Topic: 1972 CB750 - carb drain gaskets and air screw springs

[jeremyillingworth]

I'm finally getting around to reassembling my carbs after cleaning and putting in new rubber parts from the 16010-300-305 Honda gasket set.

Which are the correct gaskets for the carb bowl drains? The bike came with (not that that makes it right) rubber O rings on the drains and it seems like most online kits have them but the original manual specifies 6 mm flat washers. I tried the O rings from the kit and some of the drain screws seat nicely but other ones don't seem to screw in all the way with the O ring in place. There are flat washers in the kit that I don't have another place for. They aren't 6 mm but they fit nicely around the drain screw.

Which is the correct solution?

I also dropped and lost one of the air screw springs. Is there a part number for that?

[jeremyillingworth]

Here is a picture of the two.

[newday777]

The flat fiber washers are for the float valves.
The small flat aluminum washers are for the brass vacuum port screws
I used the small orings on the bowl drains.
There are 4 larger orings in each kit(you will have 8 extras with 4 kits)for the fuel tee.

[jeremyillingworth]

I have the fiber washers on the float valves and one set of aluminum washers on the vacuum port screws but there is a second set of differently sized aluminum washers that fit on the drain screws and I thought those might be the flat washers that are mentioned in the original manual.

[Oddjob]

Only ever seen orings on the drain screws myself. They flatten out on the flange around the hole in the float bowl, if that flange has stuff embedded onto it then that would prevent the ring from fitting correctly. Have a look at the float bowls and see if there is any corrosion etc around the hole, if so remove it and try again.

Original Honda oring kits or aftermarket? Aftermarket do makes mistakes and supply the wrong size rings to begin with.

[HondaMan]

Only the 750K0 bikes (up to some magical serial number in 1970) use the thin aluminum washers to seal the carb bowl screws. After that they had your type of brass screws, which use a 4.7x1.6 (sometimes 1.7)mm O-ring instead on each screw. It fits into the little groove on the float bowl's screw boss.

[jeremyillingworth]

Thanks for the info. I think it's interesting how small changes were made over the years.

I lost one of my drain plug O rings. Is it just a regular hardware store O ring or do I have to get it from Honda?

[newday777]

Only the 750K0 bikes (up to some magical serial number in 1970) use the thin aluminum washers to seal the carb bowl screws. After that they had you type of brass screws, which use a 4.7x1.6 (sometimes 1.7)mm O-ring instead on each screw. It fits into the little groove on the float bowl's screw boss.

On a set of 7A - II carbs I was cleaning today I found the small diameter head bowl drain screws using the other aluminum crush washer. The head is much smaller that the big head oring drain screws have.(no I didn't measure them)

[newday777]

Thanks for the info. I think it's interesting how small changes were made over the years.

I lost one of my drain plug O rings. Is it just a regular hardware store O ring or do I have to get it from Honda?

As to getting one locally, I found the right size in a box of assorted metric orings I had bought years ago at harbor freight. BTW that box of Storehouse orings I bought in 2009 when I was starting my first restoration project, a 1983 GL1100. But I've found it was pretty useless on Hondas as Honda decided to use different sizes that what are in the kit so I haven't used maybe 10 to 15 of the 400 orings in the box, in 15 years now.....
And I think I might have found one in a package of light brown vinton assortment of fuel injection orings from a auto parts store, helps board, probably from Autozone or Orielly or Pep Boys, again from years ago.

[HondaMan]

Only the 750K0 bikes (up to some magical serial number in 1970) use the thin aluminum washers to seal the carb bowl screws. After that they had you type of brass screws, which use a 4.7x1.6 (sometimes 1.7)mm O-ring instead on each screw. It fits into the little groove on the float bowl's screw boss.

On a set of 7A - II carbs I was cleaning today I found the small diameter head bowl drain screws using the other aluminum crush washer. The head is much smaller that the big head oring drain screws have.(no I didn't measure them)

Yep, those washers came back for a while in the "7A" carbs, both on some later K4-ish and the F0 carbs. Couldn't find one to save your soul then, either.

[HondaMan]

Thanks for the info. I think it's interesting how small changes were made over the years.

I lost one of my drain plug O rings. Is it just a regular hardware store O ring or do I have to get it from Honda?

If you need a few, PM me your address. I keep them around for when someone sends me carbs for rebuild.

[jeremyillingworth]

Thanks for the offer, but I think I found one that will work in the loose O ring bin at Canadian Tire.

It's all back together now and waiting for the weekend to run fresh fuel through it. Right now the carb slides seems stiff and dont' want to return on their own. Is this normal? Will it loosed up once there is some fuel going through it?

[newday777]

Loosen the top ring and find the sweet spot of the top cap for the slide to drop easily with no sticktion.
If that doesn't work, pull the slide out and polish it clean and the barrel to be sure there isn't anything that is hanging it up.

[Oddjob]

Check the slides with the cables removed. If they work ok then it's the cables hanging them up not the slides themselves.

[HondaMan]

I think you already did this, but...if the little "foot" of the slide rods in the slides wasn't removed and made shiny clean and happy, it can stick in the top cap of the carb because it doesn't slip easily forward-back around mid-throttle or so. This ultimately can wear that little rubber seal in the middle of the top cover (which doesn't hurt anything). Then it can drag on the slide rod(s), making the rack try to stick right around half-throttle.

