Hello again everyone, Lets restore a set of carbs for this old Honda! Yes I had already turned them over one time & popped all of the bowls off and blew them out with some carburetor cleaner and put them back on to test run the old bike & see how it sounded, but it was a running a little lean with the pod air filters, plus I was installing a new exhaust system and needed to re-jet anyway. But the single best reason I have found to go to this extreme is that there is almost nothing else that will make an old motorcycle run like new again the way a complete carburetor refurbishment will. In fact the only thing better is a new set of carbs, which if you pull your old ones apart & find severe damage or wear to the castings, is your only choice.
Here you see the four carburetors, along with a flathead screwdriver, a JIS screwdriver, a set of mini wrenches and a ratchet with a socket. This is not everything you need to completely disassemble them but it’s a good start. The patient is my own 1980 Honda CB650 Custom with a set of Keihin PD mechanically operated carburetors. Although these a very similar to the piston valve or round slide carbs used on a number vintage motorcycles these do have a few differences from the older ones. This will not be a complete step by step guide to restoring every single piece. If need to learn the basics of carburetor repair I suggest that you start by clicking here.
Instead I will focus on how to disassemble the set of carbs, and point out some of the different features along the way, along with a few tips and tricks to make the job more successful. Please note that successful does not mean easier. This is not rocket science but there are no shortcuts either.
The first step in getting a set of carburetors apart is to remove everything you can from the choke & throttle shafts. Here I am using a JIS screwdriver to remove the choke butterflies. What the heck is a JIS screwdriver you ask? Just click here to find out!
Please note that these screws are often punched from the backside at the factory to spread them out & keep them from coming loose & falling into a running engine. This often makes them a pain in the butt to remove.
Next take the tops off and remove the long screws that connect the slide lifters to the throttle shaft.
Be sure to inspect the shafts for damage like this before you start pulling the unit apart. At some point a previous owner had used a pair of pliers to move a sticky slide. To avoid damage to the bushings etc. use a strip of sand paper or emery cloth to remove anything you find like this prior to dis-assembly.
Once you have everything loosened or removed from the shafts it’s time to remove the brackets that hold the rack together. On this set there is one large bracket on the engine side & two small braces on the air cleaner side.
Then you can slide the two outer carburetors off the shafts and lay them out along with all of the parts that go between each carb. Lay everything out on your workbench in the order that it comes apart with everything facing the direction that it goes back together correctly. It just makes life so much easier. In fact if you have never done this before I suggest taking a copious amount of pictures at each stage of dis-assembly to help you remember if the doohickey is pointed toward the watcha-ma-callit or the thinga-ma-bob.
Here’s a shot of the two middle carbs with the vent hoses in the foreground and everything else behind them.
On this motorcycle the throttle return spring is fitted between this bracket and the number three carburetor. Carefully drive out this rolled pin to remove the return spring and remove the shaft completely from all the carbs.
Here is a shot of all the major components laid out on the bench more or less in order. Always keep the parts from each individual carburetor together, never mix them up because there are some small differences in every set even if it just wear patterns. Some machines (but not this one) may even have different jetting between different cylinders so just play it safe and don’t mix your parts.
At this point instead of a big complicated looking rack of carbs, you are now just rebuilding 4 individual carbs with only a few differences from you average
This is one thing that is found on the early emission controlled Hondas (late 70s & early 80s) is this pilot mixture screw with the tab on it to prevent it from being rotated too far in on direction or another. The float bowl must be removed first to remove these screws. Of course if you doing this for an off road, race only machine you can carefully grind the tab off before re-assembly.
Another big difference on some machines is the presence of an air cut off valve often found under a cover like this. The purpose of this valve is to temporarily restrict the flow of air through idle air circuit when the throttle is suddenly snapped shut. This helps prevent that lean backfire that occurs on some motorcycles when you suddenly let off of the gas.
These valves were later eliminated from some models but the side cover is still there, but it’s just blank. Some folks will actually remove these and solder the holes closed, then re-jet the carbs extra rich to run without them but IMHO that is not the way to go. Yes I know that at 30 to 40 dollars & up each this is a damn expensive little part, but take my word for it if you replace them you will notice difference.
One nasty little surprise is the presence of pressed in pilot / slow / idle jets. pick your terminology but all three words refer to the little brass tube you see in the picture above. If you are restoring a completely stock bike with stock mufflers and airbox then I will recommend you do everything in your power to clean it without removing it. But if you cant get it clean or if you’re like me & think stock sucks don’t fret it can be changed. As far as I know there is only one source for the pressed in jets and that is Sirius Consolidated, one of my favorite purveyors of all things for motorcycle & powersport carburetors. Click here to check them out.
If you’re planning to change them out anyway grab them with a set of pliers, twist a little to break them free & pull them out.
I finally decided to break down and try ultrasonic cleaning. This is a cheapie from Harbor Freight. For my cleaning solution I am using 1 to 1 mixture of Simple Green & water. The only real drawback to this one is that you can only get 1 carburetor in it at the time, and even then the entire carb is not submerged. I just ran every one for the maximum cleaning cycle of 480 seconds and the flipped it over and ran it for the max cycle again. Be sure to put all of the parts for each carburetor in the cleaner, including all of the internal parts, along with the fuel & air vent tubes that run between the carbs.
