81 CB750 carb rebuild kit
- Thread starter Commander_Chaos
- Start date
I just refurbed two banks of carbs for two different bikes. I bought the cheapest sets on ebay. Those cheap sets had most of the Orings that I needed, Gaskets worked well after a little bit of a stretch, long needles were too short in the sets. I used all of the original brass and long needles. I used one new accelerator pump and four new of the other diaphrams. Important to use the original brass or buy it separate from the kits. Bench sync is very important and should be done with precision. Match all of the throttle butterflies to carb #2. Pilot screws were set at 1-1/2 turns out. I may not know all of the right terms or functions of every part but I've done many different carbs and haven't been beat yet.
After making the right choices for O rings in the right places and using the original brass, I can't imagine these two bikes throttles being more crisp, smooth or more responsive than they are right now, even without vacuum syncing or further tuning.
After making the right choices for O rings in the right places and using the original brass, I can't imagine these two bikes throttles being more crisp, smooth or more responsive than they are right now, even without vacuum syncing or further tuning.
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Isn't carb 2 the one to which all other carbs are set?
Commander_Chaos
CB750 Enthusiast
You're going to get an annoying number of questions from me before this is over. I've never rebuilt anything but an Amal. I tried working on a set of these when i was young and they bitch-slapped me into submission.
Am I right in thinking that the butterflies have to be removed from at least two of the carburetors to get all four separated? It looks like the backs of the butterfly screws have been peened over to keep them in place. I'm worried about ruining the shaft.
Am I right in thinking that the butterflies have to be removed from at least two of the carburetors to get all four separated? It looks like the backs of the butterfly screws have been peened over to keep them in place. I'm worried about ruining the shaft.
You're probably right Brett. Should I correct my post? Carbs look different on the bike vs off the bike.
I removed the choke plates, no reason to remove the throttle butterflies. As I said, I may use the wrong terms for the parts.
Commander_Chaos
CB750 Enthusiast
I see. The choke is the only one with a continuous shaft and the screws aren't peened over on the outer butterfly.
None of the kits I'm looking at have the rubber diaphrams. Should i not be worrying about those?
None of the kits I'm looking at have the rubber diaphrams. Should i not be worrying about those?
The DOHC carbs so share stats for the throttle plates/ butterflies on 1 and 2, and 3 and 4. To separate those pairs, you have to disassemble the butterflies from the shaft.
I used new stainless screws on the choke plates with lock-tite, didn't peen. You'll have to inspect the diaphragms, I had no holes but some were dry and crusty.
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Throttle butterflies do not need to be removed to separate the carbs. Choke plates and shafts do have to be removed to separate the carbs.
CC, find and read the MacGregor Manual, like he says, read it twice. I was able to disassemble, make piles for cleaning, and then assemble with his instructions. At first I was very leery to just pile up the parts. At first I was anal about sorting the parts, but with his assembly instructions There were no problems leaving out parts or confusing parts.
One big help for me, and I can't stress this enough, get an impact driver, the manual type that is hit with a hammer. I used mine with the bit it came with. With the carb bank in my lap I removed all of the JIS screws with it, NO MISHAPS at all. MacGreger should have included this in the manual.
Commander_Chaos
CB750 Enthusiast
Just downloaded the manual. I guess I'll borrow an iPad to read it. I have individual bins for each carburetor plus I'm loosely reassembling them plus I shot video.
Commander_Chaos
CB750 Enthusiast
I'm doing okay with that, so far. The carbs have been soaking in kerosene for close to a year. There's also significant rust to contend with so I'm tapping the screwdriver/allen key into each fastener with a hammer which is probaby shocking them loose. I'll replace as many as possible with stainless.
I did have a bit of a heads-up before doing these. I've cleaned one bank twice before, another bank once without total disassembly but these two banks needed a total disassembly this time.
You're going to want to replace all the fuel rail orings, too!
MacGregor gives a good list for the stainless. I could not bring myself to use the stainless allens in the list for the choke plates. No way I could get those tight enough with that tiny allen wrench, so I bought the same size in Phillips stainless. I remember when one of MacGregor's rebuilt banks gave up one of its choke plate screws, that screw found its way into the engine.............what do you say after that?
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Yeah I can see that. The Philips are way easier to remove if any gets seized later, too. Good call
I'm gonna stick my neck out here because I know that many, if not all, will disagree with me here. Non adjustable plastic floats.........My float heights came out about 1.5mm different than the published and quoted heights. Too high, if I'm remembering right, too low with the carbs turned upright. So, I had new and old, OEM and aftermarket floats and float valves. They all measured the same IN ANY COMBINATION. Don't go down that rabbit hole of searching for something that just may not be there. Unless you have one or two that are way out of whack with the other ones, just use them if they don't have holes in them.
Ethanol, I'm an old guy, I grew up in an automotive environment in Michigan. Let's just say that the whole town was one big old fashioned "Speed Shop". Pre-teen I hung out with much older guys at the Honda shop and speed shops. Every spring was carb cleaning season at the Honda shop and carb cleaning was probably the main income from labor at the speed shops. That was pre-ethanol, I see no difference inside the carbs from now and then.
Ethanol, I'm an old guy, I grew up in an automotive environment in Michigan. Let's just say that the whole town was one big old fashioned "Speed Shop". Pre-teen I hung out with much older guys at the Honda shop and speed shops. Every spring was carb cleaning season at the Honda shop and carb cleaning was probably the main income from labor at the speed shops. That was pre-ethanol, I see no difference inside the carbs from now and then.
Commander_Chaos
CB750 Enthusiast
I've never had a problem with ethanol, as long as I drained the system before I put it away for the winter. I have an antique car that I don't even do that with. I just watch for unseasonably warm days and drive it so that it never sits for more than a month or so.