[newday777]

I think you already did this, but...if the little "foot" of the slide rods in the slides wasn't removed and made shiny clean and happy, it can stick in the top cap of the carb because it doesn't slip easily forward-back around mid-throttle or so. This ultimately can wear that little rubber seal in the middle of the top cover (which doesn't hurt anything). Then it can drag on the slide rod(s), making the rack try to stick right around half-throttle.

Mark
This foot at the bottom of the slide rod, held to the inside of the slide with 2 tiny JIS screws? And the foot holds the throttle needle in the slide.
I have found many of the feet and springs corroded and rusty and have cleaned them up with small brass brushes.

[HondaMan]

I think you already did this, but...if the little "foot" of the slide rods in the slides wasn't removed and made shiny clean and happy, it can stick in the top cap of the carb because it doesn't slip easily forward-back around mid-throttle or so. This ultimately can wear that little rubber seal in the middle of the top cover (which doesn't hurt anything). Then it can drag on the slide rod(s), making the rack try to stick right around half-throttle.

Mark
This foot at the bottom of the slide rod, held to the inside of the slide with 2 tiny JIS screws? And the foot holds the throttle needle in the slide.
I have found many of the feet and springs corroded and rusty and have cleaned them up with small brass brushes.

Yep, that's the one! The long rod's foot moves forward-aft about 1.2mm when passing through the middle of the slide's travel. When they get crusty they will stick right when opening upward past 1/2 throttle, and if the foot doesn't slip the upper 1/2 throttle tends to stick. So, Honda put a 200-lb return spring on the rack to make sure it would close anyway if you let the throttle go(!).

[newday777]

I think you already did this, but...if the little "foot" of the slide rods in the slides wasn't removed and made shiny clean and happy, it can stick in the top cap of the carb because it doesn't slip easily forward-back around mid-throttle or so. This ultimately can wear that little rubber seal in the middle of the top cover (which doesn't hurt anything). Then it can drag on the slide rod(s), making the rack try to stick right around half-throttle.

Mark
This foot at the bottom of the slide rod, held to the inside of the slide with 2 tiny JIS screws? And the foot holds the throttle needle in the slide.
I have found many of the feet and springs corroded and rusty and have cleaned them up with small brass brushes.

Yep, that's the one! The long rod's foot moves forward-aft about 1.2mm when passing through the middle of the slide's travel. When they get crusty they will stick right when opening upward past 1/2 throttle, and if the foot doesn't slip the upper 1/2 throttle tends to stick. So, Honda put a 200-lb return spring on the rack to make sure it would close anyway if you let the throttle go(!).

3 of the 4 rods on the last set I just cleaned some of the chrome is worn off the rods. The feet seem to be cleaner than some of the last 6 sets I've cleaned in the past month. No crust growing in the feet.

[HondaMan]

I think you already did this, but...if the little "foot" of the slide rods in the slides wasn't removed and made shiny clean and happy, it can stick in the top cap of the carb because it doesn't slip easily forward-back around mid-throttle or so. This ultimately can wear that little rubber seal in the middle of the top cover (which doesn't hurt anything). Then it can drag on the slide rod(s), making the rack try to stick right around half-throttle.

Mark
This foot at the bottom of the slide rod, held to the inside of the slide with 2 tiny JIS screws? And the foot holds the throttle needle in the slide.
I have found many of the feet and springs corroded and rusty and have cleaned them up with small brass brushes.

Yep, that's the one! The long rod's foot moves forward-aft about 1.2mm when passing through the middle of the slide's travel. When they get crusty they will stick right when opening upward past 1/2 throttle, and if the foot doesn't slip the upper 1/2 throttle tends to stick. So, Honda put a 200-lb return spring on the rack to make sure it would close anyway if you let the throttle go(!).

3 of the 4 rods on the last set I just cleaned some of the chrome is worn off the rods. The feet seem to be cleaner than some of the last 6 sets I've cleaned in the past month. No crust growing in the feet.

I've seen many of the rods lacking chrome, usually between 1/3 and 1/2 throttle or so. It doesn't seem to hurt anything.

[newday777]

I think you already did this, but...if the little "foot" of the slide rods in the slides wasn't removed and made shiny clean and happy, it can stick in the top cap of the carb because it doesn't slip easily forward-back around mid-throttle or so. This ultimately can wear that little rubber seal in the middle of the top cover (which doesn't hurt anything). Then it can drag on the slide rod(s), making the rack try to stick right around half-throttle.

Mark
This foot at the bottom of the slide rod, held to the inside of the slide with 2 tiny JIS screws? And the foot holds the throttle needle in the slide.
I have found many of the feet and springs corroded and rusty and have cleaned them up with small brass brushes.

Yep, that's the one! The long rod's foot moves forward-aft about 1.2mm when passing through the middle of the slide's travel. When they get crusty they will stick right when opening upward past 1/2 throttle, and if the foot doesn't slip the upper 1/2 throttle tends to stick. So, Honda put a 200-lb return spring on the rack to make sure it would close anyway if you let the throttle go(!).

3 of the 4 rods on the last set I just cleaned some of the chrome is worn off the rods. The feet seem to be cleaner than some of the last 6 sets I've cleaned in the past month. No crust growing in the feet.

I've seen many of the rods lacking chrome, usually between 1/3 and 1/2 throttle or so. It doesn't seem to hurt anything.

Good to know. Thanks again