Diaphragm/Vacuum Piston on VTR Carbs
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Squid
Squid
Joined: Mar 2006
Posts: 83
Diaphragm/Vacuum Piston on VTR Carbs
Can someone take a look in the Service Manual at the images of the Diaphragm/Vacuum Piston on pages 5-10 and 5-14 compared to the drawing on page 5-12. When reinstalling this part, do you want the bell to be down over the piston body or turned out towards the cover?
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World Champion
Joined: Dec 2004
Posts: 105,287
From: Fulton, MO
You want this as on 5-10 and 5-14. I have found it much easier to install the slide/diaphragm assembly into the cover first and then installing the two parts together onto the carb. I minimizes the fumble-fu*k that occurs trying to deal with the long spring. Careful, don't pinch the diaphragm. Also, use one hand to install and hold together the ***'y and the other to guide the needle into the main jet.
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Superstock
Superstock
Joined: Jun 2005
Posts: 489
From: San Antonio, TX
You want the diaphram down. Then drop the piston in slowly just until the diaphram contacts the carb mating surface. Use a finger underneath the piston to hold it in place while you seat the diaphram. You can check the fit first by placing the carb top on without the spring in place, to get a feel for how it should look when it is assembled properly.
Putting it all together along with the loooooong stock spring is a bit of a pain in the *** (here, shorter DJ springs are a god send). Using a pen, pencil, or long screwdriver shank as a guide you can compress the long spring into the cap. This will help you get the cap back on without disturbing the position of the diaphram.
Once the cap is in place , you can then remove your finger from the underside of the piston and commence to installing the screws. This is how I do it. Others here may have different methods that could work as well.
Putting it all together along with the loooooong stock spring is a bit of a pain in the *** (here, shorter DJ springs are a god send). Using a pen, pencil, or long screwdriver shank as a guide you can compress the long spring into the cap. This will help you get the cap back on without disturbing the position of the diaphram.
Once the cap is in place , you can then remove your finger from the underside of the piston and commence to installing the screws. This is how I do it. Others here may have different methods that could work as well.
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SuperSport
Joined: Jun 2005
Posts: 524
What works for me is to keep the slide at full throttle with a large bottle cap. Insert it into the throttle bore so that supports the slide from underneath.
This keeps both hands free for the spring/cap installation where I follow the same procedure as Beerhunter.
The bottle cap I found that works really well is what comes on the Honda four quart oil containers but anything that just fits in the bore will work.
Don't forget to remove it when you're done.
Kai Ju
This keeps both hands free for the spring/cap installation where I follow the same procedure as Beerhunter.
The bottle cap I found that works really well is what comes on the Honda four quart oil containers but anything that just fits in the bore will work.
Don't forget to remove it when you're done.
Kai Ju
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Squid
Squid
Joined: Jul 2007
Posts: 48
i was thinking about using some glue to hold the diaphragm in place while i put the cap/spring on... i cant get the rear carb to create vacuum and there are no tears in the diaphragm. think i need to seat it first, then install, but the thing needs to like be held in place while i put it together or it just comes out of the lip on the carb. anyone done that?
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MotoGP
Joined: Dec 2004
Posts: 2,553
From: South Texas
Kai's bottle cap sounds like a great tip. Hope I remember it next time. Does it have to be Honda brand oil or can I use Mobil 1. What about synthetic? Too early for that? 
Sorry, couldn't resist.
I put the diaphragm into an umbrella shape (for a lack of a better description) and slowly lower it into the carb until the edges stick into the valley. I use honda's silicone spray to lube the diaphragm which has a slight tacky properties and that helps to hold it in place.
Then add spring and cap together while SLOWLY but deliberately sandwiching it all together. Usually get it right the first or second time.
Sorry, couldn't resist.I put the diaphragm into an umbrella shape (for a lack of a better description) and slowly lower it into the carb until the edges stick into the valley. I use honda's silicone spray to lube the diaphragm which has a slight tacky properties and that helps to hold it in place.
Then add spring and cap together while SLOWLY but deliberately sandwiching it all together. Usually get it right the first or second time.
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Squid
Squid
Joined: Jul 2007
Posts: 48
it seemed to now work.. so i took some gasket maker meant for gasoline, used a thin layer around the edge of the diaphragm. seated it one section at a time, taped it down, let it dry, removed tape, inserted cover/spring. voila. she runs like a charm.
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