Replacing a Mikuni Carburetor with a Weber Carburetor

[BJH324JH]

So, I'm getting ready to mount the fuel pump to the truck frame, I'm just wondering before I do any of that, are the fuel filter hoses the ones you connect to the electric fuel pump?

20190922_160559.jpg

[85Ram50]

What I did, I think there is a picture of it at that link to post #37, is connect the outflow from the filter to the intake of the pump. Then the out from the pump to the hard line the outflow from the filter was connected to. The connection to the hard line is hidden in the pics I took. I also put in a new filter.

[geezer101]

Mount the pump as close as practical to the tank with the fuel filter installed between the tank and the pump. That way it screens the debris out of the fuel before it passes through the pump. Make sure the pump is shielded from getting hit by rocks and dirt etc.

[BJH324JH]

What I did, I think there is a picture of it at that link to post #37, is connect the outflow from the filter to the intake of the pump. Then the out from the pump to the hard line the outflow from the filter was connected to. The connection to the hard line is hidden in the pics I took. I also put in a new filter.

The filter has two fuel lines, which one is which?

[geezer101]

Check the filter - it should have fuel in/out marked on it. You will need to install the pump on the fuel out hose on the filter.

[85Ram50]

You can always trace the lines from under the rig to see where they go if there is no in/out marking on the filter. I cannot find a 2nd gen manual in the manuals section and I was lucky the filter for my rig is in on the bottom out at the top.

Edit- DUH, The line from the tank which is a very short distance will be the in line on the fuel filter.

[BJH324JH]

Check the filter - it should have fuel in/out marked on it. You will need to install the pump on the fuel out hose on the filter.

It unfortunately doesn't have markings. Checked all around the fuel filter and couldn't find the markings. Is it safe just by looking at it that the top hose on the fuel filter is the fuel coming in and the lower one is the outlet, where the fuel comes out after being filtered by the paper?

[geezer101]

You'll need to physically trace the fuel lines if there aren't any flow indicators on the filter. Guessing could be disastrous. The fuel pick up from the tank should be the easiest to identify.

[85Ram50]

It unfortunately doesn't have markings. Checked all around the fuel filter and couldn't find the markings. Is it safe just by looking at it that the top hose on the fuel filter is the fuel coming in and the lower one is the outlet, where the fuel comes out after being filtered by the paper?

The tank is right there, just check which line goes to the tank. That is the line into the fuel filter the other one will be going to the hard line that runs along the frame to the front. You disconnect that one from the hard line and attach it to the in line on the new pump. Connect the out line from the new pump to the hard line you just disconnected the fuel filter from.

[BJH324JH]

This is so perplexing! The airtex video demonstrated an oil pressure sensor with three terminals on it. The first terminal is power, that one is easy, it hooks up to the electric fuel pump.
The second terminal is for the starter solenoid that is powered up when the engine is cranked. Lastly, the third terminal is for ignition power. In guessing the last one is for a keyed ignition wire. What terminal on the starter do I wire the oil pressure sensor to? There is "S","B","M" terminals on the solenoid according to the manual. Here is a picture from the manual:
20190921_183729.jpg

The last terminal on the oil pressure sensor switch is for the ignition. I'm guessing a keyed terminal?

[85Ram50]

This is so perplexing! The airtex video demonstrated an oil pressure sensor with three terminals on it. The first terminal is power, that one is easy, it hooks up to the electric fuel pump.
The second terminal is for the starter solenoid that is powered up when the engine is cranked. Lastly, the third terminal is for ignition power. In guessing the last one is for a keyed ignition wire. What terminal on the starter do I wire the oil pressure sensor to? There is "S","B","M" terminals on the solenoid according to the manual. Here is a picture from the manual:
20190921_183729.jpg

The last terminal on the oil pressure sensor switch is for the ignition. I'm guessing a keyed terminal?

You have it. That link I posted explains what wires to connect to. You have to T splice the starter wire or I guess connect to the same post on the starter, and the other wire which is hot only when the key is on and supplies the power while running should be hanging loose since you removed the Mikuni. It powered the Mikuni shut off solenoid I think.

[geezer101]

^correct. The loose wire powered the fuel cut solenoid.

[SubGothius]

Connect the S(tart) terminal on the oil pressure safety switch to the S terminal on the solenoid.

Auto parts, hardware, or electrical supply stores should have a spade terminal adapter that has 1 female and 2 male spade connectors on it. The female end goes on the S terminal, then the male ends take your stock starter signal wire and the new wire to the safety switch.

(EDIT: These are apparently called "double male" terminals/adapters and come in "chair", "flat tab" and "piggy back" types -- Dorman part# 85412 comes with 2 of each for about $3 at Pep Boys or $2 at O'Reilly. Or, you could find a piggyback crimp-on terminal for your safety-switch wire, which has a female spade connector with an extra male spade forking off of it for your starter signal wire.)

