Topic: 1976 Honda cb750K

[BenelliSEI]

Next up was a “bench sync”. Here’s how I do it:

I usually prep a long shank, 1/4 drill bit by making sure the shank is smooth and burr free. I stick it in my drill press backwards and dress the shank with some Emory cloth so it is really smooth and polished.

I start by removing the idle screw/spring and park the spring off to one side. Screw the idle screw in until it touches the operating bridge and add a few turns to the screw. Adjust #2 carb so the slide just barely lifts off the bottom of the carb throat. Lightly lock down the top nut, without moving the base nut/thimble.

Next I crank the idle screw up until I can just slide in the shank end of the 1/4” bit, between the belly of the #2 carb throat and the bottom of the slide. Adjust it so the shank must be flat and centred to just drag in and out. Now using the top adjusters on the other three carbs, set them to the same height….. Lightly lock the top nuts and done. The rubber dust covers on the top adjusters are a pain, but I like the look and they keep the sliding contact faces clean.

Remember to put the spring and washer back in the idle screw! I always check the sync with my old Motion Pro sticks, once the bike is running. I’m often surprised how close this mechanical routine gets it.

[willbird]

Set the float heights and added in a fresh set of bowl gaskets. Cleaned out my small ultrasonic tank and was astounded at the amount of fine silt in the bottom. These carbs were pretty good looking to start with (I almost left them alone). I did them one at a time, rotating them regularly in the tank, for about 40 minutes each. They are spotless.

If not too much trouble can you show how you measure the float heights ? Picture worth 1000 words and all of that jazz ;-). I have been dumping my ultrasonic pretty much every time I use it. Just did first of a set of carbs, thinking to dump it when done and run the bodies all back through fresh solution.

Bill

[BenelliSEI]

Bill….. years ago I bought the white plastic gauge in the photo. I set the carbs up on a block of wood, at the angle shown. This allows the float tang to sit lightly on the valve, without compressing the needle. You can feel it’s just right when you push the floats down and they pop right back. I then place the “bridge” tool on the base of the carbs (at 90 degrees to the base)  and make sure it just passes over the floats, barely touching them. It takes a while to get the tang that touches the needle bent to exactly where it needs to be. The bridge design of the gauge also allows me to check that both floats are at the same height when finished. I’ve actually never used the “clear tube” method as this always seems to work for me…..

I think I bought it on EBay ages ago, but simple enough to make one? I also make sure the float pins are really smooth and clean. The float needs to be very free on the pin for this to work!

[BenelliSEI]

Done. Next, I’ll hook up a remote tank and make sure the floats and float valves are behaving, before mounting on the bike. This photo was before the bench sync, incase anyone notices.

[willbird]

Bill….. years ago I bought the white plastic gauge in the photo. I set the carbs up on a block of wood, at the angle shown. This allows the float tang to sit lightly on the valve, without compressing the needle. You can feel it’s just right when you push the floats down and they pop right back. I then place the “bridge” tool on the base of the carbs (at 90 degrees to the base)  and make sure it just passes over the floats, barely touching them. It takes a while to get the tang that touches the needle bent to exactly where it needs to be. The bridge design of the gauge also allows me to check that both floats are at the same height when finished. I’ve actually never used the “clear tube” method as this always seems to work for me…..

I think I bought it on EBay ages ago, but simple enough to make one? I also make sure the float pins are really smooth and clean. The float needs to be very free on the pin for this to work!

Ok that makes total sense

[denward17]

Carbs look great John!

How do you test that the floats actually stop the flow of fuel?

[HondaMan]

Bill….. years ago I bought the white plastic gauge in the photo. I set the carbs up on a block of wood, at the angle shown. This allows the float tang to sit lightly on the valve, without compressing the needle. You can feel it’s just right when you push the floats down and they pop right back. I then place the “bridge” tool on the base of the carbs (at 90 degrees to the base)  and make sure it just passes over the floats, barely touching them. It takes a while to get the tang that touches the needle bent to exactly where it needs to be. The bridge design of the gauge also allows me to check that both floats are at the same height when finished. I’ve actually never used the “clear tube” method as this always seems to work for me…..

