Yes, beaten into submission by my carbs. If you don’t already know, my new insane vehicular project can be found at xkeresto.wordpress.com and that is one reason I haven’t been “here.”
You probably thought that over the weeks since my last post I’ve been sailing all over Southern California on my blue dream machine. You would be wrong. The next weekend after I thought I was road-worthy Joe and I had a club ride planned. I was all excited to take the R75/5 on its first serious road trip. Come the morning of the ride it would not start for nothing! I ran the battery down, trying all variations of choke/no-choke/throttle/no throttle because I could not believe it wasn’t just about to fire up.
I dumped the fuel in the float bowls. No help. Do I have spark? Yep, pulled a boot, inserted a loose plug, and watched/heard it arc as I cranked (you have to provide a normal spark gap or a short to prevent damaging the Boyer electronic ignition). I pulled the plugs and they looked okay, a bit black-sooty but nothing that didn’t wipe off. Double-checked the gaps. The biggest hint I got was not much smell of gasoline when I pulled the plugs and they were not wet. After cranking that long the plugs should have gotten wet, if not flooded.
Next I messed with both carbs, pulling the jets with the bodies left on the bike. Blew some carb cleaner where I could spray. That didn’t get me started. Hmm, pulled the valve covers and did another complete valve adjustment. They had moved a little since my last check but nothing radical. On a warm afternoon I did get the bike to start again, though it still took some cranking to do it. At this point I double checked the timing. Nope. Perfect. I did away with the points so no gap to check there.
I was SO disappointed as I was forced to mount up on the FJR for the ride. If I recall correctly it was a HOG of Glendale club poker run benefit for Guide Dogs of America. Not the big annual run but just a side run. Still had hundreds of riders show up, final count was something like 400! It ended at the Route 66 Grill in Canyon Country. Those Harley groups know how to party! They had a live band, slow ride contest, beer stein holding contest and more. No pictures. I was too bummed. The R75/5 would have stood out in that ocean of chrome. Yes, Joe won stuff… again. In addition to many raffle prizes (I won some crap) they also had a jar of candy corn and a jar of something else and you had to guess how many in each. Joe guessed closest on both. There had to be two hundred other entries! Can you believe it?
Regarding my R75/5, the only thing I can think of is the Bings have a reputation for difficult cold starting and that morning was the earliest (coldest) attempt I made to date. All my confidence in this machine escaped like air from a balloon. How could I travel anywhere except from-and-to my garage if I was worried I’d run the battery down trying to start it later? Grrrr.
Okay. I was faced with several choices. I could replace some parts in my Bings, maybe even send them out for service since I had several whacks at doing it myself. Or I could replace them with a Mikuni carb conversion that several 5 United members spoke highly of, sold by Stan at Rocky Point Cycle (see link in my big list at upper right). Brad, an old dirt biker, even suggested Mikunis since those were a common swap for Bings back when.
I know in a prior post I said I would not be beaten by my carbs. I was mistaken. I am utterly vanquished by my Bings. But I got even. They’re stored in a box now.
The Mikuni carb conversion cost $375. That is less than just new parts required to attempt to fix my Bings. And owner history on Bings is spotty. Sometimes they work and sometimes they never work. It has to be some manufacturing tolerance issue because it is so widely reported. Not one person complained the Mikunis did not work. They are reported to improve throttle response, ultimate power, and reduce gas mileage. Hey now, THAT’s a tradeoff I can live with! They don’t look perfectly stock but I took photos. Decide for yourself.
Here is the kit from Stan. I broke down and got it this week, taking vacation Friday to get a jump on installation.
It includes two Mikuni VM series carbs (pre-jetted for your application), extra jets (to tune if needed), rubber intake adapters, hose clamps, a length of fuel hose, and custom throttle cables. Very complete! You have to discuss your order with Stan so he can pre-jet correctly and send you the right type of throttle cable. He sells a choke cable conversion also, another $40 IIRC, but I decided to try as is (separate choke levers on each carb) since I can always add it later. Stan developed this conversion to use on his personal motorcycle, a 1971 R75/5, so he has high confidence in the R75 application.
The carbs come stickered “Not for Aircraft Use.” There must be some tiny airplane engines out there if someone might consider using one of these on it? Stan includes a one page instruction sheet and a copy sheet of an exploded carb view but to be honest the instructions are pretty weak. I suppose you could say that if you cannot figure these out you should not be installing ‘em. But I struggled slightly anyway.
The first conundrum was “where do the throttle cables go?” The Bings have an external lever that rotates a butterfly. The Mikunis have no butterfly. The throttle cable attaches directly to the main jet’s needle piston through the lid. Here are some views of the carb with the lid off and the guts.
There is also no diaphragm that pulls up on the slide like the Bings, just one big return spring that does double-duty forcing the carb back to idle and also providing the throttle control return force. The float bowl is screwed on, presumably needing to be removed less frequently than the Bing bowl? There are three small brass hose barbs and one big one. The big one was obviously fuel input but what about the other three? Well, one is the carb overflow, something flush-mounted on the Bing body. The other two are vents above the bowl. I’m not sure why two vents are needed but, as confirmed by Brad, don’t block those vents!
Now you know we can’t get through a retrofit like this without monkey-boy showing up somewhere, right? Well, my examination of the carbs led me to think they were two of the same model and lay-out, meaning no left and no right, unlike the Bings. That is not entirely true. Yes most of the body, and for instance the choke lever, is the same on both carbs. But I failed to notice that the idle adjust screw location varied from carb-to-carb. I ended up mounting them with the adjust screws pointed toward the body of the bike, harder to adjust on a hot bike. It was not a complete disaster though because the air mixture screws did NOT vary, meaning one was always going to be harder to access no matter how I mounted them. Strange.
Here is one on the right side of the bike —
Looks okay to me. My only real complaint about the set-up is the custom throttle cable’s length adjusters (to set the 1 mm or so throttle slack on each side) are positioned on the cables so they are hidden under the tank, almost right at my rubber cable loom. It would make more sense to have them exposed on the run of cable between tank and carb.
Yeah, it was annoying to mount up the carbs again. Same twist-dance getting everything on at once. I had a problem with gas leaking at my in-line filters. They were working fine before but now they required tiny hose clamps. Oh well, that’s a hazard of using 1/4″ filters in 7 mm fuel line. One trip to the auto parts store and everything was set right. No new puddles of gas on the floor.
The moment of truth? Rrr, Rrr, Vroom! Started right up. Immediately flipped off the chokes and it idled fine at a slightly open throttle setting. I tweaked idle adjustments so cold idle wouldn’t stall and went for a warm-up ride. It ran just fine up to redline and idle rose after it warmed up. Got it back in the garage, backed off the idle, and tweaked the air/fuel screw on each side for max RPM, keeping idle near 1K RPM. When I was done I took Stan’s advice and turned the screws in a quarter turn (slightly richer) for year-round safe riding. I shut everything off and came in to write up the experience. Took me longer than I thought it would. Call it six hours of work.
I may need to tweak them some more and if I want to balance ‘em better than by-ear I’ll have to drill ports for my Carbtune but it seems unlikely I will need to.
So if you see a broken down blue R75/5 on the side of the road somewhere in So Cal, that’s probably me. Happy trails!