The way these safety switches typically work, there's an internal switch that flips whether the (P)ump terminal is connected to the (S)tart terminal or the (I)gnition terminal. At rest, P is connected to S, then oil pressure flips that to connect P to I instead. This can be handy to know in case the switch ever fails, as you can probably just swap the I wire over to the S terminal to get going again until you can replace the switch.

Ideally, you'll also want that P(pump) wire triggering a relay that sends power directly from the battery (+) terminal to the pump, rather than having power from the ignition switch go thru the safety switch directly to the pump.

[BJH324JH]

You'll need to physically trace the fuel lines if there aren't any flow indicators on the filter. Guessing could be disastrous. The fuel pick up from the tank should be the easiest to identify.

The tank is right there, just check which line goes to the tank. That is the line into the fuel filter the other one will be going to the hard line that runs along the frame to the front. You disconnect that one from the hard line and attach it to the in line on the new pump. Connect the out line from the new pump to the hard line you just disconnected the fuel filter from.

You guys are right. Guessing is not an option especially when the freaking tank is just right there in the open for me to see, like 85ram50 mentioned. I'll hunt the lines down.

[BJH324JH]

Connect the S(tart) terminal on the oil pressure safety switch to the S terminal on the solenoid.

Auto parts, hardware, or electrical supply stores should have a spade terminal adapter that has 1 female and 2 male spade connectors on it. The female end goes on the S terminal, then the male ends take your stock starter signal wire and the new wire to the safety switch.

The way these safety switches typically work, there's an internal switch that flips whether the (P)ump terminal is connected to the (S)tart terminal or the (I)gnition terminal. At rest, P is connected to S, then oil pressure flips that to connect P to I instead. This can be handy to know in case the switch ever fails, as you can probably just swap the I wire over to the S terminal to get going again until you can replace the switch.

Ideally, you'll also want that P(pump) wire triggering a relay that sends power directly from the battery (+) terminal to the pump, rather than having power from the ignition switch go thru the safety switch directly to the pump.

Thank you so much for clearing this up for me and educating me on how oil pressure sensors work. Also, thank for the tip about jumping the cables to get things working again. That may come in handy in a crappy situation.

I don't have the time to go to the auto parts store and get the spade terminal adapter you are talking about, so I just made one. Here is a picture of the one I made:
20190924_104043.jpg

As for having this setup ideally, what extra electronics would I have to buy. You mentioned a relay, is there something specific you can point me too. I don't even know what the heck a relay is and how I would wire it to the truck.

So far, the "P" (Power) terminal is hooked up and "S" (Starter Solenoid) are hooked up. I just need the "I" (Ignition) terminal hooked up. Everyone says to hook it up to the existing fuel cut off solenoid that's hanging around in the truck. Although there is other connections hanging around as well. Here is a picture of all the connections hanging around. Which one do I tap into?
20190924_131533.jpg

[85Ram50]

Do you remember which one you disconnected from the Mikuni cut off solenoid? Just directly connect to the end of it.
if not on my first gen trace the yellow with red stripe wire that connects to the temp sensor back to the harness. there are two black wires coming out at the same spot. One goes to the starter the other one goes to the Mikuni and now to my oil pressure cut off switch.

[geezer101]

Here's a ready made kit - https://www.ebay.com/itm/40A-12V-Wir...oAAOSwCU1Y1D57

Take the chintzy switch out of the equation and you have a fused 40 amp rated relay with most of the wiring to hook it up

[85Ram50]

BJH324JH- Once you figure out how to wire that relay up let me know I'd put one in if I knew how to do it right

[BJH324JH]

BJH324JH- Once you figure out how to wire that relay up let me know I'd put one in if I knew how to do it right

I am a complete beginner at this and can barely do the oil switch method. I might try geezers method, but I already started on the oil pressure switch method and want to finish it. Maybe in the future I'll give geezers relay method a try, but I'm already close to finishing. Anyways what's the advantage of using the ready made kit over using the chintzy oil pressure switch?

Now as for the last wire mentioned, I can't for the life of me figure out where the fuel cut off solenoid wire is to hook up! May anyone with a 2nd generation MM help me get this damn thing wired up already. Just out of curiosity, does the oil pressure sensor switch use "Accessory" power, or "On" power? Also, does the weber choke use "Accessory" power, or "On" power? Sorry guys for being such a freaking idiot, but I really do like doing this and wish to learn.

[85Ram50]

Now I feel terrible for having to tell you that apparently the best install of the electric pump includes the oil cut off and a relay. As I understand it the oil cut off won't shut down the pump if the motor did not stop running in a rollover crash making things quite dicey if you are trapped. If I understand correctly the relay takes care of that factor. IDK how.