I think I bought it on EBay ages ago, but simple enough to make one? I also make sure the float pins are really smooth and clean. The float needs to be very free on the pin for this to work!

I've made all my float gages from roof flashing aluminum (some old ones are steel). I have them at 27, 26, 25, and 24mm for the 750 roundtops and similar sets for the other carbs.

One thing to watch out for: the modern float valves have much stronger springs in them than the old ones. This usually results in a lower float bowl level when riding, as the older ones would bounce more and let the bowls fill a bit faster. This is why I usually set at least one side of these roundtops at 25mm instead of 26mm (also helps with the offset when on the kickstand), so as to deepen the bowls a bit when riding.

[BenelliSEI]

Carbs look great John!

How do you test that the floats actually stop the flow of fuel?

Thanks! I usually stand them up (in a tub), on the bench. Hook up my remote fuel can to the #1 and #2 tee and fill them up. Assuming they hold, test #3 and #4. Drain and install. Will post a picture.

[BenelliSEI]

Bill….. years ago I bought the white plastic gauge in the photo. I set the carbs up on a block of wood, at the angle shown. This allows the float tang to sit lightly on the valve, without compressing the needle. You can feel it’s just right when you push the floats down and they pop right back. I then place the “bridge” tool on the base of the carbs (at 90 degrees to the base)  and make sure it just passes over the floats, barely touching them. It takes a while to get the tang that touches the needle bent to exactly where it needs to be. The bridge design of the gauge also allows me to check that both floats are at the same height when finished. I’ve actually never used the “clear tube” method as this always seems to work for me…..

I think I bought it on EBay ages ago, but simple enough to make one? I also make sure the float pins are really smooth and clean. The float needs to be very free on the pin for this to work!

I've made all my float gages from roof flashing aluminum (some old ones are steel). I have them at 27, 26, 25, and 24mm for the 750 roundtops and similar sets for the other carbs.

One thing to watch out for: the modern float valves have much stronger springs in them than the old ones. This usually results in a lower float bowl level when riding, as the older ones would bounce more and let the bowls fill a bit faster. This is why I usually set at least one side of these roundtops at 25mm instead of 26mm (also helps with the offset when on the kickstand), so as to deepen the bowls a bit when riding.

+1 to that. I make sure the floats actually clear the gauge by a hair……

[willbird]

Carbs look great John!

How do you test that the floats actually stop the flow of fuel?

Thanks! I usually stand them up (in a tub), on the bench. Hook up my remote fuel can to the #1 and #2 tee and fill them up. Assuming they hold, test #3 and #4. Drain and install.

I dream up stuff while driving to and from work. One such pipe dream was a test stand that would use a graduated burette to allow detecting even small leakage. I remember as a kid my dad lapping needles and seats if they leaked.

[BenelliSEI]

I’ve often thought about a good stand for these for working on them, and testing. Never figured out a tidy solution and really they end up flipped over and around so many times during the entire process that a stand would probably just get in the way……

[newday777]

I’ve often thought about a good stand for these for working on them, and testing. Never figured out a tidy solution and really they end up flipped over and around so many times during the entire process that a stand would probably just get in the way……

Here is a stand someone had made and posted on here that looks sturdy and has a test tank mounted on it.

My idea is to not use the joist hangers, but to use a screw in each end in the center of the 2x4 so you can rotate the carb rack to either straight up to test the float valves for leaks or clear tube test, and predrilled hole for a second screw on each of the ends of the 2x4 to set to plumb for valve testing or the float setting angle. Once you find the angles you want put a fine pen line on the vertical 2x4 wood for the rotational angle you want to go back to each time.

[willbird]

I’ve often thought about a good stand for these for working on them, and testing. Never figured out a tidy solution and really they end up flipped over and around so many times during the entire process that a stand would probably just get in the way……

Here is a stand someone had made and posted on here that looks sturdy and has a test tank mounted on it.

