As an owner of an early 80s cb750, it quickly becomes obvious that the CV carburetors on these old motorcycles require the original airbox to run properly. It would seem like a minor price to pay for the reliability and dependability that we get with the design since no jetting changes are needed as we change altititude or swap exhausts. The constant vacuum (CV) design automatically compensates for the difference and keeps the bike running happily. Just don't change the airbox or use anything other than the original style airfilter.
I got my bike running perfectly so I'm either a glutton for punishment or just a dumb geek who can't leave stuff alone. "If it aint broke, fix it until it is" might be my motto. People tell me I wouldn't be happy with a new bike because there would be nothing to fix. Maybe it's just a way for me to keep my mind busy so I don't have to think how much I hate my job.
I started noticing some posts from people on cb1100f.net where they were moving over to different carburetors from a late 80s - early 90s Suzuki GSX-R750. Although these carbs are also CV, they reportedly deal with a hi flowing air filter better than the antiquated cv carbs on the Honda. There are still plenty of posts on gixxer.com advising owners to stick to the OEM airbox to ensure things work properly so the jury is still out on how these will work.
The reported improvements are instantaneous throttle response and noticeably better performance. The posts seemed to indicate that you could basically pick up a rack of carbs for $30 on ebay, slap them on and race away. I'm not naive enough to believe it, but I was intrigued enough to start researching and thinking about this as my next project.
Mikuni BST36-SS carburetor prices are all over the place. eBay has carb racks below $100 but also alot at $300 or more for rebuilt and cleaned items (which I will come to learn is not an outrageous price). These carburetors, also known as slingshot carburetors, were 36mm in diameter and featured vacuum operated semi-flat slides made of some type of lightweight composite/ceramic material. Compare this with the aluminum piston in the original carbs and speculation starts running wild with fast reaction and better design of these Mikuni CV versus the Keihin CV running on the Hondas. The black slides weigh in at 29g with needle and diaphragm while the aluminum slides weigh in at 75g.
During my research, I found what appeared to be a decently priced set of carburetors on eBay, but before I could verify that's what I needed for my project they were purchased and the auction ended. They were probably on sale for 30 minutes. I knew it must have been a good price.
Searched for a while longer and found a set for $100 after the seller's original listing ended for $125. I bought them within minutes of his relisting at the lower price and my qualifications of purchasing carbs only from a bike dismantler were satisified.
I received a well packed set of Mikuni BST36SS carbs from a 1988 oil cooled GSX-R750 california model. They were filthy and missing a few items and I could tell that the bike they came from must have been junked because the carbs needed alot of attention and could not have allowed the bike to run very well, if at all.
I disassembled the carbs, leaving them attached as a rack, took inventory and started soaking them in an old cooler in a water and pinesol (original) solution. After 2 days they look much cleaner but I decided to take an aquarium undergravel powerhead and throw it in the cooler for a poor man's ultrasonic cleaning. The water pump basically circulates the solution with the intention of dislodging any stubborn dirt particles. This seemed to help because the solution went from dirty clear to dirty opaque. NOTE: Pinesol can be harmful to certain types of plastics so care must be taken in soaking questionable materials. My cooler started bubbling in some areas so the active ingredient in pinesol must do something.
It looks like this carb project will cost somewhere in the $3-400 dollar range to "slap" them on. Any jetting changes will cost more.
For now I will be on the look-out for float gaskets, assorted throttle springs, a diaphgram spring, new needle jets, o-rings for the floats, float valve, a set of K&N pods where the filter covers 2 carbs (K&N ru-2922). Figure $100 for the parts and another $100 for the pods, $10-50 for connectors.
1-1-4-4 carb connectors for the Honda are supposed to work but the sizing was not right for the BST36ss so I'll need to rig up some heater hose, but I think I will try silicone couplers some day.
Do you have a reason you used the Pods that cover two carbs? Or was it just preference? I would like to do the same as you but with individual Pods. Great write up!
ReplyDelete