Carb Diaphragm tear late model 750 Nighthawk 1991-2003

[Bob H]

2 of the 4 carb diaphragms on my 1993 Nighthawk 750 had damage exactly like this video (different model bike similar rubber vacuum)

One had 2 small slits, other had a slit and pinhole similar to these images from his video

Pinhole in this next image

In the video above he is repairing with aftermarket rubber diaphragm, and I considered that.
Couple hours research showed mixed results with aftermarket diaphragms. Someone on Ebay is selling complete vacuum assembly (aftermarket) with customers complaining they don't fit. Some people say that can be "patched" but others say they will fail again if not replaced.

In my research, I found a bunch of AdvRider threads where older BMW carbs would develop large tears in rubber diaphragm resulting in the slide staying down (closed) even when throttle opened.

These are expensive, I was on the fence if my damages were enough to affect performance.
I looked at the Factory manual (newer edition, not the older blue-cover version) and was shocked to see this:


I had watched many videos specific to overhaul the late model 750 carbs, and only one of them mentioned to check the rubber diaphragm (credit to motorcyclemd.com for the $40 rebuild video, you have to buy his course). Cody says in the video to check them as soon as you do the tear down because if you have to order parts that will be a delay - and that they were expensive.

There is SO MUCH LABOR in doing this project, that I did not want to cut corners on materials or have the bike not run as good as it can. So I bit the bullet and bought the OEM ones from Partzilla and was at least thankful they are still available. $574.95 including tax, $133.40 for each one. If 2 are bad, I am doing them all.


Honestly I don't know that I would have noticed the slits if it wasn't for Cody's video, where he pulls the rubber and closely inspects (similar to the one I link to for a dirt bike). I had disassembled the carbs days earlier and they looked just like when I put washers under those needles 10 years ago. Yesterday I saw some warbles in the rubber but they looked intact - until you hold it up to the light and pull on the rubber.

[Bob H]

Not all Constant Velocity carbs have a rubber diaphragm. I had an older CB 750 with this style 79-80 model.

The following video is a great explanation of CV carbs, showing the "all metal" vacuum slide.
This helped me to visualize how small holes in the rubber diaphragm would impair the slide movement up as you give it throttle.
On the late model 750 that rubber membrane is all you have, there is no piston shaped slide.

[Pete in PA]

That sucks.  Glad mine were good. They seldom go bad.  Wonder what happened.

[Bob H]

... They seldom go bad.

Yeah I had not even heard of it, didn't know it was a "thing".
Over the last day, reading of other people's issues and solutions it doesn't seem that rare.
It is common enough for an industry to spring up offering a cheaper fix for the diaphragms (on many different model carburetors) and now there are knock-off's of the company that started offering the repair kits.

Mine is a 1993 with 38,000 miles which is pretty low for the year. I remember on the old forum Loki had same model with over 100,000 miles.

I am going to try to find ethanol free gas, maybe at the airport.  I could smell the gas on the diaphragm and a slight fuel residue, it does get sucked up into the chamber with the vacuum thing. Our fuel is crazy here, which makes it over $5 a gallon because it is formulated only for the land of Fruits and Nuts

[Pete in PA]

For $46 for 4 from China I'd have tried them.

They seem VERY proficient at duplication.

Couldn't find any reviews of them good or bad.

[Bob H]

For $46 for 4 from China I'd have tried them.

Yeah, $574.95 just for those parts was painful. I plan on keeping the bike, might be different if fixing it for sale.

I don't have a lot of money, and am very frugal in some areas of life.
But I will spend whatever it takes for some things, even a tool I may only use once.

Some people collect stupid Sh*t and blow money, for me it is a No Mar tire changer and Snap On torque wrench

[Pete in PA]

For $46 for 4 from China I'd have tried them.

Yeah, $574.95 just for those parts was painful. I plan on keeping the bike, might be different if fixing it for sale.

I don't have a lot of money, and am very frugal in some areas of life.
But I will spend whatever it takes for some things, even a tool I may only use once.

Some people collect stupid Sh*t and blow money, for me it is a No Mar tire changer and Snap On torque wrench

I hear you on the tire changer.  Been changing my own tires since somewhere in the early 90s when shops started the "you have to buy the tire here" BS.  Thats paid for it self MANY times over.

Just changed the rear out. Went so easy

[Gene]

To weigh in on the diagram rip/hole problem, I would think that as long as the diagram is not too damaged, why not some gasket kind of RTV to repair it. Has anyone tried it?

[Pete in PA]

To weigh in on the diagram rip/hole problem, I would think that as long as the diagram is not too damaged, why not some gasket kind of RTV to repair it. Has anyone tried it?

Im going to say it won't work.  The slide is fluctuating rapidly at every intake stroke.  Not untill near redline will it stay open.

[Bob H]

why not some gasket kind of RTV to repair it. Has anyone tried it?

There are reports of people trying it with varying success. I agree with Pete and visualize failure with the constant flexing of the rubber.
I did not even know this was a thing. Never came across it until I tugged on my diaphragms (that looked fine at a casual glance).
I saw the small splits (1/4" some 3/8") and pinholes.

I mentioned the motorcyclemd.com $40 rebuild video course. He is employed as a Honda mechanic, and says in the video that he has seen all kinds of attempted repair for those diaphragms such as Sealants, superglue, tape - he says "none of them work" and to just buy the part.

Based on my recent experience that 2 out of 4 of mine failed, that tells me that there has been a degradation of the rubber.
Maybe the ethanol gas, who knows. But if I was to patch the small splits and pinholes, ride the bike another 38,000 miles I would likely experience continued failure of the diaphragm.

I just checked what those parts cost at David Spears site, and it is a special order at about double the price I just paid at Partzilla.
The main thing is to have the bike running well, and TRUSTWORTHY as I seek out very remote canyon roads and get into places without cell service.

[mollusc]

The rubber diaphragms had failed on my Yamaha, and I tried to replace them.  Carbs are essentially the same as on our Nighthawks.  I was able to buy the rubber piece but I could never get the slides disassembled to get them on.  Ended up just replacing the slides, rubber and all.  It's a pig of a bike so I wasn't concerned about keeping original parts all in the same place.

[Bob H]

I was able to buy the rubber piece but I could never get the slides disassembled to get them on.

Must have been frustrating. I was in blissful ignorance that the rubber diaphragms can fail until this week. Now I see all these videos of people doing the replacement, using snips-wire cutters to cut the plastic ring that accompanies the rubber diaphragm. Some are pretty brutal, snipping away chunks of plastic until they can grab the ring with pliers and break it at the weakened area they had been snipping at.
This one is a Honda Shadow

Once the plastic ring & rubber diaphragm are removed, there is about 3/16 wide recessed area, but the factory diaphragm's are very thin.
The aftermarket ones appear to solve that by creating a big thick region of rubber to fill in that entire recessed area.

It appears the 84-86 700S Nighthawk uses the same carbs that are on the late model 91-2003 750.
I ordered the most reputable gasket set I could find (Randakks) that was not "as" expensive as OEM gaskets.
The package was labeled "CB700SC84-85". Fearing the wrong part, I looked at Randakk's site and those gaskets are listed for both models.

I did a search on Ebay for used 700s carbs, found several sellers and they all appear identical to the late model 750 carbs that I am rebuilding. Both are Keihin.