Replacing a Mikuni Carburetor with a Weber Carburetor

[BJH324JH]

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!

I think I may have figured it out where the fuel cut solenoid wire is. After looking at the manual for a bit and looking up what each solenoid does, I came to figure out that "Slow cut solenoid valve" is the "fuel cut solenoid valve". Just different names. Here is a picture of the fuel cut solenoid valve on the mikuni carburetor:

20190926_112601.jpg

It has red and blue wires going into a connection block, just like SubGothius mentione:

"If your Mikuni setup is like my '87, the fuel cutoff solenoid terminals are bundled into that multi-terminal connector block."

Now, there is two wires coming out the fuel cut solenoid. One is blue and the other is red. I believe one is ground and the other is power? Do I use both wires of the cut off solenoid , or just one?

[BJH324JH]

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.

Ah okay, I understand.

[BJH324JH]

Just for confirmation, the fuel return line gets blocked off?

[geezer101]

...Now, there is two wires coming out the fuel cut solenoid. One is blue and the other is red. I believe one is ground and the other is power? Do I use both wires of the cut off solenoid , or just one?

You'd only need the power wire but now you are going down the oil pressure switch path you probably won't need to isolate the power in that connection. What would happen is the power that normally would go to that solenoid would be used to trigger the relay in an ignition on/off circuit.

[BJH324JH]

You'd only need the power wire but now you are going down the oil pressure switch path you probably won't need to isolate the power in that connection. What would happen is the power that normally would go to that solenoid would be used to trigger the relay in an ignition on/off circuit.

I took out the multimeter and tested the connection where the fuel cut solenoid hooked up to and it seems to have only about 10volts. Do I need to find another keyed ignition source? Here is a picture I took of the multimeter reading:
20190926_144936.jpg

[SubGothius]

That ~11V reading should be sufficient to trigger the relay. A fully charged battery should push about 14V, so that reading may indicate your battery is nearly depleted and/or needs replacement. To confirm, see if you also get ~11V across the battery terminals directly.

BTW, I notice you also already have the new carb's electric choke wired up, so if that's already hooked to an ignition-switched +12V source, you could just extend that from the choke to I terminal, instead of using the cutoff solenoid circuit at all.

Hm, now that I think of it, prolly best to have that choke lead go to the safety switch I terminal first, then wire the P terminal to the relay trigger terminal 86, then have the relay switched output terminals 87 and 87b go to the pump and choke respectively. That way, the choke is only powered when the engine is actually running, so you won't open the choke when you've got the key on but engine off, listening to the radio or whatever.

Just for confirmation, the fuel return line gets blocked off?

Yes, so I gather from other threads here. However, if you ever get vapor lock in the summer, you might consider using that to keep fuel constantly circulating through the engine bay and back to the tank, so it never sits still enough to vaporize anywhere.

[BJH324JH]

That ~11V reading should be sufficient to trigger the relay. A fully charged battery should push about 14V, so that reading may indicate your battery is nearly depleted and/or needs replacement. To confirm, see if you also get ~11V across the battery terminals directly.

BTW, I notice you also already wired a lead to the new carb's electric choke, so you could just extend that from the choke to I terminal, instead of using the cutoff solenoid circuit.

Hm, now that I think of it, prolly best to have that choke lead go to the safety switch I terminal first, then wire the P terminal to the relay trigger terminal 86, then have the relay switched output terminals 87 and 87b go to the pump and choke respectively. That way, the choke is only powered when the engine is actually running, so you won't open the choke when you've got the key on but engine off, listening to the radio or whatever.



Yes, so I gather from other threads here. However, if you ever get vapor lock in the summer, you might consider using that to keep fuel constantly circulating through the engine bay and back to the tank, so it never sits still enough to vaporize anywhere.

I just connected one end to the choke, the other end is hanging around not connected to anything. I tested the voltage on the battery directly and it gets 12volts. I did have to recondition it with a battery charger though. The battery might need replacing. As for the relay, is there one you recommend, or is the one geezer linked the one to use?

[85Ram50]

Just for confirmation, the fuel return line gets blocked off?

I blocked mine off in the engine bay where it connected to the hard line. If it were empty when I did this I would block it off at the tank.
SubGothius seems to know a lot more than I do about wiring. I figured the electric fuel pump wiring out for the first time from the leads Geezer and Pennyman gave me and a search on YT. Some of the info already on this site also helped, some was way over my head. So that post where I describe what I did it is the first time I ever did that.

