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Mazda B2600 2.6L Mitsubishi de-smog and Weber carburetor conversion --

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goodfellow:
This was a project that I completed over six years ago to convert an old Mazda B2600 into a more roadworthy vehicle. That included getting rid of the old and rusted non-functioning smog controls.

I changed the fluids, drained the tank and managed to get the 2.6 liter Mitsubishi engine running pretty well at idle, but that was about it. When I started to accelerate, the engine would bog down and smoke like a chimney. Knowing that these engines have notoriously finicky Mikuni Solex carbs, I decided to tackle this "smogger" carb head on.

In 1988, the emission regs were catching up with this design. So to meet specs, Mazda/Mitsubishi squeezed the last bit of emission control out of this carb. These days all this control messaging is handled by a small computer. Yet, even though this truck does have an ECM, it's nothing more than a glorified fuel management system. The bulk of the emission controls are mostly handled by vacuum switches and dashpots. To that end, they wound up with a HUGE mess of vacuum lines that plagued this truck's performance ever since it was first on the road. Thankfully in 1989 Mazda moved to a fuel injection system and much of this hose crap went away.

Here is the "infamous" Mikuni Solex 2bl carburetor -- a design way past its time; even back in 1988.



Since the carb is heated by the cooling system, and the intake manifold sits lower in the engine compartment, I need to drain the radiator to avoid spilling lots of coolant into the intake when removing the carb.



Vacuum line hell --!!!!  billcat







Most of these lines are now solid and cracked from all the heat -- that causes lots of vacuum leaks down deep in these vacuum line manifolds. I don't have a vacuum hose diagram, so I just take pen and paper and label everything that's important.









Success!! It's off -- It doesn't look like much, but just getting to this point is a chore; simply because of all the brittle hoses and corroded clamps that had to be disconnected.



I can see why this thing ran so rich and smoked so much -- just look at all the raw fuel that's dumping down that intake. There's a "lake" of raw gas in that intake manifold.





Once I got the carb on the bench I quickly found the problem. This 2bl carburetor has a progressive secondary that is operated by a vacuum dashpot. So at idle the carb is effectively a single barrel unit. Under acceleration, engine vacuum to the dashpot opens up the secondary; effectively turning this thing into a 2bl carb.

See the dashpot in the upper right corner of the pic? That's the secondary throttle actuator.



Hooking up a vacuum pump to that secondary dashpot shows a little movement, but not enough to fully open the throttle plate under heavy acceleration. That's what was causing my power loss and hesitation when trying to accelerate at speed, and why the damn thing idled pretty well; the primary was working at idle, but the secondary wasn't opening under load. That's probably also why the raw gas wasn't atomized in the secondary and just filled up the manifold.



I could open the secondary manually, but the dashpot was not up to the task by itself.



So, I'm off to eBay to order the parts for a Mikuni Solex rebuild -- stay tuned.  beerdude

goodfellow:
Wow -- I'm impressed!! Look what showed up at the door a few minutes ago.

Weber goodness, and a very nice install kit to go with it.

goodfellow:
Started stripping the engine of "smog" equipment and lines today. Essentially, all the electronic fuel management will be removed, and the Weber will function like a normal manual intake system.

It's amazing how much emission control crap was on this little truck; a wonder that it drove at all. Gulp valves, metering valves, fuel solenoids, vacuum switches; the list is endless and the vacuum lines are never ending. Just look at this mess.







It did clean up the engine compartment pretty well though. Now it actually looks like a normal engine --





The emission recirculation port on the exhaust manifold was removed, and since I didn't have a plug this size, I cut the tube and squeezed the top shut in a vise and welded the seam closed. It makes a good seal and the entire manifold is tight.

ADDITIONAL NOTE: This last step wasn't necessary. The Weber kit did include two large manifold plugs for plugging the two large ports in the exhaust manifold (one on top and the other towards the rear of the manifold). The best way to remove the old lines is to use a cutting wheel or sawz-all to cut the metal tubes right above the fittings, and then heat the fittings with an O/A torch to loosen and remove them. Without that O/A heat, no amount of penetrating oil would budge those rusted fittings.