My idea is to not use the joist hangers, but to use a screw in each end in the center of the 2x4 so you can rotate the carb rack to either straight up to test the float valves for leaks or clear tube test, and predrilled hole for a second screw on each of the ends of the 2x4 to set to plumb for valve testing or the float setting angle. Once you find the angles you want put a fine pen line on the vertical 2x4 wood for the rotational angle you want to go back to each time.

I think you can buy studs that are wood screw foir say 3" then a normal thread for the remaining length ?? That would let you run the wood screw part into the end of the stud and then use wing nuts which would allow setting any angle you desired ? Probably just using all thread glued into the end of the stud with a suitable glue for wood to metal would work too, gorilla glue, 5 minute epoxy, whatever we have.

[willbird]

I’ve often thought about a good stand for these for working on them, and testing. Never figured out a tidy solution and really they end up flipped over and around so many times during the entire process that a stand would probably just get in the way……

Here is a stand someone had made and posted on here that looks sturdy and has a test tank mounted on it.

My idea is to not use the joist hangers, but to use a screw in each end in the center of the 2x4 so you can rotate the carb rack to either straight up to test the float valves for leaks or clear tube test, and predrilled hole for a second screw on each of the ends of the 2x4 to set to plumb for valve testing or the float setting angle. Once you find the angles you want put a fine pen line on the vertical 2x4 wood for the rotational angle you want to go back to each time.

I think you can buy studs that are wood screw foir say 3" then a normal thread for the remaining length ?? That would let you run the wood screw part into the end of the stud and then use wing nuts which would allow setting any angle you desired ? Probably just using all thread glued into the end of the stud with a suitable glue for wood to metal would work too, gorilla glue, 5 minute epoxy, whatever we have. Probably everybody digs into their pile of stuff too good to throw away but not good enough to keep for these kinds of projects. About every 6 months I take the "I'll use these later" wood scraps to the burn barrel LOL.

[BenelliSEI]

Heading to the shop this am. Cleaned up the original drain pipes. Hard as a rock, but they will do. You guys have me thinking about a stand (another distraction). Have a great day!

[BenelliSEI]

Float valve and fuel tee “0” ring test. All good. Dry/blow them out and installation next.

[denward17]

That's good news, don't have to open them back up.

[BenelliSEI]

That's good news, don't have to open them back up.

Seems like it! I’m going back later to measure the volume of fuel in each bowl. Curious how evenly they filled. I pulled off the line and back drained from the individual tees, so should be relatively the same?

[BenelliSEI]

Measured bowl contents. They all had about 40 ml. Not sure what’s right, but they were all the same, so we’ll move ahead!

[BenelliSEI]

Progress Today!

FYI….. The intake manifolds I bought from vintagecb750.com were absolutely useless. They are specifically listed for 1971-76 cb750K. They snapped on nicely, looked  and felt good. HOWEVER, the throat spacing for my K6 was totally wrong. I had a nice clean Honda set that I was saving for another project. Subbed them in and the carbs fit perfectly. I’m going to make sure the box and part number are correct and what I ordered. Assuming they are, they’ll be getting them back. For Cdn$122 I expect an explaination and a refund.

[Magpie]

You're setting the bar pretty high there John!!

[BenelliSEI]

You're setting the bar pretty high there John!!

Cliff….how’s yours coming along? Too bad you are so far away, they’d make a nice set out on a ride together!

[HondaMan]

I dream up stuff while driving to and from work. One such pipe dream was a test stand that would use a graduated burette to allow detecting even small leakage. I remember as a kid my dad lapping needles and seats if they leaked.

I read that too fast the first time: I was wondering who the 'brunette' was?

[Ozzybud]

Your Power Plant is looking amazing. great attention to detail.

[PeWe]

Good to have bowl bottom plugs removable.
I did not bother to do that on mine. I will at next bowl off.

I had extra bowls where I had to use a vise to hold the screws while rotating the bowl to get them loose.