[SubGothius]

D'oh! I just realized the safety switch already effectively IS a relay, just controlled by oil pressure rather than another circuit. So you can just run a wire from the battery (+) terminal directly to the switch I terminal, and wire the switch P terminal to the choke and pump. You don't even need switched ignition power, because the safety switch would only send power to the pump and choke when starting or when the engine is already running, and cuts power when the engine stops running.

[BJH324JH]

D'oh! I just realized the safety switch already effectively IS a relay, just controlled by oil pressure rather than another circuit. So you can just run a wire from the battery (+) terminal directly to the switch I terminal, and wire the switch P terminal to the choke and pump. You don't even need switched ignition power, because the safety switch would only send power to the pump and choke when starting or when the engine is already running, and cuts power when the engine stops running.

You are awesome, thank you so much for your help. This sounds like something my peanut brain can handle doing. I hope everything goes okay. My friend will have the fire extinguisher ready, just in case.

[geezer101]

The internet can be your friend - use it well. There will be plenty of schematics and videos to use as a guide (you will know what is good or bad as soon as the instructions include a fuse and relay and advise using an ignition on only power source) If some turkey is running accessories straight off the coil or the battery, you have found a "how not to" video...

[SubGothius]

I worked up some relay diagrams. Here's how it would work with the safety switch you've already got:



However, when I tackle this same project myself soon, I plan to use a different safety switch that incorporates a terminal for the low-pressure light (instead of a (S)tarter terminal), so I won't need a T-adapter for the stock oil light switch; this setup would use a different type of relay wired like so:



Edited to add:

I have found a 10 amp fuse is perfectly adequate for this. Also, I'd always put the fuse between the battery (+) and relay (not after the relay as in the second diagram above). The "N/C to starter" terminals would of course lead to the same terminal on the starter as the starter-trigger wire (the smaller wire connected to the starter).

I've also found cheap relays can fail quickly or sometimes overheat, which can cut power to the pump by surprise at inconvenient times, so I'm upgrading to this top-quality TE Connectivity 7-1393304-1 SPST relay. The SPDT equivalent would be their 6-1393302-0.

[BJH324JH]

I worked up some relay diagrams. Here's how it would work with the safety switch you've already got:

SPDT switch SPST relay.png

However, when I tackle this same project myself soon, I plan to use a different safety switch that incorporates a terminal for the low-pressure light (instead of a (S)tarter terminal), so I won't need a T-adapter for the stock oil light switch; this setup would use a different type of relay wired like so:

DPST switch SPDT relay.png

You are pretty good at this electrical stuff. This stuff boggles my mind. I was wondering if there was a way to eliminate the t fitting because it looks ugly. Glad there is a way. Thank you for the help, I'm just waiting for my power brake booster hose, so that I can start her up. I'll keep you guys updated.

[SubGothius]

Here's an eBay search you can use to find a suitable oil pressure switch with warning light terminal:
oil (switch,pressure,sender) (86281,331347,14057554,4186352,swg1708,"swg 1708")

Apparently the block threading is a 1/8"-28 BSPT (British Standard Pipe Tapered, aka ISO 7-1), rather than the more typical 1/8"-27 NPT taper, so you'll need an 1/8-27 NPT Female to 1/8-28 BSPT Male adapter like this:
https://www.ebay.com/itm/113237920183

BTW, I've noticed Standard and Wells seem to use similar part numbers starting with PS (for Pressure Switch presumably), but they don't match, so you can't assume, say, a PS122 is the same part from one mfr. to another. The OEM part nos. referenced in the eBay search above should be more reliable as a cross-reference.

[BJH324JH]

I cranked her up and nothing. I couldn't get the truck to start. In fact the pump wasn't running at all. I checked the electrical pump by hooking it up directly to the battery and I was able to get it running. Could my engine not be producing enough oil pressure and thus not causing the fuel pump to get electrical power? Truck ran fine before with old carburetor. Didn't show the oil pressure switch light on the dash with the old carburetor

[BJH324JH]

I checked the t fitting and it's as dry as can be. There is no oil going to the oil pressure sensor. Does that mean the oil pump is screwed up?

[SubGothius]

Hm, if it's been sitting a long time, the oil galleries may have drained fully and could take some time to fill up again. Try pulling the main coil wire to the distributor (so it won't fire up) and crank it over for a few bursts of about 5-10 sec. at a time each. Then see if you've got any oil in the Tee.

