When honda made xl350 r the OEM carburetor insulator, or intake manifold, listed for a mere $60. Honda founder Soichiro Honda promised that stock parts for any Honda manufactured would always be available. With his passing in 1991 the MBAs took over and scuttled that promise.
The other problem is Honda didn't sell a lot of these two-carb 350s . As a result there just aren't that many replacement parts around: OEM or aftermarket. Enter Ebay . . .
Or not. Enter a crafty parts-dealer out of Orem Utah, who buys up all the hard to find, but necessary, parts and then jacks the price to to kidnap values. Good work if you can get it. Not too good for your soul however. Not too good for us owners of old hondas.
The do-it-yourself answer is to TIG your old base plate to some aluminum tubing and clamp on some silicone hose. Connect the carbs and off you go for under $50.
In hindsight some DIY things to do and not.
Use a pro for the TIG welding. I used a friend -- a shade tree mechanic. You can see by the welds, I almost had to take that ransom payoff trip to ebay.
Cut the tubing with a cut-off disk not a hacksaw. Smearing is a TIG term you will learn otherwise.
Soak the old base plate in Acetone for a while. Maybe even a trip through the ultrasonic cleaner.
Soak it again. thirty years of fuel are deposited in that porous aluminum.
Honda parts number: 16211-KF0-010 Carburetor Insulator, Intake Manifold
I bought the silicon hose off Ebay $6 free shipping. It is made for this type of application.
Download the image to the left for the cuts. The cuts on the aluminum tubing are double bevel. That was the difficult part to figure out. In the images is a plan for the cuts. Download, print, cut-out and tape it to the tubing. Mark the tubing and cut. It isn't as critical as I first imagined. The silicon hose is quite flexible and will account for a degree of error.