Bringing the SK's back to life

Almost a year ago, I acquired a rather special set of carbs. The decision to get the SK's at that time was based only on the fact that they looked uber-cool and because they used Weber jets I figured they would, at least be serviceable. I recently wrote a post about fabricating the velocity stacks for the carbs, and now I'm going to go into a bit of detail with what it took to getting them running.

The engine the SK carbs will be used on is my 1600cc Mitsubishi 4G3x with a works Rally cam shaft #4, some mild head works and high compression pistons. Caluclations we made, so that this engine makes good torque and peak power at around 6000 RPM, dictated that we needed 30mm primary venturis, which should ideally be used in twin 40mm carbs. The SK's are 45mm and came with 36mm venturis. The carb body might be a bit too big, meaning we're giving up a bit of low end torque and tractability for street use, but it should be ok for top end power. It's a little bit of compromise to be able to run such a unique set of carbs. While the SK may use Weber jets, the primary venturies are unique to the SK. This made is necessary for us to fabricate our own using a process similar to how we made the velocity stacks.

Our 30mm venturi machined out of T6 Aluminum billet to the left compared with the original SK 36mm venturi. Note that our venturi has a more pronounced and rouned profile similar to a Weber venturi, which is more efficient at creating the venturi effect than the SK design.

The next thing that needed to be made was a throttle linkage. The SK's came with a synchonizer to mate the two carbs together, but no form of linkage at all, so we started by gutting part of linkage of a dead set of Keihin CVK carbs that I had laying around in the shop. This was ground down and modified to fit in between the synchonizer to give the cable somewhere to hold on to.

A Honda City throttle cable was next, chosen because it was just about the right length and most importantly it's business end matched to Keihin cable loop, and with a little bit of modification to shorted the protuding edges it was perfect. 

A cable stopper bracket was fabricated to hold it all in place. 

The decision to use the Keihin loop, which was small, and the fact that the Lancer originally used a mechanical linkage, not a throttle cable to actuate the carb meant the next thing we had to sort out was how to connect the cable to the accelerator pedal. Due to the small diameter of the Keihin loop, I wanted something that would open the carb progressively and after studying how we could possibly modify the original throttle pedal, we decided it would be better to just build one from scratch.

A big loop for the other end of the cable to compensate for the smallness of the carb end in order for the carbs to open more progressively. This one was fabricated from scratch.

The rest of the pedal was based on the original one, using a small mechanical linkage to move the throttle cable. 

All the parts were powdercoated before being installed in the car. 

A throttle return spring bracket was also fabricated. Adjustable mounting options let us alter the spring tension to change the feel of the carbs. 

I cleaned up the carbs with a couple of cans of carb cleaner and once all the parts were completed we were able to successfully start the car, and get the carbs synchronized. All good, except for the fact that the barrel of the No. 3 cylinder had a clogged idle circuit. To remedy this, the carbs were again totally dismantled and sent to a friend of mine for a total cleaning job which even included sand blasting the bodies to give them a brand new look.

Here's me reinstalling the carbs back into the car.

Have not been able to start the car yet to test the carbs after cleaning since we are waiting for a new Oil Cooler to install on the car. I will post an update later once we get everything running.

One last thing, not exactly part of the carbs, But just as necessary. Because of the the tight space in the car's engine bay in order for the synchronizer tool (aka Air flow meter) to fit into the barrel of the No.4 carb be had to offset our master cylinder for the AP Lockheed brake system, in order to give clearance. This was done using a custom made bell crank system. Otherwise the pot of the master cylunder would have been right in front of the velocity stack of No. 4 with only a few mm clearance in between.

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Velocity stack fabrication

A quick background. A velocity stack is that trumpet or bell mouth shaped fixture that smooths incoming air into an intake tract. This works by eliminating 'pinch zones' that would occur if the air were to enter a straight pipe. Quite popular and definitely a cool thing to have on a set of side draft carbs or throttle bodies. They enhance the induction noise the carbs make. 

Continuing from my post about my SK Racing Carbs, we recently fabricated a set of billet aluminum velocity stacks to go with them. My carbs did not come with any, and the SK being the curious Weber and Mikuni Hybrid that is meant that nothing out of the box would fit so a set had to be made. 

There are a number of shops here in the Philippines that can make a set, and most are actually quite affordable. The best deal I found was from this guy called Mang Orly, with a set of (what looks to me like) spun aluminum velocity stacks costing around 2500 Pesos. They looked pretty good too and have an option to even be anodized. I was pretty much ready to put in an order for a custom set for the SK's. But, because we can... We just had to go make our own. 

Aluminum spinning is by far the most common and efficient way to make velocity stacks. This is how original Weber stacks are made. This however requires special equipment and of course the skill to spin aluminum. Look up videos of it on YouTube, it's an art form all on it's own. We however had neither the equipment or the skills. So we had to go with the relatively more inefficient (read as expensive) method of machining our stacks out of solid billet aluminum. Machining though does produce a nicer looking product (in my opinion). 

So here's how we made it. 

It all starts with a plan. Internal diameter of 48mm to match the throat opening of the 45mm SK carbs. 76mm outside diameter to give enough wall thickness after machining to make sure they are sturdy and would have a fairly large bell mouth opening. The bigger the bell mouth the more effective it is. 38mm height due to space constraints, would have wanted at least 50mm tall, but no way that was going to fit the Lancer.  

All the machine work would be done at our factory by our very talented mechanic using a 70's vintage Harrison lathe machine. 

The first order of business was to get the material. I ordered 3 inch thick solid T6 Aluminum billet from our supplier and we made a few custom blades to shape the block of metal into our sexy stacks. This is the block of aluminium billet we started with, to it's right a custom made profiling blade for shaping the bell mouth of the stack. 1 blade for each shape and one special blade to bore out the internal diameter. I won't go into detail for the special tools made... Let's call them a trade secret, but the one picture below should give an idea of sorts.. 

A pilot hole is first drilled down the center of the billet in order to start machining the internal diameter. We used the biggest drill bit we had which I think was around 25mm. 

The using the first of our special blades machined this from the inside out to 48mm. 

This was then cut down to size for our 38mm height. 

Using the second special blade, the bell mouth was machined. While doing the actual item, I decided not to follow the plan we had drawn up because it would look nicer if the bell mouth extended all the way to the outside edge instead of rounding it out. 

Next was to shape the outside of the stack. This used a not so special blade to remove some material from the outside, and another special blade to shape the outside lip profile of the stack. It was also sanded and polished while spinning on the lathe. 

Once formed holes were drilled to match the studs on the carb. Also had to file the sides down a bit to give space for the retaining bolts to be turned down as I discovered while test fitting. 

 Repeat steps 3 more times to produce 4 identical stacks!

In the end, I'm quite happy with how the finished product turned out. They look the business and are quite impressive. The cost of the billet aluminum alone was about as much as I would have paid to have Mang Orly make a set of stacks, and if the around 8 whole days of work it took to make these stacks were factored in as cost with equivalent overhead for running the machines, it pushes the cost of making these stacks to a price range that some people will think is crazy. Although, of course solid billet stacks really do cost much more than spun ones as I've seen tuners in other countries make them. Locally I don't know of any other tuning shop that makes solid billet velocity stacks.