If that tee has any fork pointing upward, above where it screws into the block, that could be trapping an air pocket there. That's why it's best to have a Tee oriented with two forks horizontal and the other pointing down, or at least all forks level and horizontal, rather than having any fork pointing upward.

[BJH324JH]

Hm, if it's been sitting a long time, the oil galleries may have drained fully and could take some time to fill up again. Try pulling the main coil wire to the distributor (so it won't fire up) and crank it over for a few bursts of about 5-10 sec. at a time each. Then see if you've got any oil in the Tee.

If that tee has any fork pointing upward, above where it screws into the block, that could be trapping an air pocket there. That's why it's best to have a Tee oriented with two forks horizontal and the other pointing down, or at least all forks level and horizontal, rather than having any fork pointing upward.

Yes, that makes sense, I'll try that. It's been sitting for months.

[BJH324JH]

Cranked the truck multiple times, no luck. I hope it's the oil pump that needs replacing.

Great news, the truck started! To get the oil circulating everywhere in the engine again I had to remove the t fitting and plug the thread with an oil pressure switch while I pressurized the other pressure switch (The one with all the electrical connections attached to it.) with my vaccum/pressure gun.The vaccum/pressure gun basically acted as a "pressurized engine" and thus able to flip the switch inside which in turn, turned on the electric fuel pump and I was then able to start the truck and get the oil pump working. Doing this proved to be much more effective than cranking the engine for 10 seconds. That was taking too long or not working at all. I afterwards removed the oil pressure switch and it had oil in it. I then switched to the oil switch with the electrical connections on it to see if the engine itself could pressurize the switch and it got pressurized immediately and the truck started just fine. I now just have to put the t fitting back on and thread both the oil pressure sensors on it again now that the truck has oil in its gallies again. The truck runs rough, but I'm guessing that's because of the timing being off. It also doesn't stop immediately when the ignition is set to off. I think this is called dieasealing. Is this normal and safe? I want to thank Geezer101, 85ram50, SubGothius and everyone who helped get me to this point. Thank you very much!

[BJH324JH]

Okay, so I have yet to time the engine, I just couldn't wait to try out the truck. Could the transmission be out because I can barely reach above 55 going uphill on freeway. Honestly I thought this carb would fix that. That's one of the main reasons I got the carburetor.

[geezer101]

The dieselling and lack of top end power are related. Drop your idle speed by adjusting the throttle stop on the carb, then experiment with the ignition timing. The factory recommended timing settings no longer apply with the Weber and this part will be trial and error.

By the way - nice work with getting the oil pressure safety cut off working correctly

[BJH324JH]

The dieselling and lack of top end power are related. Drop your idle speed by adjusting the throttle stop on the carb, then experiment with the ignition timing. The factory recommended timing settings no longer apply with the Weber and this part will be trial and error.

By the way - nice work with getting the oil pressure safety cut off working correctly

Thank you geezer, I couldnt have done it without you guys. Seeing as the factor timing doesn't apply, do I just go by feel?

[geezer101]

I would try this - drop the idle, try to hold the engine rpm @ about 2000 and gently swing the distributor between advance and retard until the engine rpms jump by themselves, back it off a touch and try road testing it. Fingers crossed you'll see improvements in top speed and mid range (should shift nicer) and the dielselling should be tamed.

[BJH324JH]

I would try this - drop the idle, try to hold the engine rpm @ about 2000 and gently swing the distributor between advance and retard until the engine rpms jump by themselves, back it off a touch and try road testing it. Fingers crossed you'll see improvements in top speed and mid range (should shift nicer) and the dielselling should be tamed.

Thank you geezer, always appreciate your help. I don't know whether I should tackle this or let the carb shop do this. I don't know what tools to use. I think a tachometer might be needed.

[geezer101]

A cheap hand held dwell/tach meter will do the job. The dwell isn't much help on an ECM distributor system other than to tell you it's out of spec.

Here's my beef - new school shops can't (or don't understand how to) tune old school engines. There is a subtle art to digging up hidden performance in an old school and every shop I dealt with used the 'factory spec' tune. That's when I started doing my own tune ups and learning how to find that little extra. I had one bunch or muppets undo a good tune I performed on a friends Mazda 808 (40 minutes of dialling in the ignition and resetting the breaker points for optimisation) - when he picked it up from getting something else done to it, it was a slug compared to how it went in.