FBC System section describes what to do and the resistance measurements you should get. Besides minimum values you want the ohms to increase gradually and evenly as the throttle is opened and closed through its entire range. Dead spots or erratic changes in measurements are signs of a bad TPS.

Yeah, I know. I was just wondering if it's a common enough problem that it would be worth assembling all the info in one place for people without the resources or knowledge to figure out how to test it. If it is, I'll take some pics when I do it and throw one together.
08-02-2018, 07:15 PM
pennyman1

It is worth the effort, as others will need the info at some point. It can be made into a sticky so it is easy to find.