ae86andkp61 Dork
7/1/19 7:29 p.m.

I bought a customized JDM cafe racer Suzuki a few years back, and I’ve never been happy with the air intake setup.

Among other mods, it has a Keihin FCR on what is essentially a warmed-over DR350 engine. As a guy who has owned multiple DR350’s, I can tell you that the carb is a great upgrade, and that mine runs pretty well. I suspect it will run better with a proper stack and filter.

From my understanding of this bike’s wacky history, at some point in the past, the Japanese owner that was building it is a trackday bike went open stack on the FCR. Later, the Idaho farmer/moto collector I bought it from decided to filter the air...great! The bad news is that he decided to do so by TIG welding a ring onto the *inside* of the velocity stack bellmouth, making it into something resembling a nearly straight tube with some sharp square corners and weld bead in the way, and then put a tiny K&N on the outside of the ring. On top of all this, the stack is of a design that leaves the idle and slow circuits unfiltered!

I found an engineering firm in NJ that used to make well-thought-out FCR retrofit kits for two carbs on 950/990 KTM twins. I figured one of their stacks certainly wouldn’t choke my built 350 and snagged the last one in existence off their display in the company office. It even ducts filtered air into all the jets plus the throat, and the orange anodized finish even kept the mismatched homebuilt charm of the original orange stack. So far, so good.

I measured the frame plus the 3.75” air filter flange on the stack and ordered a couple filters I thought might work. Neither of them fit, primarily due to the angle of the carb plus intake port.

Here’s the old setup on the bottom and the new on the top for comparison. The new filter is the first I tried, with an offset on the flange. It was plenty short enough, but waaay too big around the base to fit in the frame.

Here’s the carb plus the new stack when viewed from above to illustrate the angle. The rubber boot between the carb and head is relaxed, but the stack is right up against the frame.

Here’s the second filter I tried (basically the smallest one K&N makes with a 3.75” flange) and you can see the angle  mismatch between the stack flange and the filter flange, as well as the angle mismatch between the carb and the cylinder head port boot. The head-side boot is stretched about as far as I’d dare at this point.

I’ve scoured the internet for a smaller filter to fit the flange, or the same size with some offset/angle but no luck. I looked for a different boot to go between the head and the carb with no luck. The cyl. head side of the carb is a “spigot” that threads into the carb body, so I looked for a 40mm OD FCR spigot with a slight angle with no luck.

My thoughts so far:

I could fabricate a metal part to go to the cylinder head intake port, and then get an aftermarket angled 40mm rubber elbow with two hose clamps. This setup would likely add to the overall length (which could be a problem) as well as reducing heat and vibration insulation for the carb when compared to the rubber setup on there now. I also suspect that an angle in the intake tract downstream of the carb could mess with performance as atomized fuel puddles when hitting the walls, but I’m no expert.

I’ve considered making a custom angled intake spigot. This would be more labor intensive, calling upon my lathe and my buddy with a TIG, plus some porting, but would alleviate all of the above concerns except the angled intake tract.

I also considered modifying the stack, which could include milling the carb-side face at an angle, and or turning the flange down to 3.5” for a smaller filter. With my four-jaw chuck on the lathe and some careful work I could even offset the flange a bit.

Any thoughts or advice?

ShawnG PowerDork
7/1/19 8:07 p.m.

I'm no engineer but my calibrated eyecrometer and fabricators spidey sense is telling me that you could probably knock 1/4" off that diameter without affecting the airflow.

Probably a bit more if you don't mind buggering up the flow a little bit.

Might help with the filter choices.

ae86andkp61 Dork
7/1/19 10:03 p.m.

In reply to ShawnG :

Great to hear, as this is where my own pull-it-outta-my-ass thoughts were headed. Thanks for the validation!

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