Being the meticulous and detailed asshole that I am even after the ultrasonic cleaning I blow everything out good with an air gun and rinse well with aerosol carb cleaner. I also use Simple Green in my parts washer & have discovered that if you don’t rinse the parts afterwards they sometimes have soap residue on them. Plus I just like to be sure that all passages are clear.
Here’s a fresh clean carburetor ready to be re-assembled with all new jets. FYI with the K&N pod filters and the Mac Exhaust I am using #42 pilot jets and #120 main jets. and put my initial pilot mixture screw setting at 2 turns out. This set up turned out to work very well with only minor adjustments after starting the engine. I actually arrived at these numbers for my initial set up after perusing hundreds of posts at the HondaCB650.com & the SOHC4.net forums taking notes and basically picking out the jet sizes most commonly reported to be working successfully in these carbs on this bike with pod filters & aftermarket exhaust. Internet research is such a great time saver but remember to put your ego aside & your bullshit detector on high. You’ll have to wade through a river of fiction to get a few drops worth of facts.
Even armed with these numbers I was prepared to pull the whole thing back apart several times to get it right, but I got lucky this time. That does not mean these jet sizes and settings will work for you & your CB650, but they are as good a starting point as any. Back in the pre-internet days I was re-jetting a moderately modified DOHC CB750F and wound up pulling the carbs off and putting them back on a dozen times before I found the perfect combination of performance and street-ability. If you are modifying your motorcycle you should be prepared to do the same.
This was not a cheap process either, I replaced every single piece of rubber & all of the jets in this set of carbs. The cost just for parts set me back over $300 bucks plus shipping. The jets, air cut off valves & the accelerator pump, all came from Sirius Consolidated, everything else is genuine Honda OEM parts, even the o-rings on the crossover pipes.
This is how the air cut off valve diaphragm, spring & o-ring fit into the body.
I preach a lot about service manuals, and yes I usually try to practice what I preach. This allowed me to verify that the float height really was supposed to be 12.6 mm like most of the fine folks on the net were saying. Trust but verify!
Lets talk about that accelerator pump thingy a little bit. It is usually mounted on the #2 carb like this one. It’s purpose is to squirt a little extra fuel into the engine if you suddenly yank the throttle open. Even though it only enriches one cylinder it helps to reduce the lean stumble that results from a sudden inrush of air when you twist the throttle as fast as you can. It’s also great to squirt a little extra fuel into the intake before starting a cold engine if needed.
It lives under this little cover on the bottom of this carburetor.
Here it is laid out on the bench so you can see how it comes apart.
This shot is just to show you the differences in the #2 float bowl & the rest. These differences are there for the accelerator pump circuit.
At the time of this writing I have found no source for exact replacement parts for the CB650 accelerator pumps. I think this one is for a CL/CB450. The diaphragm is the same but the shaft is longer. so it will have to be cut to match the stock length.
I marked it and then clamped it into a vise with rubber jaws & then filed it to the correct length. Just remember if you get over excited & cut too much off that you have just screwed yourself out of the price of 18 bottles of Guinness Extra Stout.
Modifications are always a pain in the rear. These air filters don’t clear the choke linkage so even more modifications are required. It’s a vicious cycle I tell ya, and not for the faint of heart or the flat of wallet. Modifications beget more modifications & before you know it you have a complete custom motorcycle that will hopefully run at least as well as a stocker. ;0
Just a few more tips for you, re-assembly is pretty much the reverse of dis-assembly. Make sure you wind the throttle return spring up enough to close the slides when released, and always use some form of top quality name brand thread locker on these little screws holding the butterflies in place. If these come loose they can turn your fine running engine into junk if they get into the cylinder.
This picture shows how the spring connects the left & right choke shafts together. Once you have it all back together you should try to “bench Sync” the carbs. Basically you want to get it adjusted so that it appears the all of the slides are lifting at the same time & closing the same distance. A picture of the synchronization adjuster is shown somewhere further down in this post.
Now make sure all of the clamps are on the intake boots loosely, hook up your cables, & then lube the carb spigots & the inside of the rubber boots on the head.
Now push, pull, cuss, twist, shove & pray. If you are reusing the old boots they may be hardened and make this process a little harder. sometimes heating the rubber with a heat gun helps. Just remember that if they are damaged in any way and do not seal properly you must replace them.
Here I am clamping everything down getting ready for the initial start up. I actually started the engine and let it warm up a bit before moving on the vacuum synchronization process.
Once I was satisfied that the engine was basically running okay & that there were no fuel or air leaks I shut off the engine & temporarily installed these adapters in place of the plugs on the engine side of each carburetor. Then each adapter gets a hose between it and this set of gauges.
Before you get started with this procedure it is usually a good idea to have a fan pointed at your engine & running. Sometimes this can take awhile & you don’t want to risk engine damage.
This picture shows the location of the adjuster screw in each carb. Make sure that you always tighten the lock nut securely when you are done adjusting the carbs. The number two carburetor does not have this adjustment on it, it is the baseline and you want to match the other 3 carbs to it. When you get it done all of the needles on all four vacuum gauges should rise & fall at the same time and to the same readings. Now you can put all the tops back on the carbs & begin road testing. Most run just fine, perhaps with a little tweaking of the pilot mixture screws and idle speed if needed. Just make sure that everything else, especially the ignition system is working properly too before road testing the carbs.
Ride as safe as you can & still have fun!