Alternatively, you cab just cut the tubes above the fittings and install a short piece of rubber hose on the stub, then plug the hose with a large bolt or other makeshift plug.



Don't even need the O2 sensor anymore. I'll just disconnect it at the plug and bundle the wire.



Tomorrow I'll install the Weber --

goodfellow:
Finished this Weber conversion this afternoon. There are many bits and pieces in this generic Weber conversion kit that won't be used on the Mazda 2.6 liter Mitsubishi engine.
The first change required that the throttle linkage that was installed on the Weber carburetor be substituted for a cable guide version that in the kit. This is the original solid linkage piece.....



.....and here is the cable version that is included in the kit. This one will fit the Mazda throttle cable perfectly. A simple nut holds this linkage in place.



Next comes a generic adapter plate and gasket. This is an odd sized gasket and adapter plate that can be used with several different OEM installs; hence the elongated  corner looks a bit odd, but it will fit. Countersink hex screws keep it in place.

NOTE: when installing these gaskets and plates, I used LOTS of gasket sealer to ensure that no vacuum leaks develop in this adapter plate "pancake" stack that must be installed.





Next comes the intermediate plate and gasket. The cap screws must fit below the surface plane of this plate --









Lastly, the carburetor studs and the carburetor base gasket are installed on the intermediate plate





Now before the carburetor is installed I plugged a secondary fuel line and water heater hose that were needed for the old electronic Mikuni/Solex carb. I simply plugged the hoses with a bolt and a hose clamp. You can see the plugged lines on the left and right side of this pic -- simply a bolt with a hose clamp



The carburetor is installed on the studs and the throttle cable is hooked up. The only large mod that I needed to do was to raise the throttle cable mounting clamp that was included in the Weber kit. It is mounted on the back of the carb and was sitting too low thereby binding the throttle cable. I simply welded a piece of 1/8" thick steel on top of the old bracket to lengthen it. Then I drilled another throttle cable mounting hole to accept the cable. This raised the cable to a height that allowed smooth operation.

These pics show the original (unmodified) bracket -- it simply bolts to the two rear carb mounting studs.





After I welded on a piece of 1.5" steel, it raised the bracket to the proper position to allow the cable to move freely.



Last item was finding a switched 12v power source to actuate the electric choke. I simply tapped into the existing Mikuni carb wiring harness connector and found a switched 12v source. A simple wire and spade connectors were furnished in the Weber kit, and was easy to install.









After this last electrical install, I plumbed a new fuel line (also included in the Weber kit) and I was ready to fill the radiator and start the truck.

Voila!!! She runs -- Success!!! The engine sounds much smoother and more responsive. Tomorrow I'll button up all the wiring and a make things tidy -- time to put this thing back on the road.

So this morning I installed the air cleaner, breather, and routed the hoses and wires in a more efficient manner. Lots of zip ties, rubber grommets and tape were used to tidy things up.

First the air cleaner element that came with this Weber kit is top quality. However, because of my breather fitting location I mounted the element in reverse. It fits, but on some applications it may not. The breather fitting is screwed into the bottom, and bolted on the throat of the carb.







A nice thick gasket was included in the kit.





Final routing was accomplished by going straight out the back and over the accelerator cable.





Final routing of the vacuum lines and the electric choke was nicely bundled and secured







Everything is nice and secure and the engine idles at 850RPM -- perfect. The carb doesn't have a run-up circuit that would be triggered when the AC is on, so the RPM does drop to about 700 with AC -- no problem! I can increase the RPM to 950 this becomes a problem.





That's it -- the thing runs well and all the lights/electricals work to pass DMV inspection. Now all I have to work on is the parking brake -- it's totally not functioning from having the truck sit for a decade. 

bonneyman:
Great job!

Looks ALOT like the Toyota 22R fiasco! All those feet (23 last time I replaced it all!) of silicone rubber tubing getting cooked by the heat and rattled by vibration. Just waiting for a chance to crack and leak! Arrgh!
But AZ emissions standards (and yearly smog check) won't let me touch any of that stuff.
But I have seen kits to replace the stock carb and all the tubing in states that don't have emissions checks. or if your vehicle is for off-road use only.

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