If you do not remember which wire you took off the Mikuni ??? How many loose wires do you have? What connectors are on the end? What sort of connector does the Mikuni have on it that might receive one of those wire connectors? Maybe Pennyman1 or Geezer101 will stop by.

[BJH324JH]

Now I feel terrible for having to tell you that apparently the best install of the electric pump includes the oil cut off and a relay. As I understand it the oil cut off won't shut down the pump if the motor did not stop running in a rollover crash making things quite dicey if you are trapped. If I understand correctly the relay takes care of that factor. IDK how.

If you do not remember which wire you took off the Mikuni ??? How many loose wires do you have? What connectors are on the end? What sort of connector does the Mikuni have on it that might receive one of those wire connectors? Maybe Pennyman1 or Geezer101 will stop by.

If you take a look at post #90, I took a picture of all the connections. I've just been at this on and off and forgot where things went.

[85Ram50]

I see two wires. The temp sensor wire on mine is in front of the intake manifold before the distributor.
Anyway, if you just have those two wires with different connectors on them go look at the Mikuni you removed for where one of them might fit.

[SubGothius]

For power to the I terminal, you can see if any of those loose connectors have ~12V power when the key is on and not when it's off. If your Mikuni setup is like my '87, the fuel cutoff solenoid terminals are bundled into that multi-terminal connector block. You may wish to just cannibalize the old carb side of that connector, cut the wire pair at the cutoff solenoid, and splice onto that; one of those wires should get 12V switched current, and the other is a ground. You can tell which gizmo is the solenoid when you plug it in and turn the key, you should hear it click.

A relay is a switch in one circuit that's operated by electricity in another circuit. You want a NO SPST type (normally-open single-pole single-throw), which has 4 or 5 terminals: two operate the internal switch, which dis/connects power across the other two, or some NO SPST relays have an extra switched output terminal (called a "dual make" relay). When power flows across the control terminals, that energizes an internal electromagnet that closes a switch across the switched terminals.

Why use a relay? The electromagnet draws tiny current, whereas the switch can handle high current, so you can switch a high-current circuit with another one that's low-current, dodgy/unknown, or otherwise worth protecting. The circuit across the ignition key switch to the I and then P terminals may not be strong enough to drive the pump, or you may just want to protect your ignition switch and pressure safety switch by reducing the current demand across them, so we use that circuit to operate the relay, which in turn switches current directly from the battery (+) terminal to the pump.

This page explains and illustrates more about relays:
https://www.the12volt.com/relays/spd...ive-relays.asp

Here's the basic wiring you'll use:
https://www.the12volt.com/relays/relaydiagram47.html

...where terminal 86 will take the 12V wire from your old cutoff solenoid , 85 will take the old solenoid (or any) ground, 30 will take a fused line from the battery (+) terminal, and 87 will go to the pump. If you get a dual-make relay with an 87b terminal, that allows for another trick you can do in a pinch: swap the wire from the 30 to the 87b terminal for constant power to the pump -- unswitched, so be sure to undo that swap once you reach your destination!

[geezer101]

I am a complete beginner at this and can barely do the oil switch method. I might try geezers method, but I already started on the oil pressure switch method and want to finish it. Maybe in the future I'll give geezers relay method a try, but I'm already close to finishing. Anyways what's the advantage of using the ready made kit over using the chintzy oil pressure switch?

Now as for the last wire mentioned, I can't for the life of me figure out where the fuel cut off solenoid wire is to hook up! May anyone with a 2nd generation MM help me get this damn thing wired up already. Just out of curiosity, does the oil pressure sensor switch use "Accessory" power, or "On" power? Also, does the weber choke use "Accessory" power, or "On" power? Sorry guys for being such a freaking idiot, but I really do like doing this and wish to learn.

Sorry, a bit of a misunderstanding there . I was referring to the on/off switch in that relay kit link I posted up. Do not discount yourself on effort. Auto electrics are tricky to master. Once you have hooked up a relay or 2 into a system, you get to understand how they work.

I have to point this out though. The carb is going to be full of fuel and killing the fuel pump will not be enough to shut the engine off in a worse case scenario like a roll over. You'd need to be able to shut down the ignition as well as the fuel pump. The oil pressure switch would need to be introduced into the engine system before power to the coil and fuel pump to ensure a full engine shut down.

[SubGothius]

killing the fuel pump will not be enough to shut the engine off in a worse case scenario like a roll over.

The safety switch simply prevents hosing down the scene of a crash with raw fuel in case the crash severs a fuel line. Killing the engine is besides the point, and it hardly matters for safety if the engine somehow keeps running after a rollover. Not sure a carb full of fuel can even function upside-down, for that matter.