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Carburettor slide diaphragm

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Hello guys,

 

The diaphragms on the slides are getting really old - even for the "newest" of the carburetted models, the 1997 VFR 750. Over time those diaphragms must have gotten less flexible.

 

What is the effect of this on the mixture? I have a suspicion of richer mixture on lower  RPM's.

 

What do you think? Does any have specific knowledge?

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The diaphragms lift the slide and the needle up out of the emulsion tube, so less flexbility = less lift; I doubt that would cause low end richness. What does cause low-end richness is any wear on the needle or the emulsion tube that it fits into. My RF900 suffered wear in the emulsion tube and that caused poor fuel economy and occasionally fouled the spark plugs. You can viually inspect the emulsion tubes by removing the diaphragms and slides and looking straigth down to the emulsion tube in the floor of the throat. It should look perfectly circular, but on my RF the holes wore oval due to movement of the slides allowing the neede to vibrate against the tube. 

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I was in talks to get new diaphragms made, but communication halted last year. They weren't as straightforward to replace as some other models.

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On 12/16/2019 at 9:07 PM, Terry said:

The diaphragms lift the slide and the needle up out of the emulsion tube, so less flexbility = less lift; I doubt that would cause low end richness. 

Right, less lift for a certain vacuum. That higher vacuum will then work a little harder on the slow jet circuit introducing more fuel than wanted.

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17 hours ago, YoshiHNS said:

I was in talks to get new diaphragms made, but communication halted last year. They weren't as straightforward to replace as some other models.

Unfortunately I was not aware back then - too bad that did not take off

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NRP helped me out with the fuel tap on my '86 VF500F2. 

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On 12/18/2019 at 8:00 AM, Thumbs said:

NRP have diaphragms for nearly all the Honda models 

 

http://www.nrp-carbs.co.uk/honda_carburettor_diaphragms.htm 
 

The prices are +20% tax

 

CBR600 F2/3 and VFR750 94-97 are listed as "call". I might have to call eventually.

Bookmarked. Thanks for the link.

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The CBF500 (2004-06) also use these slides - don't know how many of those are found in junkyards... 

 

 

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Other juicy comments to add about possible low-end rich mixture with old stiff diaphragms - or is it rubbish?

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Just speculation, but I'm with Terry... If that diaphragm is stiffer than designed, it will be less reactive to vacuum, meaning less lift, meaning a leaner mixture if anything.

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8 minutes ago, Egg on Leggs said:

Interesting - never tried that 🙂 Now about the diaphragms... wonder if the cure also works on silicone? I does not mention silicone (except for use to soften rubber)

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As per above advice re wear on needles & tubes. That is the most likely issue. Bikes much older than VFR’s are still running on original diaphragms. The worst issue is leaving cabs unused or with stale fuel in them. There used to be drain down kits for carbs available, where you removed the drain screws & used the kit to lead hoses to a single outboard drain point. 

Either way good lick with finding a cure for your issue.

 

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On 1/6/2020 at 2:56 PM, Marvelicious said:

Just speculation, but I'm with Terry... If that diaphragm is stiffer than designed, it will be less reactive to vacuum, meaning less lift, meaning a leaner mixture if anything.

 

On 1/7/2020 at 8:33 AM, Mohawk said:

As per above advice re wear on needles & tubes. That is the most likely issue. Bikes much older than VFR’s are still running on original diaphragms. The worst issue is leaving cabs unused or with stale fuel in them. There used to be drain down kits for carbs available, where you removed the drain screws & used the kit to lead hoses to a single outboard drain point. 

Either way good lick with finding a cure for your issue.

 

 

I took the tank and air filter off the bike and added fuel from a small gravity tank. Measured the pressure (vacuum) upstream of the butterfly. The pressure differential is far to great to be influenced by a slightly stiff diaphragm... so who the hell started this thread anyway... just asking 🙂

 

I really don't believe needle is the issue, they look fine. Same for jets.

 

I checked the springs (slide piston return springs) - one spring was a tad shorter and feels different. I don't have a Newton gauge - so I guess I will replace all 4 springs this winter.

 

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try a top speed run.  if the bike accelerates strong and reaches at least 145mph, diaphragms are a-ok.

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7 hours ago, squirrelbrains said:

try a top speed run.  if the bike accelerates strong and reaches at least 145mph, diaphragms are a-ok.

Squirrelbrains, welcome to VRFD 🙂

 

My particular issue focus on possible rich mixture at lower RPM where vacuum is just beginning to move the slide piston. After measuring pressure (as a result of polite hits from other members) I concluded my theory was rubbish!

 

Anyway, nice bikes you have in your collection... 🙂 

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Thanks.  actually i'm a returning member but needed to change my handle from squirrelman when i began using a new computer and couldn't get into the website.

 

rich mix at low speeds ? maybe someone switched your pilot jets to larger ones ? try closing an idle mix screw (i know it's not easy) while engine is running and see how far in they are in when that cylinder quits firing.  if a cyl continues running with screw all the way in, let us know, cuz that's a different problem altogether.

 

Picture1-12-2020_108.thumb.jpg.863c503cf46e36312179960db1b64beb.jpg

 

Picture1-12-2020_058.thumb.jpg.b199743ca1beed0337572b2bf31023b5.jpg

 

 

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10 hours ago, squirrelbrains said:

Thanks.  actually i'm a returning member but needed to change my handle from squirrelman when i began using a new computer and couldn't get into the website.

 

rich mix at low speeds ? maybe someone switched your pilot jets to larger ones ? try closing an idle mix screw (i know it's not easy) while engine is running and see how far in they are in when that cylinder quits firing.  if a cyl continues running with screw all the way in, let us know, cuz that's a different problem altogether.

 

Ok, I retract the new member wlecome then 🙂 You know, when I saw your handle I though "odd people with all the squirrel-names..." (no offence) as I remember you (well, the other you) from the loose float seat. Why did you give up "the other you"? Well really none of my business anyway...

 

The jets are checked and cleaned - they are 130/40. Of course I can't say if someone used abrasive wire at some point in time but I found no traces pointing to that.

Mixture screws does lower idle RPM's going lean or rich. Screws (needle part) has a good profile.

 

I did find one slide spring that feels different and is also a bit shorter. I'm trying to source some new (used) springs.

 

I never spent this much time on any carb in my life (the sneaky loose float seat) - so I want to make sure I got everything sorted and don't stop right before finish line... 🙂

 

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